Auditory thalamic organization: Cellular slabs, dendritic arbors and tectothalamic axons underlying the frequency map

A model of auditory thalamic organization is presented incorporating cellular laminae, oriented dendritic arbors and tectothalamic axons as a basis for the tonotopic map at this level of the central auditory system. The heart of this model is the laminar organization of neuronal somata in the ventral division of the medial geniculate body (MGV) of the rabbit, visible in routine Nissl stains. Microelectrode studies have demonstrated a step-wise ascending progression of best frequencies perpendicular to the cell layers. The dendritic arbors of MGV neurons are aligned parallel to the cellular laminae and dendritic tree width along the frequency axis corresponds closely with the frequency steps seen in microelectrode studies. In the laminated subdivision, tectothalamic axons terminate in the form of bands closely aligned with the laminae and dendritic arbors of thalamic relay neurons. The bands of tectothalamic axons extend in the anterior-posterior (A-P) plane forming a dorsal-ventral series of stacked frequency slabs. In the pars ovoidea region, the homologous spiraling of somata, dendritic fields and tectothalamic axons appear to represent a low-frequency area in this species. At least two types of tectothalamic terminals were found within the bands: large boutons frequently arranged in a glomerular pattern and smaller boutons arising from fine caliber axons. We propose that the rabbit is an ideal model to investigate the structural-functional basis of functional maps in the mammalian auditory forebrain.