Senses • OUTPUT: Motor Endings—Axon Terminals of Motor Neurons • Innervate Effectors (Muscle Fibers and Glands)

Home , Superior rectus muscle

PowerPoint® Lecture Slides Basic Structural Components of the PNS prepared by Leslie Hendon University of Alabama, Birmingham • INPUT: Sensory receptors—pick up stimuli from inside or outside the body • Nerves and ganglia – connecting structures C H A P T E R • Nerves—bundles of peripheral axons

Part 1 • Ganglia—clusters of peripheral neuronal cell bodies Senses • OUTPUT: Motor endings—axon terminals of motor neurons • Innervate effectors (muscle fibers and glands)

Peripheral Sensory Receptors Peripheral Sensory Receptors

• …Are structures that pick up sensory stimuli • Sensory receptors also classified according to and initiate signals in sensory axons • Location • Two main categories of sensory receptors • Type of stimulus detected • 1. Free nerve endings of sensory neurons • Structure • Monitor general sensory information • 2. Complete receptor cells—specialized epithelial cells or small neurons • Monitor most types of special sensory information

Classification by Location Classification by Stimulus Detected

• Exteroceptors—sensitive to stimuli arising from outside the body • Mechanoreceptors—respond to mechanical forces • Located at or near body surfaces • Touch, pressure, stretch, vibration, and itch • Include receptors for touch, pressure, pain, and temperature • Interoceptors—receive stimuli from internal viscera • Baroreceptors monitor blood pressure • Located in digestive tube, bladder, and lungs • Thermoreceptors—respond to temperature changes • Monitor a variety of stimuli • Chemoreceptors • Changes in chemical concentration • Respond to chemicals in solution • Taste stimuli • Stretching of tissues • Photoreceptors—respond to light • Temperature • Located in the eye Proprioceptors • • Nociceptors • Located in skeletal muscles, tendons, joints, and ligaments • Monitor degree of stretch • Respond to harmful stimuli that result in pain • Send inputs on body movement to the CNS

1 Classification by Structure Free Nerve Endings (or ‘unencapsulated’) • (Discussion here of General sensory receptors only, not special senses) • Abundant in epithelia and underlying connective tissue • General senses are widely distributed • General senses include nerve endings of sensory neurons that • Respond to pain and temperature monitor: • Monitor affective senses • Touch • Pressure • Two specialized types of free nerve endings • Vibration • Epithelial tactile complexes (Merkel discs) • Stretch • Pain • Consist of tactile epithelial cell innervated by • Temperature sensory nerve ending • Proprioception • Slowly adapting receptors for light touch • So, classification of General Sensory Receptors by structure …. • Hair follicle receptors—wrap around hair follicles • Divided into two groups • Rapidly adapting receptors • Free nerve endings • Encapsulated nerve endings

Unencapsulated Nerve Endings Encapsulated Nerve Endings • Consist of one or more end fibers of sensory neurons • Enclosed in connective tissue • Mechanoreceptors • Include four main types • Tactile (Meissner’s) corpuscles • Lamellar (Pacinian) corpuscles • Bulbous corpuscles (Ruffini endings) • Proprioceptors

Encapsulated Nerve Endings Tactile Corpuscles • Tactile (Meissner’s) corpuscles • Spiraling nerve ending surrounded by Schwann cells • Occur in the dermal papillae • Rapidly adapting receptors for discriminative touch • Occur in sensitive, hairless areas of the skin • Lamellar Corpuscles • Single nerve ending surrounded by layers of flattened Schwann cells • Occur in the hypodermis • Sensitive to deep pressure—rapidly adapting receptors • Bulbous Corpuscles • Located in the dermis and respond to pressure • Monitor continuous pressure on the skin—adapt slowly

2 Lamellar Corpuscles and Bulbous Corpuscles More Encapsulated Nerve Endings

• Proprioceptors • Monitor stretch in locomotory organs • Three types of proprioceptors: • Muscle spindles— measure the changing length of a muscle • Imbedded in the perimysium between muscle fascicles • Golgi tendon organs— monitor tension within tendons • Located near the muscles/tendon junction • Joint kinesthetic receptors--- monitor stretch within synovial joints • Sensory nerve endings within the joint capsules

Proprioceptors Structure of Proprioceptors Secondary sensory γ Efferent (motor) endings (type II fiber) fiber to muscle spindle

