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Overview of Motor Systems


Each of us has fewer than a million motor neurons with which to control muscles. Without them, we would be completely unable to interact with the outside world. With them, however, we are capable of an enormous range of complex activities, from automatic and semiautomatic movements such as postural adjustments to the characteristically human movements involved in speaking and writing. The way in which a wide variety of neural structures interact to make these activities possible is the topic of Chapters 18, Chapter 19, Chapter 20 .

Each Lower Motor Neuron Innervates a Group of Muscle Fibers, Forming a Motor Unit

Lower motor neurons, the target of central nervous system (CNS) pathways and connections involved in motor control, are arranged in the spinal cord and brainstem in groups corresponding to individual muscles (see Fig. 10.10 ). The axons of lower motor neurons leave the CNS in anterior roots (or in motor roots of cranial nerves) and divide into terminal branches widely distributed in their target muscles. Each branch ends at the single neuromuscular junction of a muscle fiber (see Figs. 8.1 and 8.11 ). The combination of one motor neuron and all the muscle fibers it innervates is referred to as a motor unit. Motor units vary tremendously in size, in a way that makes functional sense: the size of the motor units in a given muscle is related to the degree of fine control involved in the use of that muscle. For example, there may be only 2 or 3 muscle fibers in a motor unit in the stapedius, 10 in an extraocular muscle, 100 in a hand muscle, and 1000 in a large antigravity muscle such as the gastrocnemius ( Fig. 18.1 ). Even within a single muscle, however, motor units vary in size and functional properties, as described later.

Fig. 18.1, The muscle fibers of a single motor unit (type FR) in cat gastrocnemius. The general location of the group of fibers is indicated by shading on a drawing of the whole muscle (A) and a longitudinal section through the muscle indicated by the dotted line (B). On a cross section of the muscle (C), each muscle fiber in the motor unit is indicated by a dot.

Lower Motor Neurons Are Arranged Systematically

Just as there are systematic maps in sensory pathways (see Fig. 10.22 ) and in cortical areas (see Figs. 3.31 and 17.30 ), so too is there a systematic arrangement of clusters of motor neurons. In the anterior horn of the spinal cord, for example, motor neurons for axial muscles are located medial to those for more distal muscles, and those for flexors are posterior to those for extensors ( Fig. 18.2 ). This axial-distal mapping corresponds to the arrangement of descending pathways, some of which are important for postural adjustments of axial muscles, and others for the control of more distal musculature (see Fig. 18.8 ).

Fig. 18.2, Somatotopic arrangement of motor neurons at C8. The large number of motor neurons for distal muscles accounts for the lateral expansion of the anterior horn at this level.

There Are Three Kinds of Muscle Fibers and Three Kinds of Motor Units

Invertebrates have excitatory and inhibitory motor neurons, but in vertebrates, all lower motor neurons release acetylcholine onto nicotinic receptors of skeletal muscle. A single action potential in the axon of a lower motor neuron causes the release of acetylcholine at hundreds of active zones, resulting in a single action potential in the postjunctional muscle fiber (see Fig. 8.10 ). This in turn causes a twitch of the muscle fiber. Therefore force production by vertebrate muscle fibers is related to the rate of firing of lower motor neurons: successive twitches sum temporally, much the way excitatory postsynaptic potentials do ( Fig. 18.3 ).

Fig. 18.3, Relationships between action potentials in lower motor neurons (A), action potentials in a postjunctional muscle fiber (B), and force production by the muscle fiber (C).

Most muscles are called on to contract for different purposes. The gastrocnemius, for example, must contract weakly but for long periods when we stand upright, more strongly while running (which most of us cannot do for nearly as long as we can stand), and very strongly but very briefly during a jump. Corresponding to these requirements, there are three kinds of skeletal muscle fibers ( Fig. 18.4 ), each populating one of three different types of motor unit ( Fig. 18.5 ). Red fibers (type I) are thin, contain abundant mitochondria, and contract weakly and slowly but are able to sustain contractions for long periods. White fibers ( types IIa and IIb ) are larger, contain relatively few mitochondria, and contract in briefer, more powerful twitches. Type IIb fibers use glycolysis almost exclusively to fuel their contractions and fatigue very rapidly; type IIa fibers use a combination of oxidative metabolism and glycolysis and fatigue at intermediate rates. Most muscles contain all three fiber types randomly intermingled, but in proportions that vary depending on the principal function of a given muscle. a

a In domestic fowl, for example, which do a lot of standing and running but little flying, dark meat is muscle with many red fibers, and white meat is muscle with many white fibers. The flight muscles of migratory birds, in contrast, are mostly dark meat.

