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REFRENCE This Course text book in the assignment many citations thank you
Pinel, J. P. (10/2010). Biopsychology, 8th Edition [VitalSource Bookshelf version]. Retrieved from http://online.vitalsource.com/books/9781269533744
8 The Sensorimotor System How You Move
8.1 Three Principles of Sensorimotor Function
8.2 Sensorimotor Association Cortex
8.3 Secondary Motor Cortex
8.4 Primary Motor Cortex
8.5 Cerebellum and Basal Ganglia
8.6 Descending Motor Pathways
8.7 Sensorimotor Spinal Circuits
8.8 Central Sensorimotor Programs
The evening before I started to write this chapter, I was standing in a checkout line at the local market. As I waited, I furtively scanned the headlines on the prominently displayed magazines—woman gives birth to cat; flying saucer lands in cleveland shopping mall; how to lose 20 pounds in 2 days. Then, my mind began to wander, and I started to think about beginning to write this chapter. That is when I began to watch Rhonda’s movements and to wonder about the neural system that controlled them. Rhonda is a cashier—the best in the place.
The Case of Rhonda, the Dexterous Cashier
I was struck by the complexity of even Rhonda’s simplest movements. As she deftly transferred a bag of tomatoes to the scale, there was a coordinated adjustment in almost every part of her body. In addition to her obvious finger, hand, arm, and shoulder movements, coordinated movements of her head and eyes tracked her hand to the tomatoes; and there were adjustments in the muscles of her feet, legs, trunk, and other arm, which kept her from lurching forward. The accuracy of these responses suggested that they were guided in part by the patterns of visual, somatosensory, and vestibular changes they produced. The term sensorimotor in the title of this chapter formally recognizes the critical contribution of sensory input to guiding motor output.
As my purchases flowed through her left hand, Rhonda registered the prices with her right hand and bantered with Rick, the bagger. I was intrigued by how little of what Rhonda was doing appeared to be under her conscious control. She made general decisions about which items to pick up and where to put them, but she seemed to give no thought to the exact means by which these decisions were carried out. Each of her responses could have been made with an infinite number of different combinations of finger, wrist, elbow, shoulder, and body adjustments; but somehow she unconsciously picked one. The higher parts of her sensorimotor system—perhaps her cortex—seemed to issue conscious general commands to other parts of the system, which unconsciously produced a specific pattern of muscular responses that carried them out.
The automaticity of Rhonda’s performance was a far cry from the slow, effortful responses that had characterized her first days at the market. Somehow, experience had integrated her individual movements into smooth sequences, and it seemed to have transferred the movements’ control from a mode that involved conscious effort to one that did not.
I was suddenly jarred from my contemplations by a voice. “Sir, excuse me, sir, that will be $18.65,” Rhonda said, with just a hint of delight at catching me in mid-daydream. I hastily paid my bill, muttered “thank you,” and scurried out of the market.
As I write this, I am smiling both at my own embarrassment and at the thought that Rhonda has unknowingly introduced you to three principles of sensorimotor control that are the foundations of this chapter: (1) The sensorimotor system is hierarchically organized. (2) Motor output is guided by sensory input. (3) Learning can change the nature and the locus of sensorimotor control.
8.1 Three Principles of Sensorimotor Function
Before getting into the details of the sensorimotor system, let’s take a closer look at the three principles of sensorimotor function introduced by Rhonda. You will better appreciate these principles if you recognize that they also govern the operation of any large, efficient company—perhaps because that is another system for controlling output that has evolved in a competitive environment. You may find this metaphor useful in helping you understand the principles of sensorimotor system organization—many scientists find that metaphors help them think creatively about their subject matter.
The Sensorimotor System Is Hierarchically Organized
The operation of both the sensorimotor system and a large, efficient company is directed by commands that cascade down through the levels of a hierarchy (see Graziano, 2009)—from the association cortex or the company president (the highest levels) to the muscles or the workers (the lowest levels). Like the orders that are issued from the office of a company president, the commands that emerge from the association cortex specify general goals rather than specific plans of action. Neither the association cortex nor the company president routinely gets involved in the details. The main advantage of this hierarchical organization is that the higher levels of the hierarchy are left free to perform more complex functions.
Thinking Creatively
Both the sensorimotor system and a large, efficient company are parallel hierarchical systems; that is, they are hierarchical systems in which signals flow between levels over multiple paths (see Darian-Smith, Burman, & Darien-Smith, 1999). This parallel structure enables the association cortex or company president to exert control over the lower levels of the hierarchy in more than one way. For example, the association cortex can directly inhibit an eye blink reflex to allow the insertion of a contact lens, just as a company president can personally organize a delivery to an important customer.
The sensorimotor and company hierarchies are also characterized by functional segregation . That is, each level of the sensorimotor and company hierarchies tends to be composed of different units (neural structures or departments), each of which performs a different function.
In summary, the sensorimotor system—like the sensory systems you read about in Chapter 7—is a parallel, functionally segregated, hierarchical system. The main difference between the sensory systems and the sensorimotor system is the primary direction of information flow. In sensory systems, information mainly flows up through the hierarchy; in the sensorimotor system, information mainly flows down.
