#13 Flashcards
(41 cards)
The inner ear is contained within the petrous portion of the temporal bone, within a system of passages known as the bony labyrinth, comprising two main functional parts:
A. The cochlea, which detects sound and functions in hearing.
B. The vestibular system, which detects accelerations experienced by the head and functions in maintaining balance as well as eye position in response to movement
A is contained within the scala vestibule and scala tympani of the cochlear duct. A has an ionic composition similar to CSF and other extracellular fluids:
a Perilymph
Na and Cl
A is limited to the scala media and has a composition unlike
any other extracellular fluid found in the body:
A: Endolymph
K
Secreted by the a (a highly vascularized pseudo-stratified epithelium on the inner surface of the bony cochlea) directly into the scala media, the endolymph has b potential compared to the perilymph
a stria vascularis
b a positive (+80mV)
Eardrum converts
acoustic pressure waves into mechanical motion
Ossicles acts as a, ligaments act as b
levers (force increased by 30%)
fulcrums
The flow of energy from the air to fluid is
maximized by:
- Pressure gain from the ossicular chain
- Areal ratio of the tympanic membrane to
the stapes (13.4) - Areal ratio of the stapes to the oval
window
Sound energy transmission through the ossicles is regulated by two small muscles
1) Tensor tympani: attaches to the malleus, innervated by motor fibers of CN V3, thought to be important for suppression of self-generated noise (particularly from chewing).
2) Stapedius: smallest skeletal muscle in the body (about 1 mm in length), attaches to the stapes, innervated by CN VII, thought to generally dampen sensitivity upon exposure to loud sounds. Damage to CNVII eliminates this control, resulting in extreme sensitivity to sound (hyperacusis).
A is separated by membranes into compartments called scala. The scala vestibuli and scala tympani contain b and abut the oval window and round window, respectively. These two chambers communicate with each other at the c, a passage located at the apex of the basilar membrane.
a The cochlea
b perilymph
c helicotrema
Basilar membrane vibrations also cause pressure waves in the perilymph of the scala tympani. Since perilymph is a non-compressible fluid, it has to go somewhere when indented by the oval window. The force of these waves is absorbed by the a, so they are not reflected back into scala tympani.
a round window
The number of turns in the cochlea is correlated to a in various mammals
a low frequency sensitivity
More turns, more bass
There are 3 rows of outer hair cells, which actively change their length in response to vibrations in the basilar membrane. This serves to
improve the sensitivity of hearing, particularly at high frequencies. The
effect is to amplify quiet sounds more than large ones, so that a wide range of sound pressures can be reduced to a smaller range of hair displacements
X in particular are very sensitive to damage from exposure to overly loud sounds and to certain ototoxic drugs (e.g., aminoglycoside antibiotics). Once damaged, these cells cannot regenerate.
Outer hair cells
Resting potential of hair cell is x, which the favors influx
of y
x –60mV
y K+ (and some Ca2+)
X is released into the cleft,
increasing the tonic firing rate
of the spiral ganglion cell
(cochlear nerve, CN VIII).
x Glutamate
K+ ions leave the hair cell X, due to higher concentrations inside hair cell compared to perilymphatic side. Thus, the entire flow of potassium from endolymphatic to perilymphatic space is mitigated without Y
X passively
Y expenditure of ATP, a handy energy saving feature for a cell that needs to release chemical neurotransmitter even at rest (no stimulus)
Usher’s syndrome)
mutation on tip links- genetic deafness
Mechanotransduction
tip links spring-gate cation-selective channels for potassium
when stereocilia deflected into the tectorial membrane, allowing potassium to enter the hair cell
Ribbon synapse
Unlike most neurons which release neurotransmitter as the result of a regenerative action potential, bipolar spiral ganglion cells (cochlear n.) must release transmitter in response to small graded potential changes. Encoding changes in the tonic rate of transmitter release requires the release of several thousand presynaptic vesicles per second. To accomplish this, the presynaptic hair cell has a special adaption called the ribbon synapse
* Provides a shuttle zone for continuous exocytotic release of glutamate
* Capable of very high rate of sustained transmitter release over long periods
* Allows hair cells to be both reliable and temporarily precise in their transduction of
auditory stimuli
The synaptic ribbon is a specialized organelle found in
hair cells (auditory and vestibular pathways) and photoreceptor cells (visual pathway)
Only when an adequate stimulus is presented (a tone), is the average fluctuation exceeded in the complex X of the cochlear nuclei, sufficient to drive relay neurons to threshold and carry a signal further up the Y
X pectral receptive field
Y central auditory pathway
95% of spiral ganglion cells (called Type I) innervate X, the remainder (called Type II) innervate Y
X inner hair cells
Y outer hair cells
CENTRAL AUDITORY PATHWAYS
- The cochlear nerve emerges from the internal acoustic meatus and enters the brain at the pontomedullary junction, ending in the dorsal and ventral cochlear nuclei.
- Interspersed among the central projections of the cochlear nerve are specialized 2nd order neurons called cochlear root neurons. These neurons send axons in the pontine reticular formation that in turn send axons to the spinal cord, thought to be involved in the acoustic startle reflex.
- The cochlear nerve also contains some efferent fibers, called the olivocochlear bundle, which run from the superior olivary complex in the pons to the cochlear hair cells. Crossed fibers (the majority) innervate mostly the outer hair cells. This pathway is thought to modulate the frequency responses of the hair cells, protecting the cochlea against loud noises and aiding in the detection and discrimination of sounds in noise.
Clinical considerations involving the cochlear nerve
a) Symptoms: tinnitus, unilateral deafness (often associated with vestibular and facial nerve problems).
b) Examples: acoustic neuroma (a progressively enlarging Schwann cell tumor of the VIII nerve in the internal acoustic meatus); also meningioma of the cerebellopontine angle.
c) Increased acoustic startle response is seen in some patients with pontine strokes
