OBJECTIVES

This chapter should help you to:

· Name the classes of sensory receptors and the receptors in each class.
· Know the structure, function, location, and mode of transducing a stimulus into a neuronal action potential for the sensory rcceptors.
· Locate and identify the receptors in a slide or photomicrograph of an organ or tissue.
· Identify the named components of rcceptors' organs from a slide, diagram, or photomicrograph.
· Relate the microscopic structure of each receptor to its "adequate stimulus."

SYNOPSIS

I. GENERAL FEATURES OF THE SENSE ORGANS

Sense organs respond to stimuli by generating action potentials in an associated sensory (afferent) nerve cell process. Signals travel to the CNS for integration, allowing reflex or conscious reactions to environmental changes. Sensory tibers usually carry signals for only one sensory modality, eg, pain, touch, or temperature.

A. Classification: Receptors are classified by their relationship to the nervous system, their stimulus sensitivity, and the presence or absence of a capsule.


1. Relationship between the receptor and the nervous system a. Neuronal receptors. The sensory nerve is stimulated directly. Each stimulus partly depolarizes the nerve ending. Many such small generator potentials are summed to achieve threshold and fire an action potential. Examples include cutaneous (skin) recep tors and proprioceptors. b. Epithelial receptors are epithelial cells that generate a receptor potential (depolarize) in response to specific stimuli. Stimulated receptor cells release neurotransmitters to stimulate (indirectly) nearby nerve endings. Common in special sense organs, epithelial receptors include the eye's rods and cones, the car's hair cells, and the taste buds' sensory cells. c. Neuroepithelial receptors. Peripherally located neurons receive a stimulus and transmit the signal to the CNS along their own axons, eg, olfactory receptors.
2. Adequate stimulus is the stimulus to which receptors are most sensitive. Table 24-1 shows the sensory receptors classified by adequate stimulus. Receptors in one class may also respond to other stimuli at higher intensity.
3. Presence or absence of a capsule. Encapsulated receptors are surrounded by a specialized connective tissue capsule. Free nerve endings lack a capsule.

B. Distribution: Sense organ distribution maximizes detection of adequate stimuli. The receptive field is the body region in which a stimulus evokes a response. More receptors in a given area yield greater sensitivity. Touch receptors, for example, are abundant in the fingertips.

C. Adaptation: This refers to how rapidly receptors recover and respond to repeated stimuli.

II. RECEPTORS FOR SUPERFICIAL & DEEP SENSATION

A. Free Nerve Endings: These are the numerous and widely distributed peripheral dendritic branches of sensory neurons, whose soma are located mainly in craniospinal ganglia. These unencapsulated receptors are generally branches of unmyelinated or lightly myelinated fibers found in bundles beneath epithelia. As they penetrate the epithelium, they lose their myelin and branch among the epithelial cells. Branches of one nerve may cover a wide area and overlap the territories of others. Different free nerve endings have different adequate stimuli leg, pain, cold, warmth, touch, or pressure), but are structurally indistinguishable. Of the many receptors in skin, these are the most numerous. They also occur in the walls of hollow organs, where some monitor dilation and contraction. Others sense pain in the body's interior.

B. Merkel's Corpuscles: These unencapsulated touch receptors, deep in the epidermis, sense direct pressure and, via desmosomal attachments, indirect pressure. More abundant in the thick skin of the palms and soles, they have 2 main components: the specialized Merkel's cell (Chapter 18) and a specialized nerve ending that loses its Schwann's cell sheath as it penetrates the epidermal basal lamina. The nerve endings free end forms a flat Merkel's disk that contacts the Merkel's cell in synapselike junctions. The DNES-like Merkel's cell granules lack catecholamines.

C. Other Nerve Endings With Expanded Tips: These are also found in deep epidermis; some function as cold receptors, responding to local cooling.

D. Meissner's Corpuscles: These elongated ovoid mechanoreccptors (touch and superficial pressure) have a thin capsule and contain many stacked lamellae of flat Schwann's cells and fibroblasts. One or more nerve endings may enter from the base and zigzag through the stack. Commonly found in the skin's dermal papillae (Chapter 18), they are more numerous in the fingertips, palms, soles, and nipples.

