Temporal dispersion windows in cortical neurons.

A temporal dispersion window is the time required for a volley of action potentials on presynaptic axons to cross the dendritic arbor of a postsynaptic neuron. The volley produces a series of unitary postsynaptic potentials (PSPs) on the postsynaptic neuron. Temporal dispersion is, thus, one factor that can influence the integration of unitary PSPs and the production of action potentials in cortical neurons. Temporal dispersion windows for neurons in the visual cortex of the freshwater turtle, Pseudemys scripta, were estimated by characterizing geniculate afferents and the morphology of neurons in the visual cortex. Horseradish peroxidase injections in the thalamus revealed thin and unmyelinated terminal arbors that run horizontally from lateral to medial across the cortex, forming en passant synapses across the dendrites of cortical neurons. Axons with two calibers were seen, one with diameters between 0.5 and 2.0 microm, and a second with diameters below the resolution limit of the light microscope. The conduction velocity of geniculate afferents in the cortex was measured at 0.18 m/sec +/-0.04 using the latency of extracellular field potentials evoked by electrical stimulation of the lateral forebrain bundle. The positions and dendritic arbors were characterized in Golgi preparations. Seven morphologically distinct neuron types were positioned to intersect the geniculate afferents in Golgi preparations. The spatial overlap between the dendritic arbors of these cells and the geniculate afferents varied from 128 to 850 microm. Temporal dispersion windows for the seven cell types ranged from 0.7 to 4.7 msec, estimated using a geniculate fiber conduction velocity of 0.18 m/sec. Estimated conduction velocities of 0.04 m/sec for small-caliber fibers produce temporal dispersion windows of 3.2 to 21.3 m/sec.