Motor Output Is Guided by Sensory Input
Efficient companies are flexible. They continuously monitor the effects of their own activities, and they use this information to fine-tune their activities. The sensorimotor system does the same (Gomi, 2008; Sommer & Wurtz, 2008). The eyes, the organs of balance, and the receptors in skin, muscles, and joints all monitor the body’s responses, and they feed their information back into sensorimotor circuits. In most instances, this sensory feedback plays an important role in directing the continuation of the responses that produced it. The only responses that are not normally influenced by sensory feedback are ballistic movements—brief, all-or-none, high-speed movements, such as swatting a fly.
Neuroplasticity
Behavior in the absence of just one kind of sensory feedback—the feedback that is carried by the somatosensory nerves of the arms—was studied in G.O., a former darts champion.
The Case of G.O., the Man with Too Little Feedback
An infection had selectively destroyed the somatosensory nerves of G.O.’s arms. He had great difficulty performing intricate responses such as doing up his buttons or picking up coins, even under visual guidance. Other difficulties resulted from his inability to adjust his motor output in the light of unanticipated external disturbances; for example, he could not keep from spilling a cup of coffee if somebody brushed against him. However, G.O.’s greatest problem was his inability to maintain a constant level of muscle contraction:
Clinical Implications
The result of this deficit was that even in the simplest of tasks requiring a constant motor output to the hand, G.O. would have to keep a visual check on his progress. For example, when carrying a suitcase, he would frequently glance at it to reassure himself that he had not dropped it some paces back. However, even visual feedback was of little use to him in many tasks. These tended to be those requiring a constant force output such as grasping a pen while writing or holding a cup. Here, visual information was insufficient for him to be able to correct any errors that were developing in the output since, after a period, he had no indication of the pressure that he was exerting on an object; all he saw was either the pen or cup slipping from his grasp. (Rothwell et al., 1982, p. 539)
Many adjustments in motor output that occur in response to sensory feedback are controlled unconsciously by the lower levels of the sensorimotor hierarchy without the involvement of the higher levels (see Poppele & Bosco, 2003). In the same way, large companies run more efficiently if the clerks do not have to check with the company president each time they encounter a minor problem.
Learning Changes the Nature and Locus of Sensorimotor Control
When a company is just starting up, each individual decision is made by the company president after careful consideration. However, as the company develops, many individual actions are coordinated into sequences of prescribed procedures that are routinely carried out by personnel at lower levels of the hierarchy.
Neuroplasticity
Similar changes occur during sensorimotor learning (see Ashe et al., 2006; Kübler, Dixon, & Garavan, 2006). During the initial stages of motor learning, each individual response is performed under conscious control; then, after much practice, individual responses become organized into continuous integrated sequences of action that flow smoothly and are adjusted by sensory feedback without conscious regulation. If you think for a moment about the sensorimotor skills you have acquired (e.g., typing, swimming, knitting, basketball playing, dancing, piano playing), you will appreciate that the organization of individual responses into continuous motor programs and the transfer of their control to lower levels of the nervous system characterize most sensorimotor learning.
A General Model of Sensorimotor System Function
Figure 8.1 on page 194 is a model that illustrates several principles of sensorimotor system organization; it is the framework of this chapter. Notice its hierarchical structure, the functional segregation of the levels (e.g., of secondary motor cortex), the parallel connections between levels, and the numerous feedback pathways.
This chapter focuses on the neural structures that play important roles in the control of voluntary behavior (e.g., picking up an apple). It begins at the level of association cortex and traces major motor signals as they descend the sensorimotor hierarchy to the skeletal muscles that ultimately perform the movements.
FIGURE 8.1 A general model of the sensorimotor system. Notice its hierarchical structure, its functional segregation, its parallel descending pathways, and its feedback circuits.
8.2 Sensorimotor Association Cortex
Association cortex is at the top of your sensorimotor hierarchy. There are two major areas of sensorimotor association cortex: the posterior parietal association cortex and the dorsolateral prefrontal association cortex (see Brochier & Umiltà, 2007; Haggard, 2008). Posterior parietal cortex and the dorsolateral prefrontal cortex are each composed of several different areas, each with different functions (see Culham & Kanwisher, 2001; Fuster, 2000). However, there is no general consensus on how best to divide either of them for analysis or even how comparable the areas are in humans, monkeys, and rats (Calton & Taube, 2009; Husain & Nachev, 2006).
Posterior Parietal Association Cortex
Before an effective movement can be initiated, certain information is required. The nervous system must know the original positions of the parts of the body that are to be moved, and it must know the positions of any external objects with which the body is going to interact. The posterior parietal association cortex (the portion of parietal neocortex posterior to the primary somatosensory cortex) plays an important role in integrating these two kinds of information, in directing behavior by providing spatial information, and in directing attention (Britten, 2008; Husain & Nachev, 2006).
You learned in Chapter 7 that the posterior parietal cortex is classified as association cortex because it receives substantial information, input from more than one sensory system. It receives information from the three sensory systems that play roles in the localization of the body and external objects in space: the visual system, the auditory system, and the somatosensory system (see Andersen & Buneo, 2003; Macaluso, Driver, & Frith, 2003). In turn, much of the output of the posterior parietal cortex goes to areas of motor cortex, which are located in the frontal cortex: to the dorsolateral prefrontal association cortex, to the various areas of secondary motor cortex , and to the frontal eye field —a small area of prefrontal cortex that controls eye movements (see –Figure 8.2). Electrophysiological studies in macaque monkeys and functional magnetic resonance imaging (fMRI) and transcranial magnetic stimulation (TMS) studies in humans indicate that the posterior parietal cortex comprises a mosaic of small areas, each specialized for guiding particular movements of eyes, head, arms, or hands (Culham & Valyear, 2006; Fogassi & Luppino, 2005).
