MODULE 12
The AUDITORY PATHWAY
- - - A good way to start your review of auditory pathways is by looking at it, drawn on a dorsal view of the brain such as Figure 2-5. We show this in Figure 12-2. If this view is too schematic or cluttered for you, try Blumenfeld's Figure 12.16 or Fitzgerald's Figure 20.3. For a more elegant and detailed view look at Haines' Figure 21-10 - but be aware that our ventral cochlear nucleus = his anterior cochlear nucleus, and our dorsal cochlear nucleus = his posterior cochlear nucleus.
- - - The cochlear division of vestibulocochlear nerve enters the brain at the level of Cu Slide 9, and the two red lines in this view show the position and course of the fibers. Most entering first order neurons give off two collateral branches; one of these terminates in the ventral cochlear nucleus and the other passes dorsally over the inferior cerebellar peduncle to end in the dorsal cochlear nucleus. This nucleus lies on the dorsal surface of the inferior cerebellar peduncle, just lateral to the inferior vestibular nucleus. This arrangement is shown schematically in Haines' Figure 21-9. This is a good example of divergence and suggests that auditory information passes rostrally in the brain by way of two parallel pathways - each coded for a different parameter of information. This arrangement should remind you of the M and P pathways in the visual system.
- - - The projection of second order neurons, having their cell bodies in the dorsal cochlear nucleus, is termed the dorsal acoustic striae and is shown in Figure 12-2. While the dorsal acoustic striae does not stand out in the slides, its general course is shown schematically on slides 7 and 8. Fibers leaving the ventral cochlear nucleus (slide 9) pass medially just under the spinal tract of the trigeminal nerve. They represent the major output pathway from the cochlear nuclei. The majority of these fibers will run rostrally for a brief interval (slide 10) and then cross the midline in an obvious bundle known as the trapezoid body (slides 11, 12, 13).
- - - Now, let's follow the auditory pathway through the brainstem. There are several ways to do this. You can make use of the call up figure Figure 12-3 or you can click on the individual slides in the text below. But the scheme that works best for us is to click on slide 5 at the start, then use the "one slide rostral" button on each frame to go forward, keeping the slides on one slide of the screen while you scroll through the text on the other.
- - - We don't show the axons of the first order neurons entering the nuclei, but we do show how the output pathways are formed.
- - - Several nuclei are interposed along the pathway from the cochlear nucleus to the inferior colliculus. The nuclei of the trapezoid body (slides 11,12,13) are one example. In most cases we have no specific function to assign to these relay nuclei. An exception to this rule is the superior olivary nucleus (slides 11, 12, 13). This is the first structure in the auditory pathway to receive inputs from both the ipsilateral and contralateral cochlear nuclei and we know it plays an important role in the process of localizing sounds in space. This is a fascinating topic - try to read about it in Haines (Page 343 and Figure 21-11). We have simplified things in our diagrams, and represent the superior olive as a single nucleus, rather than the two structures (lateral and medial nuclei) described by most authors.
- - - Rostral to the level of the trapezoid body all the fibers of the auditory pathway are grouped within the lateral lemniscus (slides 13 - 19). Figure 12 - 1 and several of the slides attempt to show schematically that the pathway is a bilateral one (receives an input from both ears), but that the major input is from the contralateral ear. However, starting with slide 13 we show the pathway only on the right. Just as there are small nuclei scattered within the trapezoid body, so they are also present along the course of the lateral lemniscus - in this case they are called nuclei of the lateral lemniscus (slides 14, 15). If you think the lateral lemniscus is running close to space we previously set aside for the anterolateral pathway, you are right, and - at least on some slides - it would be hard to separate one from the other. All (we really mean all) the ascending fibers in the lateral lemniscus are believed to synapse within the inferior colliculus (slides 16, 17, 18).
- - - Cells within the inferior colliculus (we might call them third order neurons, but if there has been an intervening synapse along the pathway from the cochlear nuclei, then this wouldn't be strictly correct) give rise to axons which leave the nucleus and move rostrally in a compact bundle called the brachium of the inferior colliculus. This pathway runs on the surface of the midbrain, just lateral to the superior colliculus, and it is an old friend that you remember from module 2 (Figure 2-5, Figure 2-11). The brachium of the inferior colliculus is first seen on slide 19. Because of the "looping" course taken by the lateral lemniscus, both fibers entering the inferior colliculus and fibers leaving it are seen on this slide, even though the nucleus itself is not present. The fibers within the brachium have a brief course and can be seen entering the medial geniculate nucleus at the level of slide 21. This thalamic nucleus, homologous with the lateral geniculate nucleus of the visual system and lying just medial to it, is present in slides 21-24. After a synaptic relay within the nucleus, the auditory path moves laterally through the sublenticular part of the internal capsule. The fibers terminate in the auditory cortex, which is buried in the depth of the lateral sulcus on the superior aspect of the superior temporal gyrus. This part of the pathway is known as the auditory radiation, and the course these fibers take is shown diagrammatically in Figure 12-2. The auditory radiation has been drawn in on slides 26, 28 and 30. For a lateral view of the auditory radiation, see Figure 12-4.
- - - It is beyond the scope of this module to discuss how auditory information is processed as it passes through the inferior colliculus and the medial geniculate nucleus to reach the auditory cortex, but Haines has an excellent discussion of this on Pages 345-6.
- - - Because of its clinical significance and anatomical basis, you should, however, be familiar with how the cortex goes about processing sounds, when the sounds happen to be spoken words (= language). Blumenfeld discusses this on Pages 827-32, Fitzgerald does so on Pages 347-350, and Haines the topic briefly on Page 346.
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