E. Pacinian Corpuscles: These highly sensitive pressure receptors occur in deep dermis, hypo dermis, periosteum, joint capsules, and mesenteries. Their well-developed capsules consist of many layers of flat fibroblastlike cells separated by narrow fluid-filled spaces. Larger and rounder than Meissner's corpuscles, they resemble sliced onions in tissue sections. The nerve enters the capsule, loses its myelin, penetrates the core while covered by a few layers of flat Schwann's cells, and terminates near the pole opposite its entry, giving off several blunt branches.

F. Ruffini's Corpuscles: These slow-adapting mechanoreceptors are common in dermis, hypo dermis, and joint capsules. Their thin capsules surround a fluid-filled cavity containing a col lagen mesh that penetrates the capsule to anchor it in the surrounding tissue. The single nerve ending loses its Schwann cell sheath as it enters. Its many branches weave around the collagen fibers and respond to movement of the surrounding tissue.

G. End-Bulbs: These fluid-filled bulbs with thin capsules leg, Krause's end-bulbs) contain many nerve endings that enter at one pole and branch internally. They are relatively common and vary in size; most are mechanoreceptors. The largest are the genital corpuscles in the genital connec tive tissue; the smallest are in the conjunctiva. Others occur in subepithelial connective tissue of the oral and nasal cavities, in the peritoneum, and in the connective tissue around joints and nerve trunks.

H. Carotid Sinus: This baroreceptor (a type of mechanoreceptor) is discussed in Chapter II.

III. PROPRIOCEPTORS

A. Muscle Spindles: These are encapsulated fusiform (spindle-shaped; wide equator and ta pered poles) proprioceptors in striated muscles. Layers of flattened fibroblasts make up the capsule. Muscles for delicate and precise movements leg, eye muscles) require more spindles. They have both sensory and motor innervation.


1. Intrafusal fibers comprise a bundle of 2-20 specialized muscle fibers within the capsule. Oriented parallel to the extrafusal fibers--typical striated muscle outside the capsule--the shorter, narrower intrafusal fibers cross the spindle capsule from pole to pole, attaching, by their striated ends, at the poles. Their nonstriated, dilated centers lack myofilaments, contain the nuclei, and lie at the spindle's equator. The 2 types of intrafusal fibers are the nuclear chain fibers, which are short and numerous, with nuclei in rows; and the nuclear bag fibers, which are less numerous and longer (may extend beyond the capsule). The nuclei in their baglike dilated centers occur in a cluster.
2. Sensory innervation is of 2 types: a. Primary annulospiral endings show dynamic sensitivity; ie, they are more sensitive to initial muscle stretching. A single larger myelinated sensory (afferent) fiber penetrates each spindle, losing its myelin sheath inside the capsule. The unmyelinated portion branches to form spiral endings that embed in the sarcolemma around the intrafusal fibers' dilated centers. b. Secondary flower spray and annulospiral endings occur in most spindles. One or more small myelinated sensory nerves loses its myelin as it branches within the spindle. Branches may end in spirals, like primary endings, or in a flower spray pattern, where expanded tips of each branch contact the sarcolemma. Unlike primary endings, they terminate mainly on nuclear chain fibers and exhibit static sensitivity; ie, they are more sensitive to prolonged stretching.
3. Motor innervation reaches intrafusal fibers via small myelinated motor (efferent) fibers from cells in the ventral spinal cord gray matter. These gamma motor neurons contact the intrafusal fibers' striated polar regions in 2 ways. Most endings on nuclear bag fibers resemble motor end plates (Chapter 10), with small clusters of boutons terminate, Those on nuclear chain fibers form multiple boutons en passage. Stimulation by these neurons contracts the intrafusal fibers' striated poles, stretching their nonstriated centers where the sensory endings are located. Thus, motor neurons can heighten a spindle's sensitivity to further stretching of the muscle in which it is embedded.

B. Golgi Tendon Organs: These occur mainly near muscle-tendon junctions. Each has a cap sule of flat fibroblasts and is filled with collagen bundles that may extend beyond the capsule to insert in the tendon. Some extrafusal muscle fibers may attach to these bundles, allowing activation by muscle stretching or contraction. One large myclinated sensory fiber penetrates the capsule and gives off unmyelinated branches between the collagen bundles.

IV. CHEMORECEPTORS

A. Taste Buds: These ovoid chemoreceptors occur on the tongue's dorsal surface and in smaller numbers on the soft palate and epiglottis. Lingual taste buds are embedded in epithelial projections called papillae. Taste buds occur on the apical surfaces of fungiform papillae and on the lateral surfaces of foliate and circumvallate papillae, Some papillae are more sensitive to certain tastes than others, but no clear structure-function relationships have been established Taste buds communicate with the oral cavity through a small taste pore. Chemicals enter the pore to stimulate the receptors. The 5 cell types that constitute taste buds (Table 24-2) may be different stages in the life of a single cell type that undergoes continual turnover.

