The present study examined the conduction properties of 75 visual callosal axons of the awake rabbit. These axons were studied by measuring latency to antidromic activation of cell bodies following midline callosal and/or contralateral cortical stimulation. Seventy-three of 75 neurons (axon conduction velocities = 0.3 to 12.9 m/sec) demonstrated decreases in antidromic latency and threshold to a test stimulus which followed an antecedent conditioning stimulus at appropriate intervals. Control experiments indicated that (i) the latency and threshold variations resulted from prior impulse conduction along the axon, and (ii) the latency decrease reflected an increase in conduction velocity along the main axon trunk. The maximum magnitude of the latency decrease for different axons ranged from 3 to 22% of control values, and the duration ranged from 18 to 169 msec. The duration of the latency decrease was greater for slowly conducting axons than for fast conducting axons. Latency increases to an antidromic test stimulus occurred for up to several minutes following a train of antidromic conditioning pulses. Antidromic latency and threshold shifts were also observed in somatosensory callosal axons and in some corticotectal axons. © 1976.
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