Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college Sensory Physiology: 1. Distinguish between general senses and special senses. The human body has two major types of senses: special senses and general senses. Special senses have specialized sense organs and include vision (eyes), (), balance (ears), taste (tongue), and smell (nasal passages). General senses are all associated with touch and lack special sense organs. 2. Classify the general senses. The general senses are pain, temperature, touch, pressure, vibration, and proprioception. Receptors for those sensations are distributed throughout the body. 3. What are receptors and their functions? Receptors are proteins or glycoproteins that bind signalling molecules known as first messengers, or ligands. They can initiate a signaling cascade, or chemical response, that induces cell growth, division, and death or opens membrane channels. 4. What are the different types of receptors in the body? a) Nociceptors (pain receptors) b) Thermoreceptors (temperature sensors) c) Mechanoreceptors (Pressure sensors) d) Chemoreceptors (Chemicals sensors) e) Photoreceptors (Light sensors) 5. Classify the special senses. The five special senses are: 1. Olfaction (smell) 2. Gustation (taste) 3. Vision (sights) 4. Equilibrium (balance) 5. Hearing Receptors for these senses are located in specialized areas called sense organs. 6. What is transducers? A transducer is an electrical device that is used to convert one form of energy into another form. In general, these devices deal with different types of energies such as mechanical, electrical energy, light energy, chemical energy, thermal energy, acoustic energy, electromagnetic energy, and so on.

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college 7. What is biotransducers? A biotransducer is the recognition-transduction component of a biosensor system. It consists of two intimately coupled parts; a bio-recognition layer and a physicochemical transducer, which acting together converts a biochemical signal to an electronic or optical signal. The bio-recognition layer typically contains an enzyme or another binding protein such as antibody. The physicochemical transducer may be electrochemical, optical, electronic, gravimetric, pyroelectric or piezoelectric. 8. Why receptors are called biological transducers? Receptors are called transducers because they 'convert' the energy contained in the stimulus into another form of energy, specifically into some sort of membrane potential. Receptors are termed selective because each type of receptor is highly specific (selective) with respect to the type of stimulus it responds to. 9. What is Muller's law? The law of specific nerve energies, first proposed by Johannes Peter Müller. The law of specific nerve energies states that an individual's mind cannot access objects in the natural environment except through the nerves. 10. What are Weber's law and Fechner's law? The Weber–Fechner law refers to two related hypotheses in the field of psychophysics, known as Weber's law and Fechner's law. Both laws relate to human perception, more specifically the relation between the actual change in a physical stimulus and the perceived change. This includes stimuli to all senses: vision, hearing, taste, touch, and smell. Weber's law states that the smallest change in the intensity of a stimulus capable of being perceived is proportional to the intensity of the original stimulus. Fechner's law states that the subjective sensation is proportional to the logarithm of the stimulus intensity. Since Weber's law fails at low intensity, so does Fechner's law. 11. Write down the mechanism of transduction of stimuli from sensory receptors. Transduction is the process that converts a sensory signal to an electrical signal to be processed in a specialized area in the brain.

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college The most fundamental function of a sensory system is the translation of a sensory signal to an electrical signal in the nervous system. This takes place at the sensory receptor. The change in electrical potential that is produced is called the receptor potential. How is sensory input, such as pressure on the skin, changed to a receptor potential? As an example, a type of receptor called a mechanoreceptor possesses specialized membranes that respond to pressure. Disturbance of these dendrites by compressing them or bending them opens gated ion channels in the plasma membrane of the sensory neuron, changing its electrical potential. In the nervous system, a positive change of a neuron’s electrical potential (also called the membrane potential), depolarizes the neuron. Receptor potentials are graded potentials: the magnitude of these graded (receptor) potentials varies with the strength of the stimulus. If the magnitude of depolarization is sufficient (that is, if membrane potential reaches a threshold), the neuron will fire an action potential. In most cases, the correct stimulus impinging on a sensory receptor will drive membrane potential in a positive direction, although for some receptors, such as those in the visual system, this is not always the case. Sensory receptors for the various senses work differently from each other. They are specialized according to the type of stimulus they sense; thus, they have receptor specificity. For example, touch receptors, light receptors, and sound receptors are each activated by different stimuli. Touch receptors are not sensitive to light or sound; they are sensitive only to touch or pressure. However, stimuli may be combined at higher levels in the brain, as happens with olfaction, contributing to our sense of taste.

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college

Mechanoreceptor activation: (a) Mechanosensitive ion channels are gated ion channels that respond to mechanical deformation of the plasma membrane. A mechanosensitive channel is connected to the plasma membrane and the cytoskeleton by hair-like tethers. When pressure causes the extracellular matrix to move, the channel opens, allowing ions to enter or exit the cell. (b) in the human is connected to mechanosensitive ion channels. When a sound causes the stereocilia to move, mechanosensitive ion channels transduce the signal to the .

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college Olfaction and Gustation: 12. Write down the structure of olfactory organ. Olfactory system, the bodily structures that serve the sense of smell. The system consists of the nose and the nasal cavities, which in their upper parts support the olfactory mucous membrane for the perception of smell and in their lower parts act as respiratory passages. The bony framework of the nose is part of the skull, but the outer nose is supported only by bone above; lower down, its shape is kept by cartilaginous plates. The expanded lower part of the side of the nose, the ala, is formed only of skin, both externally and internally, with fibrofatty tissue between the layers. The nasal cavities are separated by a septum covered in its lower two-thirds by thick, highly vascular mucous membrane composed of columnar ciliated epithelium with masses of acinous glands embedded in it, while in its upper part it is covered by the less vascular but more specialized olfactory membrane. Near the front of the lower part of the septum a slight opening into a short blind tube, which runs upward and backward, may sometimes be found; this is the vestigial remnant of Jacobson’s organ. The supporting framework of the septum is made up of ethmoid above, vomer below, and the septal cartilage in front. The outer wall of each nasal cavity is divided into three meatuses by the overhanging turbinated bones. Above the superior turbinated bone is a space between it and the roof known as the recessus sphenoethmoidalis, into the back of which the sphenoidal air sinus opens. Between the superior and middle turbinated bones is the superior meatus, which contains the openings of the posterior ethmoidal air cells, while between the middle and inferior turbinated bones is the middle meatus, which is the largest of the three and contains a rounded elevation, the bulla ethmoidalis. Above and behind this is often an opening for the middle ethmoidal cells; below and in front runs a deep sickle-shaped gutter, the hiatus semilunaris, which communicates above with the frontal air sinus and below with the opening into the antrum of Highmore or maxillary antrum. The inferior meatus is below the inferior turbinated bone, and, when that is lifted, the valvular opening of the nasal duct is seen. The roof of the nose is narrow, and it is here that the olfactory nerves pass in through the cribriform plate. The floor is wider so that a coronal section through each nasal cavity has roughly the appearance of a right- angled triangle.

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college

Figure: Human nasal cavity

13. Describe the neural pathway of olfactory sensation. A. Olfactory Receptors: The olfactory receptors are embedded in mucous membrane of the upper part of the nasal cavity above the superior concha. The fine central processes of bipolar receptor nerve cells form the olfactory nerve fibers. Bundles of these nerve fibers pass through the openings of the cribriform plate of the ethmoid bone to enter the olfactory bulb B. Olfactory Bulb: This ovoid structure have several types of nerve cells; the mitral cells, tufted cells and granular cells. The incoming olfactory nerve fibers synapse with the dendrites of the mitral cells and form synaptic glomeruli. The olfactory bulb, in addition, receives axons from the contralateral olfactory bulb through the olfactory tract. C. Olfactory Tract: This narrow band of white matter runs from the posterior end of the olfactory bulb under the inferior surface of the frontal lobe of the brain.  It consists of the central axons of the mitral and tufted cells of the bulb and some fibers from the opposite olfactory bulb.

