Special Senses

l!1llll'ttmfiiiSpecial Senses 367

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B.Touch and pressure receptors

1. Pacinian corpuscles

a.Pacinian corpuscles are large, ellipsoid encapsulated receptors in the dermis and hypodermis and in the connective tissue of the mesenteries and joints.

b.They are especially abundant in the digits and breasts.

c.They are composed of a multilayer capsule consisting of fibroblasts and collagen and bathed in tissue fluid. The capsule surrounds an inner unmyelinated nerve terminal.

d.They often resemble a sliced onion in histological sections.

e.Function. Pacinian corpuscles perceive pressure, touch, and vibration.

2.Ruffini endings

a. Ruffini endings are encapsulated receptors in the dermis and joints.

b.They are composed of groups of branched terminals from myelinated nerve fibers and are surrounded by a thin connective tissue capsule.

c.Function. Ruffini endings function in pressure, stretching, and touch reception.

368BRS Cell Biology and Histology

3.Meissner corpuscles

a.Meissner corpuscles are ellipsoid, encapsulated receptors in the dermal papillae of thick skin, eyelids, lips, and nipples.

b.Theypossess a connective tissue capsulethatenvelops the nerve terminal and its associated Schwann cell.

c.Function. Meissner corpuscles function in fine touch perception.

4.Free nerve endings

a.Free nerve endings are unencapsulated, unmyelinated terminations in the skin in longitudinal and circular arrays around most of the hair follicles.

b.Function. Free nerve endings function in touch perception.

5.Krause end bulbs

a.are encapsulated receptors located in the dermis, oral cavity, peritoneum, conjunctiva, genital regions, connective tissues, nasal cavity, and in joints. They were thought to be cold receptors, but presently their function is unknown.

C. Temperature and pain receptors

1 . Cold receptors respond to temperatures below 25°C to 30°C. 2. Heat receptors respond to temperatures above 40°C to 42°C.

3.Nociceptors

a. Nociceptors are sensitive to pain stimuli from mechanical stress, extremes in temperature, or the presence of certain cytokines. When a stimulus is sufficiently strong to overload the normal receptor, that stimulus may trigger pain sensations by activating the nociceptors.

b. They are delicate myelinated fibers that lose their myelin before entering the epidermis.

D. Proprioceptive receptors (see Chapter 8 IV B and C)

1 . Golgi tendon organs are encapsulated mechanoreceptors sensitive to stretch and tension in tendons.

2.Muscle spindle receptors are 3 to 12 small encapsulated intrafusal muscle fibers called flower spray endings and annulospiral endings that sense differences in muscle length and tension.

A.Overview

1.The eyes, housed in the bony orbits, are the photosensitive organs responsible for vision.

2.Each eye is composed of three tunics: the tunica fibrosa (outer layer), tunica vasculosa (middle layer), and retina.

3.It receives light through the cornea. The light is focused by the lens on the retina, which contains specialized cells that encode the various patterns ofthe image for transmission to the brain via the optic nerve.

4.The eye possesses intrinsic muscles that adjust the aperture of the iris and alter the lens diameter, permitting accommodation for close vision.

5.The eye also possesses extrinsic muscles, attached to the external aspect of the eyeball, which move both eyes in a coordinated manner to perceive the desired visual fields.

6.The eyeball is continually moistened on its anterior surface with lacrimal fluid (tears) secreted by the lacrimal gland.

7.The eyeball is covered by the upper and lower eyelids, which protect its anterior surface.

B.Tunica fibrosa (see Figure 21 .2)

1.The outermost tunica fibrosa is composed of the sclera and the cornea. The sclera (the white of the eye) is an opaque, relatively avascular fibrous connective tissue layer that covers the posterior five-sixths of the eyeball. The sclera receives insertions of the extrinsic ocular muscles.

endothelium and the connective tissue stroma of the cornea. e. Corneal endothelium

(1) The corneal endothelium lines the posterior aspect of the cornea beneath Descemet membrane, where it separates the cornea from the aqueous humor in the anterior chamber of the eye.

(2) It is a simple squamous epithelium with cells that exhibit numerous pinocytic vesicles.

(3) It resorbs fluid from the stroma, thus contributing to the transparency of the cornea, contributory to light refraction.

Cornea

Because the cornea is without a blood supply and lymphatics, the cornea may be transplanted without eliciting an immune response that would result in tissue rejection. Thus, it is one of the most frequently donated tissues.

Corneal Reflex (blink response)

A wisp of cotton that has touched the cornea will cause the patient to blink. The blink is a test of the integrity of the ophthalmic division of the trigeminal nerve (Cranial nerve V).

