Motor system overview

Editor's Notes

• #3: Nevertheless, there is considerable evidence for the general motor control scheme shown in Figure 12-1. Commands for voluntary movement originate in cortical association areas. The movements are planned in the cortex as well as in the basal ganglia and the lateral portions of the cerebellar hemispheres, as indicated by increased electrical activity before the movement. The basal ganglia and cerebellum both funnel information to the premotor and motor cortex by way of the thalamus. Motor commands from the motor cortex are relayed in large part via the corticospinal tracts to the spinal cord and the corresponding corticobulbar tracts to motor neurons in the brain stem. However, collaterals from these pathways and a few direct connections from the motor cortex end on brain stem nuclei, which also project to motor neurons in the brain stem and spinal cord. These pathways can also mediate voluntary movement. Movement sets up alterations in sensory input from the special senses and from muscles, tendons, joints, and the skin. This feedback information, which adjusts and smoothes movement, is relayed directly to the motor cortex and to the spinocerebellum. The spinocerebellum projects in turn to the brain stem. The main brain stem pathways that are concerned with posture and coordination are the rubrospinal, reticulospinal, tectospinal, and vestibulospinal tracts and corresponding projections to motor neurons in the brain stem.
• #11: Muscle SpindlesMuscle spindles are small encapsulated sensory receptors that have a spindle-like or fusiform shape and are located within the fleshy part of the muscle. Their main function is to signal changes in the length of the muscle within which they reside. Changes in the length of muscles are closely associated with changes in the angles of the joints that the muscles cross. Thus, muscle spindles can be used by the central nervous system to sense relative positions of the body segments.Each spindle has three main components: (1) a group of specialized intrafusalmuscle fibers whose central regions are noncontractile; (2) large-diameter myelinated sensory endings that originate from the central regions of the intrafusal fibers; and (3) small-diameter myelinated motor endings that innervate the polar contractile regions of the intrafusal fibers (Figure 36-3A). When the intrafusal fibers are stretched, often referred to as “loading the spindle,” the sensory endings are also stretched and increase their firing rate. Because muscle spindles are arranged in parallel with the extrafusal muscle fibers that make up the main body of the muscle, the intrafusal fibers change in length as the whole muscle changes. Thus, when a muscle is stretched, the activity in the sensory endings of muscle spindles is increased. When a muscle shortens, the spindle is unloaded and the activity decreases.The motor innervation of the intrafusal muscle fibers comes from small-diameter motor neurons, called gamma motor neurons to distinguish them from the large-diameter alpha motor neurons that innervate the extrafusal muscle fibers. Contraction of the intrafusal muscle fibers does not contribute to the force of muscle contraction. Rather, activation of gamma motor neurons causes shortening of the polar regions of the intrafusal fibers. This in turn stretches the noncontractile central region from both ends, leading to an increase in firing rate of the sensory endings or to a greater likelihood that stretch of the muscle will cause the sensory ending to fire. Thus, the gamma motor neurons provide a mechanism for adjusting the sensitivity of the muscle spindles.The structure and functional behavior of muscle spindles is considerably more complicated than this simple description implies. When a muscle is stretched, there are two phases of the change in length: a dynamic phase, the period during which length is changing, and a static or steady-state phase, when the muscle has stabilized at a new length. Structural specializations within each component of the muscle spindles allow spindle afferents to signal aspects of each phase separately.There are two types of intrafusal muscle fibers: nuclear bag fibers and nuclear chain fibers. The bag fibers can be divided into two groups, dynamic and static. A typical spindle has 2 or 3 bag fibers and a variable number of chain fibers, usually about 5. Furthermore, there are two types of sensory fiber endings: a single primary ending and a variable number of secondary endings (up to 8). The primary (Ia fiber) ending spirals around the central region of all the intrafusal muscle fibers (Figure 36-3B). The secondary (group II fiber) endings are located adjacent to the central regions of the static bag and chain fibers. The gamma motor neurons can also be divided into two classes, dynamic and static. Dynamic