Brain, brain stem, and spinal cord consist of gray matter and white matter. With the Weigert stain, myelin and blood are black, gray matter is brown. Differentiate gray and white matter and compare their distribution in the different parts of the CNS.

1. White matter consists of nerve cell processes (axons), and glial cells (fibrous astrocytes, oligodendroglia, and microglia). Myelin is a component of oligodendroglia.

2. Gray matter consists of nerve cell bodies, dendrites, axons and glial cells (protoplasmic astrocytes, oligodendroglia and microglia). The term neuropil refers to that portion of gray matter containing numerous synapses and consisting of a felt work of nerve and glial cell processes. Some axons in gray matter are myelinated.

C. Identify three layers of the cerebellar gray matter.

1. Molecular layer - location of the fan-shaped dendritic tree of Purkinje cells, some small nerve cells (stellate and basket) and the terminations of cerebellar afferent axons .

2. Purkinje cell layer - a single row of large nerve cells whose axons end in cerebellar nuclei and whose dendrites fan out in the molecular layer.

3. Granular layer - numerous small nerve cells (granule cells) whose axons terminate in the molecular layer. Glomeruli are islands of synapses within the granular layer.

D. Identify three regions of spinal cord gray matter.

1. Ventral horn - location of anterior horn (final common path) or alpha motor neurons

2. Lateral horn - location of preganglionic autonomic nerve cells

3. Dorsal horn - location of nerve cells in sensory pathways

E. Meninges are the connective tissue coverings. They consist of:

1. Leptomeninges

a. Pia mater - a very thin layer of cells and collagen fibers closely applied to the outer surface of the CNS

b. Arachnoid mater - a thin membrane separating the cerebrospinal fluid space (subarachnoid space) from the dura mater. It has thin strands of cells and collagen that pass through the cerebrospinal fluid space to attach to the underlying pia mater.

2. Pachymeninges (dura mater)

a. Meningeal layer - dense irregular connective tissue that separates from the periosteum of the calvarium to form spaces for venous sinuses and the falx cerebri and cerebelli. The outer surface of the arachnoid and the inner surface of the dura are covered by a continuous simple squamous epithelium. Thus protrusions of arachnoid membrane through the dura into the venous spaces (arachnoid granulations) provide for the resorption of cerebrospinal fluid into venous blood.

b. Periosteal layer - periosteum lining the inner surface of the calvarium.

F. PNS - Nerves and ganglia

1. Nerves - axons, Schwann cells and connective tissue

2. Ganglia - nerve cell bodies and processes, capsule cells and connective tissue

II. NEURON

A. Basic structure

1. Soma (perikaryon, cyton or cell body) consists of cell nucleus plus the cytoplasm surrounding it. The nucleus is usually large with fine textured chromatin and a distinct dark nucleolus. The cytoplasm contains Nissl bodies, that are clusters of RER with free polysomes between the cisternae. Golgi complexes, microtubules and microfilaments are prominent. SER, lysosomes, lipofuscin and melanin may be present.

2. Dendrites are tapering branched processes arising from the soma and have cytoplasm similar to the soma containing small segments of RER and ribosomes. They with their "dendritic spines" that project from them increase the receptive field of the neuron.

3. Axon - a single process arising from a conical region devoid of Nissl granules, the axon hillock. Microtubules and microfilaments stream from the hillock into the initial segment, a constricted non-myelinated portion of the axon where action potentials generally arise because its membrane is specialized and has the lowest stimulation threshold. The action potential is a localized reversal of the membrane potential caused by a sudden influx of sodium ions. It triggers the opening of adjacent voltage-gated sodium channels and is thus propagated along the cell membrane. The axon is lacking RER and ribosomes, but does contain many microtubules and microfilaments and some mitochondria and SER.

Microtubules are necessary for the rapid transport of vesicles to (anterograde) and from (retrograde) the axon terminal. They carry vesicles and organelles from the soma toward the axon terminal (anterograde transport mediated by kinesin) or vesicles containing sampled extracellular material (neurotrophic factors, viruses, toxins etc. mediated by dynein) from the terminal to the soma (retrograde transport). The terminal branches of the axon are called telodendria.

