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George E. Carvell, PhD, PT   University of Pittsburgh
Peripheral Sensory Input
Descending Pathway Input
Spinal Interneuron Input
Propriospinal Tract Input
Modified from BC&P fig 13.8 p 443
SPINAL CORD: Segmental Motor Centers
Segmental Motor Centers (SMCs) are networks of Motoneurons (MNs) and Interneurons (INs) in the Ventral Horn that have dedicated their neuronal lives to make your moves look and feel graceful and controlled. There are FOUR major sources of excitatory drive to the SMCs. 1. Peripheral Afferent Input (Direct or Indirect).  2. Spinal Interneurons that are part of local spinal networks extrinsic to the SMC INs. 3. Propriospinal Tract Neurons that connect SMCs within a limb (Short Propriospinal) or provide interlimb & limb/trunk coordination (Long Propriospinal). 4. Descending Pathways from the Brainstem and Cerebral Cortex.
SKILL & SMCs: Where the Rubber Meets the Road
“The essence of most overlearned skills is the apparent ease with which a complex set of movements proceeds as if it were a simple act. The motor system chooses the minimal number of Motoneurons within the minimal set of muscles that are activated as a finely-tuned synergy with precision timing. This is made possible through the generous contributions of the SMCs connected to a well-organized sensorimotor system.”
“An overlearned skill has all the qualities one would expect of a well-governed motor assembly: speed, agility, accuracy & adaptability. For a few ‘motorically-gifted’ individuals, the events appear to unfold with deceptive ease and any errors appear to be inconsequential to the naïve outside observer. For the rest of us, our sensorimotor systems have their work cut out for them. Much effort must be expended to prevent too much activity, across too many muscles, that at times, seem to have no clue what the other actors are doing. Practice improves performance but our nervous system may lack the confidence (or ability) to
move to the next level.“ GEC ‘01