SPASTICITY-PATHOPHYSIOLOGY
SUMMARY
1. Spasticity results from a loss of descending influence on the inhibitory Ia interneuron (loss of facilitation of inhibition) & gamma motor neurons.
2. Gamma motor neuron hyperactivity ⇒ increased excitability of the muscle spindle ⇒ excessive activity of alpha motor neurones ⇒ exaggerated reflexes.
3. Note that there is normal muscle spindle function but abnormal sensory input processing.
THE SPINAL REFLEX
4. Tendon reflexes are the only examples of monosynaptic reflex arcs (2 neurons with 1 synapse). All other reflexes are 2 or more synapses.
5. Exaggeration of the tendon reflexes is due to an UMN lesion and is due to alteration in supraspinal control of the anterior horn cells which are rendered unduly excitable due to disruption of the gamma reflex loop.
Image: Dr. Appukutty Manickam.
Reference(s)
Cifu, D.X. (2020). Braddom’s physical medicine and rehabilitation. Elsevier. Get it on Amazon.
Cuccurullo, S. (2019). Physical medicine and rehabilitation board review. New York: Demosmedical. Get it on Amazon.
O’Young, B., Young, M.A. and Stiens, S.A. (2008). Physical Medicine and Rehabilitation Secrets. Mosby. Get it on Amazon.