B. Olfactory Epithelium: The main receptor for olfaction (smell), this pseudostratified col umnar epithelium is mainly restricted to the upper surface of the superior concba (turbinate bone) in the nasal cavity. The epithelium has 3 cell types, whose nuclei lie in 3 rows.


1. Olfactory cells are bipolar neurons (broad middle, narrow apex and base) derived from embryonic neural ectoderm. Their large round pale nuclei form the middle row, between the supporting and basal cell nuclei. From the cell apex, 6-20 long, nonmotile cilia extend into the nasal cavity, increasing the chemoreceptive surface. Chemicals interact with receptor sites on the cilia to elicit receptor potentials. The base of each cell tapers into an axon that carries the impulse to the brain.
2. Supporting cells are columnar, but are wider at their microvillus-covered apices. Their pale nuclei form the top row in the broad region between the surface and the row of olfactory cell nuclei. They contain RER, SER, lysosomes, lipid droplets, and a red-brown pigment that helps distinguish the olfactory region from the surrounding epithelium.
3. Basal cells are small, conical cells at the base of the olfactory epithelium. Their nuclei form the deepest row, and their narrow apical processes extend between the other cells.

C. Carotid and Aortic Bodies: See Chapter 11 for a discussion of these chemoreceptors.

II. THE EYE

This complex organ (Fig 24-1) refines and projects images onto its photosensitive retina, which then generates the signals interpreted by the brain as vision. The basic structure of the globe (eyeball) includes 3 main layers: the tunica interna (retina), tunica vasculosa, and tunica fibrosa. This arrangement is better understood in light of its embryonic origin.

A. Embryonic Development: The eye begins at the optic bulb, or optic vesicle, a hollow outgrowth of the embryonic brain. As this contacts the overlying ectoderm, 2 crucial events occur: the bulb induces the ectoderm to form the lens placode, and the portion of the bulb contacting the ectoderm invaginates to form the double-walled optic cup. The cup later forms the tunica interna (retina). Its outer layer forms the retina's pigmented epitbelium, and the inner layer forms the photosensitive neural retina. The stalk connecting the optic cup to the brain becomes the optic nerve. The lens placode thickens and invaginates to form the lens vesicle, which pinches off to form the lens and comes to lie in the mouth of the optic cup. Mesenchyme condenses around the tunica intcma to form the globe's 2 outer tunics: the outer most tunica fibrosa and the tunica vasculosa, which lies between the fibrosa and intima.

B. Tunica Fibrosa: The eye's outermost tunic has 2 main components. The anterior sixth forms the transparent cornea; the posterior five-sixths, the opaque sclera, The junction between the cornea and sclera is the limbus.,


1. Cornea, This transparent avascular disk bulging from the front of the eye has 5 layers. The anterior epithelium is outermost. A thin nonkeratinized stratified squamous epithelium, it has many free nerve endings. Bowman's membrane (anterior limiting lamina) is a cell-free, thick basement membrane composed of ground substance and reticular fibers. The stroma (substantia propria) forms the cornea's core and 90% of its thickness. It has many layers of collagen bundles oriented parallel to those in the same layer and perpendicular to those in adjacent layers. Fibroblasts lie between the layers. Descemet's membrane (posterior limit ing membrane) is a thick basement membrane differing from Bowman's membrane in posi tion and composition. It has elastin, but no elastic fibers. Its network of atypical collagen fibers is decorated with granules. The corneal endothelium (posterior epithelium) is a simple cuboidal epithelium lining the cornea's internal surface.
2. Limbus, The vessels of this highly vascular, ringlike junction between the cornea and sclera, together with the fluid in the anterior chamber, nourish the avascular cornea. Near the limbus, the stroma contains an endothelial channel, Schlemm's canal, that drains fluid from the anterior chamber toward veins in the limbus. Blocking this canal can raise intraocular pressure and cause glaucoma.
3. Sclera, This opaque white connective tissue covers the eye's posterior five-sixths. It is anchored in the orbit by the dense connective tissue of Tenon's capsule. The sclera has 3 layers. The episclera is the sclera's outermost layer of tibroelastic tissue. The substantia propria is a dense mat of collagen bundles and fibroblasts forming the sclera's thick middle layer. The ocular muscles insert here. The lamina fusca is the sclera's loose connective tissue inner layer. It contains elastic fibers and melanocytes and is separated from the choroid by the narrow perichoroidal space.

