3.1 Introduction Show
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.
Test Your Knowledge
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:
Which of these involves the body in performing more than one motor task at a time?Coordination: Coordination is the ability of the body to smoothly and successfully perform more than one motor task at the same time. Needed for football, baseball, tennis, soccer, and other sports that require hand-eye and foot-eye skills, coordination can be developed by repeatedly practicing the skill to be learned.
What ability uses senses such as sight and hearing together with body parts in performing motor tasks smoothly and accurately?Coordination. It is the ability to use the senses, such as sight and hearing, together with body parts in performing motor tasks smoothly and accurately.
Is the ability of the body to perform any movement in the shortest possible time?Speed – is the ability to perform a movement in one direction in the shortest period of time. Purpose – to measure running speed.
Which of the following skills is the ability to shift smoothly from one position to another?Agility or nimbleness is an ability to change the body's position quickly and requires the integration of isolated movement skills using a combination of balance, coordination, speed, reflexes, strength, and endurance.
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