Efferent neurons and suspected interneurons in motor cortex of the awake rabbit: axonal properties, sensory receptive fields, and subthreshold synaptic inputs.

1. Properties of antidromically identified efferent neurons within the cortical representation of the vibrissae, sinus hairs, and philtrum were examined in motor cortex of fully awake adult rabbits. Efferent neurons were tested for both receptive field and axonal properties and included callosal (CC) neurons (n = 31), ipsilateral corticocortical (C-IC) neurons (n = 34) that project to primary somatosensory cortex (S-1), and corticofugal neurons of layer 5 (CF-5) (n = 33) and layer 6 (CF-6) (n = 32) that project to and/or beyond the thalamus. Appropriate collision tests demonstrated that substantial numbers of corticocortical efferent neurons project an axon to both the corpus callosum and to ipsilateral S-1. 2. Suspected interneurons (SINs, n = 37) were also studied. These neurons were not activated antidromically from any stimulus site but did respond synaptically to electrical stimulation of the ventrolateral (VL) thalamus and/or S-1 with a burst of three or more spikes at frequencies from 600 to > 900 Hz. All of these neurons also responded synaptically to stimulation of the corpus callosum. The action potentials of these neurons were much shorter in duration (mean = 0.48 ms), than those of efferent neurons (mean = 0.90 ms). 3. CF-5 neurons differed from CC, C-IC, and CF-6 neurons in their spontaneous firing rates, axonal properties, and receptive field properties. Whereas CF-5 neurons had a mean spontaneous firing rate of 4.1 spikes/s, CC, C-IC, and CF-6 neurons all had mean values of < 1 spike/s. Axonal conduction velocities of CF-5 neurons were much higher (mean = 12.76 m/s) than either CC (1.47 m/s), C-IC (0.97 m/s), or CF-6 (mean = 1.96 m/s) neurons. A decrease in antidromic latency (the "supernormal" period) followed a single prior impulse in most CC, C-IC, and CF-6 neurons but was minimal or absent in CF-5 neurons. Although all but two CF-5 neurons responded to peripheral sensory stimulation, many CC (35%), C-IC (59%), or CF-6 (66%) neurons did not. CC, CF-5, and CF-6 neurons that did not respond to sensory stimulation had significantly lower axonal conduction velocities and spontaneous firing rates than those that responded to such stimulation. 4. Sensory receptive fields of neurons in motor cortex were considerably larger than those observed in S-1 but were similar in size to those seen in secondary somatosensory cortex (S-2).(ABSTRACT TRUNCATED AT 400 WORDS)