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college  As the olfactory tract reaches the anterior perforated substance, it divides into medial and lateral olfactory striae.  The lateral stria carries the axons to the olfactory area of the cerebral cortex.  The medial olfactory stria carries the fibers that cross the median plane in the anterior commissure to pass to the olfactory bulb of the opposite side. D. Anterior commissure: This is a small commissure that connects the two halves of the olfactory system. E. Olfactory Cortex: Includes the following regions;  Primary olfactory cortex: uncus, limen insulae (apical region of the insula) and corticomedial part of the amygdaloid body.  Secondary olfactory cortex is the entorhinal area (anterior part of the parahipocampal gyrus that lies behind the uncus  The olfactory cortex collectively called piriform lobe  Note the olfactory cortex is the one area of cortex that receives direct sensory input without an inter posed thalamic connection.

Figure: Neural pathway of olfactory sensation.

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college 14. Why does a smell seem to fade after you get used to it? Smell is the perception of odorants by our brains. Odorants are gaseous chemicals which stimulate sensory cells in our nose called olfactory sensory neurons. Just a few odorant molecules are enough to stimulate a sensory neuron which starts to rapidly fire nerve impulses to the brain. The brain processes the information and identifies the smell. If there is a constant odorant in the room our brain starts to perceive it as decreasing in intensity over time, ie the smell seems to fade. This is due to a phenomenon called sensory adaptation, which is not yet fully understood. During sensory adaptation our brain adapts, recognises the constant smell is not dangerous and stops identifying it so it is not overloaded with redundant information. Our olfactory sensory neurons also adapt to the repetitive odorant stimuli by reducing their rate of firing. Therefore we perceive the smell to be fading, allowing us to adapt to our environment and perceive new smells. 15. What is Olfactometer ? An olfactometer is an instrument used to detect and measure odor dilution. Olfactometers are used in conjunction with human subjects in laboratory settings, most often in market research, to quantify and qualify human olfaction. Olfactometers are used to gauge the odor detection threshold of substances. 16. Write down the structure of gustatory organ. Taste is associated mainly with the tongue, although there are taste (gustatory) receptors on the palate and epiglottis as well. The surface of the tongue, along with the rest of the oral cavity, is lined by a stratified squamous epithelium. In the surface of the tongue are raised bumps, called papilla, that contain the taste buds. There are three types of papilla, based on their appearance: vallate, foliate, and fungiform. The tongue is covered with papillae (a), which contain taste buds (b and c). Within the taste buds are specialized taste cells (d) that respond to chemical stimuli dissolved in the saliva and, in turn, activate sensory nerve fibers in the facial and glossopharyngeal nerves. The number of taste buds within papillae varies, with each bud containing several specialized taste cells (gustatory receptor cells) for the transduction of taste stimuli. These receptor cells release neurotransmitters when certain chemicals in ingested substances (such as food) are carried to their surface in saliva. Neurotransmitter from the

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college gustatory cells can activate the sensory neurons in the facial and glossopharyngeal cranial nerves.

Figure: Tongue and taste buds

17. Describe the neural pathway of gustatory sensation. 1. Receptors: Taste buds on tongue, lips, palatal arch and soft palate Each “bud” contains several cell types in microvilli (taste hairs) that project through taste pore. . Gustatory receptor cells communicate with cranial nerve axon endings to transmit sensation to brain. . Cranial Nerves of taste . Anterior 2/3 tongue: chorda tympani→ Facial nerve . Posterior 1/3 tongue: Glossopharyngeal nerve . • Most posterior part of the tongue: Vagus nerve 2. The first order neuron in the pathway is the geniculate ganglion of the facial nerve and inferior ganglia of the glossopharyngeal and vagus nerves 3. The second order neuron is the nucleus solitarius and its upper part enlarged and called the gustatory nucleus. The axons of the cells of the nucleus cross to the opposite side and ascend to end in the posteromedial ventral nucleus of the thalamus

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college 4. The third order neuron is the posteromedial ventral nucleus of the thalamus. The axons of the cells pass through the sensory radiation to the gustatory area in the superior wall of the posterior ramus of the lateral sulcus.

Figure: neural pathway of gustatory sensation.

18. What is aftertaste ? Aftertaste is the taste intensity of a food or beverage that is perceived immediately after that food or beverage is removed from the mouth.[1] The aftertastes of different foods and beverages can vary by intensity and over time, but the unifying feature of aftertaste is that it is perceived after a food or beverage is either swallowed or spat out and without the input of the other sensory systems. The result of the chemicals in food and drink continuing to interact with the specific taste receptor cells within our taste buds.

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college Audition & Equilibrium: 19. Write down the structure of external ear. The most-striking differences between the human ear and the ears of other mammals are in the structure of the outermost part, the . In humans the auricle is an almost rudimentary, usually immobile shell that lies close to the side of the head. It consists of a thin plate of yellow elastic cartilage covered by closely adherent skin. The cartilage is molded into clearly defined hollows, ridges, and furrows that form an irregular shallow funnel. The deepest depression, which leads directly to the external auditory canal, or acoustic meatus, is called the concha. It is partly covered by two small projections, the tonguelike in front and the behind. Above the tragus a prominent ridge, the , arises from the floor of the concha and continues as the incurved rim of the upper portion of the auricle. An inner, concentric ridge, the , surrounds the concha and is separated from the helix by a furrow, the scapha, also called the fossa of the helix. In some ears a little prominence known as Darwin’s tubercle is seen along the upper, posterior portion of the helix; it is the vestige of the folded-over point of the ear of a remote human ancestor. The lobule, the fleshy lower part of the auricle, is the only area of the that contains no cartilage. The auricle also has several small rudimentary muscles, which fasten it to the skull and scalp. In most individuals these muscles do not function, although some persons can voluntarily activate them to produce limited movements. The external auditory canal is a slightly curved tube that extends inward from the floor of the concha and ends blindly at the tympanic membrane. In its outer third, the wall of the canal consists of cartilage; in its inner two-thirds, of bone. The entire length of the passage (24 mm, or almost 1 inch) is lined with skin, which also covers the outer surface of the tympanic membrane. Fine hairs directed outward and modified sweat glands that produce earwax, or cerumen, line the canal and discourage insects from entering it.

