AFTER-POSITIVITY IN LATERAL GENICULATE BODY

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Abstract
The discharge of an action spike by the cell bodies of the principal neurons of the dorsal nucleus of the lateral geniculate body is followed by a prominent after-positivity which lasts 100-200 mseconds. This after-positivity may be elicited by either orthodromic or antidromic stimulation. After-positivity represents hyper-polarization of the cell body membrane and is associated with an increase in threshold as tested by the ability of the cell body to produce an action spike in response to either an antidromic or orthodromic stimulus. Double antidromic stimulation reveals that for about 10 mseconds after the conditioning response, in the period after the cell body spike has recovered from refractoriness and before depression due to after-positivity has begun, after-positivity develops in response to the 2nd stimulus and adds to that following the 1st. Afterward, for a period of 150-250 mseconds after-positivity in response to the 2nd stimulus is depressed and is still depressed at a time when the cell body spike has completely recovered. Repetitive firing of the cell body at high frequencies and for short periods produces a build-up in the after-positivity following each action spike, but after-positivity disappears during maintained repetitive antidromic stimulation at rates between 10 and 150/second.