In one important study (Desmurget et al., 2009), electrical stimulation was applied to the inferior portions of the posterior parietal cortexes of conscious neurosurgical patients. At low current levels, the patients experienced an intention to perform a particular action and, at high levels, felt that they had actually performed it. However, in neither case did the action actually occur.
Damage to the posterior parietal cortex can produce a variety of deficits, including deficits in the perception and memory of spatial relationships, in accurate reaching and grasping, in the control of eye movement, and in attention (Freund, 2003). However, apraxia and contralateral neglect are the two most striking consequences of posterior parietal cortex damage.
Clinical Implications
Apraxia is a disorder of voluntary movement that is not attributable to a simple motor deficit (e.g., not to paralysis or weakness) or to any deficit in comprehension or motivation (see Heilman, Watson, & Rothi, 1997). Remarkably, apraxic patients have difficulty making specific movements when they are requested to do so, particularly when the movements are out of context; however, they can often readily perform the very same movements under natural conditions, when they are not thinking about doing so. For example, an apraxic carpenter who has no difficulty at all hammering a nail during the course of her work might not be able to demonstrate hammering movements when requested to make them, particularly in the absence of a hammer. Although its symptoms are bilateral, apraxia is often caused by unilateral damage to the left posterior parietal lobe or its connections (Jax, Buxbaum, & Moll, 2006).
FIGURE 8.2 The major cortical input and output pathways of the posterior parietal association cortex. Shown are the lateral surface of the left hemisphere and the medial surface of the right hemisphere.
Contralateral neglect , the other striking consequence of posterior parietal cortex damage, is a disturbance of a patient’s ability to respond to stimuli on the side of the body opposite (contralateral) to the side of a brain lesion, in the absence of simple sensory or motor deficits (see Driver, Vuilleumier, & Husain, 2004; Luauté et al. 2006; Pisella & Mattingley, 2004). Most patients with contralateral neglect often behave as if the left side of their world does not exist, and they often fail to appreciate that they have a problem (Berti et al., 2005). The disturbance is often associated with large lesions of the right posterior parietal lobe (see Mort et al., 2003; Van Vleet & Robertson, 2006). For example, Mrs. S. suffered from contralateral neglect after a massive stroke to the posterior portions of her right hemisphere.
Clinical Implications
The Case of Mrs. S., the Woman Who Turned in Circles
Mrs. S.’s stroke had left her unable to recognize or respond to things to the left—including external objects as well as parts of her own body. For example, Mrs. S. often put makeup on the right side of her face but ignored the left.
Mrs. S.’s left-side contralateral neglect created many problems for her, but a particularly bothersome one was that she had difficulty getting enough to eat. When a plate of food was put in front of her, she could see only the food on the right half of the plate and thus ate only that much, even if she was very hungry.
After asking for and receiving a wheelchair capable of turning in place, Mrs. S. developed an effective way of getting more food if she was still hungry after completing a meal. She turns her wheelchair around to the right in a full circle until she sees the remaining half of her meal. Then, she eats that food, or more precisely, she eats the right half of that food. If she is still hungry after that, she turns once again to the right until she discovers the remaining quarter of her meal and eats half of that . . . and so on.
(“The Case of Mrs. S., the Woman Who Turned in Circles,” reprinted with the permission of Simon & Schuster Adult Publishing Group from The Man Who Mistook His Wife for a Hat and Other Clinical Tales by Oliver Sacks. Copyright © 1970, 1981, 1983, 1984, 1986 by Oliver Sacks.)
Most patients with contralateral neglect have difficulty responding to things to the left. But to the left of what? For most patients with contralateral neglect, the deficits in responding occur for stimuli to the left of their own bodies, referred to as egocentric left. Egocentric left is partially defined by gravitational coordinates: When patients tilt their heads, their field of neglect is not normally tilted with it (see the top panel of Figure 8.3 on page 196).
In addition to failing to respond to objects on their egocentric left, many patients tend not to respond to the left sides of objects, regardless of where the objects are in their visual fields (Kleinman et al., 2007). Neurons that have egocentric receptive fields and others with object-based receptive fields have been found in primate parietal cortex (Olson, 2003; Pouget & Driver, 2000). Object-based contralateral neglect is illustrated in the lower panel of Figure 8.3. In this demonstration, patients with contralateral neglect had deficits in responding to the right hand of an experimenter who was facing them, regardless of its specific location in the patients’ visual field.
FIGURE 8.3 Contralateral neglect is sometimes manifested in terms of gravitational coordinates, sometimes in terms of object-based coordinates.
You have learned in the preceding chapters that the failure to perceive an object consciously does not necessarily mean that the object is not perceived. Indeed, two types of evidence suggest that information about objects that are not noticed by patients with contralateral neglect may be unconsciously perceived. First, when objects were repeatedly presented at the same spot to the left of patients with contralateral neglect, they tended to look to the same spot on future trials, although they were unaware of the objects (Geng & Behrmann, 2002). Second, patients could more readily identify fragmented (partial) drawings viewed to their right if complete versions of the drawings had previously been presented to the left, where they were not consciously perceived (Vuilleumier et al., 2002).
