University:
Western Sydney University
Course:
MEDI 2003 | Clinical Sciences 2
Academic year:

2024
Views:

76

Pages:

3
Author:

Mariam A.

Physiology of Hearing Basilar membrane tuning  Coding of different frequencies of sound occurs in different areas of the cochlea.  The frequency of sounds detected decreases with distance from the tympanic membrane (e.g. the basilar end that is towards the tympanic membrane encodes for higher frequency sounds). o Accounted for by both width and thickness of the area.  Each distinct region of the cochlea is innervated by a certain region of neurons of the auditory nerve (tonotopic organisation). Transducing motion to sound  Hair cells are mechanoreceptors that detect movement along one axis. o There are 50 – 100 stereocillia per hair cell. o Arranged in an ordered slope (lower to higher). o Tips are connected by tip links, which aid in the entire process of transduction.  Endolymph bathes apical surface of hair cells and has high [K+], whilst perilymph bathes basolateral surface. o At rest, there is small K+ leakage.  Bending of the cilia towards the highest cilia causes depolarisation (opposite occurs when bending away from the highest cilia). o Neurotransmitter released by high cells onto nerves is thought to be glutamate.  Opening and closing of K+ channels can be faithfully followed up to 3000Hz. Intensity coding  Loudness is coded by firing rate: o More energy leads to higher basilar membrane bending and increased probability of transmitter release.  However, firing increases up to a point, then saturates. o Most neurons respond over a range of 25 – 40dB.  To detect sounds that are louder, populations of neurons with different but overlapping activation thresholds are recruited. Central auditory system  Cochlear nucleus in rostral medulla (3 divisions).      o Auditory nerve from axons of spiral ganglia. o Anteroventral, posteroventral and dorsal cochlear nuclei. o Act as filters to extract and process information. Superior olive in mid pons (2 divisions). o Some information is crossed here. Nucleus of lateral lemniscus. Inferior colliculus in midbrain. Medial geniculate of thalamus. Primary auditory cortex in superior temporal gyrus in temporal lobe. o Tonotopic organisation to correspond to organisation in cochlea.