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Afferent neurons innervate cochlear inner hair cells, at synapses where the neurotransmitter glutamate communicates signals from the hair cells to the dendrites of the primary auditory neurons. There are far fewer inner hair cells in the cochlea than afferent nerve fibers – many auditory nerve fibers innervate each hair cell.
The Outer ear consists of the pinna or auricle (visible parts including ear lobes and concha), and the auditory meatus (the passageway for sound). The fundamental function of this part of the ear is to gather sound energy and deliver it to the eardrum. Resonances of the external ear selectively boost sound pressure with frequency in the range 2 ...
The exact mechanism by which sound is transmitted by the neurons of the cochlear nerve is uncertain; the two competing theories are place theory and temporal theory. The vestibular nerve travels from the vestibular system of the inner ear. The vestibular ganglion houses the cell bodies of the bipolar neurons and extends processes to five ...
Type I neurons make up 90-95% of the neurons and innervate the inner hair cells. They have relatively large diameters, are bipolar, and are myelinated. Each type I axon innervates only a single inner hair cell, but each inner hair cell is innervated by up to 30 such nerve fibers, depending on species and location within the cochlea.
Neurons sensitive to interaural level differences (ILDs) are excited by stimulation of one ear and inhibited by stimulation of the other ear, such that the response magnitude of the cell depends on the relative strengths of the two inputs, which in turn, depends on the sound intensities at the ears.
Thus, the LSO receives excitatory input from the ipsilateral ear and inhibitory input from the contralateral ear. This is the basis of ILD sensitivity. Projections from both cochlear nuclei are primarily high frequency, and these frequencies are subsequently represented by the majority of LSO neurons (>2/3 over 2–3 kHz in cat).
The number of neurons in the spiral ganglion is estimated to be about 35,000–50,000. [1] Two apparent subtypes of spiral ganglion cells exist. Type I spiral ganglion cells comprise the vast majority of spiral ganglion cells (90–95% in cats and 88% in humans [2]), and exclusively innervate the inner hair cells. They are myelinated, bipolar ...
The central auditory system converges inputs from both ears onto neurons within the brainstem. This system contains many subcortical nuclei that collect, integrate, and analyze afferent signals from the ears, for extraction and analysis of the dimensions of sounds. The outcome is a representation of auditory space and auditory objects. [3] [11]