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college

Figure: Structure of ear

Tympanic membrane: The thin semitransparent tympanic membrane, or , which forms the boundary between the outer ear and the , is stretched obliquely across the end of the external canal. Its diameter is about 8– 10 mm (about 0.3–0.4 inch), its shape that of a flattened cone with its apex directed inward. Thus, its outer surface is slightly concave. The edge of the membrane is thickened and attached to a groove in an incomplete ring of bone, the tympanic annulus, which almost encircles it and holds it in place. The uppermost small area of the membrane where the ring is open, the pars flaccida, is slack, but the far greater portion, the pars tensa, is tightly stretched. The appearance and mobility of the tympanic membrane are important for the diagnosis of middle-ear disease, which is especially common in young children. When viewed with the otoscope, the healthy membrane is translucent and pearl-gray in colour, sometimes with a pinkish or yellowish tinge. 20. Write down the structure of middle ear. The cavity of the middle ear is a narrow air-filled space. A slight constriction divides it into an upper and a lower chamber, the tympanum () proper below and the epitympanum above. These chambers are also referred to as the atrium and the attic, respectively. The middle-ear space roughly resembles a rectangular room with four walls, a floor, and a ceiling. The outer

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college (lateral) wall of the middle-ear space is formed by the tympanic membrane. The ceiling (superior wall) is a thin plate of bone that separates the middle-ear cavity from the cranial cavity and brain above. The floor (inferior wall) is also a thin bony plate, in this case separating the middle-ear cavity from the jugular vein and the carotid artery below. The back (posterior) wall partly separates the middle-ear cavity from another cavity, the mastoid antrum, but an opening in this wall leads to the antrum and to the small air cells of the mastoid process, which is the roughened, slightly bulging portion of the temporal bone just behind the external auditory canal and the auricle. In the front (anterior) wall is the opening of the (or auditory tube), which connects the middle ear with the nasopharynx. The inner (medial) wall, which separates the middle ear from the , or labyrinth, is a part of the bony otic capsule of the inner ear. It has two small openings, or fenestrae, one above the other. The upper one is the , which is closed by the footplate of the . The lower one is the , which is covered by a thin membrane. Auditory : Crossing the middle-ear cavity is the short ossicular chain formed by three tiny bones that link the tympanic membrane with the oval window and inner ear. From the outside inward they are the (hammer), the (anvil), and the stapes (stirrup). The malleus more closely resembles a club than a hammer, and the incus looks more like a premolar tooth with uneven roots than an anvil. These bones are suspended by ligaments, which leave the chain free to vibrate in transmitting sound from the tympanic membrane to the inner ear. The malleus consists of a handle and a head. The handle is firmly attached to the tympanic membrane from the centre (umbo) to the upper margin. The head of the malleus and the body of the incus are joined tightly and are suspended in the epitympanum just above the upper rim of the tympanic annulus, where three small ligaments anchor the head of the malleus to the walls and roof of the epitympanum. Another minute ligament fixes the short process (crus) of the incus in a shallow depression, called the fossa incudis, in the rear wall of the cavity. The long process of the incus is bent near its end and bears a small bony knob that forms a loose ligament-enclosed joint with the head of the stapes. The stapes is the smallest bone in the body. It is about 3 mm (0.1 inch) long and weighs scarcely 3 mg (0.0001 ounce). It lies almost horizontally, at right angles to the process of the incus. Its base, or footplate, fits nicely in the

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college oval window and is surrounded by the elastic annular ligament, although it remains free to vibrate in transmitting sound to the labyrinth.

Figure: The auditory ossicles of the middle ear and the structures surrounding them.

Muscles: Two minuscule muscles are located in the middle ear. The longer muscle, called the tensor tympani, emerges from a bony canal just above the opening of the eustachian tube and runs backward and then outward as it changes direction in passing over a pulleylike projection of bone. The tendon of this muscle is attached to the upper part of the handle of the malleus. When contracted, the tensor tympani tends to pull the malleus inward and thus maintains or increases the tension of the tympanic membrane. The shorter, stouter muscle, called the stapedius, arises from the back wall of the middle-ear cavity and extends forward and attaches to the neck of the head of the stapes. Its reflex contractions tend to tip the stapes backward, as if to pull it out of the oval window. Thus, it selectively reduces the intensity of sounds entering the inner ear, especially those of lower frequency. Nerves: The seventh cranial nerve, called the facial nerve, passes by a somewhat circuitous route through the in the petrous portion of the

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college temporal bone on its way from the brainstem to the muscles of expression of the face. A small but important branch, the chorda tympani nerve, emerges from the canal into the middle-ear cavity and runs forward along the inner surface of the pars tensa of the membrane, passing between the handle of the malleus and the long process of the incus. Since at this point it is covered only by the tympanic mucous membrane, it appears to be quite bare. Then it resumes its course through the anterior bony wall, bringing sensory fibres for taste to the anterior two-thirds of the tongue and parasympathetic secretory fibres to salivary glands. Eustachian tube: The eustachian tube, about 31–38 mm (1.2–1.5 inches) long, leads downward and inward from the tympanum to the nasopharynx, the space that is behind and continuous with the nasal passages and is above the soft palate. At its upper end the tube is narrow and surrounded by bone. Nearer the pharynx it widens and becomes cartilaginous. Its mucous lining, which is continuous with that of the middle ear, is covered with cilia, small hairlike projections whose coordinated rhythmical sweeping motions speed the drainage of mucous secretions from the tympanum to the pharynx. The eustachian tube helps ventilate the middle ear and maintain equal air pressure on both sides of the tympanic membrane. The tube is closed at rest and opens during swallowing so that minor pressure differences are adjusted without conscious effort. During an underwater dive or a rapid descent in an airplane, the tube may remain tightly closed. The discomfort that is felt as the external pressure increases can usually be overcome by attempting a forced expiration with the mouth and nostrils held tightly shut. This maneuver, which raises the air pressure in the pharynx and causes the tube to open, is called the Valsalva maneuver, named for Italian physician-anatomist Antonio Maria Valsalva (1666–1723), who recommended it for clearing pus from an infected middle ear. 21. Write down the structure of inner ear. There are actually two labyrinths of the inner ear, one inside the other, the contained within the . The bony labyrinth consists of a central chamber called the vestibule, the three , and the spirally coiled . Within each structure, and filling only a fraction of the available space, is a corresponding portion of the membranous labyrinth: the vestibule contains the and , each

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college semicircular canal its semicircular duct, and the cochlea its . Surrounding the membranous labyrinth and filling the remaining space is the watery fluid called . It is derived from blood plasma and resembles but is not identical with the cerebrospinal fluid of the brain and the aqueous humour of the eye. Like most of the hollow organs, the membranous labyrinth is lined with epithelium (a sheet of specialized cells that covers internal and external body surfaces). It is filled with a fluid called , which has a markedly different ionic content from perilymph. Because the membranous labyrinth is a closed system, the endolymph and perilymph do not mix.

Figure: The two labyrinths of the inner ear. The bony labyrinth is partially cut away to show the membranous labyrinth within.

Vestibular system The is the apparatus of the inner ear involved in balance. It consists of two structures of the bony labyrinth, the vestibule and the semicircular canals, and the structures of the membranous labyrinth contained within them. Vestibule: The two membranous sacs of the vestibule, the utricle and the saccule, are known as the organs. Because they respond to gravitational forces, they are also called gravity receptors. Each sac has on its inner surface a single patch of sensory cells called a macula, which is about 2 mm (0.08 inch) in diameter. The macula monitors the position of the head relative to the vertical. In the utricle the macula projects from the anterior wall of that tubular sac and lies primarily in the horizontal plane. In