Dorsolateral Prefrontal Association Cortex
The other large area of association cortex that has important sensorimotor functions is the dorsolateral prefrontal association cortex . It receives projections from the posterior parietal cortex, and it sends projections to areas of secondary motor cortex , to primary motor cortex , and to the frontal eye field . These projections are shown in Figure 8.4. Not shown are the major projections back from dorsolateral prefrontal cortex to posterior parietal cortex.
Dorsolateral prefrontal cortex seems to play a role in the evaluation of external stimuli and the initiation of voluntary reactions to them (Matsumoto & Tanaka, 2004; Ohbayashi, Ohki, & Miyashita, 2003). This view is supported by the response characteristics of neurons in this area of association cortex. Several studies have characterized the activity of monkey dorsolateral prefrontal neurons as the monkeys identify and respond to objects (e.g., Rao, Rainer, & Miller, 1997). The activity of some neurons depends on the characteristics of objects; the activity of others depends on the locations of objects; and the activity of still others depends on a combination of both. The activity of other dorsolateral prefrontal neurons is related to the response, rather than to the object. These neurons typically begin to fire before the response and continue to fire until the response is complete. There are neurons in all cortical motor areas that begin to fire in anticipation of a motor activity, but those in the dorsolateral prefrontal association cortex fire first.
Evolutionary Perspective
The response properties of dorsolateral prefrontal neurons suggest that decisions to initiate voluntary movements may be made in this area of cortex (Rowe et al., 2000; Tanji & Hoshi, 2001), but these decisions depend on critical interactions with posterior parietal cortex (Brass et al., 2005; Connolly, Andersen, & Goodale, 2003; de Lange, Hagoort, & Toni, 2005).
8.3 Secondary Motor Cortex
Areas of secondary motor cortex are those that receive much of their input from association cortex (i.e., posterior parietal cortex and dorsolateral prefrontal cortex) and send much of their output to primary motor cortex (see Figure 8.5 on page 198). For many years, only two areas of secondary motor cortex were known: the supplementary motor area and the premotor cortex. Both of these large areas are clearly visible on the lateral surface of the frontal lobe, just anterior to the primary motor cortex . The supplementary motor area wraps over the top of the frontal lobe and extends down its medial surface into the longitudinal fissure, and the premotor cortex runs in a strip from the supplementary motor area to the lateral fissure.
FIGURE 8.4 The major cortical input and output pathways of the dorsolateral prefrontal association cortex. Shown are the lateral surface of the left hemisphere and the medial surface of the right hemisphere.
Identifying the Areas of Secondary Motor Cortex
The simple two-area conception of secondary motor cortex has become more complex. Neuroanatomical and neuro-physiological research with monkeys has made a case for at least eight areas of secondary motor cortex in each hemisphere, each with its own subdivisions (see Graziano, 2006; Nachev, Kennard, & Husain, 2008): three different supplementary motor areas (SMA, preSMA, and supplementary eye field), two premotor areas (dorsal and ventral), and three small areas—the cingulate motor areas —in the cortex of the cingulate gyrus. Although most of the research on secondary motor cortex has been done in monkeys, functional brain-imaging studies have suggested that human secondary motor cortex is similar to that of other primates (see Rizzolatti, Fogassi, & Gallese, 2002).
FIGURE 8.5 Four areas of secondary motor cortex—the supplementary motor area, the premotor cortex, and two cingulate motor areas—and their output to the primary motor cortex. Shown are the lateral surface of the left hemisphere and the medial surface of the right hemisphere.
Evolutionary Perspective
To qualify as secondary motor cortex, an area must be appropriately connected with association and secondary motor areas (see Figure 8.5). Electrical stimulation of an area of secondary motor cortex typically elicits complex movements, often involving both sides of the body. Neurons in an area of secondary motor cortex often become more active just prior to the initiation of a voluntary movement and continue to be active throughout the movement.
Simulate The Motor Areas of the Cerebral Cortex www.mypsychlab.com
In general, areas of secondary motor cortex are thought to be involved in the programming of specific patterns of movements after taking general instructions from dorsolateral prefrontal cortex (see Hoshi & Tanji, 2007). Evidence of such a function comes from brain-imaging studies in which the patterns of activity in the brain have been measured while the subject is either imagining his or her own performance of a particular series of movements or planning the performance of the same movements (see Kosslyn, Ganis, & Thompson, 2001; Sirigu & Duhamel, 2001).
Despite evidence of similarities among areas of secondary motor cortex, substantial effort has been put into discovering their differences. Until recently, this research has focused on differences between the supplementary motor area and the premotor cortex as originally defined. Although several theories have been proposed to explain functional differences between these areas (e.g., Hoshi & Tanji, 2007), none has received consistent support. As the boundaries between various areas of secondary motor cortex become more accurately characterized, the task of determining the function of each area will become easier—and vice versa.
Mirror Neurons
Few discoveries have captured the interest of neuroscientists as much as the discovery of mirror neurons (Lyons, Santos, & Keil, 2006). Mirror neurons are neurons that fire when an individual performs a particular goal-directed hand movement or when she or he observes the same goal-directed movement performed by another (Fadiga, Craighero, & Olivier, 2005).
Mirror neurons were discovered in the early 1990s in the laboratory of Giacomo Rizzolatti (see Rizzolatti, Fogassi, & Gallese, 2006). Rizzolatti and his colleagues had been studying a class of macaque ventral premotor neurons that seemed to encode particular goal objects; that is, these neurons fired when the monkey reached for one object (e.g., a toy) but not when the monkey reached for another object. Then, the researchers noticed something strange: Some of these neurons, later termed mirror neurons , fired just as robustly when the monkey watched the experimenter pick up the same object, but not any other—see Figure 8.6.
