Ha. Swadlow et Tp. Hicks, SOMATOSENSORY CORTICAL EFFERENT NEURONS OF THE AWAKE RABBIT - LATENCIES TO ACTIVATION VIA SUPRATHRESHOLD AND SUBTHRESHOLD RECEPTIVE-FIELDS, Journal of neurophysiology, 75(4), 1996, pp. 1753-1759
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.