B. Variations in structure

1. Multipolar - the typical neuron with many dendrites arising from the soma and a single long axon. Such cells are located in the CNS and in autonomic ganglia.

2. Bipolar - a neuron having only two processes, one at each end of an oval cyton. They are associated with the systems for smell, hearing and vision.

3. Unipolar - a cyton having a single process that branches to form a peripheral and central process. They are found in sensory ganglia.

C. Sizes - most belong to one of two types

1. Golgi Type I - axons terminate a large distance from the soma. Look for the large pyramidal cells in the cerebral cortex and the anterior horn cells.

2. Golgi Type II - small cells whose axons terminate in the immediate vicinity of the soma. Axons are very short.

D. Synapses - localized specializations of both presynaptic and postsynaptic cells separated by the synaptic cleft, a regular intercellular gap of 20-30 nm containing a fibrillar material.

1. Pre-synaptic specialization - many conical condensations of cytoplasmic proteins are found on the inner surface of the membrane. A cluster of synaptic vesicles 40-60 nm in diameter and some mitochondria are associated with this membrane specialization. .

2. Post-synaptic - condensation of protein on the inner surface of the membrane is typically thicker and more uniform than on the presynaptic membrane.

3. Impulse transmission is in only one direction and is effected by the release of a neurotransmitter from the presynaptic terminal. An action potential triggers the influx of calcium ions and this promotes the exocytosis of the contents of synaptic vesicles. Neurotransmitters may be excitatory or inhibitory. When a neurotransmitter binds to its receptor on the post-synaptic membrane ion flux is modified to either hyperpolarize (inhibit) or depolarize (excite) the membrane. Inhibition occurs when the membrane potential becomes more negative and vice versa. Inactivation of the neurotransmitter occurs by endocytosis and this conserves the plasma membrane, by break down of the transmitter or by diffusion.

Some of the many known neurotransmitters are:

a. Acetylcholine

b. Glycine

c. Glutamic acid

d. Norepinephrine (noradrenalin)

e. Serotonin

f. Gamma-aminobutyric acid (GABA)

g. Dopamine

4. Morphologic types of synapses are named according to their presynaptic and postsynaptic components. The most common is axo-dendritic, but all other combinations are known as: axo-axonic, somato-somatic, etc.

III. GLIAL CELLS -

There is no stroma (connective tissue) in the CNS except in those sites where the pia mater accompanies blood vessels into the tissue. Glial (glue) cells provide physical and metabolic support for the nerve cells. There are three types. In H&E sections glial cells are only evident by their nuclei that are usually smaller than the neuron nuclei.

A. Astrocytes - these cells have many processes but few organelles. The shape of the processes vary with the location, being long and slender in white matter (fibrous astrocytes) and short and thick with many branches in gray matter (protoplasmic astrocytes). Many of intermediate filaments made of glial fibrillary acid protein (GFAP) are in these processes that have expanded ends surrounding blood vessels or separating nerve tissue from the pia mater.

B. Oligodendrocytes - found in both gray matter, as perineuronal satellite cells, and in white matter as interfascicular cells. These cells have few processes but are rich in organelles (RER, free ribosomes, Golgi cisternae, and mitochondria). The processes expand to form myelin so that a single cell may contribute to the formation of myelin around several axons.

C. Microglia - not clearly defined as an independent cell population. When nerve tissue is damaged, they are thought to proliferate and become macrophages to clean up the debris. When filled with phagocytic vacuoles, they are called compound granular corpuscles or Gitterzellen. These cells are believed to be derived from monocytes and are considered to be part of the mononuclear phagocytic system.