FIGURE 8.6 Responses of a mirror neuron of a monkey.
Why did the discovery of mirror neurons in the ventral premotor cortex create such a stir? The reason is that they provide a possible mechanism for social cognition (knowledge of the perceptions, ideas, and intentions of others). Mapping the actions of others onto one’s own action repertoire would facilitate social understanding, cooperation, and imitation (Iacoboni, 2005; Jackson & Decety, 2004; Knoblick & Sebanz, 2006).
Support for the idea that mirror neurons might play a role in social cognition has come from demonstrations that these neurons respond to the understanding of an action, not to some superficial aspect of it (Rizzolatti et al., 2006). For example, mirror neurons that reacted to the sight of an action that made a sound (e.g., cracking a peanut) were found to respond just as robustly to the sound alone—in other words, they responded fully to the particular action regardless of how it was detected. Indeed, many ventral premotor mirror neurons fire when a monkey does not perceive the key action but has enough clues to create a mental representation of it. The researchers identified mirror neurons that fired when the experimenter reached and grasped an object on a table. Then, a screen was placed in front of the object before the experimenter reached for it. Half the neurons still fired robustly, even though the monkeys could only have imagined what was happening.
Mirror neurons have also been found in the inferior portion of the posterior parietal lobe. Some of these respond to the purpose of an action rather than to the action itself. For example, some posterior parietal mirror neurons fired robustly when a monkey grasped a piece of food only if it was clear that the food would be subsequently eaten—if the food was repeatedly grasped and then placed in a bowl, the grasping was associated with little firing. The same neurons fired strongly when the monkey watched the experimenter picking up pieces of food to eat them, but not when the experimenter picked up food and placed it in a bowl.
The existence of mirror neurons has not yet been confirmed in humans because there are few opportunities to record the firing of individual neurons in humans while conducting the required behavioral tests (Turella et al., 2007). Although it has not been possible to directly demonstrate the existence of human mirror neurons, indirect lines of evidence suggest that mirror neurons do exist in the human brain. For example, functional brain-imaging studies have found areas of human motor cortex that are active when a person performs, watches, or imagines a particular action (e.g., Rizzolatti & Fabbri-Destro, 2008; Rodriguez et al., 2008). Indeed, many researchers believe that the evidence for human mirror neurons is so strong that they have started to consider the possible role that pathology of these neurons might play in human neuropsychological disorders (see Ramachandran & Oberman, 2006).
FIGURE 8.7 The motor homunculus: the somato-topic map of the human primary motor cortex. Stimulation of sites in the primary motor cortex elicits simple movements in the indicated parts of the body. (Based on Penfield & Rasmussen, 1950.)
Evolutionary Perspective
8.4 Primary motor cortex Central fissure
The primary motor cortex is located in the precentral gyrus of the frontal lobe (see Figure 8.5 and Figure 8.7). It is the major point of convergence of cortical sensorimotor signals, and it is the major, but not the only, point of departure of sensorimotor signals from the cerebral cortex. Understanding of the function of primary motor cortex has recently undergone radical changes—see Graziano (2006). The following two subsections describe this evolution: First, we consider the conventional view of primary motor cortex function, and second, we learn the current view of primary motor cortex function and some of the evidence on which it is based.
Conventional View of Primary Motor Cortex Function
In 1937, Penfield and Boldrey mapped the primary motor cortex of conscious human patients during neurosurgery by applying brief, low-intensity electrical stimulations to various points on the cortical surface and noting which part of the body moved in response to each stimulation. They found that the stimulation of each particular cortical site activated a particular contralateral muscle and produced a simple movement. When they mapped out the relation between each cortical site and the muscle that was activated by its stimulation, they found that the primary motor cortex is organized somatotopically—that is, according to a map of the body. The somatotopic layout of the human primary motor cortex is commonly referred to as the motor homunculus (see Figure 8.7). Notice that most of the primary motor cortex is dedicated to controlling parts of the body that are capable of intricate movements, such as the hands and mouth.
It is important to appreciate that each site in the primary motor cortex receives sensory feedback from receptors in the muscles and joints that the site influences. One interesting exception to this general pattern of feedback has been described in monkeys: Monkeys have at least two different hand areas in the primary motor cortex of each hemisphere, and one receives input from receptors in the skin rather than from receptors in the muscles and joints. Presumably, this adaptation facilitates stereognosis —the process of identifying objects by touch. Close your eyes and explore an object with your hands; notice how stereognosis depends on a complex interplay between motor responses and the somatosensory stimulation produced by them (see Johansson & Flanagan, 2009).
What is the function of each primary motor cortex neuron? Until recently, each neuron was thought to encode the direction of movement. The main evidence for this was the finding that each neuron in the arm area of the primary motor cortex fires maximally when the arm reaches in a particular direction; that is, each neuron has a different preferred direction.