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college the saccule the macula is in the vertical plane and directly overlies the bone of the inner wall of the vestibule. In shape it is elongated and resembles the letter J. Each macula consists of neuroepithelium, a layer that is made up of supporting cells and sensory cells, as well as a basement membrane, nerve fibres and nerve endings, and underlying connective tissue. The sensory cells are called hair cells because of the hairlike cilia—stiff nonmotile stereocilia and flexible motile kinocilia—that project from their apical ends. The nerve fibres are from the superior, or vestibular, division of the vestibulocochlear nerve. They pierce the basement membrane and, depending on the type of hair cell, either end on the basal end of the cell or form a calyx, or cuplike structure, that surrounds it. Each of the hair cells of the vestibular organs is topped by a hair bundle, which consists of about 100 fine nonmotile stereocilia of graded lengths and a single motile . The stereocilia are anchored in a dense cuticular plate at the cell’s apex. The single kinocilium, which is larger and longer than the stereocilia, rises from a noncuticular area of the cell membrane at one side of the cuticular plate. The longest stereocilia are those closest to the kinocilium; the stereocilia decrease in length in stepwise fashion away from the kinocilium. Minute filamentous strands link the tips and shafts of neighbouring stereocilia to one another. When the hair bundles are deflected—e.g., because of a tilt of the head—the hair cells are stimulated to alter the rate of the nerve impulses that they are constantly sending via the fibres to the brainstem. Covering the entire macula is a delicate acellular structure, the otolithic, or statolithic, membrane. This membrane is sometimes described as gelatinous, although it has a fibrillar pattern. The surface of the membrane is covered by a blanket of rhombohedral crystals, referred to as otoconia or statoconia, which consist of calcium carbonate in the form of calcite. These crystalline particles, which range in length from 1 to 20 μm (1 μm = 0.000039 inch), are much denser than the membrane—their specific gravity is almost three times that of the membrane and the endolymph—and thus add considerable mass to it. The vestibular hair cells are of two types: type I cells have a rounded body enclosed by a nerve calyx, and type II cells have a cylindrical body with nerve endings at the base. They form a mosaic on the surface of the maculae, with the type I cells dominating in a curvilinear area (the striola) near the centre of the macula and the cylindrical cells around the periphery. The significance of these

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college patterns is poorly understood, but they may increase sensitivity to slight tiltings of the head.

Figure: The membranous labyrinth of the vestibular system (centre), which contains the organs of balance, and (lower left) the cristae of the semicircular ducts and (lower right) the maculae of the utricle and saccule.

Semicircular canals: The three semicircular canals of the bony labyrinth are designated according to their position: superior, horizontal, and posterior. The superior and posterior canals are in diagonal vertical planes that intersect at right angles. Each canal has an expanded end, the ampulla, which opens into the vestibule. The ampullae of the horizontal and superior canals lie close together, just above the oval window, but the ampulla of the posterior canal opens on the opposite side of the vestibule. The other ends of the superior and posterior canals join to form a common stem, or crus, which also opens into the vestibule. Nearby is the mouth of a canal called the , which opens into the cranial cavity. The other end of the horizontal canal has a separate opening into the vestibule. Thus, the vestibule completes the circle for each of the semicircular canals. Each of the three bony canals and its ampulla enclose a membranous semicircular duct of much smaller diameter that has its own ampulla. The membranous ducts and ampullae follow the same pattern as

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college the canals and ampullae of the bony labyrinth, with their openings into the utricle and with a common crus for the superior and posterior ducts. Like the other parts of the membranous labyrinth, they are filled with endolymph and surrounded by perilymph. The narrow passes from the utricle through the vestibular aqueduct into the cranial cavity, carrying excess endolymph to be absorbed by the . Each membranous ampulla contains a saddle-shaped ridge of tissue called the crista, the sensory end organ that extends across it from side to side. The crista is covered by neuroepithelium, with hair cells and supporting cells. From this ridge rises a gelatinous structure, the cupula, which extends to the roof of the ampulla immediately above it, dividing the interior of the ampulla into two approximately equal parts. Like the hair cells of the maculae, the hair cells of the cristae have hair bundles projecting from their apices. The kinocilium and the longest stereocilia extend far up into the substance of the cupula, occupying fine parallel channels. Thus, the cupula is attached at its base to the crista but is free to incline toward or away from the utricle in response to the slightest flow of endolymph or a change in pressure. The tufts of cilia move with the cupula and, depending on the direction of their bending, cause an increase or a decrease in the rate of nerve impulse discharges carried by the vestibular nerve fibres to the brainstem. Cochlea: Structure of the cochlea: The cochlea contains the sensory organ of hearing. It bears a striking resemblance to the shell of a snail and in fact takes its name from the Greek word for this object. The cochlea is a spiral tube that is coiled two and one-half turns around a hollow central pillar, the . It forms a cone approximately 9 mm (0.35 inch) in diameter at its base and 5 mm in height. When stretched out, the spiral tube is approximately 30 mm in length. It is widest—2 mm—at the point where the basal coil opens into the vestibule, and it tapers until it ends blindly at the apex. The otherwise hollow centre of the modiolus contains the cochlear artery and vein, as well as the twisted trunk of fibres of the cochlear nerve. This nerve, a division of the very short vestibulocochlear nerve, enters the base of the modiolus from the brainstem through an opening in the petrous portion of the temporal bone called the internal meatus. The spiral ganglion cells of the cochlear nerve are found in a bony spiral canal winding around the central core.

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college A thin bony shelf, the osseous spiral lamina, winds around the modiolus like the thread of a screw. It projects about halfway across the cochlear canal, partly dividing it into two compartments, an upper chamber called the scala vestibuli (vestibular ramp) and a lower chamber called the scala tympani (tympanic ramp). The scala vestibuli and scala tympani, which are filled with perilymph, communicate with each other through an opening at the apex of the cochlea, called the , which can be seen if the cochlea is sliced longitudinally down the middle. At its basal end, near the middle ear, the scala vestibuli opens into the vestibule. The basal end of the scala tympani ends blindly just below the round window. Nearby is the opening of the narrow , through which passes the perilymphatic duct. This duct connects the interior of the cochlea with the subdural space in the posterior cranial fossa (the rear portion of the floor of the cranial cavity). A smaller scala, called the cochlear duct (scala media), lies between the larger vestibular and tympanic scalae; it is the cochlear portion of the membranous labyrinth. Filled with endolymph, the cochlear duct ends blindly at both ends— i.e., below the round window and at the apex. In cross section this duct resembles a right triangle. Its base is formed by the osseous spiral lamina and the , which separate the cochlear duct from the scala tympani. Resting on the basilar membrane is the , which contains the hair cells that give rise to nerve signals in response to sound vibrations. The side of the triangle is formed by two tissues that line the bony wall of the cochlea: the stria vascularis, which lines the outer wall of the cochlear duct, and the fibrous , which lies between the stria and the bony wall of the cochlea. A layer of flat cells bounds the stria, separating it from the spiral ligament. The hypotenuse is formed by the transparent (or Reissner membrane), which consists of only two layers of flattened cells. A low ridge, the spiral limbus, rests on the margin of the osseous spiral lamina. The Reissner membrane stretches from the inner margin of the limbus to the upper border of the stria. The spiral ligament extends above the attachment of the Reissner membrane and is in contact with the perilymph in the scala vestibuli. Extending below the insertion of the basilar membrane, it is in contact with the perilymph of the scala tympani. It contains many stout fibres that anchor the basilar membrane and numerous connective-tissue cells. The structure of the spiral ligament is