IV. EPENDYMA -

A simple cuboidal or columnar epithelium lining the ventricular system of the brain and central canal of the spinal cord. Only where it covers the vascular loops of the choroid plexus does it have occluding junctions which permit these cells to control the production of cerebrospinal fluid (CSF). Elsewhere, solutes including proteins, can exchange between the nervous tissue and the CSF. The cells of the choroid plexus provide a blood-CSF barrier. The blood-brain barrier is maintained by continuous type capillaries with tight zonula occludens in most regions of the CNS.

Ependyma forms a special epithelium on the choroid plexus. Its function of CSF production can be inhibited by the sympathetics, at least in rabbits. Science 201:176-178, 1978

Peripheral Nervous System (PNS)

I. NERVES AND NERVE FIBERS - look for the following features on sections of a peripheral nerve.

A. Nerve - one or more nerve fascicles held together by dense irregular connective tissue.

1. Epineurium - irregular connective tissue that surrounds and extends between fascicles where it may contain fat cells

2. Perineurium - conspicuous sheath around individual nerve fascicles - like pia mater from which it is derived, consists of several layers of epithelial cells surrounded by a basal lamina. The perineural cells have occluding junctions between them forming layers that are separated by collagen bundles. This provides a protective barrier.

3. Endoneurium - delicate connective tissue with capillaries between nerve fibers within fascicles (bundles of nerve fibers)

B. Nerve fiber = axon + neurilemma

1. Axon - process of nerve cell surrounded by axolemma (cell membrane) - contains neurotubules, mitochondria, smooth vesicles and numerous neurofilaments.

2. Neurilemma - Schwann cell and its components (myelin) surrounding the axon. In myelinated axons, the axon is bare between the ends of Schwann cells. This is the node of Ranvier.

3. Schwann cell - parts in myelinated fibers

a. myelin - spiral of double layer of plasma membrane inside a Schwann cells surrounding an axon. The membranes are closely applied to each other. Inner mesaxon and outer mesaxon appear to suspend the myelin, and thus the axon, within the cytoplasm of the Schwann cell cytoplasm by the Schwann cell plasma membrane. Various Schwann cell cytoplasmic regions are named. Propogation of the action potential varies with the axon diameter, myelin thickness and the spacing of the nodes of Ranvier. The smallest fibers conduct at a rate of about 5 meters per second, the largest at a rate of 100 meters per second.

4. Unmyelinated nerve fiber - axon surrounded by a single layer of Schwann cell plasma membrane. Mesaxon attaches to Schwann cell plasma membrane; there are up to a dozen nerve fibers per Schwann cell. Propogation of the action potential is very slow, about 2 meters per second.

5. Neurokeratin - lace work of protein remaining from myelin when lipid has been removed in preparation for light microscopy.

II. NERVE ENDINGS

A. Motor

1. Myoneural junction (motor end plate) in skeletal muscle - consists of several axon endings from a single nerve fiber. Ask to see a special preparation that is snake muscle prepared with gold chloride to show motor endings.

a. Synaptic trough - depression in surface of muscle cell containing axonal ending covered over by Schwann cell.

b. Subneural or junctional folds - formed by infoldings of muscle plasma membrane (sarcolemma). Muscle mitochondria are concentrated in and around the folds and clefts.

c. Axon terminal - expanded end of axon containing many mitochondria and clear, membrane limited, synaptic vesicles 200-400 A in diameter. Probably store acetylcholine - the transmitter.

d. Basal laminae - Surrounds Schwann cell and muscle cell (external laminae) join and continue into junctional clefts.

2. Sympathetic nerve endings

a. Terminal boutons - terminal swellings of non-myelinated axons containing dense cored vesicles (norepinephrine). These occur very close to smooth muscle cells or glandular epithelial cells. Influence spreads to adjacent cells by diffusion of norepinephrine but especially via gap junctions between cells.

b. Synapses en passant - synaptic swellings along course of axon, otherwise like terminal boutons.

B. Sensory endings - general sense (pain, touch, etc.)

1. Free nerve endings - lacking any coverings, these fibers are associated with epithelial organs, often penetrating the basal lamina and interdigitating between epithelial cells. Numerous in cornea and around hair follicles these serve pain and touch respectively.

