Lectures 21 and 22: Special sense organs
Specific neural receptors are incorporated in a non-neural structure which enhances and refines reception of stimuli
taste buds:
olfactory epithelium: very tall, pseudostratified columnar, containing three cell types: olfactory receptor cells: bipolar neurons; nuclei in middle of epi; single dendritic process extends to free surface, terminating in small swelling producing about 12 extremely long, modified (nonmotile) cilia which lie flat on surface; single fine nonmyelinated axon at base penetrates basement membrane; bundles of axons pass via about 20 small holes of cribriform plate of ethmoid bone to reach olfactory bulbs of forebrane; supporting (sustentacular) cells: elongated with tapered bases; nuclei uppermost; long microilli extend from luminal surface; form tangled mat with cilia; bound to sensory cell by typical junctional complexes forming terminal bar; basal (stem) cells: small; conical cells;underlying CT: afferent nerve fibers & numerous serous olfactory glands (Bowmans glands)
eyeanterior and posterior chambers: anterior compartment: anterior to lens, suspensory ligament and ciliary body; posterior compartment: small posterior to lens, suspensory ligament and ciliary bodycontain aqueous humor (clear, watery fluid similar to CSF and hypotonic to plasma) secreted into posterior chamber by ciliary body and circulated through pupil to drain into scleral venous sinus (= canal of Schlemm) vitreous body: gelatinous mass consisting of vitreous humor
wall of eye (3 layers)
ciliary body (circumferential thickening beneath limbus; surrounds coronal equator of lens; attached to lens by suspensory ligament or zonule; smooth muscle controls shape of lens); bulges into eye at coronal equator of lens, between ora serrata and limbus = forward continuation of choroid layer of uveal tract; highly vascular and pigmented; continuous anteriorly with iris; lined by double layer of cuboidal epithelium; highly pigmented deep layer - forward continuation of pigmented epithelial layer of retin; outer, non-photosensitive extension of neural retina; smooth muscle derived from neural crest (?); contraction reduces tension upon suspensory ligament, increasing curvature of lens = accommodation; innervated by parasympathetic nerve fibers; ciliary processes extend into posterior chamber; rich in fenestrated capillaries; produce aqueous humor;
iris: diaphragm in front of lens from ciliary body; highly pigmented; aperture called pupil; divides anterior compartment into anterior & posterior compartments; loose, highly vascular, pigmented stroma (determines color of eye); anterior surface, irregular, discontinuous layer fibroblasts and melanocytes; lined by endothelial cells in fetus; posterior surface, relatively smooth, lined by epithelium derived from two layers of retina; superficial layer heavily pigmented; deep epi. layer transformed into myoepithelial layer = dilator pupillae radially oriented muscle; innervated by sympathetic nerve fibers; constrictor pupillae = constrictor muscle of the pupil; circumferentially oriented smooth muscle situated in stroma near free edge of iris; innervated by parasympathetic nervous system
(2) rods & cones processes = outer segment
cones: outer segment long conical structure about 2/3 length of rod; containing flattened membraneous disks; disk membrane continuous with plasma membrane; on one side spaces btwn disks continuous with extracellular environment; disks not shed; pigment receptive to blue, green and red; bodies generally continuous with inner segment; nuclei form lower row; deep to outer limiting membrane
(4) outer nuclear layer; densely packed nuclei of rod and cone cells
(5) plexiform layer: contains synaptic connections btwn short axons of photoreceptor cells and integrating neurons
neuronal interconnections in retina interposed btwn receptors and ganglion cells; also Mullers cells = neuroglia; long cytoplasmic processes embrace and may encircle retinal neurons; fill all intervening spaces;
(6) inner nuclear layer: cell bodies of integrating neurons interdigitating (bipolar) neurons (subdivided into three types: bipolar cells, horizontal cells & amacrine cells)
(7) inner plexiform layer: integrating neurons make synaptic connections with dendrites of ganglion cells
(8) optic tract neurons or ganglion cells
(9) layer of afferent fibers passing toward optic tract ganglion cells of afferent fibers passing into optic nerve;
(10) inner limiting membrane: innermost aspect of retina; basement membrane of Mullers cells; separates it from vitreous body
scleral venous sinus = canal of Schlemm: circumferential; at angle of anterior chamber; reabsorbs aqueous humor; lined by endothelium;
lens: biconvex transparent; shape varied for fine focus;
conjunctiva: = epithelium + loose vascular supporting tissue; covering exposed part of sclera and inner surface of eyelids; stratified columnar; containing goblet cells in surface layers;
lacrimal gland: secrete lysozyme and electrolytes ~ isomorphic with plasma; resemble salivary glands; lobular structure; compound tubulo-acinar form; serous glands; drain via into superior fornix to inner aspect of eye and nasal cavity via nasolacrimal duct.