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college denser behind the stria than near the upper and lower margins. The spiral ligament, like the adjacent stria, is well supplied with blood vessels. It receives the radiating arterioles that pass outward from the modiolus in bony channels of the roof of the scala vestibuli. Branches from these vessels form a network of capillaries above the junction with the Reissner membrane that may be largely responsible for the formation of the perilymph from the blood plasma. Other branches enter the stria, and still others pass behind it to the spiral prominence. From these separate capillary networks, which are not interconnected, small veins descending below the attachment of the basilar membrane collect blood and deliver it to the spiral vein in the floor of the scala tympani. At the lower margin of the stria is the spiral prominence, a low ridge parallel to the basilar membrane that contains its own set of longitudinally directed capillary vessels. Below the prominence is the outer sulcus. The floor of the outer sulcus is lined by cells of epithelial origin, some of which send long projections into the substance of the spiral ligament. Between these so-called root cells, capillary vessels descend from the spiral ligament. This region appears to have an absorptive rather than a secretory function, and it may be involved in removing waste materials from the endolymph. In humans the basilar membrane is about 30 to 35 mm in length. It widens from less than 0.1 mm near its basal end to 0.5 mm near the apex. The basilar membrane is spanned by stiff elastic fibres that are connected at their basal ends in the modiolus. Their distal ends are embedded in the membrane but are not actually attached, which allows them to vibrate. The fibres decrease in calibre and increase in length from the basal end of the cochlea near the middle ear to the apex, so that the basilar membrane as a whole decreases remarkably in stiffness from base to apex. Furthermore, at the basal end the osseous spiral lamina is broader, the stria vascularis wider, and the spiral ligament stouter than at the apex. In contrast, however, the mass of the organ of Corti is least at the base and greatest at the apex. Thus, a certain degree of tuning is provided in the structure of the cochlear duct and its contents. With greater stiffness and less mass, the basal end is more attuned to the sounds of higher frequencies. Decreased stiffness and increased mass render the apical end more responsive to lower frequencies. Beneath the fibrillar layer of the basilar membrane is the acellular ground substance of the membrane. This layer is covered in turn by a single layer of

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college spindle-shaped mesothelial cells, which have long processes arranged longitudinally and parallel, facing the scala tympani and forming the tympanic lamella that is in contact with the perilymph. Capillary blood vessels are found on the underside of the tympanic lip of the limbus and, in some species, including the guinea pig and humans, within the basilar membrane, beneath the tunnel. These vessels, called spiral vessels, do not enter the organ of Corti but are thought to supply most of the oxygen and other nutrients to its cells. Although the outer spiral vessel is seldom found in adult animals of certain species such as the dog, cat, and rat and is not found in the basilar membrane of every adult human, it is present in the human fetus. Its impressive diameter in the fetus suggests that it is an important channel for blood delivery to the developing organ of Corti.

Figure: A cross section through one of the turns of the cochlea (inset) showing the scala tympani and scala vestibuli, which contain perilymph, and the cochlear duct, which is filled with endolymph.

Organ of Corti: Arranged on the surface of the basilar membrane are orderly rows of the sensory hair cells, which generate nerve impulses in response to sound vibrations. Together with their supporting cells they form a complex neuroepithelium called the basilar papilla, or organ of Corti. The organ of Corti

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college is named after Italian anatomist Alfonso Corti, who first described it in 1851. Viewed in cross section, the most striking feature of the organ of Corti is the arch, or tunnel, of Corti, formed by two rows of pillar cells, or rods. The pillar cells furnish the major support of this structure. They separate a single row of larger, pear-shaped inner hair cells from three or more rows of smaller, cylindrical outer hair cells. The inner hair cells are supported and enclosed by the inner phalangeal cells, which rest on the thin outer portion, called the tympanic lip, of the spiral limbus. On the inner side of the inner hair cells and the cells that support them is a curved furrow called the inner sulcus. This is lined with more or less undifferentiated cuboidal cells. Each outer hair cell is supported by a phalangeal cell of Deiters, or supporting cell, which holds the base of the hair cell in a cup-shaped depression. From each Deiters’ cell a projection extends upward to the stiff membrane, the reticular lamina, that covers the organ of Corti. The top of the hair cell is firmly held by the lamina, but the body is suspended in fluid that fills the space of Nuel and the tunnel of Corti. Although this fluid is sometimes referred to as cortilymph, its composition is thought to be similar, if not identical, to that of the perilymph. Beyond the hair cells and the Deiters’ cells are three other types of epithelial cells, usually called the cells of Hensen, Claudius, and Boettcher, after the 19th- century anatomists who first described them. Their function has not been established, but they are assumed to help in maintaining the composition of the endolymph by ion transport and absorptive activity. Each hair cell has a cytoskeleton composed of filaments of the protein actin, which imparts stiffness to structures in which it is found. The hair cell is capped by a dense cuticular plate, composed of actin filaments, which bears a tuft of stiffly erect stereocilia, also containing actin, of graded lengths arranged in a staircase pattern. This so-called hair bundle has rootlets anchored firmly in the cuticular plate. On the top of the inner hair cells 40 to 60 stereocilia are arranged in two or more irregularly parallel rows. On the outer hair cells approximately 100 stereocilia form a W pattern. At the notch of the W the plate is incomplete, with only a thin cell membrane taking its place. Beneath the membrane is the basal body of a kinocilium, although no motile ciliary (hairlike) portion is present as is the case on the hair cells of the vestibular system.

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college The stereocilia are about 3 to 5 μm in length. The longest make contact with but do not penetrate the . This membrane is an acellular gelatinous structure that covers the top of the spiral limbus as a thin fibrillar layer, then becomes thicker as it extends outward over the inner sulcus and the reticular lamina. Its fibrils extend radially and somewhat obliquely to end at its lateral border, just above the junction of the reticular lamina and the cells of Hensen. In the upper turns of the cochlea, the margin of the membrane ends in fingerlike projections that make contact with the stereocilia of the outermost hair cells. The myelin-ensheathed fibres of the vestibulocochlear nerve fan out in spiral fashion from the modiolus to pass into the channel near the root of the osseous spiral lamina, called the canal of Rosenthal. The bipolar cell bodies of these neurons constitute the spiral ganglion. Beyond the ganglion their distal processes extend radially outward in the bony lamina beneath the limbus to pass through an array of small pores directly under the inner hair cells, called the habenula perforata. Here the fibres abruptly lose their multilayered coats of myelin and continue as thin, naked, unmyelinated fibres into the organ of Corti. Some fibres form a longitudinally directed bundle running beneath the inner hair cells and another bundle just inside the tunnel, above the feet of the inner pillar cells. The majority of the fibres (some 95 percent in the human ear) end on the inner hair cells. The remainder cross the tunnel to form longitudinal bundles beneath the rows of the outer hair cells on which they eventually terminate. The endings of the nerve fibres beneath the hair cells are of two distinct types. The larger and more numerous endings contain many minute vesicles, or liquid- filled sacs, containing neurotransmitters, which mediate impulse transmission at neural junctions. These endings belong to a special bundle of nerve fibres that arise in the brainstem and constitute an efferent system, or feedback loop, to the cochlea. The smaller and less numerous endings contain few vesicles or other cell structures. They are the terminations of the afferent fibres of the cochlear nerve, which transmit impulses from the hair cells to the brainstem (see The physiology of hearing: Cochlear nerve and central auditory pathways). The total number of outer hair cells in the cochlea has been estimated at 12,000 and the number of inner hair cells at 3,500. Although there are about 30,000 fibres in the cochlear nerve, there is considerable overlap in the innervation of

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college the outer hair cells. A single fibre may supply endings to many hair cells, which thus share a “party line.” Furthermore, a single hair cell may receive nerve endings from many fibres. The actual distribution of nerve fibres in the organ of Corti has not been worked out in detail, but it is known that the inner hair cells receive the majority of afferent fibre endings without the overlapping and sharing of fibres that are characteristic of the outer hair cells. Viewed from above, the organ of Corti with its covering, the reticular lamina, forms a well-defined mosaic pattern. In humans the arrangement of the outer hair cells in the basal turn of the cochlea is quite regular, with three distinct and orderly rows; but in the higher turns of the cochlea the arrangement becomes slightly irregular, as scattered cells form fourth or fifth rows. The spaces between the outer hair cells are filled by oddly shaped extensions (phalangeal plates) of the supporting cells. The double row of head plates of the inner and outer pillar cells cover the tunnel and separate the inner from the outer hair cells. The reticular lamina extends from the inner border cells near the inner sulcus to the Hensen cells but does not include either of these cell groups. When a hair cell degenerates and disappears as a result of aging, disease, or noise- induced injury, its place is quickly covered by the adjacent phalangeal plates, which expand to form an easily recognized “scar.”

