NEUROSCIENCE: AUDITORY SYSTEM -- Cochlear Innervation and CNS Pathways

COCHLEAR INNERVATION AND CNS PATHWAYS

A. Primary Afferent Neurons

1. Cell bodies located in spiral ganglion

2. Most (95%) of afferent neurons terminate on inner hair cells, with about 10 afferent neurons per hair cell

3. Axons branch on entering the cochlear nucleus and make synapses in both dorsal and ventral parts of the nucleus

B. CNS Pathways (==>)

1. Core Pathway:

cochlea Þ spiral ganglion (cell body) Þ c.n. VIII Þ dorsal cochlear nucleus (synapse) Þ decussate and form lateral lemniscus Þ inferior colliculus (synapse) Þ medial geniculate of thalamus (synapse) Þ primary auditory cortex (temporal lobe)

a. most direct pathway
b. monaural (one ear), contralateral
c. tonotopic organization (individual neurons respond best to a limited range of frequencies – see above); tuning becomes sharper (more selective) as the pathway ascends

2. Belt Pathway:

cochlea Þ spiral ganglion (cell body) Þ c.n. VIII Þ ventral cochlear nuclei (synapse) Þ superior olivary nucleus (synapse), ipsilateral and contralateral via trapezoid body Þ lateral lemniscus Þ inferior colliculus (synapse) Þ medial geniculate of thalamus (synapse) Þ primary auditory cortex

a. binaural (contribution from both ears)
b. certain neurons in the superior olivary nucleus can compare input from the two ears, aiding in sound localization
c. less tonotopic organization; neurons may respond to "character" or timing of sounds

Note: Some ventral cochlear axons follow the Core pathway

Note: Some ascending fibers have synapses in the Lateral Lemniscus nucleus

Note: Because of the binaural representation in the ascending pathways, deafness in one ear only is not usually associated with lesions of 2nd and higher order neurons in the ascending auditory pathway

3. Cross connections at several levels permit sound localization by comparing sound timing and intensity from the two ears

a. superior olivary nuclei
b. inferior colliculus commissure
c. corpus callosum

Note: Sound localization depends upon comparing the response of the two ears for (1) intensity (particularly important for high frequencies) and (2) timing or phase (particularly important for low frequencies)

4. Inferior colliculus (midbrain)

a. relay to thalamus
b. initial processing: some neurons respond only to sounds originating a given direction, some only to tones of a certain duration, etc.

5. Medial geniculate of the thalamus

a. relay to the auditory cortex
b. processing of complex sound patterns; e.g. sonar echo location in the bat