α Efferent Primary (motor) fiber sensory to extrafusal endings muscle fibers (type Ia Extrafusal fiber) muscle Muscle fiber spindle Intrafusal Connective muscle tissue capsule fibers

Sensory fiber

Tendon organ Tendon

The Special Senses The Chemical Senses: Taste and Smell

• Taste, smell, sight, hearing, and balance • Characteristics of Special sensory receptors • Taste—gustation • Localized—confined to the head region • Smell—olfaction • Receptors are not free endings of sensory neurons • Receptors—classified as chemoreceptors • Special receptor cells • Respond to chemicals • Are neuron-like epithelial cells or small peripheral neurons • Food dissolved in saliva • Transfer sensory information to other neurons • Airborne chemicals that dissolve in fluids of in afferent pathways (i.e., cranial nerves) the nasal mucosa

3 Taste—Gustation Taste Buds

• Taste receptors • Collection of 50–100 epithelial cells • Occur in taste buds • Contain two major cell types • Most are found on the surface of the tongue • Gustatory epithelial cells • Located within tongue papillae • Basal epithelial cells • Two types of papillae (with taste buds) • Contain long microvilli—extend through a • Fungiform papillae taste pore to the surface of the epithelium • Vallate papillae • Cells in taste buds replaced every 7–10 days

Taste Buds Taste Sensation and the Gustatory Pathway

Connective Epiglottis tissue Gustatory hair • Five basic qualities of taste Taste fibers of cranial Palatine tonsil nerve • Sweet, sour, salty, bitter, and umami Lingual tonsil • “Umami” is elicited by glutamate

Vallate papilla • The “taste map” is a myth Stratified Basal Gustatory Taste squamous epithelial epithelial pore epithelium cells cells of tongue • All taste modalities can be elicited from all areas containing taste buds

Fungiform papillae Taste bud (a) Taste buds associated with (b) Enlarged section of a (c) Enlarged view of a taste bud fungiform and vallate papillae vallate papilla (micrograph, 160X)

Gustatory Pathway Gustatory Pathway from Taste Buds

Gustatory cortex • Taste information reaches the cerebral cortex (in insula)

• Primarily through the facial (VII) and Thalamic nucleus glossopharyngeal (IX) nerves (ventral • Some taste information through the vagus nerve (X) posteromedial nucleus) Pons Solitary nucleus • Sensory neurons synapse in the medulla in medulla oblongata • Relay neurons located in the solitary nucleus Facial nerve (VII) Vagus nerve (X) • Impulses are then transmitted to the thalamus and Glossopharyngeal ultimately to the gustatory area of the cerebral nerve (IX) cortex in the insula • Smell contributes to taste perception

4 Smell (Olfaction) Smell (Olfaction)

• Olfactory receptors are part of the olfactory • Cell bodies of olfactory sensory neurons epithelium • Located in olfactory epithelium • Olfactory epithelium is pseudostratified • Have an apical dendrite that projects to the epithelial columnar and contains three main cell types: surface • Ends in a knob from which olfactory cilia radiate • Olfactory sensory neurons • Olfactory cilia act as receptive structures for smell Supporting epithelial cells • • Mucus captures and dissolves odor molecules • Basal epithelial cells

Smell (Olfaction) Olfactory Receptors

Olfactory • Axons of olfactory epithelium Olfactory Mitral cell epithelium tract (output cell) • Gather into bundles— these are the filaments/axons Glomeruli Olfactory bulb of the olfactory nerve (C.N. I) Olfactory tract Cribriform plate Olfactory bulb • These axons pass through the cribriform plate of the of ethmoid bone Filaments of ethmoid bone olfactory nerve Lamina propria Nasal connective tissue conchae Olfactory Axon • Attach to the olfactory bulbs and synapse with gland Basal cell mitral cells Olfactory Route of sensory neuron • Mitral cells transmit impulses along the olfactory inhaled air Olfactory Supporting cell (a) epithelium tract to Dendrite Olfactory cilia 1. Limbic system Mucus

Route of inhaled air 2. Piriform lobe of the cerebral cortex containing odor molecules (b)

Disorders of the Chemical Senses The Eye and Vision

• Anosmia—absence of the sense of smell • Visual organ—the eye • Due to injury, colds, allergies, or zinc deficiency • 70% of all sensory receptors are in the eyes • Uncinate fits—distortion of smells or olfactory hallucinations • 40% of the cerebral cortex is involved in • Often result from irritation of olfactory pathways processing visual information • After brain surgery or head trauma; sometimes • Anterior one-sixth of the eye’s surface is precedes seizures in olfactory cortex (auras) visible

