Spinal Reflexes
The spinal reflexes are stereotyped responses to specific stimuli. They include the myotatic or tendon reflexes, the cutaneous trunci reflex (incorrectly called the “panniculus reflex”), the perineal reflex, and several others.
As their names imply, spinal reflexes depend on the integrity of local spinal cord segments; dorsal and ventral nerve roots; and peripheral nerves, neuromuscular junctions, and muscles. Lesions of descending pathways in the spinal cord that are located cranial to the spinal components of the peripheral nerves to the limbs being tested result in normal to increased spinal reflexes and are commonly referred to as upper motor neuron lesions. Lesions in the gray matter of the spinal cord segments at the level of the reflex arc, the ventral roots, or peripheral motor nerves cause diminished spinal reflexes and are commonly referred to as lower motor neuron lesions.It is appropriate at this point to define the terms upper motor neuron and lower motor neuron. Upper motor neurons are nerve cells contained completely within the CNS, with cell bodies in the brain and axons that terminate at synapses within the brain or spinal cord. Diseases affecting upper motor neurons result in normal to increased spinal reflexes, variable severity of weakness, and variably increased muscle tone (spasticity). The nerve cell bodies of somatic lower motor neurons lie in the nuclei of cranial nerves in the brainstem or in the ventral horn gray matter of the spinal cord. The axons exit the CNS, course within the peripheral or cranial nerves, and terminate at neuromuscular junctions. Diseases affecting lower motor neurons result in decreased spinal reflexes, ataxia, moderate to severe weakness, decreased muscle tone (flaccidity), and rapid, pronounced atrophy of the denervated muscles.