DORSAL & VENTRAL COCHLEAR NUCLEI
This point contains an extremely cursory outline of auditory pathways. My goal here is to help you identify several auditory nuclei in the brain stem. You will learn more about these pathways later in the course. TRUST ME!!
The two cochlear nuclei lie dorsal lateral (dorsal cochlear nucleus) and ventral lateral (ventral cochlear nucleus) to the inferior cerebellar peduncle at the rostral pole of the medulla (they drape the inferior cerebellar peduncle). The cell groups related to the other division of C.N. VIII, the vestibular nuclei, lie medial to the inferior cerebellar peduncle.
The primary input to both cochlear nuclei is from the auditory portion of C.N. VIII. The axons making up this division of C.N. VIII consist of the central processes of neurons that lie in the spiral or cochlear ganglion (lies in the modiolus [bony core] of the cochlea). The peripheral processes of neurons within the cochlear ganglion end upon the hair cells comprising the organ of Corti. We will not discuss the organ of Corti at this time.
Since you will have a series of lectures on the auditory system later in this course I will give you a very INCOMPLETE overview of ascending auditory pathways. Lets trace an ascending pathway from the ventral cochlear nucleus. The axon courses rostrally to reach the pons where it travels in the lateral lemniscus. The axon can travel in the lateral lemniscus until it reaches the inferior colliculus (midbrain) where it synapses. Cell in the inferior colliculus project to the medial geniculate body of the thalamus via the brachium (arm) of the inferior colliculus. The medial geniculate body projects to primary auditory cortex in the temporal lobe.
The ascending axon from the ventral cochlear nucleus can also give off a collateral to a structure called the superior olive. Axons of cells in the superior olive then cross in what is called the trapezoid body, enter the opposite lateral lemniscus and eventually reach the inferior colliculus. You can take it to auditory cortex from here.
As far as neurological deficits involving lesions of the auditory nuclei and pathways, you should know that a lesion of the auditory portion of the C.N. VIII (nerve) results in deafness in the IPSILATERAL ear. Also, a lesion involving the dorsal and ventral cochlear nuclei results in deafness in the ipsilateral ear. Only subtle deficits result from unilateral lesions of such structures as the lateral lemniscus, superior olive, inferior colliculus, medial geniculate body and auditory cortex (areas 41 and 42). SO I WILL USE THE TERM SUBTLE AUDITORY DEFICITS WHEN PROBLEM SOLVING WITH LESIONS INVOLVING AUDITORY STRUCTURES OTHER THAN THE AUDITORY NERVE AND DORSAL AND VENTRAL COCHLEAR NUCLEI.
1.) cells of origin of the auditory division of C.N. VIII lie in the spiral or cochlear ganglion.
2.) all axons of the auditory nerve end in the dorsal and ventral cochlear nuclei.
3.) the trapezoid body and superior olive are parts of the auditory system and lie in the pons.
4.) the lateral lemniscus terminates in the inferior colliculus of the midbrain, while the brachium of the inferior colliculus terminates in the medial geniculate body of the thalamus.