C.Tunica( vasculosa (uvea, the middle tunic) is composed of three parts: choroid, ciliary body, and iris. see Figure 21 .2)

1.Choroid

a.The choroid is the highly vascular, pigmented layer of the eye on the posterior wall of the eyeball; its loose connective tissue contains many melanocytes.

b.It is loosely attached to the tunica fibrosa.

c.Its deepest layer, Bruch membrane extends from the ora serrata to the optic disk and separates the choriocapillary layer from the pigment epithelium. It is composed of two thin layers of collagen fibers with an intervening layer of elastic fibers. Both layers of collagen fibers are covered by basal laminae.

d.The choriocapillary layer contains the blood capillaries derived from the choroid stroma that supply the retina.

2.Ciliary body

a.The ciliary body is the wedge-shaped anterior expansion of the choroid.

b.It completely encircles the lens and separates the ora serrata from the iris.

c.It is lined on its inner surface bytwolayers( of cells: an outer pigmented columnar epithelium rich in melanin and an inner nonpigmented simple columnar epithelium.

(1 ) Ciliary processes

(a)are radially arranged extensions about 70) of the ciliary body.

(c)are covered by two epithelial layers. The nonpigmented inner layer transports( components from the plasma filtrate( in the) posterior chamber and thus forms the aqueous humor, which flows to the anterior chamber via the pupillary aperture see Figure 21 .2).

(d)possess suspensory ligaments zonulae that arise from the processes and insert

into the capsule of the lens, serving to anchor it in place.

(2)Ciliary muscle

(a)is attached to the sclera and ciliary body in such a manner that its contractions stretch the ciliary body and release tension on the suspensory ligament and lens. Ciliary muscle contraction permits the lens to become more convex, allowing the

l!1llll'ttmfiiiSpecial Senses 375

(2)They phagocytize the shed tips of the outer segments of rods.

(3)They synthesize melanin, which absorbs light after the rods and cones have been stimulated.

(4)serve as the blood-retinal barrier.

3.The photoreceptor layer consists of neurons (photoreceptor cells) called rods and cones.

a. Rods (sensitive to light of low intensity) (Figure 21 .5)

(1 ) Overview

(a)Rods possess four regions-outer and inner segments, a nuclear region, and a synaptic region.

(b)They may synapse with bipolar cells, giving rise to summation.

(c)They possess an incomplete cilium terminating in a basal body within the inner segment.

(2)Outer segments of rods

(a)consist mainly of hundreds offlattened membranous disks that contain rhodopsin, the visual pigment located in the discs of rods and discs of cones.

(b)eventually shed their disks, which are subsequently phagocytized by the pigment epithelial cells.

(3)Inner segments of rods possess mitochondria, glycogen, polyribosomes, and proteins, which migrate to the outer segments to be incorporated into the membranous disks.

(4)Photoreception by rods is initiated by the interaction of light with rhodopsin (visual purple), which is composed of the integral membrane protein opsin bound to retinal, the aldehyde form ofvitamin A.

(a)The retinal moiety of rhodopsin absorbs light in the visible range.

(b)Retinal dissociates from opsin. This reaction, called bleaching, eventually closes the Na+ channels, thus permitting diffusion of bound Ca2+ ions into the cytoplasm of the outer segment of a rod cell.

(c)Excess Ca2+ acts to hyperpolarize the cell because Na+ is prevented from entering the cell.

(d)Ionic alterations in the rod generate electrical activity, which is relayed to other rods via gap junctions.

(e)Dissociated retinal and opsin reassemble by an active process in which Muller and pigment epithelial cells also participate.

(f)Ca2+ is recaptured by the membranous disks, leading to reopening of the Na+ channels and reestablishment of the normal resting membrane potential.

b. Cones (sensitive to light of high intensity) (Figure 21 .5)

(1 ) Cones are much less numerous than rods, but produce greater visual acuity than do rods; thus, the macula is populated only by cones.

(2)They are generally similar in structure to rods and mediate photoreception in the same way, with the following exceptions:

(a)The membranous disks in the outer segments of cones are invaginations of the plasma membrane, whereas in rods they are not.

(b)The proteins synthesized in the inner segments of cones are passed to the entire outer segment, whereas in rods they are added only to newly forming disks.

(c)Cones possess iodopsin in their disks. The amount of this photopigment varies in different cones, making them differentially sensitive to red, green, or blue light.

(d)Each cone synapses with a single bipolar neuron, whereas each rod may synapse with several bipolar neurons.

Age-related macular degeneration (AMD) is a disease of the retina located in the central region of the macula, hence affecting central vision.

