Investigation of excitatory unit responses arising at different times after application of an electrical stimulus to the cerebral cortex

Abstract

Short local electrical stimulation of the cerebral cortex is known to be accompanied (in the waking or weakly anesthetized animal) by a prolonged neuronal response. Some workers consider that the main component of the response is inhibitory [28,29], but most accept the presence of both excitatory and inhibitory components in the response [21,23]. During application of a single stimulus to the cortex an excitatory response may arise more than once in the same neuron: For instance, most investigators distinguish between shortlatency and postinhibitory excitatory responses. Short-latency responses, as may be judged from data in the literature and the present writers' previous observations, exhibit sufficiently wide dispersion of their latent periods [3,4,7,8,12,13].In this investigation an attempt was made to show that the process of excitation evoked in the cerebral cortex by a single application of an electrical stimulus is heterogeneous in nature, i.e., that excitatory responses arising successively in the neuron are triggered by volleys arriving along different pathways and, possibly, reaching synaptic imputs in different locations. EXPERIMENTAL METHODExperiments were carried out on waking rabbits lightly secured in a frame. By means of capillary microelectrodes filled with 2 M potassium citrate solution, potentials were recorded from single neurons in the sensomotor cortex (extra-and intracellular derivation; coordinates: x = 2.5-3 mm, y = 2.5-3 mm); the electrocorticogram (ECoG) was recorded at the same time .at a distance of about 1.5 mm from the site of insertion of the microelectrode. To record the ECoG and stimulate the brain, nichrome wires, about 80 ~ in diameter, glued together, were used. Surface (the electrode tips lay on the pia mater or immediately below it), deep (the electrode tips were at a depth of 1.5-1.8 mm, in the lower layers of the cortex), or transcortical (the tip of one electrode was on the surface, that of the other in the depth of the cortex) stimulation was used. Electrical stimuli (ES) were applied at different distances from the area of derivation: ES-I was 0.3-0.6, ES-2 2-2.5, and ES-3 and ES-4 were 8-10 mm away. The distribution of the voltages and intensities of stimulation arising under such conditions are given in Figs. 53 and 54 of the monograph by Egorov and Kuznetsova [2]. The duration of the electrical stimulus was 0.1-0.5 msec, and the strength of the current for "close" stimuli varied from 0.i to 0.7 mA and for "distant" from 0.3 to 3 mA. Stimuli applied several times at a frequency of 0.1-0.5 Hz were regarded as "single" and those applied with a frequency of 2 Hz or more as in "repetitive."