C. Tunica Vasculosa (Uvea): The middle tunic of the eye, this has 3 major components: the choroid (posterior), ciliary body, and iris (anterior).


1. Choroid, The choroid lies between the sclera and the retina's pigmented epithelium and has 4 layers. The suprachoroidal lamina resembles the sclera's lamina fusca, from which it is separated by the perichoroidal space. The vascular lamina is a loose connective tissue with many whorllike veins that converge to form 4 larger vortex veins which exit the back of the eye through the sclera. The choriocapillary layer (choriocapillaris) is a layer of fenestrated sinusoids embedded in loose connective tissue. Bruch's membrane, the choroid's inner layer, is the basement membrane of the retina's pigmented epithelium.
2. Ciliary body. The ciliary body extends forward from the choroid as a ringlike triangular thickening at the level of the lens. It has the same layers as the choroid, minus the choriocapillaris. Its 2 structural specializations are the ciliary processes and ciliary muscles. a. The ciliary processes are irregular epithelium-covered connective tissue outgrowths of the ciliary body extending toward the lens. They serve as origins for the fibers of the circular ligament of Zinn (zonule) that insert in the edge of the lens to anchor it. The 2 layers of pigmented epithelium covering these processes derive from the layers of the optic cup. The inner ciliarv epithelium borders the internal cavity of the eye. Its cells have the basolateral plasma membrane infoldings typical of ion- and water-transporting cells. They secrete aqueous humor which flows through the pupil to the anterior cham ber. From here, the fluid penetrates the tissue near the limbus to reach Schlemm's canal (V.B.2). The deeper, simple columnar epithelial layer derives from the optic cup's outer layer. b, The ciliary muscles comprise 3 groups of smooth muscle bundles near the junction of the ciliary body and sclera. Contracting all groups pulls the ciliary body and choroid for ward, releasing tension on the zonule and allowing the lens to round up for near vision. Relaxing all groups increases tension on the zonule, flattening the lens to focus on distant objects. Adjusting individual muscles allows focusing on objects at intermediate distances.
3. Iris. This structure controls the amount of light reaching the retina and gives the eye its color. It projects as a flat ring from the ciliary body, in front of the lens, leaving a circular opening at its center, the pupil. The iris includes the most anterior extensions of the tunica vasculosa and tunica interna, forming the border between the anterior and posterior cham bers. The anterior chamber lies between the cornea and the iris; the posterior chamber lies between the iris and the lens-zonule complex. a. Layers. The anterior surface of the iris is rough, with pigment cells and fibroblasts. Its stroma is a poorly vascularized connective tissue with fibroblasts and melanocytes. The vascular stratum, between the stroma and posterior surface, has many blood vessels. The posterior surface is smooth, heavily pigmented, and continuous with the double- layered epithelium covering the ciliary processes. b, Involuntary muscles. The sphincter pupillae is a ring of smooth muscle in the pupillary margin that contracts under sympathetic control to partly close the pupil. The dilator pupillae fibers extend like spokes between the ciliary body and pupillary margin. These contract under parasympathetic control to open the pupil.

D. Tunica Interna (Retina): This derivative of the optic cup (V.A) is considered an extension of the CNS. Its nonphotosensitive anterior portion Forms part of the ciliary body and iris. Its posterior portion is a highly specialized photoreceptor. The junction between its anterior and posterior parts, the ora serrata, lies behind the ciliary body. The posterior portion of the retina is further divisible into 2 layers on the basis of structure, function, and embryonic origin.


1. Pigmented epithelium. This melanin-rich simple cuboidal epithelium derives from the optic cup's outer wall. It rests on a thick elastic basement membrane (Bruch's membrane) that separates it from the choroid. The cells' many apical microvilli embrace the outer segments of the rods and cones in the overlying neural retina. While not a photoreceptor, this layer is crucial to vision. It absorbs light that has passed through the photosensitive layer, ensuring that light stimulates the rods and cones only on its first pass. Its cells have basal plasma membrane infoldings and mitochondria typical of ion- and water-transporting cells. They also phagocytose and degrade the vesicles shed by the rods' outer segments.
2. Neural retina. This highly organized, exquisitely sensitive photoreceptor derives from the optic cup's inner wall. Its structure and function are considered below (V.E-H). While the pigmented epithelium extends to the ciliary body and iris, the neural retinal extends only to the ora serrata.

