Reaching to grasp is of fundamental importance to primate motor behavior and requires coordinating hand preshaping with limb transport and grasping. We aimed to clarify the role of cerebellar output via the magnocellular red nucleus (RNm) to the control of reaching to grasp. Rubrospinal fibers originating from RNm constitute one pathway by which cerebellar output influences spinal circuitry directly. We recorded discharge from individual forelimb RNm neurons while monkeys performed a reach-to-grasp task and two tasks that were similar to the reach-to-grasp task in trajectory, amplitude, and direction but did not include a grasp. One of these, the device task, elicited reaches while holding a handle, and the other, the free-reach task, elicited reaches that did not require any specific hand use for task performance. The results demonstrate that coordinated whole-limb reaching movements are associated with large discharge modulations of RNm neurons predominantly when hand use is included. Therefore RNm neurons can at best only make a minor contribution to the control of reaching movements that lack hand use. We evaluated relations between the discharge of individual RNm neurons and electromyographic (EMG) activity of forelimb muscles during the reach-to-grasp task by comparing times of peak RNm discharge to times of peak EMG activity. The results are consistent with the view that RNm discharge may contribute to EMG activity of both distal and proximal muscles during reaching to grasp especially digit extensor and limb elevation muscles. Relations between the discharge of individual RNm neurons and movements of the metacarpi-phalangeal (MCP), wrist, elbow, and shoulder joints during individual trials of task performance were quantified by parametric correlation analyses on a subset of neurons studied during the reach-to-grasp and free-reach tasks. The results indicate that MCP extensions were consistently preceded by bursts of RNm discharge, and strong correlations were observed between parameters of discharge and the duration, velocity, and amplitude of corresponding MCP extensions. In contrast, relations between discharge and movements of proximal joints were poorly represented, and RNm discharge was not related to the speed of limb transport. Based on our data and those of others, we hypothesize that cerebellar output via RNm is specialized for controlling hand use and conclude that RNm may contribute to the control of hand preshaping during reaching to grasp by activating muscle synergies that produce the appropriate MCP extension at the appropriate phase of limb transport.

The purpose of these experiments was to define the topography of cuneate and spinal projections to the forelimb representation in the rostral dorsal accessory olive (rDAO). We were interested in determining whether the spinal and cuneate inputs constitute a homogeneous afferent source, and whether there is evidence that they serve different functional roles. We were also interested in determining whether the somatotopy of rDAO is the result of a point-to-point projection from its afferent sources, or whether the projection suggests a reorganization of afferents at the olive. Single unit recording was used to identify specific regions of rDAO, and the topography of inputs to the identified regions was determined by using wheat germ agglutinin-horseradish peroxidase (WGA-HRP) as a tracer. The results from retrograde tracing were confirmed by using WGA-HRP as an anterograde tracer from input sources. The cuneate and spinal neurons providing input to rDAO constitute two distinct neural populations. One consists of cells in the caudal cuneate nucleus and lamina VI of the rostral two cervical segments, the other consists of cells in the rostral cuneate nucleus. The cells in the caudal cuneate nucleus and the rostral cervical segments are large, multipolar neurons that form a single column of rDAO input cells. The column of cells projects to the contralateral rDAO in a topographic fashion with rostral regions of the column projecting to rostral rDAO, which contains cells that respond to somatosensory stimulation of the contralateral shoulder, trunk, and proximal forelimb. Caudal regions of the column project to caudal rDAO, which contains cells that respond to stimulation of the distal forelimb. Despite this topography, there is a large degree of overlap in the terminations from neighboring regions of the input column, indicating that a major reorganization occurs at the rDAO. The projection from the rostral cuneate nucleus arises from small neurons that project bilaterally to rDAO, and the input from the rostral cuneate nucleus lacks a clear topography. We propose that input from the cell column is responsible for the somatosensory sensitivity of rDAO neurons, whereas input from rostral cuneate is most likely modulatory, probably inhibitory, in nature.

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Classical studies of the cat rubrospinal tract describe dense terminations in spinal laminae V-VII and an absence of any significant projection to lamina IX. In contrast, our recent studies, utilizing the anterograde transport of wheat germ agglutinin conjugated with horseradish peroxidase, have demonstrated a consistent and circumscribed area of label in lamina IX at caudal cervical segments. The present study was undertaken to determine the distribution of rubrospinal terminals among motor neurons in lamina IX as well as to identify the likely target muscles of those motor neurons located near rubrospinal terminals. We injected wheat germ agglutinin-horseradish peroxidase into the red nucleus and unconjugated horseradish peroxidase into selected forearm muscles of the same side of the body. The locations of rubrospinal terminals showing anterograde label on one side of the spinal cord could then be compared with the locations of motor neurons showing retrograde label on the opposite side of the cord. The results demonstrated a clear focus of rubrospinal terminals in the lateral and dorsal portions of the ventral horn beginning at C8 and extending through rostral T1. No other segments of the spinal cord showed a focus of rubrospinal terminations in lamina IX. Retrogradely labeled motor neurons from the muscle injections showed that the rubrospinal terminals overlap extensively with motor neuronal pools supplying distal forearm muscles. Several lines of evidence indicate that the terminals are from rubrospinal fibers and are not due to transneuronal transport.