Babinski reflex and corticospinal tract lesion

Muscle SpindlesOrganizationGamma motor neurons innervate the intrafusal muscle fibers.Activation ofgamma neuronsShortening of the polar regionsof the intrafusal fibersStretches the noncontractilecenter regionsIncrease firing of thesensory endingsTherefore, the gamma motor neurons provide amechanism for adjusting the sensitivity of the musclespindles.2 types of motor fibers•Static gamma motor neuron – innervate both staticbag and chain fibers•Dynamic gamma motor neuron – innervate onlydynamic bag fibers5Gamma motor neurons: Static and Dynamic ResponsesIa sensory fiber responseStretch alone: small dynamic response to musclestretch, a moderate increase in steady-state firing.Static gamma motor neuron stimulated: steady-stateresponse increase, dynamic response decrease.Dynamic gamma motor neuron stimulated: dynamicresponse markedly enhanced, steady-state responsegradually returns to original level.Increase in activity of dynamic gamma motor neuronsincrease the dynamic sensitivity of the type Ia endingsbut have no influence on the type II endings.Increase in activity of static gamma motor neuronsincrease the tonic level of activity in both the type Ia andII endings, decrease the dynamic sensitivity of type Iaendings.6