E. Cells of the Neural Retina: Ten rctinal layers are distinguishable (Fig 24-2), but only 3 layers of retinal neurons receive, integrate, and transmit visual signals to the brain as nerve impulses. These are the photoreceptor cells (rods and cones), bipolsr cells, and ganglion cells.


1. Rods and cones are best understood by their similarities and differences. Their basic similarities are discussed here. Their differences are detailed in Table 24-3. Both rod and cone cells are photoreceptors found deep in the retina, next to the pigmented epithelium. Light must penetrate the more superficial layers to reach and stimulate them. Each has 2 struc- turally different segments. The outer segment, the specialized dendritic (receptive) portion of each cell, contains a stack of flat membrane limited vesicles and the visual pigment. This segment's distal tip is embraced by pigment ep ithelial cell microvilli. The inner segment is rich in polyribosomes and glycogen and is separated from the outer segment by a constric tion. Basal bodies near the constriction anchor one or 2 intracytoplasmic cilia that traverse the constriction into the outer segment. Mitochondria near the constriction provide energy for the visual process.
2. Bipolar cells lie in the middle of the neural retina and comprise 2 populations of inter neurons, relaying visual signals from the photoreceptors to the ganglion cells. Each diffuse bipolar cell synapses with ganglion cells and 2 or more photoreccptors (mostly rods). Each monosynaptic bipolar cell synapses with a single cone and a single ganglion cell, perhaps accounting for the cones' greater visual acuity.
3. Ganglion cells lie close to the inner surface of the globe and have large cell bodies and nuclei. Their dendrites make synaptic contact with the bipolar cells. Their axons form the layer of nerve fibers covering the retina's inner surface. They converge to exit the eye via the optic nerve, carrying visual signals to regions of the brain responsible for vision.
4. Other cell types in the neural retina include 2 minor populations of neurons and important glial cells. Horizontal cells and amacrine cells are neurons whose functions may include integrating visual signals before they reach the brain. The processes of horizontal cells terminate near the synapses between photoreceptor and bipolar cells; those of the amacrine cells terminate near synapses between bipolar and ganglion cells. Glial cells include astro cytes, microglia, and the large, highly branched Miiller cells that span the entire width of the neural retina and embrace the processes of the retinal neurons.

F. Fovea Centralis: Directly opposite the center of the lens, the retina's fovea lies at the center of a small yellowish disk called the macula lutea. Because it has the greatest concentration of cones, it is the rctinal region with the greatest visual acuity. The lateral displacement of all retinal layers except the photorcceptors makes it the retina's thinnest region. It lacks blood vessels and is nourished by the underlying choriocapillaris.

G. Optic Disk and Retinal Blood Supply: Also known as the papilla, nerve head, and blind spot, the optic disk is the site where the ganglion cells' axons converge at the back of the eye and exit to form the optic nerve. Lacking photorcccptors, it is insensitive to light. Retinal vessels enter and exit the eye through the optic disk's center and branch over the retina's internal surface. Capillaries penetrate the retina's inner layers except near the fovea. The retina's outer layers are supplied by diffusion from vessels in the choriocapillaris.

H. Optic Nerve: This consists of ganglion cell axons that converge to leave the eye at the optic disk. Once in the nerve, the axons acquire myelin from oligodendrocytes. As it leaves the eye, the nerve acquires a sheath of dura mater that is continuous with the sclera, as well as arachnoid and pia mater. It contains the retinal artery and vein at its core.

I. Vitreous Body: This transparent, gellike body consists mostly of water and hyaluronic acid and fills the large vitreous space between the lens and the retina. It contains some fibrils around the periphery that form its capsule. It contains a few macrophages and hyalocytes, stellate cells with oval nuclei that produce the fibrils and hyaluronic acid. During development, the central artery extends from the optic disk through the vitreous to the lens as the byaloid artery. It later degenerates, leaving the narrow hyaloid canal.

