Pathway of Hearing & Connections of Cochlear Nuclei
The cochlear nuclei are cell stations in the pathway of hearing. The first neurons of this pathway are located in the spiral ganglion, present in intimate relationship to the cochlea. These neurons are bipolar. Their peripheral processes reach the hair cells in the spiral organ of Corti (which is the end organ for hearing). The central processes of the neurons form the cochlear nerve, and terminate in the dorsal and ventral cochlear nuclei. The neurons in these nuclei are, therefore, second order neurons. Their axons pass medially in the dorsal part of the pons. Most of them cross to the opposite side, but some remain uncrossed.
The crossing fibres of the two sides form a conspicuous mass of fibres called the trapezoid body. Some crossing fibres run separately in the dorsal part of the pons and do not form part of the trapezoid body. (These fibres constitute the so called intermediate acoustic striae and dorsal acoustic striae).
The large majority of fibres from the cochlear nuclei terminate in the superior olivary complex (made up of a number of nuclei). Third order neurons arising in this complex form an important ascending bundle called the lateral lemniscus. Some cochlear fibres that do not relay in the superior olivary nucleus join the lemniscus after relaying in scattered groups of cells lying within the trapezoid body: these cells constitute the trapezoid nucleus (nucleus of the trapezoid body). Still other cochlear fibres relay in cells that lie within the lemniscus itself: these neurons form the nucleus of the laterallemniscus. The fibres of the lateral lemniscus ascend to the midbrain and terminate in the inferiorcolliculus. Fibres arising in the colliculus enter the inferior brachium to reach the medial geniculate body. Some fibres in the lemniscus reach this body without relay in the inferior colliculus. Fibres arising in the medial geniculate body form the acoustic radiation which ends in the acoustic area of the cerebral cortex. It may be stressed that each lateral lemniscus carries impulses arising in both the right and left cochleae.
Some further facts about the auditory pathway are as follows.
a. The spiral ganglion contains two types of cells, type I and type II. Type I cells form about 95% of the population. They innervate inner hair cells. The outer hair cells are innervated by type II cells.
b. The cochlear nuclei contain many types of neurons. The number of neurons in these nuclei greatly exceeds that of cochlear nerve fibres (which are about 25,000). This is explained by the fact that one nerve fibre may synapse with several neurons; and by the view that many neurons of the nuclei are interneurons. This may also be correlated with the observation that the number of fibres in the lateral lemniscus is much greater than that in the cochlear nerve.
Neurons receiving fibres from different parts of the spiral organ are arranged in a definite sequence (in the ventral nucleus).
c. The cochlear nerves may also carry efferent fibres. These fibres take origin in the pons in the vicinity of the superior olivary complex and travel to the spiral ganglion where they end in relation to both type I and type II cells. Such fibres could have an inhibitory or excitatory influence on transmission of auditory impulses.
d. Fibres arising in the dorsal cochlear nucleus pass mainly through the dorsal and intermediate acoustic striae. They project directly to the inferior colliculus of the opposite side. Fibres from the ventral cochlear nucleus pass through the trapezoid body to reach the superior olivary complex.
e. The superior olivary complex is made up of several nuclei. The main nuclei are the lateral superior olivary nucleus and the medial (or accessory) superior olivary nucleus. A retro-olivary group of nuclei is also described. The medial nucleus receives fibres from both cochleae and may play a role in localising the direction of sound (by calculating the time difference in arrival of inputs from the right and left cochleae). The retro-olivary group is believed to be the source of efferent fibres to the spiral ganglion. This group receives descending fibres from the inferior colliculus.
f. Some fibres beginning in the trapezoid nuclei may reach some cranial nerve nuclei through the medial longitudinal fasciculus ; and may be involved in reflex control of the stapedius, the tensor tympani, and the muscles that move the eyeballs.
g. Some fibres from the nucleus of the lateral lemniscus reach the superior colliculus. They help to coordinate visual and auditory stimuli.