Chapter 12 Senses Nervous System III - Senses Sensory Receptors • specialized cells or multicellular structures that collect information from the environment

General Senses • stimulate neurons to send impulses along sensory fibers to • receptors that are widely distributed throughout the the brain body Sensation • skin, various organs and joints • a feeling that occurs when brain becomes aware of sensory Special Senses impulse • specialized receptors confied to structures in the head Perception • eyes and ears • a person’s view of the stimulus; the way the brain interprets the information 1 2

Receptor Types Sensations

Chemoreceptors Sensation • respond to changes in chemical concentrations occurs when the brain becomes aware of sensory impulses Pain receptors (Nociceptors) • respond to tissue damage Perception Thermoreceptors occurs when the brain interprets sensory impulses • respond to changes in temperature

Mechanoreceptors Projection • respond to mechanical forces process in which the brain projects the sensation back to the apparent source Photoreceptors

• respond to light 3 it allows a person to pinpoint the region of 4 stimulation

1 Sensory Adaptation General Senses

• ability to ignore unimportant stimuli • senses associated with skin, muscles, joints, and viscera • involves a decreased response to a particular stimulus • three groups from the receptors (peripheral adaptations) or along the CNS pathways leading to the cerebral cortex (central • exteroceptive senses – senses associated with body surface; adaptation) touch, pressure, temperature, pain

• sensory impulses become less frequent and may cease • visceroceptive senses – senses associated with changes in viscera; blood pressure stretching blood vessels, ingesting a meal • stronger stimulus is required to trigger impulses • proprioceptive senses – senses associated with changes in 5 muscles and tendons 6

Touch and Pressure Senses Touch and Pressure Receptors

Free nerve endings Meissner’s corpuscles • common in epithelial • Small, oval masses of flattened tissues connective tissue cells in connective • simplest receptors tissue cells in a connective tissue • sense itching sheath with 2 or more sensory nerve fibers branching into each corpusule Pacinian corpuscles • abundant in hairless portions of • Relatively large, ellipsoidal skin; lips structures composed of connective • detect fine touch; distinguish tissue fivers and cells between two points on the skin • common in deeper subcutaneous tissues, tendons, and ligaments • detect heavy pressure and vibrations 7 8

2 Sense of Pain Temperature Senses

• free nerve endings Warm receptors • sensitive to temperatures above 25oC (77o F) • widely distributed • unresponsive to temperature above 45oC (113oF) • nervous tissue of brain lacks pain receptors Cold receptors • sensitive to temperature between 10oC (50oF) and 20oC • stimulated by tissue damage, chemical, mechanical forces, (68oF) or extremes in temperature

Pain receptors • adapt very little, if at all • respond to temperatures below 10oC • respond to temperatures above 45oC 9 10

Visceral Pain Referred Pain

• may occur due to sensory impulses from two regions following a common nerve pathway to brain • pain receptors are the only receptors in viscera whose stimulation produces sensations

• pain receptors respond differently to stimulation

• not well localized

• may feel as if coming from some other part of the body • known as referred pain

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3 Regulation of Pain Impulses Pain Nerve Pathways

Thalamus Chronic pain fibers Acute pain fibers • allows person to be aware • C fibers • A-delta fibers of pain • thin, myelinated • thin, unmyelinated Pain Inhibiting Substances • conduct impulses more • enkephalins • conduct impulses Cerebral Cortex slowly • serotonin rapidly • judges intensity of pain • associated with dull, • endorphins • associated with • locates source of pain aching pain sharp pain • produces emotional and • difficult to pinpoint • well localized motor responses to pain

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Proprioceptors Visceral Senses

• mechanoreceptors-respond to mechanical stress on Receptors are found on internal organs tissues Free nerve endings • send information to spinal cord and CNS about body position and length and tension of muscles Lamellated corpuscles

• Main kinds of proprioreceptors • Pacinian corpuscles – in joints • muscle spindles – in skeletal muscles* • Golgi tendon organs – in tendons*

*stretch receptors-respond to stretching in a tissue 15 16

4 Special Senses Sense of Smell

• sensory receptors are within large, complex sensory Olfactory Receptors organs in the head • chemoreceptors • respond to chemicals dissolved in liquids • smell in olfactory organs Olfactory Organs • taste in taste buds • contain olfactory receptors and supporting epithelial cells • hearing and equilibrium in ears • cover parts of nasal cavity, superior nasal conchae, • sight in eyes and a portion of the nasal septum

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Olfactory Nerve Pathways Olfactory Stimulation

• olfactory organs located high in the nasal cavity above the usual pathway of inhaled air Once olfactory receptors are stimulated, nerve impulses travel through • olfactory receptors undergo sensory adaptation • olfactory nerves olfactory bulbs olfactory rapidly tracts limbic system (for emotions) and olfactory cortex (for interpretation) • sense of smell drops by 50% within a second after stimulation Olfactory Code • hypothesis • odor that is stimulated by a distinct set of receptor cells and its associated receptor proteins 19 20

5 Sense of Taste Taste Receptors

75-80% of taste comes from smell

Taste Buds • organs of taste • located on papillae of tongue, roof of mouth, linings of cheeks and walls of pharynx Taste Receptors • chemoreceptors • taste cells – modified epithelial cells that function as receptors • taste hairs –microvilli that protrude from taste cells; sensitive parts of taste cells 21 22