2. Encapsulated - specialized connective tissue capsule varies with different types.

a. Vater-Pacinian corpuscle - large 2-4 mm onion ring arrangement of flattened cells surrounding a central core, that receives one or two myelinated nerves and blood vessels. It is a pressure receptor found in deep dermis, mesenteries, joints where it serves for proprioception.

b. Meissner - probably touch receptors in dermal papillae.

C. Sensory endings - muscle control

1. Neuromuscular spindle - parts enclosed by connective tissue capsule include:

a. Intrafusal fibers - very small cross striated muscle fibers having nuclei accumulated in center as a cluster (nuclear bag) or as a chain (nuclear chain).

b. Annulospiral endings (primary afferents) - termination of myelinated sensory fiber around nuclear bag and central portion of nuclear chain fibers. It signals rate and extent of muscular lengthening.

c. Flower spray endings - thinner afferent axons that terminate on contractile portions of nuclear-chain intrafusal fibers and sense elongation.

d. Gamma motor axons innervate intrafusal fibers with a motor end plate on each side of the nuclear region to alter the effective length of the spindle.

2. Golgi tendon organs - complex structures at junctions of tendons and muscles - probably sense pain and tension.

III. GANGLIA

Sensory Ganglion

1. Nerve cell bodies (Unipolar ganglion cells, no synapses)

2. Dorsal root, peripheral processes

3. Dorsal root, central processes

A. Sensory - dorsal root and cranial nerve sensory ganglia - unipolar cell bodies (no synapses) surrounded by rather complete layer of capsule cells.

B. Autonomic - multipolar cells with a very thin and inconspicuous layer of capsule cells. Synapses between pre- and postganglionic cells.

IV. DEGENERATION AND REGENERATION OF PERIPHERAL NERVES. Note: if injury is so close to the soma that it dies, it is not replaced; typically there is no regeneration.

A. Degeneration - proximal to injury only 1 or 2 internodes - this is primary. Secondary or Wallerian degeneration occurs simultaneously throughout the length of the axon distal to injury. Axon degeneration occurs first, followed by fragmentation of myelin. Macrophages clean up the debris.

B. Regeneration - axons grow out from the proximal stump at the rate of 4 mm/day - if in line with distal stump, they follow the connective tissue, especially that of the endoneurium and Schwann cells, to the original site of innervation.

C. Retrograde chromatolysis - Nissl granules become dispersed in the soma whose axon is injured.

HISTOLOGY OF THE EYE

I. SUPPORTING LAYERS

A. Sclera - tough, white in color. Consists mostly of collagen fibers arranged in irregular sheets and on its exposed surface has stratified squamous nonkeratinized epithelium. Average thickness 0.5 mm but about 1 mm thick posteriorly. The lamina cribrosa is the region where fascicles of the optic nerve exit. This region is called the optic disk and is the location of the blind spot. The sclera is continuous with the meninges, covering the optic nerve, especially the dura.

B. Cornea - transparent because it is relatively dehydrated so that the fluid ground substance has the same refractive index as the collagen fibers. Contains no vessels, has only free nerve endings (pain). Major bending of light takes place at air-corneal interface.

1. Layers (external to internal)

a. Corneal epithelium is stratified squamous non-keratinized and contains numerous pain nerve fibers. Its surface cells have a finely wrinkled membrane that holds tear fluid and a protective coating of lipids from the Meiobomian glands in the eyelids. This epithelium regenerates rapidly.

b. Bowman's membrane is a thick, tough, acellular membrane. Regeneration results in a scar.

c. Stroma or substantia propria. Many layers of highly ordered collagen embedded in chondroitin sulfate.

d. Descemet's membrane is a basement membrane with an unusual form of collagen. It is tough providing some protection from penetration to the aqueous humor

e. Descemet's endothelium is like mesothelium in its morphology. Removes water from substantia propria to maintain transparency.