5 Accessory Structures of the Eye Accessory Structures of the Eye

Levator palpebrae • Eyebrows—coarse hairs on the superciliary arches • Conjunctiva— superioris muscle Orbicularis • Eyelids (palpebrae)—separated by the palpebral transparent mucous oculi muscle Eyebrow fissure membrane Tarsal plate Palpebral • Meet at the medial and lateral angles (canthi) • Palpebral conjunctiva conjunctiva Tarsal glands • Lacrimal caruncle—reddish elevation at the medial Cornea canthus • Bulbar conjunctiva Palpebral fissure • Tarsal plates—connective tissue within the eyelids • Conjunctival sac Eyelashes • Tarsal glands—modified sebaceous glands Bulbar conjunctiva Conjunctival sac Orbicularis oculi muscle (b) Lateral view; some structures shown in sagittal section

Accessory Structures of the Eye Extrinsic Eye Muscles

Lacrimal apparatus— • Six muscles that control movement of the eye • Lacrimal sac keeps the surface of the Lacrimal gland • Originate in the walls of the orbit eye moist Excretory ducts • Insert on outer surface of the eyeball of lacrimal glands • Lacrimal gland— • Annular ring or common tendinous ring— origin of the Lacrimal punctum produces lacrimal Lacrimal canaliculus four rectus muscles fluid The six extrinsic eye muscles are: Nasolacrimal duct • • Lacrimal sac— • Lateral rectus and medial rectus fluid empties into Inferior meatus of nasal cavity • Superior rectus and inferior rectus nasal cavity Nostril • Superior oblique (“down and out”) and inferior oblique (“up and out”)

• Strabismus: misalignment of the eyes (“cross-eyes” or “squint-eyes”)

Extrinsic Eye Muscles Summary of Muscle Actions

Superior oblique muscle

Trochlea Trochlea Muscle Action Controlling cranial nerve Superior oblique tendon Superior Superior rectus oblique Superior rectus muscle Lateral Medial Lateral rectus Moves eye laterally VI (abducens) rectus rectus Lateral rectus muscle Medial rectus Moves eye medially III (oculomotor) Superior rectus Elevates eye and turns it medially III (oculomotor) Inferior rectus Depresses eye and turns it medially III (oculomotor) Inferior oblique Elevates eye and turns it laterally III (oculomotor) Inferior Inferior Superior oblique Depresses eye and turns it laterally IV (trochlear) oblique rectus

Common Inferior Inferior tendinous rectus oblique (c) Summary of muscle actions and innervating cranial nerves ring muscle muscle (a) Lateral view of the right eye (b) Anterior view of the right eye

6 Anatomy of the Eyeball

Ora serrata

Ciliary body Sclera • Components of the eye serve to... Choroid Ciliary zonule • Protect and support the photoreceptors (suspensory Retina ligament) Macula lutea • Gather, focus, and process light into precise images Cornea Fovea centralis Iris • Basic parts: Pupil Posterior pole Optic nerve • Anterior pole—most anterior part of the eye Anterior pole • Posterior pole—most posterior part of the eye Anterior segment • Internal cavity—contains fluids (humors) (contains aqueous humor) • External walls—composed of three tunics from superficial to deep : Lens 1. Fibrous layer (outer tunic) Central artery • Scleral venous and vein of sinus • 2. Vascular layer (middle tunic) the retina Posterior segment Optic disc • 3. Neural or inner layer (nervous tunic/inner tunic) (contains vitreous humor) (blind spot) (a) Diagrammatic view. The vitreous humor is illustrated only in the bottom part of the eyeball.