As its name implies, it is a disease of aging, occurring usually after the age of 50 years. There are two forms: dry and wet AMD. Dry macular degeneration, the most common, is where the central vision is affected so that objects become very blurred. This is caused by cellular debris (drusen) that appears as yellow spots deposited between the choroids and the retina. Wet macular degeneration, the most

which the tears flow over the cornea and conjunctiva, keeping them moist. Tears then enter the lacrimal puncta, leading to the lacrimal canaliculi.

b.Lacrimal canaliculi are lined by a stratified squamous epithelium and unite to form a common canaliculus, which empties into the lacrimal sac.

c.The lacrimal sac is lined by a pseudostratified ciliated columnar epithelium.

d.The nasolacrimal duct is the inferior continuation of the lacrimal sac and is also lined by a pseudostratified ciliated columnar epithelium. The duct empties into the floor of the nasal cavity.

The ear consists of three parts: the external ear, which receives sound waves; the middle ear, through which sound waves are transmitted; and the internal ear, where sound waves are transduced into nerve impulses. The vestibular organ, responsible for equilibrium, is also located in the inner ear.

A. External ear (Figure 21 .7)

3.Tympanic membrane (eardrum)

a.The tympanic membrane is covered byskin on its external surface andby a simple cuboidal epithelium on its inner surface.

b.It possesses fibroelastic connective tissue interposed between its two epithelial coverings.

c.Function. The tympanic membrane transmits sound vibrations that enter the ear to the ossicles in the middle ear.

Posterior semicircular canal

Stapes

Facial nerve

380BRS Cell Biology and Histology

(b)They possess 50 to 100 elongated, rigid stereocilia (sensory microvilli) arranged in rows and a single cilium (kinocilium). These cilia extend from the apical surface of the hair cells into the otolithic membrane. They function in the detection of linear acceleration.

(c)Types of vestibular hair cells include the following:

1 . Type I hair cells (bulbar), which are almost completely surrounded by a cup­ shaped afferent nerve ending

2.Type II hair cells (columnar), which make contact with small afferent terminals containing synaptic vesicles

(3)Supporting cells are generally columnar and possess a round basal nucleus, many microtubules, and an extensive terminal web.

(4)The otolithic membrane is a gelatinous layer of glycoprotein that contains small calcified particles called otoliths or otoconia.

b.Semicircular ducts are continuous with and arise fromthe utricle. The three semicircular ducts are perpendicular to each other so that they can detect angular acceleration of the head in three-dimensional space (see Figure 21 .7).

(1 ) The ampullae are dilated regions ofthe semicircular ducts near their junctions with the

utricle.

(2)Cristae ampullares

(a)are specialized sensory regions within the ampullae ofthe semicircular ducts.

(b)are similar to maculae but have a thicker, conical glycoprotein layer (cupula), which does not contain otoliths.

c.The endolymphatic duct ends in the expanded endolymphatic sac.

d.Endolymphatic sac

(1 ) The endolymphatic sac has an epithelial lining containing electron-dense columnar cells, which have an irregularly shaped nucleus, and electron-lucent columnar cells,

which possess long microvilli, many pinocytic vesicles, and vacuoles.

(2)It contains phagocytic cells (macrophages, neutrophils) in its lumen.

(3)Function. The endolymphatic sac may function in resorption of endolymph.

e.Cochlear duct (Figures 21 .8 and 21 .9) (1 ) Overview

(a)The cochlear duct is a specialized diverticulum of the saccule that contains the spiral organ of Corti.

(b)It is bordered above by the scala vestibuli and below by the scala tympani of the bony cochlea. These scalae, which contain perilymph, communicate with each other at the helicotrema at the apex of the cochlea.

(2)Vestibular membrane

(a)is composed of two layers of flattened squamous epithelium separated by an intervening basement membrane.

(b)helps maintainthe high ionic gradients between the perilymph in the scala vestibuli and the endolymph in the cochlear duct.

(3)Stria vascularis

(a)is a vascularized pseudostratified epithelium that lines the lateral aspect of the cochlear duct.

(b)is composed of basal, intermediate, and marginal cells.

(c)may secrete endolymph.

(4)Spiral prominence

(a)The spiral prominence is an epithelium-covered protuberance that extends the length of the cochlear duct. This epithelium is continuous with that of the stria vascularis and is reflected onto the basilar membrane, where it follows an indentation to form the external spiral sulcus.

(b)Its cells become cuboidal and continue onto the basilar membrane, where they are known as the cells of Claudius, which overlie the polyhedral cells of Boettcher.

(5)Basilar membrane

(a)is a thick layer of amorphous material containing keratin-like fibers.

(b)extends from the spiral ligament to the tympanic lip of the limbus spiralis.

(c)has two zones: the medial zona arcuata and the lateral zona pectinata.