Glossary of HISTOLOGY OF THE EAR - 53

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1. perilymph
2. endolymph

A. Perilymph
1) Within osseus labyrinth; outside membranous labyrinth
2) Low in potassium, high in sodium
3) In direct communication via duct with subarachnoid space

B. Endolymph:
1) High in potassium ions, low in sodium ions
2) In communication via duct with endolymphatic sac in subdural space
3) Endolymph formed in stria vascularis and other sites (ion transport)
4) Endolymph absorbed in the endolymphatic sac
What is the modiolus?

The cochlea wraps around a bony protuberance of the petrous bone known as the modiolus. The spiral ganglion (containing bipolar sensory nerves) and nerve fibers of the cochlear nerve are found within the modiolus

Name the labeled structures?

1. Spiral ganglion

2. Cochlear nerve
Describe the cochlear duct?

Cochlear duct (scala media)
1. Part of membranous labyrinth: endolymph-filled
2. Boundaries
a. Stria vascularis & spiral ligament form outer edge
Stria vascularis is unusual epithelium as it is vascularized.
Stria vascularis is pseudostratified columnar epithelium
b. Vestibular membrane (of Reissner) forms “roof” (separating scala media from
scala vestibuli.
c. Basilar membrane forms “floor” (separating scala media from scala tympani
3. Contains organ of Corti, which sits on basilar membrane
Describe the supporting cells of the organ of corti?

Supporting cells
a. Pillar cells
b. Inner & outer phalangeal cells
c. Other supporting cells – don’t worry about names
d. Region known as spiral limbus
1. Relate the apices of hair cells and plalangeal processes?

2. Compare the basolateral sides and apical sides of hair cells?

3. Describe the role of stria vascularis and phalangeal cells in ion transport?

1. Tight junctions between the apices of hair cells and the phalangeal processes of the phalangeal cells form the impermeable reticular lamina (you needn’t learn this term).

2. Although part of the scala media, the basolateral sides of hair cells is bathed in
fluid of perilymph-like composition; the apical side of the hair cells is bathed in endolymph

3. Potassium pumped into endolymph by stria vascularis; enters hair cells when they depolarize; the potassium is released into intercellular space on basolateral side of hair cells; picked up by phalangeal cells and transported back to stria vascularis via gap junctions between cells.
What happens if there is a mutation in the gap junction protein connexin 26?
Mutations in the gap junction protein connexin 26 can cause deafness, as can
mutations compromising the tight junctions of the reticular lamina.
What is the role of inner hair cells?

Inner hair cells transduce sound.
a. The sensitivity of their afferent nerve endings is modulated by axoaxonal
synapses from efferent nerves originating in superior olive
b. When depolarized in response to sound, the inner hair cell releases
neurotransmitter onto nerve endings of sensory nerves located in the spiral
What is the neurotransmitter of inner hair cells?
The neurotransmitter of the inner hair cell is glutamate. Overexposure to loud sounds can damage the peripheral processes of the spiral ganglion cells.
What is the role of outer hair cells?

Outer hair cells are motile and modulate sensitivity to sound. .
a. Their resting membrane potential is regulated by efferent nerves from the
superior olive.
b. The outer hair cells shorten and lenthen in response to sound.
c. Outer hair cell movement is believed to amplify sound in the vicinity of the
associated inner hair cells.
1. What is the name of the motor molecule used by outer hair cells?

2. movement of outer hair cells also requires what?
1. The motor molecule is prestin, which uses a direct voltage-to-force conversion. This is known as electromotility.

2. Movement also requires the actin-spectrin subcortical lattice, which stiffens the membrane.
What is the ampulla?

Each semicircular canal has a small dilation, the ampulla, where it joins the utricle. Within each ampulla is a sensory area, the crista ampullaris. Cells included in the crista ampullaris are:
a. Hair cells (type I and type II) with stereocilia; unlike in cochlea, each also has one kinocilium (a cilium). Recall that stereocilia are like microvilli in structure, only longer.
b. Supporting cells

There are also efferent nerves to the crista ampullaris, which modulate its sensitivity.
What is the cupula?

Hair cell stereocilia and kinocilia project into glycoprotein-containing gelatinous substance, the cupula. When the head rotates, the endolymph sloshes, bending the stereocilia and the
Describe the utricle and saccule? What is the name of their sensory area?

Each is endolymph filled. The area of the bony labyrinth they occupy is known as the vestibule.
Each has a sensory area, the macula and each macula has type I and type II hair cells and supporting cells
Describe the role of the otolithic membrane?

The hair cells each have a number of stereocilia and one kinocilium. Stereocilia and kinocilia are embedded in the otolithic membrane. The otolithic membrane has otoliths (literally, “ear stone”) made of calcium carbonate. When the head accelerates linearly, the otolithic membrane lags behind, bending the cilia and changing the activity of the vestibular nerve.

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