1. The Fibrous Layer 2. The Vascular Layer • Most external layer of the eyeball • The middle coat of the eyeball • Composed of two regions of connective tissue • Composed of choroid, ciliary body, and iris • Sclera—posterior five-sixths of the tunic • Choroid—vascular, darkly pigmented membrane • White, opaque region • Forms posterior five-sixths of the vascular tunic • Brown color—from melanocytes • Provides shape and an anchor for eye muscles • Prevents scattering of light rays within the eye • Cornea—anterior one-sixth of the fibrous tunic • Choroid corresponds to the arachnoid and pia maters • Clear collagen fiber sheets sandwiched • Ciliary body—thickened ring of tissue, which encircles the lens between thin epithelial layers • Composed of ciliary muscle (smooth muscle) • Avascular, but heals quickly • Ciliary processes—posterior surface of the ciliary body • Richly supplied with nerve endings • Ciliary zonule (suspensory ligament): extend from ciliary processes and attached around entire circumference of the lens • Limbus—junction between sclera and cornea • Scleral venous sinus—allows aqueous humor to drain

The Iris (third part of vascular layer) 3. The Inner Layer (Retina)

• Iris • Retina—the deepest tunic • Visible colored part of the eye • Composed of two layers: • Attached to the ciliary body • Pigmented layer—single layer of melanocytes • Composed of smooth muscle (2 muscles): • Neural layer—sheet of nervous tissue • Sphincter pupillae muscle (parasympathetic innervation / Contains three main types of neurons ACh) • • Dilator pupillae muscle (sympathetic innervation /NE) • Photoreceptor cells • Pupil—the round, central opening • Bipolar cells • Pupillary light reflex • Ganglion cells • Protective response of pupil constriction when a bright Also present: 2 other neuron types: light is flashed in the eye • • Horizontal cells • Amacrine cells

7 Microscopic Anatomy of the Retina The Inner Layer (continued)

Photoreceptors Bipolar Choroid Ganglion cells Rod Outer segments cells Cone Nuclei of of rods and cones • Photoreceptor cells send signals to bipolar cells ganglion cells • Bipolar cells send signals to ganglion cells to generate nerve impulses in ganglion cell axons

• Axons from ganglion cells run along internal surface of the retina

Amacrine cell Horizontal cell Pathway of signal output Pigmented Axons of Nuclei Nuclei of Pigmented Pathway of light layer of retina ganglion cells of bipolar rods and layer of • Converge posteriorly to form the optic nerve cells cones retina (b) Cells of the neural layer of the retina (c) Photomicrograph of retina

Posterior Aspect of the Eyeball Photoreceptors

Neural layer of retina • Two main types Pigmented layer of • Rod cells—more sensitive to low levels of Pathway of light retina Choroid light and to movement Sclera Optic disc • Allow vision in dim light • Cone cells—operate best in bright light Central artery and vein of retina Optic nerve • Enable high-acuity, color vision • Considered neurons

(a) Posterior aspect of the eyeball

Photoreceptors Photoreceptors Process of bipolar Light Light cell Light

Synaptic Inner terminals fibers • Rods and cones have an inner and outer Rod cell Rod body cell body segment Nuclei Cone cell body Outer segments are receptor regions where Mitochondria • Outer fiber Connecting cilia light absorbing pigments are present Inner segment

Apical • Light particles/energy modify the visual microvillus pigment and generate a nerve impulse

Outer Outer segment Discs containing visual pigments Discs being phagocytized Pigmented layer Pigmented layer

Pigment cell Melanin nucleus granules

8 Photoreceptors Regional Specializations of the Retina • Ora serrata retinae • Photoreceptors • Neural layer ends at the posterior margin of the ciliary body • Vulnerable to damage by light or heat • Pigmented layer covers ciliary body and posterior • Cannot regenerate if destroyed surface of the iris • Continuously renew and replace their outer • Macula lutea— contains mostly cones segments • Fovea centralis— contains only cones • Region of highest visual acuity • Optic disc— area of no receptors (where ganglion cell axons exit the eye as C.N.II) and therefore a “blind spot”

View of the Eye Blood Supply of the Retina

Ora serrata • Retina receives Ciliary body Sclera Central artery and vein Choroid blood from two emerging Ciliary zonule from the (suspensory Retina sources optic disc ligament) Macula lutea Macula Cornea • Outer third of the lutea Fovea centralis Optic disc Iris retina—supplied Posterior pole Pupil Retina Optic nerve by capillaries in Anterior pole the choroid Anterior segment (contains • Inner two-thirds of aqueous humor) the retina— Lens Central artery supplied by central Scleral venous and vein of sinus the retina artery and vein of Posterior segment Optic disc the retina (contains vitreous humor) (blind spot) (a) Diagrammatic view. The vitreous humor is illustrated only in the bottom part of the eyeball.

