Investigation of neuronal connections in the dorsocaudal tegmentum of the cat by a microstimulation method

Unit responses in the caudal part of the tegmentum were investigated by a microstimulation method in the mesencephalic cat. The thresholds of appearance of direct and synaptic responses with latencies of 0.8-1.4 and 1.1-2.0 msec were found to depend on the distance from the electrode to the cell recorded. Responses with a low threshold (0.2-1.1 μV) were found much more often in neurons located 6.3-7.0 mm from the surface of the inferior colliculus than in more dorsal or more ventral zones. The relationship between the threshold I, in μA, of the direct response of the low-threshold cells and their distance r, in μ, from the stimulating electrode is approximated satisfactorily by the equation I=3.3·10^-4r^1.8+0.2. The curve of I as a function of r for synaptic responses is usually more sloping and it has minima for responses recorded not near the cell. The index of synaptic response of some cells rose with an increase in the frequency of stimulation to 20-60/sec.