Figure: structure of organ of Corti

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college Endolymph and perilymph: The perilymph, which fills the space within the bony labyrinth surrounding the membranous labyrinth, is similar, but not identical, in composition to other extracellular fluids of the body, such as cerebrospinal fluid. The concentration of sodium ions in the perilymph is high (about 150 milliequivalents per litre), and that of potassium ions is low (about 5 milliequivalents per litre), as is true of other extracellular fluids. Like these fluids, the perilymph is apparently formed locally from the blood plasma by transport mechanisms that selectively allow substances to cross the walls of the capillaries. Although it is anatomically possible for cerebrospinal fluid to enter the cochlea by way of the perilymphatic duct, experimental studies have made it appear unlikely that the cerebrospinal fluid is involved in the normal production of perilymph. The membranous labyrinth is filled with endolymph, which is unique among extracellular fluids of the body, including the perilymph, in that its potassium ion concentration is higher (about 140 milliequivalents per litre) than its sodium ion concentration (about 15 milliequivalents per litre). The process of formation of the endolymph and the maintenance of the difference in ionic composition between it and perilymph are not yet completely understood. The Reissner membrane forms a selective barrier between the two fluids. Blood-endolymph and blood-perilymph barriers, which control the passage of substances such as drugs from the blood to the inner ear, appear to exist as well. Evidence indicates that the endolymph is produced from perilymph as a result of selective ion transport through the epithelial cells of the Reissner membrane and not directly from the blood. The secretory tissue called the stria vascularis, in the lateral wall of the cochlear duct, is thought to play an important role in maintaining the high ratio of potassium ions to sodium ions in the endolymph. Other tissues of the cochlea, as well as the dark cells of the vestibular organs, which must produce their own endolymph, are also thought to be involved in maintaining the ionic composition of the endolymph. Because the membranous labyrinth is a closed system, the questions of flow and removal of the endolymph are also important. The endolymph is thought to be reabsorbed from the endolymphatic sac, although this appears to be only part of the story. Other cochlear and vestibular tissues may also have important roles in regulating the volume and maintaining the composition of the inner-ear fluids.

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college 22. Describe the mechanism of hearing. Hearing is the process by which the ear transforms sound vibrations in the external environment into nerve impulses that are conveyed to the brain, where they are interpreted as sounds. Sounds are produced when vibrating objects, such as the plucked string of a guitar, produce pressure pulses of vibrating air molecules, better known as sound waves. The ear can distinguish different subjective aspects of a sound, such as its loudness and pitch, by detecting and analyzing different physical characteristics of the waves. Pitch is the perception of the frequency of sound waves—i.e., the number of wavelengths that pass a fixed point in a unit of time. Frequency is usually measured in cycles per second, or hertz. The human ear is most sensitive to and most easily detects frequencies of 1,000 to 4,000 hertz, but at least for normal young ears the entire audible range of sounds extends from about 20 to 20,000 hertz. Sound waves of still higher frequency are referred to as ultrasonic, although they can be heard by other mammals. Loudness is the perception of the intensity of sound—i.e., the pressure exerted by sound waves on the tympanic membrane. The greater their amplitude or strength, the greater the pressure or intensity, and consequently the loudness, of the sound. The intensity of sound is measured and reported in decibels (dB), a unit that expresses the relative magnitude of a sound on a logarithmic scale. Stated in another way, the decibel is a unit for comparing the intensity of any given sound with a standard sound that is just perceptible to the normal human ear at a frequency in the range to which the ear is most sensitive. On the decibel scale, the range of human hearing extends from 0 dB, which represents a level that is all but inaudible, to about 130 dB, the level at which sound becomes painful. In order for a sound to be transmitted to the central nervous system, the energy of the sound undergoes three transformations. First, the air vibrations are converted to vibrations of the tympanic membrane and ossicles of the middle ear. These in turn become vibrations in the fluid within the cochlea. Finally, the fluid vibrations set up traveling waves along the basilar membrane that stimulate the hair cells of the organ of Corti. These cells convert the sound vibrations to nerve impulses in the fibres of the cochlear nerve, which transmits them to the brainstem, from which they are relayed, after extensive processing, to the primary auditory area of the cerebral cortex, the ultimate centre of the

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college brain for hearing. Only when the nerve impulses reach this area does the listener become aware of the sound.

Figure: The mechanism of hearing. Sound waves enter the outer ear and travel through the external auditory canal until they reach the tympanic membrane, causing the membrane and the attached chain of auditory ossicles to vibrate. The motion of the stapes against the oval window sets up waves in the fluids of the cochlea, causing the basilar membrane to vibrate. This stimulates the sensory cells of the organ of Corti, atop the basilar membrane, to send nerve impulses to the brain.

23. Describe the auditory pathways and its centres. A. Receptors:  Cristae ampularis; 3 neuroepithelial receptors in the ampulla of the semicircular ducts on each side  Macula utriculi receptors in the utricle  Macula sacculi receptors in the sacule B. The 1st order vestibular afferents arise in bipolar cells of vestibular (Scarpa's) ganglion, which is in the distal portion of the internal auditory meatus.  The axons travel in the vestibular portion of the 8th cranial nerve and enter the brain stem at the pontomedullary junction. A few of the vestibular afferents go directly to the cerebellum through the inferior cerebellar peduncle. The cerebellum coordinates the movements that maintain balance.

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college C. The 2nd order neuron is the vestibular nuclei: the inferior, medial, lateral (Deiter’s) and superior vestibular nuclei. Ascending fibers from the vestibular nuclei cross to the opposite side to end in the posteromedial ventral nucleus of the thalamus D. The posteromedial ventral nucleus of the thalamus is the 3rd order neuron the axons of these cells pass through the optic radiations to reach the vestibular area which lies behind the lower part of sensory area, opposite the sensory area of the face E. Other connections of vestibular nuclei  The main projections from these nuclei are to the spinal cord (controlling head and body position), to the three, extraocular motor nuclei (III, IV, VI, controlling eye movements), to the thalamus (VPI, eventually reaching the cortex and conscious perception of movement and gravity), and to the cerebellum (coordinating postural adjustments).  The main descending tracts are the lateral vestibulospinal tract from the lateral vestibular nucleus and the medial vestibulospinal tract from the medial vestibular nucleus.  The lateral vestibular tract starts in the lateral vestibular nucleus and descends the length of the spinal cord on the same side. This pathway helps us walk upright.  The medial vestibular tract starts in the medial vestibular nucleus and extends bilaterally through mid-thoracic levels of the spinal cord in the MLF. This tract affects head movements and helps integrate head and eye movements.  In summary, remember that the lateral vestibulo-spinal tract is ipsilateral and long; the medial vestibulo-spinal tract is bilateral but shorter.  The main ascending tracts are from the superior and medial vestibular nuclei to the extraocular muscles through the medial longitudinal fasciculus (MLF).

