Motor control is a broad term that describes the general ability of a person to initiate and direct muscle function and voluntary movements. Motor control is a concept that is distinct from the many involuntary muscle actions of the body, such as shivering when cold or flinching when an object is directed at a person without warning. A related expression, "motor skills," refers to the ability to perform specific physical movements; motor control is also the acquisition and development of a series of distinct motor skills. Motor control is divided into two subsets. Gross motor control is the ability of a human to move a large muscle group or segment of the anatomy; the waving of an arm is an example of this type of movement. Fine motor control is the ability to manipulate precise movement, such as handwriting. All motor control is an integrated product of three aspects of the human anatomy: muscles, bones, and the central nervous system. The voluntary motor system, also known as the somatic nervous system, is the structure that permits and creates motor control. The system takes its name from the part of the brain known as the motor cortex, from which the signals to initiate movement originate. The impulse from the motor cortex travels along pathways through the brainstem into the spinal cord. The nerve cells of the spinal cord connect to a vast and intricate network to control the skeletal muscle movement. Motor neurons, the specialized mechanisms that communicate to the muscles, are a continuation from the nerve roots that branch out from each vertebra in the spinal column to the muscle over which control is required. There are a number of pathways essential to the function of the voluntary motor system, of which the pyramidal system is the best known and the most extensive. The voluntary, or somatic, motor system that provides the body with motor control is in contrast to the autonomic system, which begins with the regulation directed by the distinct regions of the brain, including the hypothalamus. The hypothalamus regulates the function of many of the essential bodily systems, including heart rate, blood pressure, and electrolytic balance. The hypothalamus communicates much of its direction to these involuntary structures by way of the chemical signals, hormones, that are directed to the glandular network headed by the thyroid gland. Every healthy person will be capable of both gross motor control and fine motor control. In many sports, athletic success is measured in the fine distinctions between athletes in terms of their coordination (particularly their hand-eye coordination), balance, and overall body control. Many aspects of motor control are hereditary; others are linked to the body type of the individual. As an example, a 5 ft 10 in (1.7 m) point guard on a basketball team is expected to be able to execute complex physical movements, such as dribbling the ball with either hand at full speed under defensive pressure. The 6 ft 10 in (2 m) basketball forward is not likely to be able to move with the same grace and speed as the guard. With practice, the taller and less coordinated athlete could achieve improvements in this particular skill, but it is unlikely that he or she could surpass the smaller and quicker player. Body type and heredity aside, all athletes have the capacity to improve their motor control through the practice and the repetition of distinct motor skills. In many sports, the drills that form the basis of improved motor control ability are collateral to the sport itself. Cross training techniques are often employed to enhance a particular motor ability that is desired for a sport in an athlete. A notable example is the use of jumping rope in sports such as boxing; the repeated coordination of the athlete's footwork and hands in the act of skipping improves the athlete's overall coordination. American football has a time-honored training technique where players are required to move at full speed while negotiating a series of tires placed in a pattern; this drill builds the ability of the body to coordinate a jump vertically with a movement laterally to avoid falling into the obstacle, a non-contact simulation of the agile movements required on the playing field. "Muscle memory" is a muscular attribute linked to the development of motor skills. When an athlete is sidelined from an activity due to injury, the athlete will return more quickly to his or her previous level of motor ability due to the memory preserved in the nervous system as to how the motion stressed the subject muscle or structure. A physical injury to any aspect of the voluntary motor system will impair motor control. A concussion or damage to the spine or spinal column is a frequent cause of such injuries. When a nerve becomes pinched or otherwise damaged through trauma, such as a carpal tunnel nerve fracture in the wrist, the pathway for the major nerve ending into the muscles of the hand, there will be similar limitations of movement. Motor control can be significantly impaired though stresses imposed on other bodily systems. When athletes become dehydrated, they will commonly sustain an imbalance in their electrolyte levels, particularly that of the mineral sodium. A sodium deficiency will impair the ability of a nervous system transmission to be communicated to the working muscle. see also Hormones; Nervous system; Sport performance.
3.1 Introduction The previous chapters discussed the lower levels of the motor hierarchy (the spinal cord and brainstem), which are involved in the low-level, “nuts and bolts” processing that controls the activity of individual muscles. Individual alpha motor neurons control the force exerted by a particular muscle, and spinal circuits can control sophisticated and complex behaviors such as walking and reflex actions. The types of movements controlled by these circuits are not initiated consciously, however. Voluntary movements require the participation of the third and fourth levels of the hierarchy: the motor cortex and the association cortex. These areas of the cerebral cortex plan voluntary actions, coordinate sequences of movements, make decisions about proper behavioral strategies and choices, evaluate the appropriateness of a particular action given the current behavioral or environmental context, and relay commands to the appropriate sets of lower motor neurons to execute the desired actions. 3.2 Motor Cortex Comprises the Primary Motor Cortex, Premotor Cortex, and Supplementary Motor Area
The motor cortex comprises three different areas of the frontal lobe, immediately anterior to the central sulcus. These areas are the primary motor cortex (Brodmann’s area 4), the premotor cortex, and the supplementary motor area (Figure 3.1). Electrical stimulation of these areas elicits movements of particular body parts. The primary motor cortex, or M1, is located on the precentral gyrus and on the anterior paracentral lobule on the medial surface of the brain. Of the three motor cortex areas, stimulation of the primary motor cortex requires the least amount of electrical current to elicit a movement. Low levels of brief stimulation typically elicit simple movements of individual body parts. Stimulation of premotor cortex or the supplementary motor area requires higher levels of current to elicit movements, and often results in more complex movements than stimulation of primary motor cortex. Stimulation for longer time periods (500 msec) in monkeys results in the movement of a particular body part to a stereotyped posture or position, regardless of the initial starting point of the body part (Figure 3.2). Thus, the premotor cortex and supplementary motor areas appear to be higher level areas that encode complex patterns of motor output and that select appropriate motor plans to achieve desired end results.