eyelid: dense fibro-elastic plate = tarsus; skeletal muscle of orbicularis oculi & levator palpebrae (in upper lid) lie above tarsal plate; separated from it by layer of supporting tissue; = forward continuation of sub-aponeurotic layer (tendon-like) of scalp (in upper lid)
tarsal glands = Meibomian gland: 12 to 30 within tarsal plate; oriented vertically and opening at free margin of eyelid via minute foramina; modified sebaceous glands;
glands associated with eyelashes
sebaceous ciliary gland = gland of Zeis
tympanic membrane: ear drum; pars flaccida superior; small triangular area; pars tensa; thin fibrous membrane: cuticular layer (thin, hairless skin; epidermis devoid of ridges; dermis of plump fibroblasts and fine vascular network); fibrous layer (outer layer of fibers radiating from center of membrane toward circumference; inner inner layer of fibers disposed circumferentially at periphery; fibers of unique composition); mucous layer (inner; continuation of modified respiratory-type epithelium; single layer cuboidal cells devoid of cilia and goblet cells; lamin propria thinwith own blood supply; also invests ossciles, small muscles and nerves of middle ear cavity);
ossicles (malleus, incus & stapes); formed by endochondral ossification; articulate via synovial joints; communicate with vestibule (perilymph) of internal ear through oval window; malleus and incus pivot on tiny ligaments attached to wall; tensor tympani & stapedius dampen excessive vibrations;
communicates with nasopharynx via auditory (Eustacian) tube (canal); lined by typical pseudostratified respiratory epithelium with numerous glblet cells particularly toward pharyngeal end; salpingo-pharyngeus, tensor palati and levator veli palati muscles connected to fibrocartilaginous part (lower) of tube cause it to dialge during swallowing.
communicates with air spaces within mastoid part of temporal bone; lined by simple squamous or cupoidal epithelium;
communicates with scala tympani (perilymph) via round window
internal
vestibule: central space; gives rise to other parts; contains two components of membraneous labyrinth: utricle (posterior) & saccule (anterior) ; separated from middle ear by thin bony plate contining oval (surrounded by annular ligament) and round windows = secondary tympanic membrane
semicircular canals: from posterior aspect of vestibule; two vertical at right angles; one nearly horizontal
cochlear: extends from anterior aspect of vestibule; modiolus forms central bony axis; osseous spiral lamina spicule spiralling outwards from modiolus; supports spiral limbus: thickened mass tissue above free edge; attached to apex of scala media; scala vestibuli & scala tympani lined byunspecialized squamous epithelium of mesodermal origin
semicircular membraneous duct: filled with endolymph; continuous at both ends with utricle; an ampulla lies at one end of each duct; crista ampullaris (elongated epithelial structure on ridge arising from membraneous wall at right angles to direction of flow; receptor cells [both goblet- and columnar-shaped] with numerous sterocilia and single cilium situated at margin nearest utricle; embedded in gelatinous glycoprotein = cupula) supported by single layer of columnar cells continuous with cuboidal epi lining membraneous labyrinth) containing sensory receptors with axons converging on vestibular nerve
scala vestibule & scala tympani on either side of scala media; continuous through helicotrma at tip of cochlea
membraneous component of cochlear: arises from saccule; triangular canal divides osseous space into 3 spiral components
scala media: contains endolymph; organ of Corti = spiral organ; contains receptors for sound;
vestibular (Reissners) membrane separates scala media from scala vestibuli; extremely delicate fibrous tissue lined by simple squamous epithelium on both sides;
spiral ganglion: contains bipolar primary sensory cells; axons (?) fibers ramify into dendrites and make synaptic contact with several sensory cells; secondary sensory cells and inhibitory neurons from brain stem; efferent fibers (leave spiral ganglia) pass to center of modiolus to form cochlear nerve. auditory component of VIII cranial nerve
last revised: 04-09-03