Figure 1 shows the major auditory pathways. Nerve fibers from the spiral ganglion of Corti enter the dorsal and ventral cochlear nuclei located in the upper part of the medulla. At this point, all the fibers synapse, and second-order neurons pass mainly to the opposite side of the brain stem to terminate in the superior olivary nucleus. A few second-order fibers also pass to the superior olivary nucleus on the same side.
Fig1. Auditory nervous pathways.
From the superior olivary nucleus, the auditory pathway passes upward through the lateral lemniscus. Some of the fibers terminate in the nucleus of the lateral lemniscus, but many fibers bypass this nucleus and travel on to the inferior colliculus, where all or almost all the auditory fibers synapse. From there, the pathway passes to the medial geniculate nucleus, where all the fibers do synapse. Finally, the pathway proceeds by way of the auditory radiation to the auditory cortex, located mainly in the superior gyrus of the temporal lobe.
Several important points should be noted. First, signals from both ears are transmitted through the pathways of both sides of the brain, with a preponderance of transmission in the contralateral pathway. In at least three places in the brain stem, crossing over occurs between the two pathways: (1) in the trapezoid body, (2) in the commissure between the two nuclei of the lateral lemnisci, and (3) in the commissure connecting the two inferior colliculi.
Second, many collateral fibers from the auditory tracts pass directly into the reticular activating system of the brain stem. This system projects diffusely upward in the brain stem and downward into the spinal cord and activates the entire nervous system in response to loud sounds. Other collaterals go to the vermis of the cerebellum, which is also activated instantaneously in the event of a sudden noise.
Third, a high degree of spatial orientation is maintained in the fiber tracts from the cochlea all the way to the cortex. In fact, there are three spatial patterns for termination of the different sound frequencies in the cochlear nuclei, two patterns in the inferior colliculi, one precise pattern for discrete sound frequencies in the auditory cortex, and at least five other less precise patterns in the auditory cortex and auditory association areas.
Firing Rates at Different Levels of the Auditory Pathways. Single nerve fibers entering the cochlear nuclei from the auditory nerve can fire at rates up to at least 1000 per second, with the rate being determined mainly by the loudness of the sound. At sound frequencies up to 2000 to 4000 cycles/sec, the auditory nerve impulses are often synchronized with the sound waves, but they do not necessarily occur with every wave.
In the auditory tracts of the brain stem, the firing is usually no longer synchronized with the sound frequency except at sound frequencies below 200 cycles/sec. Above the level of the inferior colliculi, even this synchronization is mainly lost. These findings demonstrate that the sound signals are not transmitted unchanged directly from the ear to the higher levels of the brain; instead, information from the sound signals begins to be dissected from the impulse traffic at levels as low as the cochlear nuclei. We will have more to say about this subject later, especially in relation to perception of direction from which sound comes.
