Neural Basis of Spasticity

Spasticity arises after a neurological injury; hence it clearly has a neural basis. However, it has many definitions and manifestations, and the underlying mechanisms are still not clearly understood. This chapter reviews the definitions of spasticity, the time course of its development, and the types of neural injury that may cause it by disinhibiting inhibitory brainstem pathways as well as by facilitating excitatory brainstem pathways that result in an excitatory-inhibitory imbalance in the spinal cord interneuronal network. The descending pathways modulate persistent inward currents via serotonin and norepinephrine, which provide a low-level depolarizing synaptic drive to the resting motoneuron pool resulting in increased afferent sensitivity and can account for hyperreflexia. However, the abnormal brainstem descending inputs and persistent inward currents cannot fully account for other spasticity-related motor impairments, such as muscle stiffness.