A clinically important reflex is the muscle stretch reflex. If a skeletal muscle is rapidly stretched, such as by tapping its tendon with a rubber hammer, the muscle will contract involuntarily. This reflex requires a signal to travel from the muscle to the central nervous system in somatosensory axons, and then another signal to travel back to the stretched muscle in lower motor neuron axons, so that nerve lesions may cause a decreased response, which is called hyporeflexia.
Dysfunction of either upper or lower motor neurons may cause weakness, but there are distinguishing signs that may also occur, called motor neuron signs, or which may occur without weakness. These signs are often helpful to distinguish weakness from peripheral versus central nervous system lesions. The lower motor neuron signs include: hyporeflexia (diminished muscle stretch reflexes), atrophy (diminished muscle bulk), fasciculations (involuntary muscle twitches), and hypotonia (diminished muscle tone). Tone is the resistance of muscles to stretch when the patient is not trying to contract the muscles. The upper motor neuron signs include: hyperreflexia (increased muscle stretch reflexes), a type of hypertonia (increased muscle tone), and the extensor plantar response (also called Babinski’s sign). The extensor plantar response involves the toes extending up when the foot is scraped with a blunt object; normally this causes the toes to flex down.
A diffuse abnormality of skeletal muscle is called myopathy, which may cause a syndrome of diffuse weakness. An abnormality of one or more nerves is called neuropathy, which may cause sensory, lower motor neuron, or autonomic dysfunction of the parts of the body connected to the involved nerve or nerves. A diffuse abnormality of the nerves is called polyneuropathy, which may cause a diffuse syndrome. A focal abnormality of a single nerve is called mononeuropathy, which may cause a focal syndrome, usually of part of just one arm or leg. The term distal means away from the center of the body, while proximal means toward the body center. Common mononeuropathies often just affect the distal parts of limbs, such as one hand.
The term unilateral means one side of the body or some structure, and bilateral means both sides. Each of the many levels of the spinal cord has bilateral spinal nerves that pass through the spine, which then branch repeatedly to form most of the innumerable small nerves that spread throughout the tissues of the limbs and torso. The term ipsilateral means the same side of the body or some structure, and contralateral means the other side. Each spinal nerve carries somatosensory axons for an ipsilateral strip of skin, called a dermatome, and lower motor neuron axons for an ipsilateral group of muscles, called a myotome. A focal abnormality of one spinal nerve is called radiculopathy, which may cause somatosensory abnormalities of that dermatome, or lower motor neuron abnormalities of that myotome.
Most cranial nerves are attached to the brain, mainly to the brainstem. Most cranial nerves connect to tissues of the head or neck, but some connect to tissues of the limbs or torso as well, particularly for many autonomic functions. Cranial neuropathies may cause sensory, lower motor neuron, or autonomic dysfunction of the structures they connect to. In addition to somatosensation, several cranial nerves are involved in the other senses.