An electrophysiological study of single somatosensory neurons in rat granular cortex serving the limbs: a laminar analysis

Abstract

1. Recordings were made from 545 neurons in somatosensory granular cortex of anesthetized Sprague-Dawley rats. Of this sample, 32% were active spontaneously. Active neurons were not distributed uniformly throughout cortex but were most common in layer V. The highest mean spontaneous discharge frequency also was found in this layer. Cells with the lowest rates of spontaneous activity were located immediately above and below. One subset of spontaneously active neurons was characterized by an unusually high discharge frequency modulated by somatic stimulation. 2. Only 25.8% of the 534 neurons tested in granular cortex could be activated by somatic stimuli. Only 9.4% had cutaneous receptive fields, and 2.4% received deep inputs. The remainder (14.0%) were driven by higher intensity stimuli and could not be classified unequivocally as either cutaneous or deep. The 50 neurons with cutaneous receptive fields were located in the middle third of the cortex, and those with the largest receptive fields were found most superficially. Neurons driven by somatic stimuli were found most frequently in layer Vb, where 44.5% of the sample confirmed histologically to be in layer Vb could be excited. 3. The large proportion of neurons lacking demonstrable somatic inputs was attributed to the use of iontophoretically administered glutamate, which allowed the detection of many unresponsive neurons. This proportion was not reduced by the use of nitrous oxide and halothane as an anesthetic. 4. Neurons activated only by deep inputs were found on the medial and rostral edge of the hindlimb granular cortex, suggesting that deep and cutaneous inputs may be segregated in this species. 5. Electrical stimuli applied to the foot pads activated a sample of neurons differing from those driven by natural somatic stimuli in terms of depth, spontaneous activity, probability of somatic input, and probability of activation by the pyramidal tract. 6. Pyramidal tract neurons tended to be located in layer Vb, were active spontaneously, and had evidence of somatic inputs, although most required relatively intense stimuli to be excited. Other neurons activated synaptically from the pyramidal tract were located in the layers immediately above and below the pyramidal tract neurons. These cells were divided into two groups on the basis of action-potential latency, action-potential shape, and sensitivity to acetylcholine.