Internal Chambers and Fluids Internal Chambers and Fluids • The lens and ciliary zonules divide the eye • Posterior segment (cavity) Posterior Iris segment Cornea Lens (contains • Filled with vitreous humor Lens epithelium vitreous Lens humor) • Clear, jelly-like substance Cornea 2 • Transmits light Corneal epithelium • Supports the posterior surface of the lens Corneal endothelium Aqueous humor

• Helps maintain intraocular pressure Anterior chamber Anterior Ciliary zonule Anterior segment (suspensory • segment Posterior chamber (contains ligament) • Divided into anterior and posterior chambers 1 Aqueous humor is aqueous formed by filtration from humor) 3 1 • Anterior chamber—between the cornea and iris the capillaries in the ciliary Scleral venous Ciliary processes. sinus processes 2 Aqueous humor flows Corneoscleral • Posterior chamber—between the iris and lens junction Ciliary from the posterior chamber body through the pupil into the Ciliary • Filled with aqueous humor anterior chamber. Some also muscle flows through the vitreous Bulbar • Renewed continuously humor (not shown). conjunctiva 3 Aqueous humor is Sclera • Formed as a blood filtrate reabsorbed into the venous blood by the scleral venous • Supplies nutrients to the lens and cornea sinus.

9 The Lens The Eye as an Optical Device

• A thick, transparent, biconvex disc • Structures in the eye bend light rays • Held in place by its ciliary zonule • Light rays converge on the retina at a single focal point • Light bending structures (refractory media) are interfaces • Lens epithelium—covers anterior surface of between media: the lens • The lens, cornea, and humors • Lens fibers form the bulk of the lens • Accommodation—curvature of the lens is adjustable • New lens fibers are continuously added • Allows for focusing on nearby objects • Lens enlarges throughout life

PLAY Vision

The Eye as an Optical Device The Eye as an Optical Device Sympathetic activation Nearly parallel rays from distant object Myopic eye (nearsighted) Hyperopic eye (farsighted) Lens Focal point

Eyeball Eyeball Ciliary zonule too long too short Ciliary muscle Inverted image

(a) Lens is flattened for distant vision. Sympathetic input Uncorrected Uncorrected relaxes the ciliary muscle, tightening the ciliary zonule, Focal point is in front of retina. Focal point is behind retina. and flattening the lens.

Parasympathetic activation Divergent rays Inverted from close object image

Concave lens moves focal Convex lens moves focal point further back. point forward. Corrected Corrected

(b) Lens bulges for close vision. Parasympathetic input contracts the ciliary muscle, loosening the ciliary zonule, allowing the lens to bulge. Copyright © 2011 Pearson Education, Inc. Figure 16.13 Copyright © 2011 Pearson Education, Inc. Figure 16.14

Visual Pathways Visual Pathways to the Cerebral Cortex • Pathway begins at the retina ... • Light activates photoreceptors Most visual information travels to the cerebral • • Photoreceptors signal bipolar cells cortex • Bipolar cells signal ganglion cells • Responsible for conscious “seeing” • Axons of ganglion cells exit eye as the optic • Other pathways travel to nuclei in the nerve (C.N. II) • After optic chiasm, where partial decussation occurs, midbrain and diencephalon optic tracts send axons to ... • Lateral geniculate nucleus (LGN) of the thalamus, where axons synapse with thalamic neurons • Thalamic neuron axons form optic radiation & reach the primary visual cortex in occipital lobe

10 Visual Pathways to the Brain and Visual Fields Visual Pathways to Other Parts of the Brain Both eyes Fixation point

• Some axons from the optic tracts ... Left eye only • Branch to midbrain & synapse in the ...

Right eye only • Superior colliculi Right eye Left eye Optic nerve Supra- • Pretectal nuclei chiasmatic nucleus Pretectal Optic chiasma nucleus Other branches from the optic tracts Optic tract • • Project to the suprachiasmatic nucleus

Uncrossed (ipsilateral) fiber Crossed Lateral (contralateral) fiber geniculate Optic nucleus of radiation thalamus

Superior Occipital colliculus lobe (primary visual cortex) (a) The visual fields of the two eyes overlap considerably. Note that fibers from the lateral portion of each retinal field do not cross at the optic chiasma. Copyright © 2011 Pearson Education, Inc. Figure 16.15a Copyright © 2011 Pearson Education, Inc.

