In primates, multiple corticospinal projections from the sensorimotor cortex operate in concert to regulate voluntary action. We examined the soma distributions of all those corticospinal neuron populations projecting to different zones in the cervical and more caudal spinal segments in the macaque that are labeled with retrogradely transported fluorescent tracers; 2-4 differentiable dyes were injected into different sites in the cervical spinal cord of each of 11 monkeys. Lamina V of the cerebral cortex, in which all corticospinal neuron somas were located, was unfolded with computer assistance to form a flat surface, and local soma densities were displayed on this plane as contour and 3-D maps. At least nine discrete, somatotopically organized corticospinal projections were identified. Three separate corticospinal projections originated in frontal cortex. The first projected mostly from area 4 (approximately 35% of the total contralateral neuron population), but also from the adjacent dorsolateral area 6a alpha (approximately 6% of total). The second large corticospinal projection (approximately 15% of total) originated in the supplementary motor area and a third small projection (approximately 2.6% of total) projected from the "postarcuate" cortex. Two separate corticospinal neuron populations were identified in areas 24 (approximately 6% of total) and 23 (approximately 4% of total) of the cingulate cortex. Thus, nearly 70% of the contralateral corticospinal projection originated in frontal and cingulate cortex. At the boundary between the primary motor and somatosensory cortex there was a sharp change in the pattern of projections. Only approximately 2.2% of the contralateral corticospinal projection originated in area 3a, rising to approximately 9% in areas 3b/1, and approximately 13% in areas 2/5. The projections from SII and insula totaled 3.4%. Ipsilateral and contralateral corticospinal projection patterns were similar, but the ipsilateral projection was only approximately 8.1% of that from the contralateral cortex. Each corticospinal neuron population had terminals in the intermediate zone of all spinal segments; additionally, there were ventral horn projections from the primary motor and cingulate cortex, and dorsal horn projections from the somatosensory cortex. Recognizing a number of separate populations of corticospinal neurons in the frontal, parietal, and insular cortex, each with unique thalamic and cortical inputs, and each of which has continuous access to all spinal motoneuron populations, underlines the importance of cortical and spinal connections linking them and coordinating their action. No coherent model of the cortical control of limb movements that incorporates this functional anatomy yet exists.
We used several fluorescent dyes (Fast Blue, Diamidino Yellow, Rhodamine Latex Microspheres, Evans Blue, and Fluoro-Gold) in each of eight macaques, to examine the patterns of thalamic input to the sensorimotor cortex of macaques 12 months or older. Inputs to different zones of motor, premotor, and postarcuate cortex, supplementary motor area, and areas 3b/1 and 2/5 in the postcentral cortex, were examined. Coincident labeling of thalamocortical neuron populations with different dyes (1) increased the precision with which their soma distributions could be related within thalamic space, and (2) enabled the detection by double labeling, of individual thalamic neurons that were common to the thalamic soma distributions projecting to separate, dye-injected cortical zones. Double-labeled thalamic neurons projecting to sensorimotor cortex were rarely seen in mature macaques, even when the injection sites were only 1-1.5 mm apart, implying that their terminal arborizations were quite restricted horizontally. By contrast, separate neuron populations in each thalamic nucleus with input to sensorimotor cortex projected to more than one cytoarchitecturally distinct cortical area. In ventral posterior lateral (oral) (VPLo), for example, separate populations of cells sent axons to precentral medial, and lateral area 4, medial premotor, and postarcuate cortex, as well as to supplementary motor area. Extensive convergence of thalamic input even to the smallest zones of dye uptake in the cortex (approximately 0.5 mm3) characterized the sensorimotor cortex. The complex forms of these projection territories were explored using 3-dimensional reconstructions from coronal maps. These projection territories, while highly ordered, were not contained by the cytoarchitectonic boundaries of individual thalamic nuclei. Their organization suggests that the integration of the diverse information from spinal cord, cerebellum, and basal ganglia that is needed in the execution of complex sensorimotor tasks begins in the thalamus.
Human subjects were required to differentiate grating surfaces of alternating grooves and ridges by moving a finger back and forth across the surface. Their discriminative capacities were measured, as well as the movement and force profiles that they selected. To measure discrimination, a forced choice paradigm was used in which three surfaces were presented on each trial. Two surfaces were the same (standards) and the subject was required to indicate which of the three surfaces (the comparison) differed from the other two. Two series of surfaces were used with standards whose spatial periods were 770 and 1002 mu, respectively. Subjects were able to discriminate, at the 75% correct level, two gratings which differed in spatial period by the order of 5%. When tangential movement between the surface and the finger was eliminated, and only radial contact permitted, discrimination was degraded and the 75% correct levels increased to the order of 10%. Subjects were free to choose their own patterns of finger movement and of contact force between finger and surface. Movement was measured cinematographically. For all subjects movement patterns were close to sinusoidal, with frequencies in the range of 4.0 Hz and with mean velocities of the order of 160 mm/s. Patterns of contact force were measured by a force transducer. For all subjects the force varied rhythmically in synchrony with movement, but the patterns and magnitudes varied with the subject. Gratings were scaled for perceived roughness by a magnitude estimation technique: the relationship between perceived roughness and grating period was monotonic.
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