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college

Figure: auditory pathway

24. What is decibel ? The decibel is a relative unit of measurement corresponding to one tenth of bel. The decibel (dB) is a logarithmic unit used to measure sound level. It is also widely used in electronics, signals and communication. The dB is a logarithmic way of describing a ratio. The ratio may be power, sound pressure, voltage or intensity or several other things. 25. What is frequency discrimination? Frequency discrimination refers to the ear's ability to perceive the difference between two pure tones of different frequencies, but the same sound level, presented sequentially.. ... This relative threshold is true

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college for frequencies between 200 and 5000Hz. At more extreme frequencies, pitch sensitivity is greatly decreased. 26. What is meant by the term loudness of sound? Loudness refers to how loud or soft a sound seems to a listener. The loudness of sound is determined, in turn, by the intensity, or amount of energy, in sound waves. The unit of intensity is the decibel (dB). As decibel levels get higher, sound waves have greater intensity and sounds are louder.

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college Vision: 27. Describe the structures of the eye. The structures of the eyes are complex. The orbit is the bony cavity that contains the eyeball, muscles, nerves, and blood vessels, as well as the structures that produce and drain tears. Each orbit is a pear-shaped structure that is formed by several bones.

Figure: Structure of eye

The outer covering of the eyeball consists of a relatively tough, white layer called the sclera (or white of the eye). Near the front of the eye, in the area protected by the eyelids, the sclera is covered by a thin, transparent membrane (conjunctiva), which runs to the edge of the cornea. The conjunctiva also covers the moist back surface of the eyelids and eyeballs. Light enters the eye through the cornea, the clear, curved layer in front of the iris and pupil. The cornea serves as a protective covering for the front of the eye and also helps focus light on the retina at the back of the eye. After passing through the cornea, light travels through the pupil (the black dot in the middle of the eye). The iris—the circular, colored area of the eye that surrounds the pupil— controls the amount of light that enters the eye. The iris allows more light into the eye (enlarging or dilating the pupil) when the environment is dark and allows less light into the eye (shrinking or constricting the pupil) when the environment is bright. Thus, the pupil dilates and constricts like the aperture of a camera lens as the amount of light in the immediate surroundings changes.

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college The size of the pupil is controlled by the action of the pupillary sphincter muscle and dilator muscle. Behind the iris sits the lens. By changing its shape, the lens focuses light onto the retina. Through the action of small muscles (called the ciliary muscles), the lens becomes thicker to focus on nearby objects and thinner to focus on distant objects. The retina contains the cells that sense light (photoreceptors) and the blood vessels that nourish them. The most sensitive part of the retina is a small area called the macula, which has millions of tightly packed photoreceptors (the type called cones). The high density of cones in the macula makes the visual image detailed, just as a high-resolution digital camera has more megapixels. Each photoreceptor is linked to a nerve fiber. The nerve fibers from the photoreceptors are bundled together to form the optic nerve. The optic disk, the first part of the optic nerve, is at the back of the eye. The photoreceptors in the retina convert the image into electrical signals, which are carried to the brain by the optic nerve. There are two main types of photoreceptors: cones and rods. Cones are responsible for sharp, detailed central vision and color vision and are clustered mainly in the macula. Rods are responsible for night and peripheral (side) vision. Rods are more numerous than cones and much more sensitive to light, but they do not register color or contribute to detailed central vision as the cones do. Rods are grouped mainly in the peripheral areas of the retina. The eyeball is divided into two sections, each of which is filled with fluid. The pressure generated by these fluids fills out the eyeball and helps maintain its shape. The front section (anterior segment) extends from the inside of the cornea to the front surface of the lens. It is filled with a fluid called the aqueous humor, which nourishes the internal structures. The anterior segment is divided into two chambers. The front (anterior) chamber extends from the cornea to the iris. The back (posterior) chamber extends from the iris to the lens. Normally, the aqueous humor is produced in the posterior chamber, flows slowly through the pupil into the anterior chamber, and then drains out of the eyeball through outflow channels located where the iris meets the cornea.

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college The back section (posterior segment) extends from the back surface of the lens to the retina. It contains a jellylike fluid called the vitreous humor. 28. What is Pupilary reflex ? The pupillary light reflex (PLR) or photopupillary reflex is a reflex that controls the diameter of the pupil, in response to the intensity (luminance) of light that falls on the retinal ganglion cells of the retina in the back of the eye, thereby assisting in adaptation of vision to various levels of lightness/darkness. A greater intensity of light causes the pupil to constrict (miosis/myosis; thereby allowing less light in), whereas a lower intensity of light causes the pupil to dilate (mydriasis, expansion; thereby allowing more light in). Thus, the pupillary light reflex regulates the intensity of light entering the eye. 29. What is Argyll Robertson pupil ? Argyll Robertson pupils (AR pupils or, colloquially, "prostitute's pupils") are bilateral small pupils that reduce in size on a near object (i.e., they accommodate), but do not constrict when exposed to bright light (i.e., they do not react to light). Argyll Robertson pupil is found in late-stage syphilis, a disease caused by the spirochete Treponema pallidum. 30. Describe the structure of Retina. Retina has ten following layers..... 1) The pigmented layer: It comprises retinal pigmented epithelium. It is involved in photoreceptor metabolism and that it comprises which captures light not picked up by the photoreceptors. 2) The photoreceptor cell layer of rods and cones: Photoreceptor cells. It is involved in light capture and phototransduction; the phototransduction cascade occurs here, which transforms light into neural signal. The photoreceptor cell segments are metabolically dependent upon the pigmented epithelium for photoreceptor regeneration and waste disposal. This layer divides into: a. An outer segment b. An inner segment.

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college 3) External limiting membrane 4) Outer nuclear layer: It has photoreceptor cell bodies.  Cones have a large outer, conical segment; they provide high-resolution color vision.  Rods have a small, narrow cylindrical outer segment; they provide low- resolution dim-light ("night") vision.  Cones predominate in central vision (within the fovea), whereas rods predominate in peripheral vision (outside of the macula). 5) Outer plexiform layer: It comprises a thin synaptic zone. 6) Inner nuclear layer: It comprises retinal interneuronal cell bodies. This layer specifically comprises: a) Bipolar cells, which, as we see have two poles, so they can pass forward electrical signal from the photoreceptor cells to the ganglion cells. b) Horizontal and Amacrine cells, which enhance visual contrast. It is well recognized that the visual system relies more on visual contrast than the overall level of illumination for visual perception. The visual system attends to the borders between light and dark areas or color differences more so than light intensity. As long as we can read the page of a book comfortably, we perceive the words on it just the same in varying levels of illumination; it is the contrast of the ink from the page that makes the largest impression in our mind. c) Müller glial cells extend across the retina. Their proximal endings form the inner limiting membrane (as we'll soon see) and their distal processes help form the external limiting membrane. 7) Inner plexiform layer: It comprises a thick synaptic zone. 8) Ganglion cell layer: It comprises ganglion cell bodies. The ganglion cell dendrites help form the inner plexiform layer, and the axons form the nerve fiber layer. 9) Nerve fiber layer: It comprises axons of the ganglion cells, which are unmyelinated. 10) Inner limiting membrane: It forms from the basal lamina of Müller glial cells.