Like the somatosensory cortex of the postcentral gyrus, the primary motor cortex is somatotopically organized (Figure 3.3). Stimulation of the anterior paracentral lobule elicits movements of the contralateral leg. As the stimulating electrode is moved across the precentral gyrus from dorsomedial to ventrolateral, movements are elicited progressively from the torso, arm, hand, and face (most laterally). The representations of body parts that perform precise, delicate movements, such as the hands and face, are disproportionately large compared to the representations of body parts that perform only coarse, unrefined movements, such as the trunk or legs. The premotor cortex and supplementary motor area also contain somatotopic maps.
One might predict that the motor cortex “homunculus” arises because neurons that control individual muscles are clustered together in the cortex. That is, all of the neurons that control the biceps muscle may be located together, and all of the neurons that control the triceps may be clustered nearby, and the neurons that control the soleus muscle may be clustered in a region further removed. Electrophysiological recordings have shown that this is not the case, however. Movements of individual muscles are correlated with activity from widespread parts of the primary motor cortex. Similarly, stimulation of small regions of primary motor cortex elicits movements that require the activity of numerous muscles. Thus, the primary motor cortex homunculus does not represent the activity of individual muscles. Rather, it apparently represents the movements of individual body parts, which often require the coordinated activity of large groups of muscles throughout the body. 3.3 Cortical Afferents and Efferents The motor cortex exerts its influence over muscles by a variety of descending routes (Figure 3.4). Some of the descending pathways reviewed in the last chapter can be influenced by motor cortex output. Thus, in addition to the direct cortical innervation of alpha motor neurons via the corticospinal tract, the following cortical efferent pathways influence the remaining descending tracts:
The cortex can also influence the processing of the side loops of the motor hierarchy. The corticostriate tract innervates the caudate nucleus and putamen of the basal ganglia. The corticopontine tract and cortico-olivary tract innervate important inputs to the cerebellum. Finally, cortical areas can influence other cortical areas, directly via corticocortical pathways and indirectly via the corticothalamic pathways (Figure 3.5). Most of these pathways are bi-directional. Thus, motor cortex receives input from other cortical areas, directly and indirectly through the thalamus, and it receives input from the cerebellum and basal ganglia, always through the thalamus.
3.4 Motor Cortex Cytoarchitecture Like all parts of the neocortex, the primary motor cortex is made of six layers (Figure 3.6). Unlike primary sensory areas, primary motor cortex is agranular cortex; that is, it does not have a cell-packed granular layer (layer 4). Instead, the most distinctive layer of primary motor cortex is its descending output layer (Layer 5), which contains the giant Betz cells. These pyramidal cells and other projection neurons of the primary motor cortex make up ~30% of the fibers in the corticospinal tract. The rest of the fibers come from the premotor cortex and the supplementary motor area (~30%), the somatosensory cortex (~30%), and the posterior parietal cortex (~10%).
3.5 Encoding of Movement by Motor Cortex Primary Motor Cortex As discussed above, the primary motor cortex does not generally control individual muscles directly, but rather appears to control individual movements or sequences of movements that require the activity of multiple muscle groups. Alpha motor neurons in the spinal cord, in turn, encode the force of contraction of groups of muscle fibers using the rate code and the size principle. Thus, in accordance with the concept of hierarchical organization of the motor system, the information represented by motor cortex is a higher level of abstraction than the information represented by spinal motor neurons. What is encoded by the neurons in primary motor cortex? Clues have come from recording the activity of these neurons as experimental animals perform different motor tasks. In general, primary motor cortex encodes the parameters that define individual movements or simple movement sequences.
Premotor Cortex The premotor cortex sends axons to the primary motor cortex as well as to the spinal cord directly. It performs more complex, task-related processing than primary motor cortex. Stimulation of premotor areas in the monkey at a high level of current produces more complex postures than stimulation of the primary motor cortex. The premotor cortex appears to be involved in the selection of appropriate motor plans for voluntary movements, whereas the primary motor cortex is involved in the execution of these voluntary movements.
Supplementary Motor Area The supplementary motor area (SMA) is involved in programming complex sequences of movements and coordinating bilateral movements. Whereas the premotor cortex appears to be involved in selecting motor programs based on visual stimuli or on abstract associations, the supplementary motor area appears to be involved in selecting movements based on remembered sequences of movements.
Association Cortex The fourth level of the motor hierarchy is the association cortex, in particular the prefrontal cortex and the posterior parietal cortex (Figure 3.14). These brain areas are not motor areas in the strict sense. Their activity does not correlate precisely with individual motor acts, and stimulation of these areas does not result in motor output. However, these areas are necessary to ensure that movements are adaptive to the needs of the organism and appropriate to the behavioral context.
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Betz cells are most abundant in layer...
Betz cells are most abundant in layer...
Betz cells are most abundant in layer...
Betz cells are most abundant in layer...
Betz cells are most abundant in layer...
Betz cells are most abundant in layer...
A corticospinal neuron in primary motor cortex can do all of the following EXCEPT:
A corticospinal neuron in primary motor cortex can do all of the following EXCEPT:
A corticospinal neuron in primary motor cortex can do all of the following EXCEPT:
A corticospinal neuron in primary motor cortex can do all of the following EXCEPT:
A corticospinal neuron in primary motor cortex can do all of the following EXCEPT:
A corticospinal neuron in primary motor cortex can do all of the following EXCEPT:
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