Motor systems

(physiology) Any portion of the nervous system that regulates and controls the contractile activity of muscle and the secretory activity of glands.

Those portions of nervous systems that regulate and control the contractile activity of muscle and the secretory activity of glands. Muscles and glands are the two types of organ by which an organism reacts to its environment; together they constitute the machinery of behavior. Cardiac muscle and some smooth muscle and glandular structures can function independently of the nervous system but in a poorly coordinated fashion. Skeletal muscle activity, however, is entirely dependent on neural control. Destruction of the nerves supplying skeletal muscles results in paralysis, an inability to move. The somatic motor system includes those regions of the central nervous system involved in controlling the contraction of skeletal muscles in a manner appropriate to environmental conditions and internal states. See also Gland; Muscle.

Skeletal muscle

The nerve supply to skeletal muscles of the limbs and trunk is derived from large nerve cells called motoneurons, whose cell bodies are located in the ventral horn of the spinal cord. Muscles of the face and head are innervated by motoneurons in the brainstem. The axons of the motoneurons traverse the ventral spinal roots (or the appropriate cranial nerve roots) and reach the muscles via peripheral nerve trunks. In the muscle, the axon of every motoneuron divides repeatedly into many terminal branches, each of which innervates a single muscle fiber. The region of innervation, called the neuromuscular junction or motor end plate, is a secure synaptic contact between the motoneuron terminal and the muscle fiber membrane. See also Synaptic transmission.

Since synaptic transmission at the neuromuscular junction is very secure, an action potential in the motoneuron will produce contraction of every muscle fiber that it contacts. For this reason, the motoneuron and all the fibers it innervates form a functional unit called the motor unit. The number of muscle fibers in a single motor unit may be as small as six (for intrinsic eye muscles) or over 700 (for motor units of large limb muscles). In general, muscles involved in delicate rapid movements have fewer muscle fibers per motor unit than large muscles concerned with gross movements.

Components of skeletal motor system

Motoneurons are activated by nerve impulses arriving through many different neural pathways. Some of their neural input originates in peripheral receptor organs located in the muscles themselves, or in receptors in skin or joints. Many muscle receptors discharge in proportion to muscle length or tension; such receptors have relatively potent connections to motoneurons, either direct monosynaptic connections or relays via one or more intemeurons. Similarly, stimulation of skin and joints, particularly painful stimulation, can strongly affect motoneurons. Such simple segmental pathways constitute the basis for spinal reflexes. The other major source of input to motoneurons arises from supraspinal centers. The illustration shows the main nervous system centers involved in controlling the input to motoneurons.

Schematic diagram of the major components of the vertebrate motor systems. Arrows indicate the main neural connections between regions.

Segmental circuits

At the spinal level, the motoneurons and muscles have a close reciprocal connection. Afferent connections from receptors in the muscles return sensory feedback to the same motoneurons which contract the muscle. Connections to motoneurons of synergist and antagonist muscles are sufficiently potent and appropriately arranged to subserve a variety of reflexes. In animals with all higher centers removed, these segmental circuits may function by themselves toproduce simple reflex responses. Under normal conditions, however, the activityof segmental circuits is largely controlled by supraspinal centers. Descendingtracts arise from two major supraspinal centers: the cerebral cortex and thebrainstem.

Brainstem

The brainstem, which includes the medulla and pons, is a major and complexintegrating center which combines signals descending from other higher centers,as well as afferent input arising from peripheral receptors. The descendingoutput from brainstem neurons affects motor and sensory cells in the spinalcord. Brainstem centers considerably extend the motor capacity of an animalbeyond the stereotyped reflex reactions mediated by the spinal cord. Incontrast to segmental reflexes, these motor responses involve coordination ofmuscles over the whole body. Another major motor function of the brainstem ispostural control, exerted via the vestibular nuclei of the ear.

Besides neurons controlling limb muscles, the brainstem also contains anumber of important neural centers involved in regulating eye movements. Theseinclude motor neurons of the eye muscles and various types of interneurons thatmediate the effects of vestibular and visual input on eye movement.

Cerebellum

Another important coordinating center in the motor system is the cerebellum,an intricately organized network of cells closely interconnected with thebrainstem. The cerebellum receives a massive inflow of sensory signals fromperipheral receptors in muscles, tendons, joints, and skin, as well as fromvisual, auditory, and vestibular receptors. Higher centers, particularly thecerebral cortex, also provide extensive input to the cerebellum via pontinebrainstem relays. The integration of this massive amount of neural input in thecerebellum somehow serves to smooth out the intended movements and coordinate the activity of muscles. Without the cerebellum, voluntary movements become erratic, and the animal has difficulty accurately terminating and initiating responses. The output of the cerebellum affects primarily brainstem nuclei, but it also provides important signals to the cerebral cortex.

Basal ganglia

At another level of motor system are the basal ganglia. These massive subcortical nuclei receive descending input connections from all parts of the cerebral cortex. Their output projections send recurrent information to the cerebral cortex via the thalamus, and their other major output is to brainstem cells.

Cerebral cortex

At the highest level of the nervous system is the cerebral cortex, which exerts control over the entire motor system. The cerebral cortex performs two kinds of motor function: certain motor areas exert relatively direct control over segmental motoneurons, via a direct corticospinal pathway, the pyramidal tract, and also through extrapyramidal connections via supraspinal motor centers. The second function, performed in various cortical association areas, involves the programming of movements appropriate in the context of sensory information, and the initiation of voluntary movements on the basis of central states. Cortical language areas, for example, contain the circuitry essential to generate the intricate motor patterns of speech. Limb movements to targets in extrapersonal space appear to be programmed in parietal association cortex. Such cortical areas involved in motor programming exert their effects via corticocortical connections to the motor cortex, and by descending connections to subcortical centers, principally basal ganglia and brainstem.

As indicated in the illustration, the motor centers are all heavily interconnected, so none really functions in isolation. In fact, some of these connections are so massive that they may formfunctional loops, acting as subsystems within the motor system. For example,most regions of the cerebral cortex have close reciprocal interconnections withunderlying thalamic nuclei, and the corticothalamic system may be considered toform a functional unit. Another example is extensive connection from cerebralcortex to pontine regions of the brainstem, controlling cells that project tothe cerebellum, which in turn projects back via the thalamus to the cerebralcortex. Such functional loops are at least as important in understanding motorcoordination as the individual centers themselves. See also Brain; Nervous system (vertebrate).