Journal of Neurophysiology, Vol 58, Issue 5 1035-1051, Copyright © 1987 by APS
Retinal W-cell input to the upper superficial gray layer of the cat's superior colliculus: a conduction-velocity analysis
D. M. Berson
Section of Neurobiology, Brown University, Providence, Rhode Island 02912.
1. I have used several methods to estimate the conduction velocities of
retinal afferents innervating the upper 50-100 micron of the stratum
griseum superficiale (the upper SGS). The measurements were based on a
unitary extracellular potential unique to this sublamina, which was first
described by McIlwain (28). He termed it the juxtazonal potential (JZP),
and showed that it results when a single spike invades the terminal arbor
of a single retinal afferent to the upper SGS, triggering synchronous
excitatory postsynaptic potentials in postsynaptic collicular cells. 2.
Individual unitary JZPs were evoked at fixed latencies by weak shocks to
the optic disk, chiasm, or tract. When the same JZP could be evoked in
isolation from two stimulus sites, the conduction velocity of the axon
triggering the JZP was estimated by dividing the conduction time between
the stimulating electrodes (i.e., the "latency difference") into the
distance separating these electrodes. This "latency-difference method"
lacked general utility, however, since the same JZP could only rarely be
evoked in isolation from two stimulus sites. 3. This limitation was
circumvented by means of a collision method. When a stimulus that evoked a
JZP in isolation was preceded by a sufficiently intense conditioning shock
to a second, more central stimulus site, the conditioning stimulus caused
the JZP to fail in an all-or-none fashion. It was assumed that when the JZP
failed, the conditioning stimulus had exceeded the spike threshold of the
axon mediating the JZP and that an antidromic action potential had collided
with the orthodromic spike initiated at the peripheral stimulus site.
Assessment of the critical interstimulus interval for producing such a
collision, together with measurements of the axon's refractory period and
the interelectrode conduction distance, permitted an estimate of the
conduction velocity of the JZP-triggering axon. Conduction-velocity
estimates generated in this way closely matched those based on the
latency-difference technique when both methods could be applied. 4.
Conduction velocities of 31 JZP-triggering axons analyzed by the collision
method ranged from 2.9 to 6.8 m/s [4.6 +/- 1.0 (mean +/- SD)]. Comparable
estimates were obtained for such axons by alternative methods based on the
absolute latencies of electrically evoked JZPs or of the field potential to
which they contribute. The conduction velocities of JZP-triggering axons
fell within the range reported for retinal W-cells and entirely outside
those of X- and Y-cells, confirming earlier evidence for W-cell input to
the upper SGS (7, 15, 18, 28).(ABSTRACT TRUNCATED AT 400 WORDS)