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college

Figure: Histological structure of retina

31. What is Photopic and Scotopic vision ? Scotopic vision uses only rods to see, meaning that objects are visible, but appear in black and white, whereas photopic vision uses cones and provides colour. 32. Describe the visual pathway The visual pathway includes..... Axons of ganglion cells of the retina → optic nerve → optic chiasm (partial decussation) → optic tract → lateral geniculate nucleus (thalamic relay nucleus for vision) → corona radiata (optic radiation) → primary visual cortex a) The retina: . Receptors are the rod (cylindrical processes) and cons (conical processes) of the retina . The axons of the cells synapse with the dendrites of the bipolar cells . Bipolar cells are the first order neuron in this pathway . The axons of bipolar cells synapse with the dendrites of ganglion cells . The ganglion cells is the second order neuron in this pathway, the axons of the ganglion cells forming the optic nerve . The retina can be divided by a horizontal line bisecting the fovea into 2 halves; temporal and nasal halves

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college . The fibers from the nasal half cross to the opposite side. While the fibers from the temporal half pass through the optic chiasma without crossing. b) Optic Nerve: . The unmyelinated axons of ganglion cells collect at the optic disk and exit from the eye, about 3 or 4 mm to the nasal side of its center, as the optic nerve. . They pierce the sclera in a region called the lamina cribrosa. Here they acquire myelin sheaths and forming the optic nerve. . The optic nerve is actually a tract of the CNS and has meningeal coverings, the subarachnoid space around the optic nerve communicates with subarachnoid space generally. . The optic nerve leaves the orbital cavity through the optic canal and unites with the optic nerve of the opposite side to form the optic chiasma. c) The optic chiasma: . The optic chiasma is a flattened bundle of nerve fibers situated at the junction of the anterior wall and floor of the third ventricle just anterior to the infundibular stalk. . The superior surface is attached to the lamina terminalis, and inferiorly, it is related to the hypophysis cerebri, from which it is separated by the diaphragma sellae. . The anterolateral corners of the chiasma are continuous with the optic nerves, and the posterolateral corners are continuous with the optic tracts . All fibers from the nasal half of each retina cross to the contralateral optic tract. . All fibers from the temporal half of each retina pass through the lateral portions of the chiasm without crossing and enter the ipsilateral optic tract. d) Optic Tract: . The optic tract emerges from the optic chiasma and passes posterolaterally around the cerebral peduncle. . Each optic tract contains:

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college 1) The fibres arising in the temporal retina of the ipsilateral eye (same side); 2) The fibres of the nasal retina of the contralateral eye (opposite side) . Most of the fibers now terminate by synapsing with nerve cells in the lateral geniculate body. A few of the fibers pass to the pretectal nucleus and the superior colliculus of the midbrain and are concerned with light reflexes e) Lateral Geniculate Body: . The lateral geniculate body is a small, oval swelling projecting from the pulvinar of the thalamus. . It consists of six layers of cells, on which synapse the axons of the optic tract. The axons of the nerve cells within the geniculate body leave it to form the optic radiation . The fibers of the optic radiation are the axons of the nerve cells of the lateral geniculate body that passes posteriorly through the retrolenticular part of the internal capsule and terminates in the visual cortex f) Visual cortex: . The visual cortex (area 17) occupies the upper and lower lips of the calcarine sulcus on the medial surface of the cerebral hemisphere. . The visual association cortex (areas 18 and 19) is responsible for recognition of objects and perception of color.

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college

Figure: Visual pathway

33. What is accommodation in eye ? Accommodation: In medicine, the ability of the eye to change its focus from distant to near objects (and vice versa). This process is achieved by the lens changing its shape. Accommodation is the adjustment of the optics of the eye to keep an object in focus on the retina as its distance from the eye varies. It is the process of adjusting the focal length of a lens. 34. What is Visual acuity ? Visual acuity is acuteness or clearness of vision, especially form vision, which is dependent on the sharpness of the retinal focus within the eye, the sensitivity of the nervous elements, and the interpretative faculty of the brain. Many humans have one eye that has superior visual acuity over the other. Visual acuity is measured by your ability to identify letters or numbers on a standardized eye chart from a specific viewing distance. Visual acuity is tested one eye at a time, with the help of a standardized Snellen eye chart.

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college 35. What are Positive and negative after-image? An afterimage is an image that continues to appear in the eyes after a period of exposure to the original image. There are two major types of afterimages: positive afterimages and negative afterimages. Positive Afterimages: In a positive afterimage, the colors of the original image are maintained. Essentially, the afterimage looks the same as the original image. You can experience a positive afterimage yourself by staring at a very brightly lit scene for a period of time and then closing your eyes. For the briefest of moments, you will continue to "see" the original scene in the same colors and brightness. Negative Afterimages: In a negative afterimage, the colors you see are inverted from the original image. For example, if you stare for a long time at a red image, you will see a green afterimage. In some instances, the colors of the original stimulus are retained. This is known as a positive afterimage. In other cases, the colors may be reversed. This is known as a negative afterimage. 36. What is light adaptation? Light adaptation is the adjustment of the eyes when we move from darkness into an area that is illuminated. During this adjustment period the sensitivity of the retina decreases. The process including contraction of the pupil and decrease in rhodopsin by which the eye adapts to conditions of increased illumination. The cones of the eyes begin reacting to the brightness of the light and become more active than the rods of the eye. This increases the accuracy of vision and the sensation of color.

37. What is Dark adaptation? Dark Adaptation is the process by which our eyes adjust to darkness after being exposed to light. Dark adaptation is made possible by the dilation of our pupils and changes in the rods and cones of our retinas. Rods detect form and motion, and cones detect color. So in darkness, our rods became more active than cones of the eye. For example, when we move from a bright, sunny area outside to a relatively dark room inside, it is difficult to see at first. But gradually our eyes recover and become more sensitive to the dim light indoors.

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college 38. What is Colour vision ? Colour vision is an ability of animals to perceive differences between light composed of different wavelengths (i.e., different spectral power distributions) independently of light intensity. Each region of the retina has cone photoreceptors containing photopigments of different spectral sensitivities. Since each photopigment absorbs light across the spectrum, the output of a single cone cannot specify either the wavelength or the intensity of light, nor allow one to distinguish the spectral characteristics of an object from those of its illuminant. Colour vision is a function of the cones. The widely accepted theory is that there are three types of cones, each containing chemicals that respond to one of the three primary colours (red, green and violet). White light stimulates all three sets of cones; any other colour stimulates only one or two sets. The brain can then interpret the impulses from these cones as various colours. 39. What is the meaning of Colour blindness? Color-blindness is the inability to distinguish the differences between certain colors. This condition results from an absence of color-sensitive pigment in the cone cells of the retina, the nerve layer at the back of the eye. Color-blindness is the result of a disorder of one or more sets of cones. The great majority of people with some degree of deficiency lack either red or green cones, and cannot distinguish between those two colours. Complete colour vision deficiency (monochromatic vision), in which none of the sets of colour cones works, is very rare. Most deficiencies of colour vision are inherited, usually by male children through their mothers from a grandfather with the condition. 40. What is Electroretinogram ? Electroretinogram is a graphic record of electrical activity of the retina used especially in the diagnosis of retinal conditions. It shows the electrical response of the eye's light-sensitive cells, called rods and cones. These cells are part of the retina (the back part of the eye). 41. What is perimetry ? Perimetry is the measurement of the limits of the visual field of a person. A perimetry test (visual field test) is an eye examination that can detect dysfunction in central and peripheral vision which may be caused by various medical conditions such as glaucoma, stroke, pituitary disease, brain tumours or other neurological deficits.

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college 42. What do you mean by Binocular vision ? Binocular vision is a type of vision in which an animal having two eyes is able to perceive a single three-dimensional image of its surroundings. Binocular vision is the ability to maintain visual focus on an object with both eyes, creating a single visual image. Lack of binocular vision is normal in infants. Adults without binocular vision experience distortions in depth perception and visual measurement of distance.

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