Current View of Primary Motor Cortex Function
Recent efforts to map the primary motor cortex have used a new stimulation technique—see Graziano (2006). Rather than stimulating with brief pulses of current that are just above the threshold to produce a reaction, investigators have used longer bursts of current (e.g., 0.5 second), which are more similar to the duration of a motor response, at slightly higher intensities. The results were amazing: Rather than eliciting the contractions of individual muscles, these currents elicited complex natural-looking response sequences. For example, stimulation at one site reliably produced a feeding response: The arm reached forward, the hand closed as if clasping some food, the closed hand was brought to the mouth, and finally the mouth opened. These recent studies have revealed a crude somatotopic organization—that is, stimulation in the face area tended to elicit face movements. However, the elicited responses were complex species-typical movements, which often involved several parts of the body (e.g., hand, shoulder, and mouth), rather than individual muscle contractions. Also, sites that moved a particular body part overlapped greatly with sites that moved other body parts (Sanes et al., 1995). That is why small lesions in the hand area of the primary motor cortex of humans (Scheiber, 1999) or monkeys (Scheiber & Poliakov, 1998) never selectively disrupt the activity of a single finger.
The conventional view that many primary motor cortex neurons are tuned to movement in a particular direction has also been challenged. In the studies that have supported this view, the monkey subjects were trained to make arm movements from a central starting point so that the relation between neural firing and the direction of movement could be assessed. However, there is another reasonable interpretation of the results of these studies. Graziano (2006) recognized this alternative interpretation when he recorded from individual primary motor cortex neurons as monkeys moved about freely, rather than as they performed simple, learned arm movements from a set starting point. The firing of many primary motor cortex neurons was most closely related to the end point of a movement, not to the direction of the movement. If a monkey reached toward a particular location, primary motor cortex neurons sensitive to that target location tended to become active regardless of the direction of the movement that was needed to get to the target.
Thinking Creatively
The importance of the target of a movement, rather than the direction of a movement, for the function of primary motor cortex was also apparent in Graziano’s stimulation studies. For example, if stimulation of a particular cortical site causes the left elbow to bend to a 90° angle, opposite responses will be elicited if the arm was initially straight (180° angle) and if it was bent halfway (45° angle)—but the end point will always be the same. Stop for a moment and consider the implications of this finding, because they are as important as they are counterintuitive. First, the finding means that the signals from every site in the primary motor cortex diverge greatly, so each particular site has the ability to get a body part (e.g., an arm) to a target location regardless of the starting position. Second, it means that the sensorimotor system is inherently plastic. So far, I have not said much about neuroplasticity, which is a major theme of this book, but it will soon start to play a central role. Here you have learned that each location in the primary motor cortex can produce innumerable patterns of muscle contraction required to get a body part from any starting point to a target location. The key point is that the route that neural signals follow from a given area of primary motor cortex is extremely plastic and is presumably determined at any point in time by somatosensory feedback (Davidson et al., 2007).
Neuroplasticity
The neurons of the primary motor cortex play a major role in initiating body movements. With an appropriate interface, could they control the movements of a machine (see Craelius, 2002; König & Verschure, 2002; Taylor, Tillery, & Schwartz, 2002)? Belle says, “yes.”
Belle: The Monkey That Controlled a Robot with Her Mind
In the laboratory of Miguel Nicolesis and John Chapin (2002), a tiny owl monkey called Belle watched a series of lights on a control panel. Belle had learned that if she moved the joystick in her right hand in the direction of a light, she would be rewarded with a drop of fruit juice. On this particular day, Nicolesis and Chapin demonstrated an amazing feat. As a light flashed on the panel, 100 microelectrodes recorded extracellular unit activity from neurons in Belle’s primary motor cortex. This activity moved Belle’s arm toward the light, but at the same time, the signals were analyzed by a computer, which fed the output to a laboratory several hundred kilometers away, at the Massachusetts Institute of Technology. At MIT, the signals from Belle’s brain entered the circuits of a robotic arm. On each trial, the activity of Belle’s primary motor cortex moved her arm toward the test light, and it moved the robotic arm in the same direction. Belle’s neural signals were directing the activity of a robot.
Belle’s remarkable feat raises a possibility. Perhaps one day injured people will routinely control wheelchairs, prosthetic limbs, or even their own paralyzed limbs through the power of their thoughts (Lebedev & Nicolesis, 2006; Scherberger, 2009). Indeed, some neuroprosthetic devices for human patients are currently being evaluated (see Pancrazio & Peckham, 2009; Patil, 2009).
Clinical Implications
Effects of Primary Motor Cortex Lesions
Extensive damage to the human primary motor cortex has less effect than you might expect, given that this cortex is the major point of departure of motor fibers from the cerebral cortex. Large lesions to the primary motor cortex may disrupt a patient’s ability to move one body part (e.g., one finger) independently of others, may produce astereognosia (deficits in stereognosis), and may reduce the speed, accuracy, and force of a patient’s movements. Such lesions do not, however, eliminate voluntary movement, presumably because there are parallel pathways that descend directly from secondary motor areas to sub-cortical motor circuits without passing through primary motor cortex.
Clinical Implications
8.5 Cerebellum and Basal Ganglia
The cerebellum and the basal ganglia (see Figure 3.21 on page 65 and Figure 3.29 on page 70) are both important sensorimotor structures, but neither is a major part of the pathway by which signals descend through the sensorimotor hierarchy. Instead, both the cerebellum and the basal ganglia interact with different levels of the sensorimotor hierarchy and, in so doing, coordinate and modulate its activities. The interconnections between sensory and motor areas via the cerebellum and basal ganglia are thought to be one reason why damage to cortical connections between visual cortex and frontal motor areas does not abolish visually guided responses (Glickstein, 2000).
