How do hair cells transduce mechanical vibrations into electrical signals? (disc

Question

How do hair cells transduce mechanical vibrations into electrical signals? (discuss the role of potassium. stereocilia, Ca channels, and neurotransmitter release) Which cranial nerve carries impulses for hearing? How do we determine the location of a sound? What reflex is initiated by way of impulses that reach the inferior colliculus? (refer back to the chapter on the brain) Where is the primary auditory area of the cortex located? Does the right hemisphere of the brain receive impulses from the right ear, left ear, or both? Explain. Equilibrium Explain the difference between dynamic and static equilibrium: In which structure are the utricle and saccule located? What is the name of the structure within the utricle and saccule that contain receptors for static equilibrium? What is the otolithic membrane and how does it function in equilibrium? Where are the ampulla located and what type of equilibrium do they detect?

Explanation / Answer

1.  Upon stimulation of the hair cells by sound, the potential difference across its surface membrane changes dependending on the amplitude of motion of the hair-bundles. Stereocilia are organelles which respond to the fluidic movement of the hair bundle for hearing and balance. The displacement of the hair bundle in the direction of the tallest stereocilium, leads to the opening of more transduction channels thus, causing depolarization with the entry of K+ into the cell. This depolarization now opens voltage-gated Ca2+ channels in the hair cell membrane, and this Ca2+ influx will then cause release of neurotransmitters from the basal end of the cell onto the auditory nerve endings. These processes finally lead to the transduction of mechanical signals to electrical impulses.It takes place in the cochlea of the inner ear.

2. Vestibulocochlear nerve

3. When the saound in the horizontal dimension, i.e., coming from the right and left sides, neurons in the brain are specially tuned to comparing the intensity and timing differences of the sound between the two ears and thus determines the location of the sound (the higher intensity of sound between the two ears determines the direction). If the sound is in the vertical dimension, i.e., coming from front or back or above and below, the frequency of sound is analysed by the size of the head and the pinna. The pinnae are shaped not only to collect sound, but also to change the frequency of sound (frequencies either get enhanced or attenuated). The Doppler effect is the perception of the change in the distance from the sound when the sound or the person hearing it is in motion.

4. The sensory impulses to the inferior colliculus initiates the movement of the head and thus, aid in sound localization. (the olivocochlearreflex)

5. Primary auditory area of the cotex is located on the superior temporal gyrus of the temporal lobe and receives impulses from the ventral division of the medial geniculate complex.

6. The auditory signals are carried by cranial nerve VIII (vestibulocochlear nerve) which controls the inner ear, and cranial nerve X, (vagus nerve) which controls the outer ear. Since cranial nerves receive and transmit impulses from both the left and right hemispheres, then the primary auditory cortices receive input from both ears, though majority of the signal input is from the opposite ear.

7. Static equilibrium is the sensation that allows us to understand the position of our head positioned relative to gravity.Dynamic equilibrium tells us the direction our head is moves in three-dimensional space, thus giving us the sense of rotation. The semicircular canals, attached to the vestibule help in determining dynamic equilibrium.

8. Vestibule of the inner ear

9. The otolithic membrane is a fibrous structure which is located in the vestibule of the inner ear. and has a critical role in static equilibrium. Otolith crystals or otoconia are present in the otolithic membrane, which cause the otoliths to get pulled towards gravity or away from the direction of movement during the position of head relative to gravity.This movement bends the hairs of the hair cells which then transmit information about position of the head,through cranial nerve VIII (vestibular branch) to the cerebellum.

10. The ampulla (cupula) are present on the semicircular canals of the inner ear and they bend towards the fluid flow alongwith the hair cells. Their function is to transmit the movement information to the cerebellum via the vestibulocochlear nerve.

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