J. Lens: This is a transparent, elastic, biconvex structure of epithelial origin. Nourished by aqueous humor, it has neither blood nor nerve supply. It is suspended by the zonule of the ciliary body behind the pupil. Ciliary muscle contraction changes the lens' curvature to focus on objects near or far, a process called accommodation. It has 3 main components:


1. The lens capsule is an elastic and transport basal lamina that covers the entire lens and prevents wandering cells from penetrating it. It consists mainly of fine type III and IV collagen fibrils embedded in a glycoprotein- and glycosaminoglycan-rich matrix.
2. Subcapsular epitbelium, The height of this low cuboidal epithelium beneath the capsule on the anterior lens surface increases to columnar near the lens equator, where cell division occurs. Its cells contain few organelles and form the lens fibers.
3, Lens fibers are long, narrow, hexagonal, specialized epithelial cells that make up most of the lens. During differentiation, they lose their nuclei, fill with proteins called crystallins, and develop a variety of plasma membrane specializations including junctional complexes and ridgelike processes.

K. Accessory Structures of the Eye:


1. Conjunctiva. This structure has 2 parts. The bulbar conjunctiva is a thin nonkeratinized stratified squamous epithelium covering the eye's anterior surface to the cornea. The pal pebral conjunctiva is a stratified columnar epithelium covering the inner surface of the eyelids. The conjunctiva is underlain by a loose, vascular lamina propria containing many lymphocytes, plasma cells, and macrophages.
2. Eyelids (palpebra) are 5-layered skin folds that protect the eyes. The thin skin on the surface lacks hairs except at the free margin. It is underlain by loose connective tissue containing the orbicularis oculi muscles that close the eyes. The dense connective tissue core, or tarsal plate, provides flexible support and harbors the tarsal (meibomian) glands. These release oily secretions from openings in the free margins that prevent the opposing lids from sticking and slow tear evaporation. A thin lamina propria and overlying palpebral conjunctiva cover the internal surface. The loosely coiled sweat glands of Moll and the sebaceous glands of Zeiss that accompany the eyelashes also open on the free margins. Near the top of the upper lid, the levator palpebrae superioris muscle inserts into the skin and top of the tarsal plate to open the lid.
3. Lacrimal apparatus. This system of glands and ducts provides tears to lubricate and protect the eyes. The lacrimal glands are tear-secreting compound tubuloalveolar glands located supcrolaterally in the bony orbits. The secretory units are surrounded by myoepithclial cells and divided into lobes by connective tissue. The columnar secretory cells have pale granlnes and secrete antibacterial lysozyme, Tears are released behind the upper eyelid and flow over the eye's anterior surface. The excess drains through the lacrimal puncta, one small hole in the free margin of each lid near the medial palpebral angle. Tears enter by capillary attraction and, with the aid of pumping action provided by the orbicularis oculi muscles, follow the lacrimal canaliculi into a short duct that empties into the lacrimal sac, This dilation of the lacrimal drainage system delivers tears to the nasolacrimal duct with the aid of gravity. This duct empties through a bony canal into the nasal cavity through the inferior meatus.

L. Brief Summary of Light Path and Vision: Light penetrates the tear layer and then the transparent cornea (V.B.I). Crossing the anterior chamber (V.C.3), a limited amount of light passes through the pupil (V.C.3) and across the lens (V.J), which focuses the image and projects it through the vitreous (V.I) onto the retina (V.D). The central part of the image focuses on the fovea of the macula lutea (V.F). Here, light penetrates and excites the photoreceptors (V.E. i), before it is finally absorbed by the pigment epithelium (V.D.1). The bleaching of the visual pigments in the excited rods and cones (Table 24-3) generates a receptor potential (I.A. l.b) that is transmitted to the bipolar cells (V.E.2), which integrate and relay the signal to the ganglion cells (V.E.3), which then transmit the signal to the brain via the optic nerve (V.H).

VI. THE EAR

This collection of structures for hearing and balance (Fig 24-3) has 3 major components: the external, middle, and internal ears.

A. External Ear: The 3 major components of the external ear are the auricle, external auditory meatus, and tympanic membrane. The auricle (pinna) is a funnellike plate of elastic cartilage sandwiched between 2 layers of skin. Moditied apocrine sweat glands in the skin, the ceruminous glands, secrete the waxy substance cerumin, The auricle collects and focuses sound waves toward the tubelike external auditory meatus. This canal, which leads to the tympanic membrane, is surrounded by elastic cartilage along its outer third and by bone along its inner two-thirds. Sounds gathered by the auricle and carried inward by the meatus vibrate the tympanic membrane (eardrum), which covers the internal oriface of the meatus. The membrane's 3 layers are the outer epidermis, middle dense connective tissue, and inner cuboidal epithelium.

