SOMATOSENSORY CORTICAL EFFERENT NEURONS OF THE AWAKE RABBIT: LATENCIES TO ACTIVATION VIA SUPRA- AND SUBTHRESHOLD RECEPTIVE FIELDS. Swadlow, Harvey A. and T. Philip Hicks. The University of Connecticut and The University of North Carolina at Greensboro.
APStracts 2:0011N, 1996.
SUMMARY AND CONCLUSIONS
1. Latencies to peripheral sensory stimulation were examined in four classes of antidromically identified efferent neurons in primary somatosensory cortex (S1) of awake rabbits. Both suprathreshold responses (action potentials) and subthreshold responses were examined. Subthreshold responses were examined by monitoring the thresholds of efferent neurons to juxtasomal current pulses (JSCPs) delivered through the recording microelectrode (usually 1-3 æA). Using this method, EPSPs and IPSPs are manifested as decreases and increases in threshold, respectively. Efferent populations examined include callosal (CC) neurons, ipsilateral corticocortical (C-IC) neurons, and descending corticofugal neurons of layer 5 (CF-5) and layer 6 (CF-6). Very brief, air- puffs (rise and fall times 0.6 ms) were delivered to the receptor 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 did not. CC and CF-6 neurons that yielded no suprathreshold response to the stimulus had lower axonal conduction velocities than those that did respond (p < .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 neurons (43%). The subthreshold responses usually consisted of an initial excitatory component (a decrease in the threshold to the JSCP) and a subsequent long- duration (> 80 ms) inhibitory component. A few neurons (1 CC, 1 C-IC and 5 CF- 6) showed an initial short- latency inhibitory response in the absence of any excitatory component. 3. Some CC and C-IC neurons yielded supra- and/or subthreshold responses to peripheral stimulation at latencies of 6.1- 7 ms. All such neurons were found at intermediate cortical depths (thought to correspond to deep layer 2-3 through layer 5). It is argued that such latencies are indicative of monosynaptic activation via thalamic afferents. Very superficial CC and C-IC neurons, and all CF-6 neuron responded at latencies of > 7 ms. CF-5 neurons all 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 results indicate that cortical cell type as well as laminar position are important factors that determine the sequence of intracortical neuronal activation following peripheral sensory stimulation. 

Received 5 July 1995; accepted in final form 17 November 1995.
APS Manuscript Number J427-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 22 January 96