Disorders of the Eye and Vision Embryonic Development of the Eye

• Age-related macular degeneration (AMD) • Eyes develop as outpocketings of the brain • Involves the buildup of visual pigments in the retina • By week 4, optic vesicles protrude from the • Retinopathy of prematurity diencephalon • Blood vessels grow within the eyes of premature infants Optic cup Ectoderm Retina • Vessels have weak walls—causes hemorrhaging and Invaginating Optic cup Lens blindness lens placode vesicle

• Trachoma—contagious infection of the conjunctiva Optic stalk Optic stalk • Presbyopia: “old eye”– loss of lens plasticity with Optic fissure age (b) Week 4, late. Optic vesicles invaginate Optic fissure to form the optic cups. The overlying surface ectoderm thickens to form the (c) Week 5. Lens placode invaginates lens placode. and forms the lens vesicle.

Embryonic Development of the Eye Embryonic Development of the Eye Mesenchyme invades Developing neural Lens layer of the retina vesicle • Ectoderm thickens and forms a lens placodes Developing pigmented layer of the retina Central • By week 5, a lens vesicle forms artery Surface ectoderm

• Internal layer of the optic cup becomes (d) Week 6. The neural and pigmented layers of the retina differentiate from the optic cup. Central artery reaches the interior • Neural retina of the eye. Mesenchyme derived from neural crest invades.

• External layer becomes Sclera and Iris choroid Corneal Vitreous epithelium • Pigmented retina humor Central Fused eyelids artery Cornea • Optic fissure—pathway for blood vessels (fibrous part) Lens Deteriorating internal blood vessels (e) Week 7. Mesenchyme surrounds and invades the optic cup to form the fibrous and vascular layers and the vitreous humor. Lens vesicle forms the lens. Surface ectoderm forms the corneal epithelium and the conjunctiva.

11 The Ear: Hearing and Equilibrium Structure of the Ear

• The ear—receptor organ for hearing and External ear Middle Internal ear equilibrium ear (labyrinth)

• Composed of three main regions: Auricle (pinna) • Outer ear—functions in hearing • Middle ear—functions in hearing Helix • Internal ear—functions in both hearing and equilibrium

Lobule

External acoustic Tympanic Pharyngotympanic meatus membrane (auditory) tube (a) The three regions of the ear

The Outer (External) Ear The Middle Ear

• Composed of • Composed of • The auricle (pinna) • The tympanic cavity • Helps direct sounds • A small, air-filled space • Located within the petrous portion of the • External acoustic meatus temporal bone • Lined with skin • Medial wall is penetrated by • Contains hairs, sebaceous glands, • Oval window and ceruminous glands • Round window • Tympanic membrane • Pharyngotympanic tube (auditory or Eustachian tube) • Forms the boundary between the • Links the middle ear and pharynx external and middle ear

Structures of the Middle Ear The Middle Ear

• Ear ossicles—smallest View Oval window bones in the body (deep to stapes) Semicircular • Malleus—attaches to Entrance to mastoid canals Superior Malleus Incus Epitympanic recess antrum in the the eardrum epitympanic recess Vestibule • Incus—between the Lateral Malleus malleus and stapes (hammer) Anterior Incus Vestibular • Stapes—vibrates Auditory (anvil) nerve against the oval window ossicles Stapes Cochlear • Tensor tympani (C.N V) (stirrup) nerve and stapedius (C.N. VII) Cochlea Tympanic • Two tiny skeletal membrane Pharyngotympanic muscles in the middle Round window (auditory) tube ear cavity

(b) Middle and internal ear Pharyngotym- Tensor Tympanic Stapes Stapedius panic tube tympani membrane muscle muscle (medial view)

12 The Internal Ear The Internal Ear

Oval window • Internal ear—also called the labyrinth (deep to stapes) Semicircular Entrance to mastoid canals • Lies within the petrous portion of the antrum in the epitympanic recess Vestibule temporal bone Malleus (hammer) Incus Vestibular Bony labyrinth—a cavity consisting of three Auditory • (anvil) nerve ossicles parts Stapes Cochlear (stirrup) nerve

• Semicircular canals Tympanic Cochlea membrane • Vestibule Pharyngotympanic Round window (auditory) tube • Cochlea

(b) Middle and internal ear

The Internal Ear The Internal Ear • Membranous labyrinth • Series of membrane-walled sacs and ducts • Fit within the bony labyrinth • Consists of three main parts • Semicircular ducts • Utricle and saccule • Cochlear duct • Filled with a clear fluid—endolymph • Confined to the membranous labyrinth • Bony labyrinth is filled with perilymph • Continuous with cerebrospinal fluid

