SOMATOSENSORY CORTICAL EFFERENT NEURONS OF THE AWAKE RABBIT - LATENCIES TO ACTIVATION VIA SUPRATHRESHOLD AND SUBTHRESHOLD RECEPTIVE-FIELDS

1. Latencies to peripheral sensory stimulation were examined in four c lasses of antidromically identified efferent neurons in the primary so matosensory cortex (S1) of awake rabbits. Both suprathreshold response s (action potentials) and subthreshold responses were examined. Subthr eshold responses were examined by monitoring the thresholds of efferen t neurons to juxtasomal current pulses (JSCPs) delivered through the r ecording microelectrode (usually 1-3 mu A). Through the use of this me thod, excitatory postsynaptic potentials (EPSPs) and inhibitory postsy naptic potentials (IPSPs) were manifested as decreases and increases i n threshold, respectively. Efferent populations examined included call osal (CC) neurons, ipsilateral corticocortical (C-IC) neurons, and des cending corticofugal neurons of layer 5 (CF-5) and layer 6 (CF-6). Ver y brief air puffs (rise and fall times 0.6 ms) were delivered to the r eceptor periphery via a high-speed solenoid valve. 2. Whereas all CF-5 neurons had demonstrable suprathreshold excitatory and/or inhibitory responses to peripheral stimulation, most CC, C-IC, and CF-6 neurons d id not. CC and CF-6 neurons that yielded no suprathreshold response to the stimulus had lower axonal conduction velocities than those that d id respond (P < 0.0001 in both cases). However, subthreshold receptive fields could be demonstrated in many of the otherwise unresponsive CC (81%), C-IC (88%), and CF-6 (43%) neurons. The subthreshold responses usually consisted of an initial excitatory component (a decrease in t he threshold to the JSCP) and a subsequent long-duration (>80 ms) inhi bitory component. A few neurons (1 CC, 1 C-IC, and 5 CF-6) showed an i nitial short latency inhibitory response in the absence of any excitat ory component. 3. Some CC and C-IC neurons yielded supra- and/or subth reshold responses to peripheral stimulation at latencies of 6.1-7 ms. All such neurons were found at intermediate cortical depths (thought t o correspond to deep layer 2-3 through layer 5). It is argued that suc h latencies are indicative of monosynaptic activation via thalamic aff erents. Very superficial CC and C-IC neurons, and all CF-6 neurons res ponded at latencies of >7 ms. All CF-5 neurons responded at latencies of >8 ms, although many were found at the same depth as the deeper CC and C-IC neurons that responded at monosynaptic latencies. These resul ts indicate that cortical cell type as well as laminar position are im portant factors that determine the sequence of intracortical neuronal activation after peripheral sensory stimulation.