II. VASCULAR LAYERS - Uvea

A. Epichoroid (suprachoroid) elastic fibers and chromatophores with melanin.

B. Choroid - highly elastic; pigment in chromatophores; 3 layers.

1. Vessel layer

2. Choriocapillaris (capillary layer) nourishes pigment epithelium and rods and cones. Fenestrated capillaries.

3. Bruch's (basal or glassy) membrane - basement membrane of retina. Provides a blood-retinal (brain) barrier. Blood vessels do not pass through this layer.

C. Ciliary body and iris are supported by the c.t. elements of the choroid, but are covered by epithelial components derived from the retinal layer; therefore, these will be presented under the next section (III).

III. INNER (RETINAL) LAYER

A. Retina - 10 layers from inside out are:

1. Inner limiting membrane - expanded ends of Muller cell processes; a special form of neuroglia. A basal lamina separates them from the vitreous body.

2. Nerve fiber layer - axons of optic nerve arise from next layer.

3. Ganglion cell layer - cytons of optic nerve axons.

4. Inner plexiform layer - processes and synapses between amacrine, bipolar, and ganglion cells.

5. Inner nuclear layer - cytons of bipolar, amacrine, and horizontal cells, and nuclei of Müller cells.

6. Outer plexiform layer - rod and cone axons meet bipolar dendrites - horizontal cell processes.

7. Outer nuclear layer - rod and cone cell bodies.

8. External limiting membrane - expanded peripheral processes of Muller cells forming junctional complexes with rod and cone processes.

9. Layer of rods and cones.

a. Inner segment - Golgi, SER & RER - mitochondria and basal body of cilium.

b. Outer segment - modified cilium; cones are stacks of flat vesicles attached to plasmalemma; turnover of cone disks is not well understood. It appears that new disks are formed at the base, but proteins are incorporated into the disk membranes diffusely. Rod disks are flat, discoidal membranous sacs that are free in the cytoplasm. They develop in the inner part of the outer segment from the plasma membrane and they migrate toward the pigment epithelium that phagocytoses the disks as they are shed.

c. Rhodopsin (retinal +opsin) is the visual pigment in rods. It mediates the conversion of light to neural energy. It is the most sensitive of the visual pigments but does not differentiate between colors (cones have pigments sensitive to red, green and blue). The retina serving the peripheral vision contains only rods, centrally only cones. Between those extremes they are mixed together with an increasing proportion of cones near the fovea centralis.

10. Pigment epithelium - removes by phagocytosis fragments shed by rods and reduces light scattering. tight junctions between them contribute to the blood-retina barrier. They are derived from the outer layer of the embryonic optic cup and in detached retina are easily separated from the rods and cones.

B. Retina - macula lutea (yellow spot) - contains fovea centralis - consists of three types of cones that are optimally sensitive to red, green and blue light.

C. Retina - convergence

1. Macula lutea - 1:1 (one bipolar/cone and one ganglion cell/bipolar).

2. At periphery - convergence is high; many rods/ganglion cell.

D. Retina - blood supply - retinal artery enters with the optic nerve and branches over inner surface of retina - rod and cone cells depend on diffusion from the choroid, but the inner layers (except in the region of the fovea) contain capillaries from the retinal artery.

E. Ciliary body and iris

1. Ciliary body

a. The middle coat of the eye, the vascular layer, contributes smooth muscle - the ciliary muscle. This muscle has meridionally and radially oriented fibers (sympathetic innervation), that are firmly anchored anteriorly to the scleral spur. Posteriorly it is free to pull on the choroid. A bundle of circularly arranged smooth muscle fibers (parasympathetic control) surround the attachment of the zonular fibers.

b. The inner layer of the eye (retinal layer) becomes attenuated anteriorly at the ora serrata to form a double layer of cuboidal epithelium that covers the ciliary body. The outer layer is pigmented (melanin); the inner is not. Basement membrane is on both sides of the double layer of epithelium on the ciliary processes. The nonpigmented cells form the blood-aqueous barrier and produce the aqueous humor that has a composition much like that of cerebrospinal fluid. They also form the zonule fibers (These consist of oxytalan.) of the suspensory ligament. The aqueous humor circulates from the posterior chamber through the pupil to the anterior chamber. It is reabsorbed at the iridial angle through spaces of Fontana into the venous canal of Schlemm.

