1.1 What is Motor Control?
Much of the brain and nervous system is devoted to the processing of sensory input, in order to construct detailed representations of the external environment. Through vision, audition, somatosensation, and the other senses, we perceive the world and our relationship to it. This elaborate processing would be of limited value, however, unless we had a way to act upon the environment that we are sensing, whether that action consist of running away from a predator; seeking shelter against the rain; searching for food when one is hungry; moving one’s lips and vocal cords in order to communicate with others; or performing the countless other varieties of actions that make up our daily lives. In some cases the relationship between the sensory input and the motor output are simple and direct; for example, touching a hot stove elicits an immediate withdrawal of the hand (Figure 1.1). Usually, however, our conscious actions require not only sensory input but a host of other cognitive processes that allow us to choose the most appropriate motor output for the given circumstances. In each case, the final output is a set of commands to certain muscles in the body to exert force against some other object or forces (e.g., gravity). This entire process falls under the subject of motor control. 1.2 Some Necessary Components of Proper Motor Control
These are some of the many components of the motor system that allow us to perform complex movements in a seemingly effortless way. The brain has evolved exceedingly complex and sophisticated mechanisms to perform these tasks, and researchers have only scratched the surface in understanding the principles that underlie the brain’s control of movement. 1.3 Motor Control Requires Sensory Input One of the major principles of the motor system is that motor control requires sensory input to accurately plan and execute movements. This principle applies to low levels of the hierarchy, such as spinal reflexes, and to higher levels. As we shall see throughout this material on the motor system, our abilities to make movements that are accurate, properly timed, and with proper force depend critically on the sensory input that is ubiquitous at all levels of the motor system hierarchy. 1.4 Functional Segregation and Hierarchical Organization The ease with which we make most of our movements belies the enormous sophistication and complexity of the motor system. Engineers have spent decades trying to make machines perform simple tasks that we take for granted, yet the most advanced robotic systems do not come close to emulating the precision and smoothness of movement, under all types of conditions, that we achieve effortlessly and automatically. How does the brain do it? Although many of the details are not understood, two broad principles appear to be key concepts toward understanding motor control:
The motor system hierarchy consists of 4 levels (Figure 1.2): the spinal cord, the brain stem, the motor cortex, and the association cortex. It also contains two side loops: the basal ganglia and the cerebellum, which interact with the hierarchy through connections with the thalamus.
1.5 The Spinal Cord: The First Hierarchical Level The spinal cord is the first level of the motor hierarchy. It is the site where motor neurons are located. It is also the site of many interneurons and complex neural circuits that perform the “nuts and bolts” processing of motor control. These circuits execute the low-level commands that generate the proper forces on individual muscles and muscle groups to enable adaptive movements. The spinal cord also contains complex circuitry for such rhythmic behaviors as walking. Because this low level of the hierarchy takes care of these basic functions, higher levels (such as the motor cortex) can process information related to the planning of movements, the construction of adaptive sequences of movements, and the coordination of whole-body movements, without having to encode the precise details of each muscle contraction. 1.6 Motor Neurons Alpha motor neurons (also called lower motor neurons) innervate skeletal muscle and cause the muscle contractions that generate movement. Motor neurons release the neurotransmitter acetylcholine at a synapse called the neuromuscular junction. When the acetylcholine binds to acetylcholine receptors on the muscle fiber, an action potential is propagated along the muscle fiber in both directions (see Chapter 4 of Section I for review). The action potential triggers the contraction of the muscle. If the ends of the muscle are fixed, keeping the muscle at the same length, then the contraction results on an increased force on the supports (isometric contraction). If the muscle shortens against no resistance, the contraction results in constant force (isotonic contraction). The motor neurons that control limb and body movements are located in the anterior horn of the spinal cord, and the motor neurons that control head and facial movements are located in the motor nuclei of the brainstem. Even though the motor system is composed of many different types of neurons scattered throughout the CNS, the motor neuron is the only way in which the motor system can communicate with the muscles. Thus, all movements ultimately depend on the activity of lower motor neurons. The famous physiologist Sir Charles Sherrington referred to these motor neurons as the “final common pathway” in motor processing.