Taste Sensations Taste Nerve Pathways

Five Primary Taste Sensations • sweet – stimulated by carbohydrates Sensory impulses from taste receptors travel along • sour – stimulated by acids • cranial nerves to • salty – stimulated by salts • medulla oblongata to • bitter – stimulated by many organic compounds • thalamus to • umani - stimulated by certain amino acids • gustatory cortex (for interpretation)

Spicy foods activate pain receptors

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6 Hearing External Ear

• auricle Ear – organ of hearing • collects sounds waves • external auditory meatus • lined with ceruminous glands • carries sound to tympanic Three Sections membrane • External • terminates with tympanic • Middle membrane • Inner • tympanic membrane • vibrates in response to sound waves • tympanic reflex-two small skeletal muscles protect the 25 26 ear from loud noices

Middle Ear Auditory Tube • tympanic cavity • air-filled space in temporal bone • eustachian tube • auditory ossicles • connects middle ear to • vibrate in response to tympanic throat membrane • helps maintain equal • malleus, incus, and stapes pressure on both sides • oval window of tympanic membrane • opening in wall of tympanic • usually closed by cavity valve-like flaps in throat • stapes vibrates against it to move fluids in inner ear

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7 Inner Ear Equilibrium

Three Parts of Labyrinths • cochlea Static Equilibrium Dynamic Equilibrium • functions in hearing • vestibule (utricle • semicircular canals • semicircular canals and saccule) • sense rotation and • functions in • sense position of movement of head and equilibrium head when body is body • vestibule • functions in not moving equilibrium

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Eyelid Sight

• palpebra • composed of four layers • skin Visual Accessory Organs • muscle • eyelids • connective tissue • conjunctiva • lacrimal apparatus • orbicularis oculi - closes • extrinsic eye muscles • levator palperbrae superioris – opens • tarsal glands – secrete oil onto eyelashes • conjunctiva – mucous membrane; lines eyelid and covers portion of eyeball

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8 Lacrimal Apparatus Extrinsic Eye Muscles

• lacrimal gland Superior rectus • lateral to eye • rotates eye up and • secretes tears medially • canaliculi • collect tears Inferior rectus • lacrimal sac • rotates eye down • collects from canaliculi • nasolacrimal duct and medially • collects from lacrimal sac • empties tears into nasal Medial rectus cavity • rotates eye medially

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Extrinsic Eye Muscles Structure of the Eye

Lateral rectus • rotates eye laterally • hollow • spherical • wall has 3 layers Superior oblique • outer fibrous tunic • rotates eye down and • middle vascular tunic laterally • inner nervous tunic

Inferior oblique • rotates eye up and laterally 35 36

9 Outer Tunic Middle Tunic

Iris Cornea • anterior portion • anterior portion • pigmented • transparent • controls light intensity by controlling the size of the • light transmission pupil • light refraction Ciliary body • anterior portion Sclera • pigmented • posterior portion • holds lens • opaque, white • moves lens for focusing • protection Choroid coat • provides blood supply • pigments absorb extra light 37 38

Anterior Portion of Eye Lens

• filled with aqueous humor

• transparent • biconvex • lies behind iris • largely composed of lens fibers • elastic • held in place by suspensory ligaments of ciliary body

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10 Ciliary Body Accommodation

• forms internal ring around front of eye • changing of lens shape to view objects • ciliary processes – radiating folds • ciliary muscles – contract and relax to move lens

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Inner Tunic Posterior Cavity

• retina • contains vitreous humor – thick gel that holds retina • contains visual receptors flat against choroid coat • continuous with optic nerve • ends just behind margin of the ciliary body • composed of several layers • macula lutea – yellowish spot in retina • fovea centralis – center of macula lutea; produces sharpest vision • optic disc – blind spot; contains no visual receptors • vitreous humor – thick gel that holds retina flat against choroid coat

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11 Light Refraction Types of Lenses

Refraction Convex lenses cause Concave lenses cause • bending of light light waves to converge light waves to diverge • occurs when light waves pass at an oblique angle into mediums of different densities

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Focusing On Retina Visual Receptors • as light enters eye, it is refracted by • convex surface of cornea Rods Cones • convex surface of lens • long, thin projections • short, blunt projections • image focused on retina is upside down and reversed from • contain light sensitive • contain light sensitive left to right pigment called rhodopsin pigments called • hundred times more erythrolabe, chlorolabe, sensitive to light than cones and cyanolabe • provide vision in dim light • provide vision in bright • produce colorless vision light • produce outlines of objects • produce sharp images • produce color vision • 3 types: red, green and 47 blue 48

12 Stereoscopic Vision Life-Span Changes

• provides perception of distance and depth • results from formation of two slightly different retinal Age related hearing loss due to images • damage of hair cells in organ of Corti • degeneration of nerve pathways to the brain • tinnitus

Age-related visual problems include • dry eyes • floaters (crystals in vitreous humor) • loss of elasticity of lens • glaucoma • cataracts

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Clinical Application

Refraction Disorders

• concave lens corrects • convex lens corrects nearsightedness farsightedness

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