The Internal Ear The Cochlea • A spiraling chamber in the bony labyrinth • Coils around a pillar of bone—the modiolus Temporal • Spiral lamina—a spiral of bone in the modiolus bone Facial nerve The cochlear nerve runs through the core of the modiolus Vestibular nerve • Semicircular ducts in semicircular canals Superior vestibular • Has three areas/ chambers: scala media, scala vestibuli. And Anterior ganglion scala tympani Posterior Inferior vestibular Lateral ganglion • The cochlear duct (scala media)—contains the Cochlear nerve Cristae ampullares in the membranous Maculae receptors for hearing Spiral organ (of Corti) ampullae • Lies between two chambers Utricle in Cochlear duct in vestibule cochlea • The scala vestibuli Saccule in Stapes in Round window • The scala tympani vestibule oval window • The vestibular membrane—the roof of the cochlear duct • The basilar membrane—the floor of the cochlear duct

13 The Cochlea The Cochlea

Modiolus Cochlear nerve, Vestibular membrane Osseous spiral lamina division of the Tectorial membrane Spiral vestibulocochlear Scala nerve (VIII) Cochlear duct vestibuli ganglion Spiral ganglion (scala media; (contains Osseous spiral contains perilymph) lamina endolymph) Vestibular membrane Stria vascularis Cochlear duct (scala media) Spiral organ Helicotrema (of Corti) Scala (a) Basilar tympani (contains membrane perilymph) Tectorial membrane Inner hair cell Vestibular membrane Osseous spiral lamina Tectorial membrane Hairs (stereocilia) Spiral Afferent nerve Scala Cochlear duct ganglion (b) fibers vestibuli Outer hair cells (scala media; (contains contains perilymph) endolymph) Supporting cells Stria vascularis Fibers of cochlear Spiral organ nerve (of Corti) Scala tympani Basilar (contains membrane perilymph)

Basilar membrane

The Cochlea The Anatomy of the Cochlea

Modiolus Cochlear nerve, division of the vestibulocochlear • The cochlear duct (scala media) (continued) nerve (VIII) Spiral ganglion Osseous spiral • The Spiral organ (of Corti)— is the receptor epithelium for lamina Vestibular hearing membrane Cochlear duct (scala media) • Consists of: Helicotrema

• Supporting cells (a) Osseous spiral Tectorial Inner hair • Inner and outer hair cells (receptor cells) Vestibular membrane lamina membrane cell Tectorial membrane Hairs Afferent Spiral (stereocilia) Scala ganglion nerve fibers • Inner hair cells are the receptors that transmit Cochlear duct vestibuli Outer hair (scala media; (contains cells contains perilymph) vibrations of the basilar membrane endolymph) Supporting cells Stria vascularis Fibers of • Outer hair cells actively tune the cochlea and cochlear Spiral organ nerve amplify the signal (of Corti) Scala tympani Basilar (contains membrane perilymph) Basilar membrane (c) (b) Copyright © 2011 Pearson Education, Inc. Copyright © 2011 Pearson Education, Inc. Figure 16.20a–c

The Role of the Cochlea in Hearing The Vestibule

Auditory ossicles

Malleus Incus Stapes Cochlear nerve 1 Sound waves vibrate the • The central part of the bony labyrinth tympanic membrane. Scala vestibuli 2 Auditory ossicles vibrate. Oval • Lies medial to the middle ear window Helicotrema Pressure is amplified. 4a Scala tympani 3 Pressure waves created by the Cochlear duct stapes pushing on the oval window move through fluid in the scala • Utricle and saccule —suspended in perilymph 2 3 Basilar vestibuli. membrane 4b 4a Sounds with frequencies below hearing travel through the • Two egg-shaped parts of the membranous 1 helicotrema and do not excite hair cells. labyrinth 4b Sounds in the hearing range go through the cochlear duct, vibrating the basilar membrane and deflecting hairs on inner hair cells. • House the macula—a patch of sensory

Tympanic Round epithelium membrane window

14 The Vestibule The Maculae in the Internal Ear Macula of utricle Macula of saccule • Macula—contains receptor cells • Monitor the position of the head when the head is still Kinocilium Otolithic Otoliths membrane • Contains columnar supporting cells Stereocilia Hair bundle • Receptor cells here are also called hair cells • Synapse with the vestibular nerve • Tips of hair cells are embedded in otolithic membrane

