Origin of afferents to physiologically defined regions of the medial geniculate body of the cat: ventral and dorsal divisions.

To study the origin of afferents to the medial geniculate body (MGB), single unit recordings were first conducted to define physiologically a given region in this auditory thalamic nucleus. Horseradish peroxidase (HRP) was then injected extracellularly, in order to label retrogradely the neurons whose axon terminals end in this region. The principal inputs to the MGB are coming from various nuclei of the brain stem, the auditory cortex and the reticular complex of the thalamus. The ventral division receives its cortical inputs principally from the primary (AI) and the posterior (PAF) auditory cortical fields, with a quantitatively smaller contribution of the secondary (AII) and anterior (AAF) cortical fields. On the other hand, the dorsal division receives a majority of its cortical inputs from AII, with a less important contribution of AI and PAF. The auditory cortex sends roughly as many axons to these two divisions as does the brain stem, mainly the inferior colliculus (IC). The analysis of ascending inputs to the same regions of the MGB reveals that, on the average, 88%, 7% and 5% of them are coming from the ipsilateral IC, the contralateral IC and other nuclei of the brain stem, respectively.