The pyramidal cells in layer V of rat auditory cortex: Anatomy, physiology, origins and characteristics of their inhibitory inputs

Auditory cortex is the last in a series of structures in the ascending auditory pathway that begins with the cochlear nucleus. It is also the major source of descending projections to both the thalamus and the midbrain. Cortical layer V is particularly interesting, as is a target of ascending projections, it comprises a portion of the corticocortical and corticothalamic projections, and it is the sole source of subthalamic cortical projections. We have studied the anatomy and physiology of auditory cortex, focusing on layer V, at the level of single cells using a number of rat brain slice preparations and in vivo anatomical techniques. Within the context of other, more well characterized cortical areas, we have attempted to further the understanding of auditory cortical layer V, its inputs, and its projections.; Using intracellular recording techniques, we have found that the light microscopic cellular anatomy, extracortical projections, and basic intrinsic physiology of rat auditory cortical cells are similar to those observed in other cortical areas. We have also shown that the thalamocortical and corticocortical inputs are physiologically similar to those in other primary sensory cortices. We have directly demonstrated a difference in the inhibitory inputs to the two pyramidal cell types (intrinsically bursting (IB) and regular spiking (RS)) in layer V. We have explored this difference further using whole-cell voltage clamp techniques. Voltage clamp recordings have revealed that while IB and RS cells receive similar amounts of inhibitory input, the most active inputs in IB cells appear to be concentrated in their dendrites, while the active inputs to RS cells are concentrated near the soma. These combined findings indicate that, in addition to having different sets of extracortical projections, RS and IB cells process their excitatory and inhibitory inputs in fundamentally different ways. Whole-cell voltage clamp recordings have also revealed that IPSC's in auditory cortical layer V cells appear to be far faster than those observed in pyramidal cells in other neocortical areas, or hippocampus. This may reflect a need for rapid processing of information in auditory cortex.