B. Middle Ear: This lies in a cavity in the temporal bone. Its connection with the auditory meatus is closed by the tympanic membrane. It communicates with the nasopharynx through the auditory (eustacbian) tube and with the mastoid air cells. The auditory tube is surrounded by elastic cartilage. Its walls are collapsed except during swallowing, when they separate to allow pressure in the middle ear cavity to equilibrate with the environment.


1. Windows. The medial bony wall of the middle ear cavity has 2 membrane-covered openings, the oval and round windows, lying at the border between the middle and internal ears (Fig 24-3).
2. Auditory ossicles, The main functional components of the middle ear are 3 uniquely shaped small bones that span the middle ear cavity from the tympanic membrane to the oval window membrane. The malleus (hammer), incus (anvil), and stapes (stirrup) transmit vibrations from the tympanic membrane to the fluid in the inner ear. Small muscles limit ossicle movement to limit damage from loud noises.
3. Mucosa, The simple squamous to cuboidal lining of the middle ear contains some mucous or seromucous secretory cells. A thin lamina propria binds the lining to the periosteum of the walls and ossicles. Near the auditory canal the lining changes to the pseudostratified columnar that lines the canal and nasopharynx.

C. Internal Ear (Labyrinth, Vestibulocochlear Apparatus): This consists of 2 mechanorecep tors, the cochlea (concerned with hearing) and the vestibule (concerned with equilibrium).


1. Embryonic development begins with a thickened ectodermal disk (otic placode) on each side of the head. Each placode invaginates to form an otic vesicle, which undergoes further outpocketing to form a series of interconnected chambers and canals, the membranous labyrinth, Once the chambers form, their lining differentiates to form the special cells and sensory organs described below. Mesenchyme condenses around the membranous labyrinth to form the bony labyrinth.
2. General organization. The internal ear consists of a complex of bony cavities and canals, the bony labyrinth, which houses the delicate membranous labyrinth and its organs of hearing and balance. The bony labyrinth consists of 2 interconnected compartments, the vestibule and cochlea. The space between the membranous and bony labyrinths contains a fluid called perilymph, The membranous labyrinth lies within and conforms to the shape of the bony labyrinth. Its interconnected chambers and canals contain a fluid called endo lymph., Small portions of its simple squamous endothelial lining develop into a sensory epithelium in which the cells rise gradually to a columnar shape. Despite their origin as outpocketings of the same otic vesicle, the cochlear and vestibular receptors are structurally and functionally distinct and will be considered separately below.

D. Vestibular Organs: Components of the bony labyrinth associated with balance include the vestibule and the semicircular canals. The membranous part of each region includes a special sensory organ composed of 2 major cell types. The hair cells are the receptor cells. Each has several long stereocilia and one true cilium extending from its apical surface. The goblet-shaped type I hair cells are surrounded by afferent nerve endings. The columnar type II hair cells contact both afferent and efferent endings on their basal and lateral surfaces. The columnar supporting cells have basal nuclei, lie between the hair cells, and produce the glycoprotein-rich gelatinous layer that covers the sensory epithelium and bulges into the membranous labyrinth's lumen.


1. The vestibule is an oblong cavity in the inner ear, housing 2 saclike membranous labyrinth components concerned with equilibrium, the utricle and saccule, In the wall of each is a sensory macula, an ovoid button of sensory epithelium covered by a gelatinous layer into which the hair cells' stereocilia and cilia extend. In both the utricle and saccule, small rocklike crystals of calcium carbonate and protein, the otolitbs (statoliths), cover the mac ula's gelatinous layer. Changes in head position change endolymph flow, moving the otoliths. The movements are transmitted through the gelatinous layer, displacing the hair cell processes and stimulating the associated nerve endings. a. The utricle is the largest membranous component of the vestibular system. This kidney shaped sac connects with the semicircular canals through their ampullae and with the saccule via the narrow utriculosaccular duct. b, The saccule is spheric and smaller. It communicates with the cochlear duct through the short narrow ductus reuniens and with the utricle through the utricosaccular duct. c, The endolymphatic duct is a tubular evagination of the utriculosaccular duct, terminat ing as a blind expansion, the endolymphatic sac, The sac has a tall columnar epithelial lining and is surrounded by vascular connective tissue. Duct and sac functions may include producing endolymph and clearing debris from it.
2. The semicircular canals are 3 thin bony canals in the temporal bone, oriented in 3 planes at right angles to each other, that communicate with the vestibule by small openings. They contain the 3 semicircular ducts of the membranous labyrinth, which communicate with the utricle. The superior, lateral, and posterior ducts leave the utricle, beginning as dilated ampullae, At its termination, the lateral duct connects directly with the utricle, while the superior and posterior ducts converge to form a single wider duct, which then joins the utricle. Each ampulla contains a sensory crista ampullaris, whose hair cells and supporting cells resemble those in the maculae, but are arranged in transverse ridges (cristae) rather than buttonlike bulges (maculae). The conical gelatinous layer of each crista is termed a cupula, and it lacks otoliths.

