Glycosaminoglycan Chains and Axon Regeneration

Specific sensory input to neocortex arrives in layer 4, whereas nonspecific input, such as information about salience or novelty, is thought to arrive in layer 1. This scheme might imply a slower arrival of inputs to layer 1. In this issue, Zhu and Zhu examine this question using paired whole-cell recording in vivo from layer 1 nonpyramidal cells and the dendrites of the output pyramidal neurons in layer 5. They measured the latency of EPSCs evoked by natural stimulation of whiskers. Surprisingly, the latency in layer 1 was 5–7 msec, the same as in layer 4. As expected, layer 1 cells had larger receptive fields responding to 6 –15 whiskers. Because concurrent signals in layers 1 and 4 lower the threshold for dendritic calcium action potentials and thus enhance burst firing in the output cells in layer 5, the authors postulate that the concurrent inputs may serve as a coincidence detector.