Motor neurons are not merely the conduits of motor commands generated from higher levels of the hierarchy. They are themselves components of complex circuits that perform sophisticated information processing. As shown in Figure 1.3, motor neurons have highly branched, elaborate dendritic trees, enabling them to integrate the inputs from large numbers of other neurons and to calculate proper outputs. Two terms are used to describe the anatomical relationship between motor neurons and muscles: the motor neuron pool and the motor unit.
If a muscle is required for fine control or for delicate movements (e.g., movement of the fingers or hands), its motor units will tend to have small innervation ratios. That is, each motor neuron will innervate a small number of muscle fibers (10-100), enabling many nuances of movement of the entire muscle. If a muscle is required only for coarse movements (e.g., a thigh muscle), its motor units will tend to have a high innervation ratio (i.e., each motor neuron innervating 1000 or more muscle fibers), as there is no necessity for individual muscle fibers to undergo highly coordinated, differential contractions to produce a fine movement. 1.7 Control of Muscle Force A motor neuron controls the amount of force that is exerted by muscle fibers. There are two principles that govern the relationship between motor neuron activity and muscle force: the rate code and the size principle.
Figure 1.6 demonstrates how the size principle governs the amount of force generated by a muscle. Because motor units are recruited in an orderly fashion, weak inputs onto motor neurons will cause only a few motor units to be active, resulting in a small force exerted by the muscle (Play 1). With stronger inputs, more motor neurons will be recruited, resulting in more force applied to the muscle (Play 2 and Play 3). Moreover, different types of muscle fibers are innervated by small and larger motor neurons. Small motor neurons innervate slow-twitch fibers; intermediate-sized motor neurons innervate fast-twitch, fatigue-resistant fibers; and large motor neurons innervate fast-twitch, fatigable muscle fibers. The slow-twitch fibers generate less force than the fast-twitch fibers, but they are able to maintain these levels of force for long periods. These fibers are used for maintaining posture and making other low-force movements. Fast-twitch, fatigue-resistant fibers are recruited when the input onto motor neurons is large enough to recruit intermediate-sized motor neurons. These fibers generate more force than slow-twitch fibers, but they are not able to maintain the force as long as the slow-twitch fibers. Finally, fast-twitch, fatigable fibers are recruited when the largest motor neurons are activated. These fibers produce large amounts of force, but they fatigue very quickly. They are used when the organism must generate a burst of large amounts of force, such as in an escape mechanism. Most muscles contain both fast- and slow-twitch fibers, but in different proportions. Thus, the white meat of a chicken, used to control the wings, is composed primarily of fast-twitch fibers, whereas the dark meat, used to maintain balance and posture, is composed primarily of slow-twitch fibers.
1.8 Muscle Receptors and Proprioception The motor system requires sensory input in order to function properly. In addition to sensory information about the external environment, the motor system also requires sensory information about the current state of the muscles and limbs themselves. Proprioception is the sense of the body’s position in space based on specialized receptors that reside in the muscles and tendons. The muscle spindle signals the length of a muscle and changes in the length of a muscle. The Golgi tendon organ signals the amount of force being applied to a muscle. Muscle Spindles Muscle spindles are collections of 6-8 specialized muscle fibers that are located within the muscle mass itself (Figure 1.7). These fibers do not contribute significantly to the force generated by the muscle. Rather, they are specialized receptors that signal (a) the length and (b) the rate of change of length (velocity) of the muscle. Because of the fusiform shape of the muscle spindle, these fibers are referred to as intrafusal fibers. The large majority of muscle fibers that allow the muscle to do work are termed extrafusal fibers. Each muscle contains many muscle spindles; muscles that are necessary for fine movements contain more spindles than muscles that are used for posture or coarse movements.
1.9 Types of Muscle Spindle Fibers
There are 3 types of muscle spindle fibers, characterized by their shape and the type of information they convey (Figure 1.8).
1.10 Sensory Innervation of Muscle Spindles Because the muscle spindle is located in parallel with the extrafusal fibers, it will stretch along with the muscle. The muscle spindle signals muscle length and velocity to the CNS through two types of specialized sensory fibers that innervate the intrafusal fibers. These sensory fibers have stretch receptors that open and close as a function of the length of the intrafusal fiber.
