The spinal cord is the basic reflex center for somatic movements. There are a large number of a and r motor neurons in the anterior horn of the spinal cord. a motor neuron excitation can cause muscle contraction activity. r motor neurons innervate the muscle spindle and contract at the same time, and their afferent fibers are class I fibers that extend into the spinal cord, which excite a motor neuron and cause muscle contraction. If the a motor neuron excitability is increased, causing the muscle to contract continuously and the muscle tone to increase, it will lead to muscle spasm. There are two common causes: 1, the central a-motor neuron in the spinal cord has a weakened inhibitory effect; 2, the posterior spinal nerve root releases increased excitatory transmitters.