The relationship between axonal and perisynaptic conduction times in the retinogeniculate pathway of cats

We examined the degree to which conduction velocity differences between X- and Y-cells are preserved during transmission through the retinogeniculate synaptic zone. This analysis focussed upon two measures: axon time; and perisynaptic time. The first is the time required for an action potential to travel from the optic chiasm to the point of major branching in the optic tract. The second is the sum of terminal invasion time, synaptic delay, and the time required for postsynaptic processes which precede the generation of an action potential. The X- and Y-cell distributions of axonal conduction times differed markedly. In contrast, the X- and Y-cell distributions of perisynaptic conduction times overlapped extensively. Therefore, the overlap in optic chiasm latency distributions between geniculate X- and Y-cells arises primarily from the perisynaptic zone. Further, on a unit-by-unit basis, we observed small but significant negative correlations between axon conduction times and perisynaptic times within both the X- and Y-cell classes. That is, units innervated by faster conducting afferents tended to have longer perisynaptic processing times, and vice versa. This sort of relationship may act to enhance within-class synchrony in postsynaptic activity.