2. Iris

a. The middle coat - stroma, spiraled blood vessels and chromatophores. It also has a substantial sphincter of circular smooth muscle differentiated from the epithelial layer during development.

b. The retinal layer - the tenuous dilator muscles are a component of basal part of the pigment epithelial layer and might be called myoepithelial cells. The double layer of epithelium is continuous on the posterior surface of the iris with that under #1b above, only both layers are pigmented.

c. The anterior surface of the iris is not covered by epithelium but with an incomplete layer of fibroblasts and pigment cells. They form a rough surface with irregular ridges and grooves.

d. Parasympathetic stimulation causes constriction of the pupil in response to light and in accommodation for near vision the simultaneous contraction of the ciliary muscle allowing the lens to round up. Sympathetic stimulation causes contraction of the dilator pupillae.

IV. LENS

A. Structure

1. Capsule - basal lamina and reticular fibers

2. Anterior epithelium - simple cuboidal under capsule at the equator they become columnar.

3. Lens substance

a. Cortex - lens fibers develop from elongate cells at the equator - directed from anterior to posterior a process that continues slowly through life.

b. Nucleus - center - hard; individual fibers not readily distinguished.

B. Accommodation

1. Focused for far vision (infinity). The lens is pulled on by the oxytalan fibers of the ciliary zonule and this causes the lens to be flattened. The ciliary zonule is pulled on (in turn) by the very elastic choroid. The ciliary muscle is relaxed.

2. Focused for near vision. The ciliary muscle is anchored to the sclera anteriorly and when it contracts it pulls on the choroid. This takes the tension off the ciliary zonule and the lens is free to assume a more globular shape. With age this capacity to "round up" is lost and presbyopia is the result.

V. OPTIC NERVE - made up of the axons of ganglion cells. Covered with meninges. Contains glial cells.

A. In cross-section can be distinguished from peripheral nerve by the septa. These septa cause small angular fascicles to be formed instead of rounded fascicles surrounded by perineural epithelium. Also, the overall round shape is often helpful.

B. In longitudinal section, optic nerve has round to oval glial cell nuclei; no Schwann cells are present. Peripheral nerve has the elongated, fusiform or spindle-shaped nuclei of Schwann cells and fibrocytes.

HISTOLOGY OF THE EAR

I. DIVISIONS OF THE EAR

A. External

1. Pinna - thin skin and elastic cartilage

2. External auditory meatus

a. Outer 1/3 - cartilaginous support

b. Inner 2/3 - bony support

c. Glands - sebaceous and ceruminous; the latter are apocrine and secrete wax

B. Tympanic membrane

1. Epithelia - external - thin stratified squamous keratinized; internal - simple squamous

2. C.T. - outer radial and inner circular collagen fibers

C. Middle

1. Tympanic cavity - contains ossicles (malleus, incus and stapes) covered by simple squamous and some ciliated columnar cells.

2. Communications - anteriorly with auditory tube; posteriorly with mastoid air cells.

3. Muscles - tensor tympani and stapedius

D. Inner

1. Membranous labyrinth containing endolymph, consists of lateral, anterior, and posterior semicircular canals; arising from utricle; that is connected to the saccule by the utriculosaccular duct; that gives rise to the endolymphatic duct and sac. The ductus reuniens provides the endolymphatic communication from the saccule to the cochlear duct, that is also part of the membranous labyrinth. Study these relationships using diagrams in your text. The potasium-rich endolymph is produce by the stria vascularis, a special capillary-containing epithelium in the lateral wall of the cochlear duct (scala media).

2. Perilymph surrounds the membranous labyrinth. It is similar to CSF in composition and communicates with arachnoid space through the perilymphatic duct.

