Therefore an important determinate of recruitment susceptibility and motor-unit activity appears to be related to the intrinsic properties of the motor neurons ( Henneman and Mendell 1981). These observations suggest that synaptic inputs, with some exceptions, are broadly distributed across a motor neuron population ( Binder et al. Although little is known about the projection frequency of spinal interneurons, one estimate suggests that collaterals from individual Ia inhibitory interneurons may terminate in multiple motor nuclei and contact ≤20% of the motor neurons within these nuclei ( Jankowska 1992). Neurophysiological studies, although indirect, have demonstrated relatively extensive divergence of corticospinal inputs both within ( Mantel and Lemon 1987) and across spinal motor nuclei (Asunuma et al. Such findings are consistent with the possibility that these specialized descending inputs may contact a large proportion of neurons in a motor nucleus ( Porter and Lemon 1993). Some anatomical evidence indicates that terminal arbors of individual collaterals from corticomotoneuronal cells are distributed in longitudinal cylinders coterminous with motor nuclei in lamina IX of the spinal cord ( Lawrence et al. Much less is known about the distribution of individual descending and spinal interneuronal inputs to motor neurons. Subsequent anatomical studies confirmed the existence of extensive ramification of individual Ia afferents throughout longitudinally oriented motor nuclei in the spinal cord ( Brown and Fyffe 1978). The remarkable finding of that investigation was that individual afferent fibers made synaptic contact with most, and in many cases all, homonymous motor neurons as well as with large numbers of motor neurons supplying synergistic muscles. In a landmark study, Mendell and Henneman ( 1971) used intracellular recordings to determine the proportion of motor neurons within a spinal motor nucleus that received synaptic input from single Ia afferent fibers. Consequently, last-order synaptic projections are not distributed uniformly across the entire pool of motor neurons innervating ED but are segregated to supply subsets of motor neurons innervating different compartments.Ī fundamental issue that underlies the coordination of activity among a population of neurons relates to how synaptic input is distributed across the ensemble. The degree of synchrony for motor-unit pairs within the same compartment (CIS = 0.7 ± 0.3 mean ± SD) was significantly greater than for motor-unit pairs in different compartments (CIS = 0.4 ± 0.22). Cross-correlation histograms were generated for all of the motor-unit pairs and the degree of synchronization between two units was assessed using the index of common input strength (CIS). One hundred and forty-five different motor-unit pairs were recorded in the human ED of nine subjects during weak voluntary contractions. Therefore the purpose of this study was to evaluate the degree of motor-unit synchrony both within and across compartments of ED. Given the unique architecture of ED, it is unclear if synaptic inputs are broadly distributed across the entire pool of motor neurons innervating ED or segregated to supply subsets of motor neurons innervating different compartments. For example, extensor digitorum (ED) is a multi-compartment muscle that extends digits 2–5. Our particular interest relates to the organization of extrinsic finger muscles that give rise distally to multiple tendons, which insert onto all the fingers. The extent of synchrony thus allows insight as to how the inputs to motor neurons are distributed. Short-term synchronization of active motor units has been attributed in part to last-order divergent projections that provide common synaptic input across motor neurons.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |