Excitatory and Inhibitory Outputs from the Inferior Colliculus to the Thalamus

The major output of the central nucleus of the inferior colliculus is provided by a large number of neurons that project primarily to the ventral division of the medial geniculate body (ipsilateral) via the brachium of the inferior colliculus and the medial division of the medial geniculate (bilateral). These include GABAer-gic neurons (see later discussion). The central nucleus also has commissural projections to the contralateral inferior colliculus and descending projections to the ventromedial periolivary region of the superior olive (ipsilateral) and to the cochlear nucleus (bilateral).

The major outputs of the dorsal and lateral cortices are ascending projections to the dorsal and medial divisions of the medial geniculate body (ipsilateral). Dorsal cortex sends outputs to the dorsal and deep dorsal nuclei. Lateral cortex projections also terminate in the dorsal nucleus and in the margins surrounding the ventral division. Unlike the central nucleus, the dorsal and lateral cortices contribute few if any ascending projections to the ventral division of the medial geniculate. Thus, the dorsal and lateral cortices of the colliculus are unlikely to directly influence the projections from the thalamus to the primary auditory cortex of the telencephalon.

GABAergic projections ascend to the thalamus from the inferior colliculus in parallel with non-GABAergic projections. These parallel projections have not been confirmed in primates, but such projections would be consistent with their presence in diverse species like rats and cats. It is unusual for GABAergic cells to participate in ascending sensory projections to the thalamus, and it is not seen in the visual or somatosensory systems. Brain slice experiments in rodents have shown that short-latency, inhibitory postsynaptic potentials that use GABA-A receptors are produced in neurons of the medial geniculate after electrical stimulation of brachium axons. These short-latency potentials can precede the excitatory potentials from the central nucleus and may regulate the onset of thalamic excitation from the inferior colliculus. Many neurons in the medial geniculate receive both excitatory and inhibitory inputs from the inferior colliculus. However, some neurons in the medial geniculate receive only excitatory inputs from the midbrain.

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