Cerebellum
The functional complexity of the cerebellum is suggested by its structure (see Apps & Hawkes, 2009). For example, although it constitutes only 10% of the mass of the brain, it contains more than half of the brain’s neurons (Azevedo et al., 2009).
The cerebellum receives information from primary and secondary motor cortex, information about descending motor signals from brain stem motor nuclei, and feedback from motor responses via the somatosensory and vestibular systems. The cerebellum is thought to compare these three sources of input and correct ongoing movements that deviate from their intended course (see Bastian, 2006; Bell, Han, & Sawtell, 2008). By performing this function, it is believed to play a major role in motor learning, particularly in the learning of sequences of movements in which timing is a critical factor (D’Angelo & De Zeeuw, 2008; Jacobson, Rokni, & Yarom, 2008).
The consequences of diffuse cerebellar damage for motor function are devastating. The patient loses the ability to control precisely the direction, force, velocity, and amplitude of movements and the ability to adapt patterns of motor output to changing conditions. It is difficult to maintain steady postures (e.g., standing), and attempts to do so frequently lead to tremor. There are also severe disturbances in balance, gait, speech, and the control of eye movement. Learning new motor sequences is particularly difficult (Shin & Ivry, 2003; Thach & Bastian, 2004).
The traditional view that the function of the cerebellum is limited to the fine-tuning (see Apps & Garwicz, 2005) and learning of motor responses has been challenged. This challenge has been based on functional brain images of activity in the cerebellums of healthy volunteers recorded while they performed a variety of non-motor cognitive tasks (see Strick, Dum, & Fiez, 2009), from the documentation of cognitive deficits in patients with cerebellar damage (e.g., Fabbro et al., 2004; Hoppenbrouwers et al., 2008), and from the demonstrated connection of the cerebellum with cognitive areas such as the prefrontal cortex (Ramnani, 2006). Various alternative theories to the traditional view have been proposed, but the most parsimonious of them tend to argue that the cerebellum functions in the fine-tuning and learning of cognitive responses in the same way that it functions in the fine-tuning and learning of motor responses (e.g., Doya, 2000).
Clinical Implications
Basal Ganglia
The basal ganglia do not contain as many neurons as the cerebellum, but in one sense they are more complex. Unlike the cerebellum, which is organized systematically in lobes, columns, and layers, the basal ganglia are a complex heterogeneous collection of interconnected nuclei.
The anatomy of the basal ganglia suggests that, like the cerebellum, they perform a modulatory function (see Kreitzer, 2009). They contribute few fibers to descending motor pathways; instead, they are part of neural loops that receive cortical input from various cortical areas and transmit it back to the cortex via the thalamus (see McHaffie et al., 2005; Smith et al., 2004). Many of these loops carry signals to and from the motor areas of the cortex (see Nambu, 2008).
Simulate Major Pathways of the Basal Ganglia www.mypsychlab.com
Theories of basal ganglia function have evolved in much the same way that theories of cerebellar function have changed. The traditional view of the basal ganglia was that they, like the cerebellum, play a role in the modulation of motor output. Now, the basal ganglia are thought to be involved in a variety of cognitive functions in addition to their role in the modulation of motor output (see Graybiel, 2005; Graybiel & Saka, 2004; Strick, 2004). This expanded view of the function of the basal ganglia is consistent with the fact that they project to cortical areas known to have cognitive functions (e.g., prefrontal lobes).
In experiments on laboratory animals, the basal ganglia have been shown to participate in learning to respond correctly in order to obtain reward and avoid punishment, a type of response learning that is often acquired gradually, trial by trial (see Joshua, Adler, & Bergman, 2009; Surmeier, Plotkin, & Shen, 2009). However, the basal ganglia’s cognitive functions do not appear to be limited to this form of response learning (e.g., Ravizza & Ivry, 2001).
Scan Your Brain
Are you ready to continue your descent into the sensorimotor circuits of the spinal cord? This is a good place for you to pause to scan your brain to evaluate your knowledge of the sensorimotor circuits of the cortex, cerebellum, and basal ganglia by completing the following statements. The correct answers are provided at the end of the exercise. Before proceeding, review material related to your incorrect answers and omissions.
1. Visual, auditory, and somatosensory input converges on the ______ association cortex.
2. A small area of frontal cortex called the frontal ______ plays a major role in the control of eye movement.
3. Contralateral neglect is often associated with large lesions of the right ______ lobe.
4. The ______ prefrontal cortex seems to play an important role in initiating complex voluntary responses.
5. The secondary motor area that is just dorsal to the pre-motor cortex and is largely hidden from view on the medial surface of each hemisphere is the ______.
6. Most of the direct sensory input to the supplementary motor area comes from the ______ system.
7. Most of the direct sensory input to the premotor cortex comes from the ______ system.
8. The ______ cortex is the main point of departure of motor signals from the cerebral cortex to lower levels of the sensorimotor hierarchy.
9. The foot area of the motor homunculus is in the ______ fissure.
10. Although the ______ constitutes only 10% of the mass of the brain, it contains more than half of the brain’s neurons.
11. The ______ are part of neural loops that receive input from various cortical areas and transmit it back to various areas of motor cortex via the thalamus.