• Membrane contains crystals of Hair cells Supporting calcium carbonate called otoliths Vestibular cells nerve fibers (a) Copyright © 2011 Pearson Education, Inc. Copyright © 2011 Pearson Education, Inc. Figure 16.22a

The Maculae in the Internal Ear The Semicircular Canals Otolithic Otoliths Hair Force of membrane cell gravity • Lie posterior and lateral to the vestibule • Anterior and posterior semicircular canals • Lie in the vertical plane at right angles • Lateral semicircular canal • Lies in the horizontal plane

Head upright Head tilted (b)

The Semicircular Canals The Semicircular Canals

• Semicircular duct—snakes through each Temporal bone Facial nerve semicircular canal Vestibular nerve Semicircular ducts in semicircular canals Superior vestibular • Membranous ampulla—located within bony Anterior ganglion Posterior Inferior vestibular ampulla Lateral ganglion Cochlear nerve Cristae ampullares • Houses a structure called a crista ampullaris in the membranous Maculae ampullae Spiral organ (of Corti) • Cristae contain receptor cells of rotational Utricle in Cochlear duct in vestibule cochlea acceleration Saccule in Stapes in Round window vestibule oval window • Epithelium contains supporting cells and receptor hair cells

15 Structure and Function of the Crista Ampullaris Structure and Function of the Crista Ampullaris

Cupula

Crista ampullaris Endolymph

Hair bundle (kinocilium plus stereocilia)

Fibers of Crista Hair cell Section of Cupula Flow of endolymph Membranous ampulla, vestibular ampullaris nerve labyrinth filled with endolymph Supporting Fibers of vestibular nerve cell

(a) Anatomy of a crista ampullaris in a semicircular canal (b) Scanning electron micrograph of a crista ampullaris (45X) At rest, the cupula stands upright. During rotational acceleration, endolymph As rotational movement slows, moves inside the semicircular canals in the endolymph keeps moving in the direction opposite the rotation (it lags direction of the rotation, bending the behind because of inertia). Endolymph flow cupula in the opposite direction from bends the cupula and excites the hair cells. acceleration and inhibiting the hair cells. (c) Movement of the cupula during rotational acceleration and deceleration

Equilibrium and Auditory Pathways Auditory Pathway from the Organ of Corti

Medial geniculate nucleus of thalamus • The equilibrium pathway

• Transmits information on the position and Primary auditory movement of the head cortex in temporal lobe Inferior colliculus • Most information goes to lower brain centers Lateral lemniscus Superior olivary Midbrain (reflex centers) nucleus (pons- medulla junction) • Cortical input to posterior insula Cochlear nuclei

• The ascending auditory pathway Medulla Vibrations • Transmits information from cochlear receptors Vestibulocochlear nerve Vibrations to the cerebral cortex Spiral ganglion of cochlear nerve • To the Superior temporal gyrus Bipolar cell Spiral organ (of Corti)

Disorders of Equilibrium and Hearing Disorders of Equilibrium and Hearing

• Motion sickness—carsickness, seasickness • Deafness • Popular theory for a cause—a mismatch of • Conduction deafness sensory inputs • Sound vibrations cannot be conducted to • Meniere’s syndrome—equilibrium is greatly the inner ear disturbed • Ruptured tympanic membrane, otitis • Excessive amounts of endolymph in the media, otosclerosis membranous labyrinth • Sensorineural deafness • Results from damage to any part of the auditory pathway

16 The Special Senses Throughout Life The Special Senses Throughout Life

• Smell and taste • Photoreceptors—fully formed by 25 weeks • Sharp in newborns • All newborns are hyperopic • In the fourth decade of life • By 3 months—image can be focused on the • Ability to taste and smell declines retina • By 6 months—depth perception is present

The Special Senses Throughout Life The Special Senses Throughout Life

• With increased age • In the newborn • The lens loses its clarity • Responses to sounds are reflexive • The dilator muscles of the iris become • Low-pitched and middle-pitched sounds can inefficient be heard • Visual acuity is dramatically lower in people • In the elderly over 70 • Hair cells are gradually lost • Ability to hear high-pitched sounds fades • Presbycusis—gradual loss of hearing with age