gamma motor neurons innervate the dynamic bag fibers, while the static gamma motor neurons innervate the static bag and the chain fibers.This duality of structure is reflected in a duality of function. The steady-state or tonic discharge of both primary and secondary sensory endings signals the steady-state length of the muscle. The primary endings are, in addition, highly sensitive to the velocity of stretch, allowing them to provide information about the speed of movements. Because they are highly sensitive to small changes, primary endings provide quick information about unexpected changes in length, useful for generating quick corrective reactions.Increases in activity of dynamic gamma motor neurons increase the dynamic sensitivity of the primary endings but have no influence on the secondary endings. Increases in activity of static gamma motor neurons increase the tonic level of activity in both primary and secondary endings, decrease the dynamic sensitivity of primary endings, and can prevent the silencing of primary activity when a muscle is released from stretch (Figure 36-3C). Thus, the central nervous system can independently adjust the dynamic and static sensitivity of the sensory fibers from muscle spindles.Box 36-2 Selective Activation of Sensory Fibers from MuscleSensory fibers are classified according to their diameter. Axons with larger diameters conduct action potentials more rapidly. Because each class of receptors gives rise to afferent fibers with diameters within a restricted range, this method of classification distinguishes to some extent the fibers that arise from the different groups of sensory receptors. The main groups of sensory fibers from muscle are listed in Table 36-1 (see Chapter 24 for the classification of sensory fibers from skin and joints).The organization of reflex pathways in the spinal cord has been established primarily by electrically stimulating the sensory fibers and recording evoked responses in different classes of neurons in the spinal cord. This method of activation has three advantages over natural stimulation. The timing of afferent input can be precisely established, the central responses evoked by different classes of sensory fibers can be assessed by grading the strength of the electrical stimulus, and certain classes of receptors can be activated in isolation (impossible in natural conditions).The strength of electrical stimuli required to activate a sensory fiber is measured relative to the strength required to activate the largest afferent fibers since the largest fibers have the lowest threshold for electrical activation. Thus group I fibers are usually activated over the range of one to two times the threshold of the largest afferents (with Ia fibers having, on average, a slightly lower threshold than Ib fibers). Most group II fibers are activated over the range of 2-5 times the threshold, while the small group III and IV fibers require stimulus strengths in the range of 10-50 times the threshold for activation.Reciprocal innervation of opposing muscles is not the only useful mode of coordination. Sometimes it is advantageous to contract the prime mover and the antagonist at the same time. Such co-contraction has the effect of stiffening the joint and is most useful when precision and joint stabilization are critical. An example of this phenomenon is the co-contraction of flexor and extensor muscles of the elbow immediately before catching a ball. The Ia inhibitory interneurons receive both excitatory and inhibitory signals from all of the major descending pathways (Figure 36-5A). By changing the balance of excitatory and inhibitory inputs onto these interneurons, supraspinal centers can reduce reciprocal inhibition and enable co-contraction, thus controlling the relative amount of joint stiffness to meet the requirements of the motor act.The activity of spinal motor neurons is also regulated by another important class of inhibitory interneurons, the Renshaw cells (Figure 36-5B). Renshaw cells are excited by collaterals of the axons of motor neurons, and they make inhibitory synaptic connections to several populations of motor neurons, including the same motor neurons that excite them, and to the Ia inhibitory interneurons. The connections of Renshaw cells to motor neurons form a negative feedback system that may help stabilize the firing rate of the motor neurons, while the connections to the Ia inhibitory interneurons may regulate the strength of reciprocal inhibition to antagonistic motor neurons. In addition, Renshaw cells receive significant synaptic input from descending pathways and distribute inhibition to task-related groups of motor neurons and Iainterneurons. Thus, it is likely that they contribute to establishing the pattern of transmission in divergent group Ia pathways according to the motor task.
