Cerebral Palsy – Pathophysiology of Spasticity
• Primary afferent I a fibers surrounding intrafusal fibers of the muscle spindle are excited when a muscle is stretched.
• The I a fiber makes a monosynaptic excitatory connection with alpha motoneurons of its muscle of origin, and it similarly connects with alpha motoneurons of synergistic muscles
. • The I a fiber also monosynaptically connects with an inhibitory interneuron that projects directly to the alpha motoneurons of antagonist muscles
. • When a muscle is stretched, excitation of homonymous and synergistic motoneurons, combined with inhibition of antagonists, subserves the mechanism of reciprocal inhibition.
• There is evidence for impairment of this mechanism in the UMN syndrome