The pathophysiology of XXXX .

@entity4372 @entity2003 @entity508 @entity924 @entity4681 @entity3519 @entity717
Spasticity is only one of several components of the @entity4681 , known collectively as the 'positive' phenomena, that are characterized by muscle overactivity. Other components include @entity924 , clonus, the clasp-knife phenomenon, flexor and extensor spasms, a Babinski sign, and @entity3519 . Spasticity is a form of @entity4372 due to hyperexcitable tonic stretch reflexes. It is distinguished from @entity2003 by its dependence upon the speed of the muscle stretch and by the presence of other positive UMN signs. Hyperactive spinal reflexes mediate most of these positive phenomena, while others are due to disordered control of @entity717 or abnormal efferent drive. An @entity4681 disturbs the balance of supraspinal inhibitory and excitatory inputs, producing a state of net disinhibition of the spinal reflexes. These include proprioceptive (stretch) and nociceptive (flexor withdrawal and extensor) reflexes. The clinical syndrome resulting from an @entity4681 depends more upon its location and extent, and the time since it occurred, than on the pathology of the lesion. However, the change in spinal reflex excitability cannot simply be due to an imbalance in supraspinal control. The delayed onset after the lesion and the frequent reduction in reflex excitability over time, suggests plasticity in the central nervous system. Knowledge of the electrophysiology and neurochemistry of spinal reflexes, together with the action of antispasticity drugs, helps us to understand the pathophysiology of @entity508 .