Fig. 18.4, Demonstration of fiber types in cross sections of human skeletal muscle biopsies, and some characteristic changes that accompany neuropathology. (A) Muscle fibers, each with nuclei (small red dots) at their periphery, are grouped into fascicles (Gomori trichrome stain); part of one fascicle is outlined. (B) Staining for an oxidative enzyme (NADH dehydrogenase) found in mitochondria and the sarcoplasmic reticulum differentiates type I (I) and type II (II) fibers, which are more or less randomly interspersed with one another. (C) Staining for myofibrillar ATPase (pH 9.4) also differentiates type I (I) and type II (II) fibers. (D) Denervated muscle fibers atrophy, becoming small and angular in cross section (arrows). (E) After partial loss of a muscle's lower motor neuron inputs, the remaining motor neurons sprout new endings that reinnervate nearby muscle fibers. Each reinnervated muscle fiber assumes the physiological properties of the other fibers in its newfound motor unit, with the result that fiber types become grouped rather than being interspersed. Here such fiber type grouping is shown at low magnification with an ATPase stain (pH 9.4).

Fig. 18.5, S (first column), FR (second column), and FF (third column) motor units of cat gastrocnemius, showing the anatomical components (A), twitch response to a single stimulus (B), and responses to intermittent bursts of action potentials (C) for each. The same time and force scale applies to all three twitches in (B).

All the muscle fibers in a motor unit are of a single type, with the result that there are three types of motor units (see Fig. 18.5 ). The smallest lower motor neurons innervate type I fibers, the largest innervate type IIb fibers, and motor neurons of intermediate size innervate type IIa fibers. The properties of each motor unit type can be predicted from the properties of the muscle fibers: type S ( s low-twitch) motor units produce small amounts of force for prolonged periods, type FF ( f ast-twitch, f atigable) units produce large amounts of force for brief periods, and type FR ( f ast-twitch, fatigue- r esistant) produce moderate amounts of force that can be sustained for moderate amounts of time (see Fig. 18.5 ).

Motor Units Are Recruited in Order of Size

The association of different muscle fiber types with motor neurons of different sizes is the basis of an elegantly simple mechanism for grading the force of muscle contraction. If two neurons have the same density of channels in their surface membranes, the smaller of the two neurons will have fewer total channels and a greater resistance to transmembrane current flow. Therefore a given amount of synaptic current will cause a greater membrane potential change in the smaller neuron, making the smaller neuron more easily excitable. As the synaptic drive reaching the anterior horn increases, motor neurons reach threshold in order of increasing size (the size principle ). S units are recruited first, and as they fire faster and faster, FR units are added. As the FR units increase their firing rate, FF units are added. This sequence is required to smoothly increase the force of muscle contraction, beginning with small increases from the background level of tone and ending with brief maximal contractions ( Fig. 18.6 ). The elegant part is that it happens automatically, in all movements, simply by virtue of the increasing size of the motor neurons in the three types of motor unit.

Fig. 18.6, Recruitment of motor units in order of size. (A) Graphic indication of the buildup of force in cat gastrocnemius during normal activities as motor units are recruited in order of size. (B) Firing rates of 60 motor units in a human forearm muscle (extensor digitorum) during isometric contraction of increasing force; each line represents a single motor unit. Force production increases by individual units firing more rapidly and, simultaneously, by additional units being recruited. Note that the force scale is logarithmic, and later-recruited units provide greater increments of force. FF, Fast-twitch, fatigable; FR, fast-twitch, fatigue-resistant.

Motor Control Systems Involve Both Hierarchical and Parallel Connections

The inputs that determine the level of activity of lower motor neurons can be divided very broadly into three overlapping classes ( Fig. 18.7 ):

  • 1.

    Built-in patterns of neural connections

  • 2.

    Descending pathways that modulate the activity of motor neurons; these effects may be either direct or indirect, by way of influences on built-in neural subsystems. Collectively, the neurons that give rise to these descending pathways are upper motor neurons.

  • 3.

    Higher centers that influence the activity of descending pathways

Fig. 18.7, Major components and schematic connections involved in motor control. The cerebellum and basal nuclei participate in motor control primarily by influencing the output from cerebral cortex to the brainstem and spinal cord. (These connections are discussed in greater detail in Chapters 19 and 20 .) Each also has additional outputs to brainstem nuclei (relatively minor for the basal nuclei, more substantial for the cerebellum). The association cortex, basal nuclei, and cerebellum play vital roles in the choice, design, and monitoring of movement but have no direct effect on lower motor neurons (LMN). For this reason, damage to structures in the lower box but not the upper box causes movement disorders in which weakness is prominent. (As indicated in Fig. 18.8 , not all upper motor neurons [UMNs] live in the cerebral cortex.)

Damage to either upper or lower motor neurons (or muscle) causes weakness and a distinctive set of accompanying symptoms and signs. Damage to higher centers also causes distinctive movement abnormalities (e.g., involuntary movements, incoordination, difficulty initiating movement) but is not accompanied by substantial weakness.

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