The cerebral cortex is interconnected with two major subcortical structures: the basal ganglia and the cerebellum. How and where cerebellar circuits interact with basal ganglia circuits has been a longstanding question. Using transneuronal transport of rabies virus in macaques, we found that a disynaptic pathway links an output stage of cerebellar processing, the dentate nucleus, with an input stage of basal ganglia processing, the striatum.

. Motion in the visual scene is processed by direction-selective neurons in primary visual cortex. These cells receive inputs that differ in space and time. What are these inputs? A previous single-unit recording study in anesthetized monkey V1 proposed that the two major streams arising in the primate retina, the M and P pathways, differed in space and time as required to create direction selectivity. We confirmed that cortical cells driven by P inputs tend to have sustained responses. The M pathway, however, as assessed by recordings in layer 4C␣ and from cells with high contrast sensitivity, is not purely transient. The diversity of timing in the M stream suggests that combinations of M inputs, as well as of M and P inputs, create direction selectivity. I N T R O D U C T I O NDirection-selective (DS) neurons discriminate the direction of moving stimuli by obtaining inputs with receptive fields that differ in space and in time. For one direction of motion, the inputs fire at about the same time (in phase) because the spatial and temporal differences cancel each other. In the opposite direction, the input activity is not as synchronous (out of phase). That is, stimulus direction is translated into relative timing. A wide variety of mechanisms can convert these timing differences into divergent postsynaptic activities.Robust direction selectivity depends on approximately quarter-cycle phase differences ("spatiotemporal quadrature," in one direction the quarter cycles subtract to 0, and in the other they sum to a half-cycle). Ideally, a DS cell would obtain inputs that differ by a quarter cycle. Spatially, such inputs exist in the form of simple cells with overlapping receptive fields that differ purely in phase, such as even and odd symmetric fields with ON-OFF-ON and ON-OFF arrangements, respectively, for example. This has led some investigators to examine whether DS cells might receive inputs from non-DS cells that are in spatiotemporal quadrature. DeValois et al. (2000) showed that non-DS cells of these types exist in macaque V1. On the other hand, Peterson et al. (2004) argued that this scheme does not seem to work for cat V1.Actual, as opposed to ideal, DS cells do not receive inputs from just two non-DS cells that are in approximate spatiotemporal quadrature. Instead, multiple inputs converge on DS cells, including both inhibitory and excitatory cortical inputs as well as direct excitation from the LGN. The spatial relationships among these inputs could vary in position as well as in spatial phase. We understand how these positional differences among receptive fields originate in the spatial distribution of retinal cells. The more compelling question is, where do the different timings arise?In the cat, these timings originate in the retina, where a range of sustained and transient responses is generated. The distribution of timing is extended in the LGN, where additional temporal phase differences are created at low temporal frequencies (Saul and Humphrey 1990). Low temporal frequencies are of interest bec...

1. We applied morphological, pharmacological, electrophysiological, and computer simulation techniques to analyze the origin of impulse initiation in amphibian retinal ganglion cells. 2. Morphological studies of retinal ganglion cells in the mudpuppy (Necturus maculosus) and larval tiger salamander (Ambystoma tigrinum) were carried out with the use of either retrograde or intracellular labeling with horseradish peroxidase. These studies identified a characteristic thinning of the axon that begins after the initial segment of axon emerges from the ganglion cell soma or primary dendrite. Morphometric analysis of the thin segment revealed an average length of 74 microns with a standard deviation of 22 microns. For 20 conventionally placed ganglion cells, the length of the thin segment could not be correlated with soma size, initial segment diameter, or distance from the optic disk. There was also little or no correlation for seven displaced ganglion cells. The diameter of the thin segment was below reliable estimation by light microscopy. 3. We studied the possible significance of the thin axonal segment for ganglion cell impulse generation through a combination of electrophysiological recordings (intracellular and whole-cell recordings) together with computer modeling experiments. 4. Electrophysiological experiments are consistent with the idea that the thin segment and cell soma are less excitable than the initial segment region, which appears to be the principal site of initiation of the nerve impulse. The initial segment is that portion of the axon that is bounded by the soma (or proximal dendrite) at its origin and the thin segment at its distal end. 5. Computer simulations of impulse activation were carried out with the use of two different anatomic constraints: one class of simulations did not take into account the thin segment and assumed uniform cylinder conditions, whereas the other class of simulations included a model of the thin axonal segment. These comparative simulations indicate that the thin segment must contain a relatively high density of voltage-gated Na+ channels and support impulse traffic to account for physiological observations on orthodromic and antidromic impulse propagation. In addition, to match the physiological recordings, it is necessary for both the initial segment and the soma compartments to contain moderately high levels of Na+ channels. 6. Our physiological and simulation studies are consistent with the idea that the nerve impulse is normally initiated in the initial segment of axon and then spreads to activate a somatic impulse in the retrograde direction and the axonal impulse in the anterograde direction.(ABSTRACT TRUNCATED AT 400 WORDS)

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