3. Bony labyrinth contains the perilymph and membranous labyrinth.

4. Internal auditory meatus conveys eighth cranial nerve to inner ear.

II. MACULAE

A. Functions - static position sense relative to gravity and linear acceleration.
Bending of hair cell processes is the stimulus.

B. Structure - epithelium and otolithic membrane.

1. Epithelium - 2 cell types

a. Sustentacular cells - extend from basal lamina to surface. Hair cells do not reach the basement membrane but are supported by the sustentacular cells. Apical cuticular plate = terminal web.

b. Hair cells - 2 types based on shape and innervation. Both have terminal web extending into long microvilli and a single cilium. Regeneration may occur. (Science 259:1616-1621, 1993)

1. Type I - goblet shaped surrounded at base by single sensory nerve ending.

2. Type II - columnar shaped; multiple, both afferent and efferent, nerve endings at base.

2. Otolithic membrane - gelatinous cuticle surrounding hair cell processes and containing otoconia (calcium carbonate crystals).

III. CRISTAE AMPULLARIS - one for each semicircular canal.

A. Function - senses angular acceleration by detecting the differential movement of the membranous semi-circular canals and their contained endolymph. The transversely oriented cupula partially blocks the lumen so that turning motions in the plane of the semicircular canal causes the membranous canal to move relative to its contained fluid. This bends the cupula with its enclosed hair cell processes triggering a neural response.

B. Structure - epithelium and cupula

1. Epithelium - much like that of maculae. Epithelium covers a transverse ridge and hair cell processes extend into cupula.

2. Cupula - gelatinous; transverse to axis of canal; surrounds hair cell processes

IV. COCHLEA - Study text diagrams and any prepared sections that are available.

A. Osseous - 21/2 spiral turns - base directed toward the internal auditory meatus receives cochlear nerve.

1. Modiolus - conical shaped osseous core contains spiral ganglion. The osseous spiral lamina, like threads on a wood screw, projects out from the modiolus. With the attached cochlear duct, the osseous spiral lamina subdivides the osseous spiral canal into the apical scala vestibuli and a basal scala tympani. The scalae communicate with each other at the apex through the helicotrema. At the base of the cochlea, the scala vestibuli opens into the osseous vestibuli, and scala tympani ends blindly at the round window.

B. Cochlear duct - membranous cochlea

1. Triangular in cross-section, with apex of triangle attached to osseous spiral lamina and base fused to periosteum of peripheral wall of osseous spiral canal. The thin vestibular membrane is next to the scala vestibuli. The thick basilar membrane stretches between the osseous spiral lamina centrally and the spiral ligament peripherally next to the scala tympani.

2. Spiral limbus - on apical side of osseous spiral lamina provides: 1) central attachment of vestibular membrane; 2) formation of tectorial membrane; 3) with organ of Corti peripherally forms internal spiral sulcus.

3. Organ of Corti from internal spiral sulcus out, consists of:

a. Border cells

b. Inner hair cells; similar to Type I vestibular hair cells, but lack cilium.

c. Inner phalangeal cells

d. Inner tunnel formed by inner and outer pillar cells.

e. Space of Nuel

f. Three to 5 rows of outer phalangeal cells supporting outer hair cells, that are elongated and similar to Type II vestibular hair cells, but lack cilium.

g. Outer tunnel

h. Hensen's cells, supporting cells

i. Claudius' cells, supporting cells

Sustentacular (phalangeal) cells are more elaborate than in the maculae. They contain many microtubules that shape phalangeal cells to clasp apical end of hair cells.

4. Stria vascularis - at least two kinds of cells: those near the lumen having elaborate basal and lateral infoldings and numerous mitochondria. Many capillaries pass among these cells - probably source of endolymph, that is rich in K+ (140 meq/L) and poor in Na+ (26 meq/L). Endolymph is probably absorbed by columnar epithelium lining endolymphatic sac.

[Top]

Main Navigation

CNS

Nervous Tissue and the Central Nervous System