Last week we took a quick break from passages with discrete questions. This week we look at Passage 3 that covers the pathway in which we hear with the ear.
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[01:42] Passage 3
Deflection of the tympanic membrane transfers vibrations to the ossicles, whose function is to amplify the vibration pressure delivered to the oval window, a small hole in the cochlea. Without this amplification, it is estimated that all but 0.1% of the incoming sound energy would be reflected upon contact with the cochlear fluid.
Inner hair cells are the mechanotransduction cells responsible for converting the basilar membrane movement into auditory information. Figure 2 shows their signal transmission pathway, The hair cells depend on stereocilia, microtubule-based projections. The tips of neighboring stereocilia are connected via actin-based linkages, and basilar membrane movement can either strain or relax the tensions of these linkages. When strained, the linkages activate non-specific cation channels, which causes an influx of ions that depolarize the hair cell. As a result of the depolarization, voltage-gated calcium channels are opened, which subsequently causes the package and release of neurotransmitters into the synaptic cleft. The synapse connects the inner hair cell to spiral ganglion cells, which in turn connect to the auditory nerve that relays the information to our brain for further processing.
[04:23] Question 14
In order to amplify the pressure exerted on the cochlear fluid at the oval window, the oval window must be:
(A) larger than the tympanic membrane.
(B) smaller than the tympanic membrane.
(C) more rigid than the tympanic membrane.
(D) less rigid than the tympanic membrane.
The correct answer here is B. There are a couple of ways you can solve this. This involves a little bit of understanding of physics. It’s this idea that if you’re going to exert the same force or pressure, where the pressure is equal to force over area, the way to amplify that pressure is to exert over a really small area.
Another way to distinguish this is that the passage defines the oval window as the small hole in the cochlea.
[07:16] Question 15
Which of the following bones has a footplate that acts as a piston, making contact with the cochlear fluid at the oval window?
The correct answer is D. The A and C are bones in the wrist. If you see a word and not familiar with it, it’s probably not something you should pick. The Stapes is the last bone of the three and it directly makes contact with the cochlear fluid.
[08:50] Question 17
The stereocilia of inner ear cells project into the endolymph fluid, where there is a high K+ and Cl- content but a low Na+ content. Based on this information and the information in the passage, which of the following ions would contribute most to the depolarization of the cell if the tip links between stereocilia were strained?
(A) I only
(B) II only
(C) I and II only
(D) I and III only
The correct answer here is A. Cl and Na are coming in because where all the stuff is potentially but they’re anions.
[12:10] Question 18
Which of the following is most likely the excitatory neurotransmitter released into the synaptic cleft between the inner ear hair cells and the spiral ganglion?
The correct answer here is D since they say excitatory neurotransmitter. And GABA is the classic inhibitory neurotransmitter and glutamate is the excitatory one. Serotonin is brain-focused and only comes up on the MCAT in terms of reward pathway and depression. Acetylcholine is the classically involved with muscle contraction.
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