Because of their patterns of innervation onto the three types of intrafusal fibers, Group Ia and Group II afferents respond differently to different types of muscle movements. Figure 1.9 shows the responses of each type of afferent to a linear stretch of the muscle. Initially, both Group Ia and Group II fibers fire at a certain rate, encoding the current length of the muscle. During the stretch, the two types differ in their responses. The Group Ia afferent fires at a very high rate during the stretch, encoding the velocity of the muscle length; at the end of the stretch, its firing decreases, as the muscle is no longer changing length. Note, however, that its firing rate is still higher than it was before the stretch, as it is now encoding the new length of the muscle. Compare the response of the Group Ia afferent to the Group II afferent. The Group II afferent increases its firing rate steadily as the muscle is stretched. Its firing rate does not depend on the rate of change of the muscle; rather, its firing rate depends only on the immediate length of the muscle.
1.11 Gamma Motor Neurons Although intrafusal fibers do not contribute significantly to muscle contraction, they do have contractile elements at their ends that are innervated by motor neurons.
Motor neurons are divided into two groups. Alpha motor neurons innervate extrafusal fibers, the highly contracting fibers that supply the muscle with its power. Gamma motor neurons innervate intrafusal fibers, which contract only slightly. The function of intrafusal fiber contraction is not to provide force to the muscle; rather, gamma activation of the intrafusal fiber is necessary to keep the muscle spindle taut, and therefore sensitive to stretch, over a wide range of muscle lengths. This concept is illustrated in Figure 1.10. If a resting muscle is stretched, the muscle spindle becomes stretched in parallel, sending signals through the primary and secondary afferents. A subsequent contraction of the muscle, however, removes the pull on the spindle, and it becomes slack, causing the spindle afferents to cease firing. If the muscle were to be stretched again, the muscle spindle would not be able to signal this stretch. Thus, the spindle is rendered temporarily insensitive to stretch after the muscle has contracted. Activation of gamma motor neurons prevents this temporary insensitivity by causing a weak contraction of the intrafusal fibers, in parallel with the contraction of the muscle. This contraction keeps the spindle taut at all times and maintains its sensitivity to changes in the length of the muscle. Thus, when the CNS instructs a muscle to contract, it not only sends the appropriate signals to the alpha motor neurons, it also instructs gamma motor neurons to contract the intrafusal fibers appropriately; this coordinated process is referred to as alpha-gamma coactivation. 1.12 Golgi Tendon Organ
The Golgi tendon organ is a specialized receptor that is located between the muscle and the tendon (Figure 1.7). Unlike the muscle spindle, which is located in parallel with extrafusal fibers, the Golgi tendon organ is located in series with the muscle and signals information about the load or force being applied to the muscle. A Golgi tendon organ is made up of a capsule containing numerous collagen fibers (Figure 1.11). The organ is innervated by primary afferents called Group Ib fibers, which have specialized endings that weave in between the collagen fibers. When force is applied to a muscle, the Golgi tendon organ is stretched, causing the collagen fibers to squeeze and distort the membranes of the primary afferent sensory endings. As a result, the afferent is depolarized, and it fires action potentials to signal the amount of force. Figure 1.12 illustrates the difference in information conveyed by muscle spindles and Golgi tendon organs. At the resting position, the Ia afferents of spindles in the triceps muscle fire at a steady rate to encode the present length of the muscle, and the Ib afferents of the Golgi tendon organs of the biceps muscle fire at a low rate. When a light object (a balloon) is placed in the hand, there is little change in the firing rate of either afferent. When the hand starts to rise, however, the triceps muscle is stretched, and the Ia afferent fibers increase their firing rate as a function of muscle length. The Ib fibers do not change appreciably, because the balloon does not add much load to the muscle. What if a heavy object (a bowling ball) were placed in the hand instead? Because a heavy load is now placed on the biceps, the Ib afferents fire vigorously. Note that the Ia afferent is not affected, as the muscle length has not changed. When the arm begins to rise, however, the Ia afferents fire, just as with the balloon.
In summary,
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Types of fibers contained within muscle spindles include...
Types of fibers contained within muscle spindles include...
Types of fibers contained within muscle spindles include...
Types of fibers contained within muscle spindles include...
Types of fibers contained within muscle spindles include...
Types of fibers contained within muscle spindles include...
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Muscle force is controlled in part by...
Muscle force is controlled in part by...
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