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T C Cope

Publications and source records attributed to T C Cope.

42 records · Page 3Linked to original sources
PubMed1984

Analysis of individual Ia-afferent EPSPs in a homonymous motoneuron pool with respect to muscle topography.

The spike-triggered averaging technique (26) was used to determine whether the synaptic input from medial gastrocnemius (MG) Ia-afferent fibers to homonymous motoneurons is "topographically weighted" (22) by means of differences in projection frequency, excitatory postsynaptic potential (EPSP) amplitude, or a combination of both factors. Motoneurons were classified as either "same branch" or "other branch," depending on whether a Ia-afferent fiber and motor axon were contained in the same or different intramuscular nerve branches. No difference was found in the projection frequency of Ia-afferents to the same branch and other branch motoneurons (95 versus 94%, respectively). The mean EPSP amplitude was larger in the same branch group of motoneurons (92 +/- 8 (SE) microV; n = V; n = 97) than in the other branch group (77 +/- 7 microV; n = 79). This difference was most striking in high-rheobase (greater than or equal to 10 nA) motoneurons, for which the mean EPSP amplitude in the same branch group was 82 +/- 12 microV (n = 48), whereas that in the other branch group was 52 +/- 5 microV (n = 37). In 60 cases it was possible to compare the EPSPs produced by a same branch afferent and an other branch afferent in the same motoneuron. The same branch afferent produced the larger EPSP in 73% (44/60) of the cases. Moreover, the mean ratio of the same branch to the other branch EPSP amplitudes was 1.7, which was both statistically significant and consistent with analogous results from our preceding study of aggregate EPSPs (22). Mean rise times and half-widths of EPSPs in the same branch group were not significantly different from those in the other branch group. Furthermore, no significant differences in rise times or half-widths between the two groups were evident when motoneurons were segregated according to their rheobase values. This suggests that the segregation of Ia-afferent and motor axons across the intramuscular nerve branches is not reflected in the locations of Ia terminals on the motoneuron somadendritic surface and that other factors must account for observed EPSP amplitude differences. Our data suggest that the topographic weighting of homonymous Ia-afferent input to cat MG motoneurons is mediated by a gradient of EPSP amplitude rather than by a gradient of Ia connectivity and also suggest that the effect is most prominent in high-rheobase motoneurons.

Afferent Pathways↗
PubMed1980

Selectivity in synaptic changes caudal to acute spinal cord transection.

The amplitude of monosynaptic EPSPs produced by the action of single semitendinosus (ST) Ia afferent fibers in ST motoneurons display little or no increase in amplitude within hours of a spinal cord transection performed at the thirteenth thoracic segment. This finding is in marked contrast to results obtained at the medial gastrocnemius (MG) Ia fiber-motoneuron synapse, where EPSPs have been shown to be enlarged, even in preparations with normal ST EPSPs. The increase in EPSP amplitude reported previously at the MG Ia-motoneurons synapse is selective and therefore is probably not a direct consequence of injury to the isolated segment of the spinal cord.

Acute Disease↗
PubMedDate not supplied

Orderly recruitment among motoneurons supplying different muscles.

Virtually all movements involve the recruitment of motor units from multiple muscles. Given the functional diversity of motor units (motoneurons and the muscle fibers they supply), the effective production of specific movements undoubtedly depends upon some principle(s) to organize the ensemble of active motor units. The principle acting to organize the recruitment of motor units within muscles is the size principle, whereby the first motor units to be recruited have the smallest values for axonal conduction velocity and contractile force, and are the slowest to contract and fatigue. Here we consider the possibility that the size principle applies in the recruitment of motor units across muscles, i.e., that regardless of their muscles of origin, active motor units are recruited in rank order, for example, from low to high conduction velocity. The benefits of orderly recruitment across muscles could be similar to the acknowledged advantages of orderly recruitment within muscles. One benefit is that the neural process involved in organizing active motor units would be simplified. In a muscle-based scheme, the size principle would organize only those motor units within individual muscles, leaving the nervous system with the additional task of coordinating the relative activities of motor units from different muscles. By contrast, in an ensemble-based scheme, orderly recruitment of all motor units according to the size principle would automatically coordinate motor units both within and across motor nuclei. Another potential benefit is the provision for movements with smooth trajectory, the result of interleaving the divergent torque contributions made by motor units from muscles that differ in their orientations about joints. Otherwise, if order were restricted within muscles, the torque trajectory of a joint would change unevenly as participating muscles begin contracting at different times and grade activity at different rates. These considerations support speculation that motor units recruited from co-contracting muscles are collectively recruited according to the size principle.

Cell Size↗