• #13: The Spinal Cord Circuitry Underlying Muscle Stretch ReflexesThe local circuitry within the spinal cord mediates a number of sensory motor reflex actions. The simplest of these reflex arcs entails the response to musclestretch, which provides direct excitatory feedback to the motor neurons innervating the muscle that has been stretched (Figure 16.9). As already mentioned, the sensory signal for the stretch reflex originates in muscle spindles, sensory receptors embedded within most muscles (see previous section and Chapter 9). The spindles comprise 8–10 intrafusal fibers arranged in parallel with the extrafusal fibers that make up the bulk of the muscle (Figure 16.9A). Large-diameter sensory fibers, called Ia afferents, are coiled around the central part of the spindle. These afferents are the largest axons in peripheral nerves and, since action potential conduction velocity is a direct function of axon diameter (see Chapters 2 and 3), they allow for very rapid adjustments in this reflex arc when the muscle is stretched. The stretch imposed on the muscle deforms the intrafusal muscle fibers, which in turn initiate action potentials by activating mechanically gated ion channels in the afferent axons coiled around the spindle. The centrally projecting branch of the sensory neuron forms monosynaptic excitatory connections with the α motor neurons in the ventral horn of the spinal cord that innervate the same (homonymous) muscle and, via local circuit neurons, inhibitory connections with the α motor neurons of antagonistic (heteronymous) muscles. This arrangement is an example of what is called reciprocal innervation and results in rapid contraction of the stretched muscle and simultaneous relaxation of the antagonist muscle. All of this leads to especially rapid and efficient responses to changes in the length or tension in the muscle (Figure 16.9B). The excitatory pathway from a spindle to the α motor neurons innervating the same muscle is unusual in that it is a monosynaptic reflex; in most cases, sensory neurons from the periphery do not contact the lower motor neuron directly but exert their effects through local circuit neurons.This monosynaptic reflex arc is variously referred to as the “stretch,” “deep tendon,” or “myotatic reflex,” and it is the basis of the knee, ankle, jaw, biceps, or triceps responses tested in a routine neurological examination. The tap of the reflex hammer on the tendon stretches the muscle and therefore excites a volley of activity from the muscle spindles in the afferent axons. The afferent volley is relayed to the α motor neurons in the brainstem or spinal cord, and an efferent volley returns to the muscle (see Figure 1.5). Since muscles are always under some degree of stretch, this reflex circuit is normally responsible for the steady level of tension in muscles called muscle tone. Changes in muscle tone occur in a variety of pathological conditions, and it is these changes that are assessed by examination of tendon reflexes.In terms of engineering principles, the stretch reflex arc is a negative feedback loop used to maintain muscle length at a desired value (Figure 16.9C). The appropriate muscle length is specified by the activity of descending pathways that influence the motor neuron pool. Deviations from the desired length are detected by the muscle spindles, since increases or decreases in the stretch of the intrafusal fibers alter the level of activity in the sensory fibers that innervate the spindles. These changes lead in turn to adjustments in the activity of the α motor neurons, returning the muscle to the desired length by contracting the stretched muscle and relaxing the opposed muscle group, and by restoring the level of spindle activity to what it was before.The smaller γ motor neurons control the functional characteristics of the muscle spindles by modulating their level of excitability. As already described, when the muscle is stretched, the spindle is also stretched and the rate of discharge in the afferent fibers increased. When the muscle shortens, however, the spindle is relieved of tension, or “unloaded,” and the sensory axons that innervate the spindle might therefore be expected to fall silent during contraction. However, they remain active. The γ motor neurons terminate on the contractile poles of the intrafusal fibers, and the activation of these neurons causes intrafusal fiber contraction—in this way maintaining the tension on the middle (or equatorial region) of the intrafusal fibers where the sensory axons terminate. Thus, co-activation of the α and γ motor neurons allows spindles to function (i.e., send information centrally) at all muscle lengths during movements and postural adjustments