FoundationalConceptsNeuroscience

called the spiral ganglion, one cluster for each ear. These are so-called
bipolar neurons, with a single myelinated (for efficient conduction)
dendrite receiving the signal from the hair cell and a myelinated axon
carrying the signal into the brainstem. In the brainstem’s medulla,
axons of the auditory nerve synapse with cells in a region called the
cochlear nucleus. Neurons of the cochlear nucleus send axons to cells
in regions of the pons called the superior olive and the lateral lemnis-
cus. The various brainstem auditory centers all send axons into the
inferior colliculus in the midbrain. The inferior colliculus projects to
the nearby medial geniculate nucleus (MGN) of the thalamus. And the
MGN sends axons into the temporal lobe of the cerebral cortex, to a re-
gion called the primary auditor cortex, or Al.
There is extensive interconnectivity between the various brain-
stem auditory nuclei, including between nuclei on opposite sides of
the brainstem. One important function of this bilateral connectivity
is to compare the arrival times and other qualities of sounds between
the two ears, underlying our ability to spatially locate the sources of
sound in the auditory environment. As with the visual system, there
is extensive signaling in both directions—that is, neural signals are
sent from the cortex to the MGN, from the MGN to the inferior collicu-
lus, and so on, all the way back to the hair cells in the cochlea.
The vast majority of the hair cells sending signals to the spiral
ganglion and thence to the cochlear nucleus are the so-called inner
hair cells, of which there are approximately thirty-five hundred per
cochlea. Another population of hair cells, called the outer hair cells,
numbering about twelve thousand cells per cochlea, make far fewer
connections to the spiral ganglion but receive much more efferent
input from the brainstem. It is currently believed that the outer hair
cells may be minimally involved in the direct detection and trans-
duction of the auditory signal and instead function to change the