E. Cochlea: This snailshell-like spiral canal houses the cochlear duct; the part of the mem branous labyrinth concerned with hearing. The cochlea's screwlike bony core, the modiolus, houses the auditory nerve cell bodies of the spiral ganglion. The thread of the screwlike modiolus is a spiral bony shelf, the osseous spiral lamina, which supports the auditory epithelium (spiral organ of Corti),


1. The cochlear duct uncoiled. The general structure of the auditory portion of the mem branous labyrinth is more easily understood if one pictures it removed from the bony cochlea and uncoiled (Fig 24-3). It then appears as a tube within a tube. The outer tube has a simple squamous epithelial lining bound tightly to the cochlea's bony walls. It begins at the oval window, ends at the round window, and contains the perilymph. The inner tube is the cochlear duct. Its cavity, the scala media, is filled with endolymph,
2. The cochlear duct in situ. Within the coiled cochlea, a section through a single turn (Fig 24-4) shows that the cochlear duct has a triangular shape, with a roof (the vestibular or Reissner's membrane, separating the scala media from the scala vestibuli), a lateral wall (mainly the stria vascularis, an unusual epithelium covering many capillaries that together produce the endolymph that fills the duct), and a floor, which includes the spiral organ of Corti and the spiral lamina on which it rests. a. The spiral lamina has both bony and membranous parts. The osseous spiral lamina is the thread of the screwlike modiolus. The membranous spiral lamina extends across the cochlear canal from the edge of the thread to the spiral crest on the lateral wall. At its core lies the thin, fibrous basilar membrane. The organ of Corti lies on the membranous portion of the spiral lamina. b, The spiral organ of Corti is highly sensitive to vibration. It is anchored by the epithelium-covered connective tissue spiral limbus to the osseous spiral lamina. The glycoprotein-rich tectorial membrane extends from the limbus to cover the sensory cells' apices as the gelatinous layer covers the vestibular maculae. The spiral organ of Corti has 2 major cell types, supporting and sensory cells. (I) The supporting cells occur in 2 groups and are termed the inner and outer pillar cells, Their broad bases contain their nuclei, and their elongated apices contain tonofilament bundles. They underlie and support the sensory cells and form the walls of a channel between the 2 groups of sensory cells called the inner tunnel. (2) The sensory cells, as in the vestibular system, are called hair cells. inner hair cells form a single row between the inner tunnel and the internal spiral tunnel formed by the tectorial membrane as it bridges the space between the limbus and the organ of Corti. These goblet-shaped cells have apical stereocilia and many basal mitochon dria. Outer hair cells form 3 parallel rows lateral to the inner tunnel and rest on columnar supporting cells. Outer hair cells have basal nuclei and mitochondria and are more columnar than inner hair cells. The 100 or so stereocilia on each cell are arranged in a V or W pattern and penetrate the cuticular plate, an expansion of the outer pillar cell. The tips of the stereocilia are covered by the tectorial membrane. Cochlear hair cells lack true cilia. They are innervated by the dendritic processes that pass from the bipolar neurons of the spiral ganglia in the modiolus through the spiral limbus and basilar membrane to reach them.
3. Hearing. Sounds collected by the auricle traverse the auditory meatus and vibrate the tympanic membrane. This moves the ossicles, and movement of the stapes in the oval window transmits vibrations to the perilymph in the scala vestibuli. The perilymph carries these vibrations through the helicotrema and into the scala tympani. Vibrations in the perilymph cause the delicate membranous spiral lamina and associated organ of Corti to move in relation to the tectorial membrane, displacing the hair cell stereocilia. Movement of the stereocilia generates an action potential in the bipolar spiral ganglion cell processes. Neural signals generated in the cochlea are carried to the brain through the cochlear nerve (the collected axons of the bipolar cells). Cochlear sensitivity is tonotopic--localized for sounds of different frequencies. The organ of Corti in the basal cochlea responds best to high frequencies, and that in the apex responds best to low frequencies.