neurobio 918 ch 11 of Bear's book: the auditory and vestibular systems part 1 (frequency cochlea p
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what are the three ossicles in order (from the tympanic membrane to the oval window)?
the malleus, the incus, and the stapes
describe the pathway that info from the inner ear takes in the CNS?
once a neural response to sound is generated in the inner ear, the signal is transferred to and processed bt a series of nuclei in the brain stem. Output from these nuclei is sent to a relay in the thalamus, the medial geniculate nucleus. Finally, the MGN projects primary auditory cortex, or A1, located in the temporal lobe.
what is the pinna?
the visible portion of the ear, which consists of cartilage covered by skin, forming a sort of funnel
what is the auditory canal?
the entrance to the internal ear
what structure is located at the end of the auditory canal?
the tympanic membrane
what is the purpose of the ossicles?
to amplify the sound
how do the ossicles amplify sound?
to understand the process, consider the definition of pressure. The pressure on a membrane is defined as the force pushing it divided by its surface area. The pressure at the oval window will become greater than the pressure at the tympanic membrane if (1) the force on the oval window membrane is greater than that on the tympanic membrane, or (2) the surface area of the oval window is smaller than the area of the tympanic membrane. The middle ear uses both mechanisms. It increases pressure at the oval window by altering both the force and the surface area.
The force at the oval window is greater because the ossicles act like levers. Sound causes large movements of the tympanic membrane, which are transformed into smaller but stronger vibrations of the oval window. And the surface area of the oval window is much smaller than that of the tympanic membrane.
what is the attenuation reflex?
when the two muscles in the inner ear-- the tensor tympani muscle and the stapedius muscle-- contract, the chain of ossicles becomes much more rigid, and sound conduction to the inner ear is greatly diminished.
what are the three canals within the cochlea called (from top to bottom)?
from top to bottom:
and scala tympani
what structure in the cochlea contains auditory receptor neurons?
the organ of corti
name the three membranes in the scala media
the tectorial membrane, Reissner's membrane (the top wall of the scala media), and the basilar membrane (located at the base of the scala media)
what are two differing structural properties of either end of the basilar membrane?
first, the membrane is wider at the apex than at the base. Second, the stiffness of the membrane decreases from base to apex
hair cells form synapses on neurons whose cell bodies are located in the ____ ______ within the modiolus. What does this structure lead into?
hair cells form synapses on neurons whose cell bodies are located in the spiral ganglion within the modiolus. Axons from the spiral ganglion enter the auditory nerve, which projects to the brain
explain what occurs in the hair cell when the stereocilia are bent in either direciton
when the stereocilia bend in one direction, the hair cell depolarizes, and when the stereocilia bend in the other direction, the hair cell hyperpolarizes
what is the TRPA1 channel? what is tip link?? explain in-depth.
recent experiments revealed that ther eis a special type of cation channel, the TRPA1 channel, on the tips of the stereocilia. It is likely that TRPA1 channels are induced to open and close by the bending of the stereocilia, thereby generating changes in receptor potential.
each channel is connected by an elastic filament, called a tip link, to the wall of the adjacent cilium. When the cilia are straight, the tension on the tip link holds the channel to a partially opened state, allowing a small leak of K ions from the endolymph into the hair cell. Displacement of the cilia in one direction increases tension on the top link, increasing the inward K current. Displacement in the opposite direction relives tension on the tip link, thereby allowing the channel to close completely, preventing inward K flow.
opening K channels hyperpolarizes or depolarizes hair cell?
explain how spiral ganglion neurons communicate with the two different types of hair cells-- inner and outer.
each inner hair cell feeds about 10spiral ganglion neurites. The inner hair cells make up the bulk of the communication that goes on between hair cells and the spiral ganglion. the situation is opposite with outer hair cells. Because they outnumber their spiral ganglion cells, one spiral ganglion fiber synapses with numberous outer hair cells
what is the purpose of outer hair cells?
outer hair cells seem to act like tiny motors that amplify the movement of the basilar membrane during low-intensity sound stimuli. Because of this action, the outer hair cells on the basilar membrane are referred to as the cochlear amplifier.
describe the path that info takes in the auditory pathway in the CNS.
from the spiral ganglion, axons project to the ventral cochlear nucleus and the dorsal cochlear nucleus. Cells in the ventral cochlear nucleus send out axons that project to the superior olive on both sides of the brain stem. Axons of the olivary neurons ascend the lateral lemniscus of the midbrain. Many efferents of the dorsal cochlear nucleus follow a route similar to the pathway from the ventral cochleaer nucleus but the dorsal path bypasses the superior olive. Although there are other routes from the cochleaer nuclei to the inferior colliculus, with additional intermediate relays, all ascending auditory pathways converge onto the inferior colliculus. The neurons in the inferior colliculus send out axons to the medial geniculate nucleus (MGN) of the thalamus, which in turn projects to auditory cortex
true or false? each cochlear nucleus receives input from both ears.
false. each cochlear nucleus receives input from just one ear on the ipsilateral side; all other auditory nuclei in the brain stem receive input from both ears.
This explains the clinically important fact that the only way by which brain stem damage can produce deafness in one ear is if a cochlear nucleus (or auditory nerve) on one side is destroyed
information about sound intensity is coded in what two ways?
the firing rates of neurons and the number of active neurons.
as the sound stimulus gets more intense, the basilar membrane vibrates with greater amplitude, causing the membrae potential of the activated hair cells to be more depolarized or hyperpolarized. As a result, the nerve fibers with which the hair cells synapse fire action potentials at greater rates. The auditory nerve fiber fires faster to the same sound frequencies when the intensity is increased. In addition, more intense stimuli produce movemnts of the basilar membrane over a greater distance, which leads to the activation of more hair cells. In a single auditory nerve fiber, this increase in the number of activated hair cells causes a broadening of the frequency range to which the fiber responds. The loadness we perceive is thought to be correlated with the number of active neurons in the auditory nerve and their firing rates