The word motor in this context refers to control of skeletal muscle contraction for movement, tone, and posture. There is another classification system for neurons carrying these types of information that I do not use, but some people do. This system uses the terms: general somatic efferent or somatomotor for motor to most of the skeletal muscles of the body; special visceral efferent or branchiomotor for motor to some of the skeletal muscles of the head and neck (this distinction relates to certain embryonic structures these muscles develop from); general visceral efferent or visceromotor for autonomic neurons that control smooth muscle, cardiac muscle, and gland cells.
Lower motor neuron axons exit the brainstem or spinal cord and travel in nerves of the peripheral nervous system to connect to and control skeletal muscle cells. Upper motor neuron axons travel in the central nervous system, mostly from an area called the primary motor cortex, to the brainstem or spinal cord, where they connect to and control lower motor neurons. Many parts of the brain influence the activity of the upper motor neurons in the primary motor cortex, particularly association motor cortices, sensory cortices, the cerebellum, and the basal ganglia.
Some motor activities are voluntarily, meaning they are under conscious volitional control, while others are involuntary. Motor reflexes are an important example of involuntary motor activity; these involve motor responses to stimuli that do not require conscious involvement. Many motor reflexes involve lower motor neurons responding to input more or less directly from sensory neurons, as opposed to voluntary motor activity where the lower motor neurons are controlled by upper motor neurons from the primary motor cortex.
Dysfunction of upper motor neurons, lower motor neurons, or skeletal muscles cells may cause weakness. There are many terms for patterns of weakness on examination. Paresis means weakness, while plegia and paralysis both mean the complete absence of volitional movement. Palsy is another term for weakness that does not distinguish between paresis and plegia. Hemiparesis/plegia refers to unilateral weakness/paralysis. Paraparesis/plegia refers to bilateral leg weakness/paralysis. Quadri- (or tetra-) paresis/plegia refers to weakness/paralysis of all four limbs. Diparesis/plegia refers to weakness/paralysis of all four limbs that is worse in the legs than in the arms. Monoparesis/plegia refers to weakness/paralysis of one limb.
All the muscles of the body have names, but they can also be referred to singly or in groups by what movements they cause when they contract. Flexor muscles cause flexion, which means bending joints. Extensor muscles cause extension, which means straightening joints. Abductor muscles cause abduction, which means moving something away from the midline. Adductor muscles cause adduction, which means moving something toward the midline. For the hand, abduction means spreading the digits apart, and adduction means moving the digits toward each other. Opposition means pushing the end of the thumb against the end of the fingers. The hand can also rotate by distal arm movements. When the hand is held horizontally, turning the palm up is called supination, and turning the palm down is called pronation. Ankle movements have special names: moving the foot toward the knee is called dorsiflexion; moving the foot away from the knee is called plantarflexion; moving the foot laterally is called eversion; and moving the foot medially is called inversion. For neck movements: moving the head anteriorly (chin down) is called flexion; moving the head posteriorly (chin up) is called extension; moving the head laterally (ear down) is called lateral flexion; and head rotation is called right or left for the direction the face is moving.
Most joints have opposing muscle groups that cause the opposite movement; these are called antagonist muscles or muscle groups. For example, the elbow has antagonist muscle groups, one of which causes flexion while the other causes extension. Usually, the balance of the amount of contraction of antagonist muscle groups determines the position, or posture, of the joint when it is not moving.
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