Stretch-evoked motor unit activation patterns in the decerebrate cat

Stan M. Dacko

doi:10.17918/00007569

The way in which muscle forces are generated and controlled for functional primarily on the recruitment and firing pattern of motor units within a muscle or groups of muscles. There is a wide range of anatomic and unit properties that covary with specific unit type. It is possible then, that the nervous system exploits these attributes in recruiting motor units. For most functional activities involving the cat hindlimb, there is a "usual" progression of motor unit activation as synergist muscles are mutually excited. Under certain conditions however, a divergence of whole muscle activity between anatomic synergists is seen and may reflect an alternative scheme of motor unit recruitment based on physiologic properties such as motor unit type. The force at which a motor unit is recruited, known as recruitment threshold, and is commonly used as a measure for comparing the relative excitability of units within a motor pool. A large deviation in recruitment threshold for motor units during repeated contractions would suggest that the recruitment process is variable and cannot be predicted by a particular level of muscle force. Therefore, the purpose of this dissertation was to study the organization of motor unit activation during stretch-evoked contractions under steady-state conditions in isolated muscle and during periods of divergent whole muscle behavior, specifically to investigate if the "usual" motor unit recruitment pattern is violated. Behavior of type-identified motor units from the ankle extensors in the cat was studied via intra-axonal penetration during 2-8mm stretch-hold-release perturbations. Force from selected hindlimb muscles and motor unit firing rate responses were recorded. Relationships between unit activity and muscle force were analyzed to assess organizational schemes. There was no evidence for selective recruitment based on motor unit type across motor pools other than the "usual" pattern, during conditions where divergent whole muscle behavior was seen. Instead motor unit recruitment was specific and ordered for each muscle within a synergist pair. The force threshold for recruitment under relatively stable conditions was minimally variable for most units. Mechanical elements contributing to muscle force only partially explain the variation in recruitment thresholds. Under all conditions examined, the "usual" motor recruitment pattern was consistently observed without any evidence for alternative recruitment schemes.

Stretch-evoked motor unit activation patterns in the decerebrate cat

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Stretch-evoked motor unit activation patterns in the decerebrate cat

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