12. Although both are considered to be motor structures, damage to the ______ or the ______ produces cognitive deficits.
Scan Your Brain answers:
(1) posterior parietal,
(2) eye field,
(3) parietal,
(4) dorsolateral,
(5) supplementary motor area,
(6) somatosensory,
(7) visual,
(8) primary motor,
(9) longitudinal,
(10) cerebellum,
(11) basal ganglia,
(12) cerebellum; basal ganglia.
8.6 Descending Motor Pathways
Neural signals are conducted from the primary motor cortex to the motor neurons of the spinal cord over four different pathways. Two pathways descend in the dorsolateral region of the spinal cord, and two descend in the ventromedial region of the spinal cord. Signals conducted over these pathways act together in the control of voluntary movement (see Iwaniuk & Whishaw, 2000). Like a large company, the sensorimotor system does not work well unless there are good lines of communication from the executive level (the cortex) to the office personnel (the spinal motor circuits) and workers (the muscles).
Dorsolateral Corticospinal Tract and Dorsolateral Corticorubrospinal Tract
One group of axons that descends from the primary motor cortex does so through the medullary pyramids—two bulges on the ventral surface of the medulla—then decussates and continues to descend in the contralateral dorsolateral spinal white matter. This group of axons constitutes the dorsolateral corticospinal tract . Most notable among its neurons are the Betz cells —extremely large pyramidal neurons of the primary motor cortex.
Most axons of the dorsolateral corticospinal tract synapse on small interneurons of the spinal gray matter, which synapse on the motor neurons of distal muscles of the wrist, hands, fingers, and toes. Primates and the few other mammals (e.g., hamsters and raccoons) that are capable of moving their digits independently have dorsolateral corticospinal tract neurons that synapse directly on digit motor neurons (see Porter & Lemon, 1993).
A second group of axons that descends from the primary motor cortex synapses in the red nucleus of the midbrain. The axons of neurons in the red nucleus then decussate and descend through the medulla, where some of them terminate in the nuclei of the cranial nerves that control the muscles of the face. The rest continue to descend in the dorsolateral portion of the spinal cord. This pathway is called the dorsolateral corticorubrospinal tract (rubro refers to the red nucleus). The axons of the dorsolateral corticorubrospinal tract synapse on interneurons that in turn synapse on motor neurons that project to the distal muscles of the arms and legs.
The two divisions of the dorsolateral motor pathway—the direct dorsolateral corticospinal tract and the indirect dorsolateral corticorubrospinal tract—are illustrated schematically in Figure 8.8.
FIGURE 8.8 The two divisions of the dorsolateral motor pathway: the dorsolateral corticospinal tract and the dorsolateral corticorubrospinal tract. The projections from only one hemisphere are shown.
Ventromedial Corticospinal Tract and Ventromedial Cortico-brainstem-spinal Tract
Just as there are two major divisions of the dorsolateral motor pathway, one direct (the corticospinal tract) and one indirect (the corticorubrospinal tract), there are two major divisions of the ventromedial motor pathway, one direct and one indirect. The direct ventromedial pathway is the ventromedial corticospinal tract , and the indirect one—as you might infer from its cumbersome but descriptive name—is the ventromedial cortico-brainstem-spinal tract .
The long axons of the ventromedial corticospinal tract descend ipsilaterally from the primary motor cortex directly into the ventromedial areas of the spinal white matter. As each axon of the ventromedial corticospinal tract descends, it branches diffusely and innervates the interneuron circuits in several different spinal segments on both sides of the spinal gray matter.
The ventromedial cortico-brainstem-spinal tract comprises motor cortex axons that feed into a complex network of brain stem structures. The axons of some of the neurons in this complex brain stem motor network then descend bilaterally in the ventromedial portion of the spinal cord. Each side carries signals from both hemispheres, and each neuron synapses on the interneurons of several different spinal cord segments that control the proximal muscles of the trunk and limbs.
Which brain stem structures interact with the ventromedial cortico-brainstem-spinal tract? There are four major ones: (1) the tectum , which receives auditory and visual information about spatial location; (2) the vestibular nucleus , which receives information about balance from receptors in the semicircular canals of the inner ear; (3) the reticular formation , which, among other things, contains motor programs that regulate complex species-typical movements such as walking, swimming, and jumping; and (4) the motor nuclei of the cranial nerves that control the muscles of the face.
The two divisions of the descending ventromedial pathway—the direct ventromedial corticospinal tract and the indirect ventromedial cortico-brainstem-spinal tract—are illustrated in Figure 8.9 on page 206.
Comparison of the Two Dorsolateral Motor Pathways and the Two Ventromedial Motor Pathways
The descending dorsolateral and ventromedial pathways are similar in that each is composed of two major tracts, one whose axons descend directly to the spinal cord and another whose axons synapse in the brain stem on neurons that in turn descend to the spinal cord. However, the two dorsolateral tracts differ from the two ventromedial tracts in two major respects:
• The two ventromedial tracts are much more diffuse. Many of their axons innervate interneurons on both sides of the spinal gray matter and in several different segments, whereas the axons of the two dorsolateral tracts terminate in the contralateral half of one spinal cord segment, sometimes directly on a motor neuron.
• The motor neurons that are activated by the two ventromedial tracts project to proximal muscles of the trunk and limbs (e.g., shoulder muscles), whereas the motor neurons that are activated by the two dorsolateral tracts project to distal muscles (e.g., finger muscles).
Because all four of the descending motor tracts originate in the cerebral cortex, all are presumed to mediate voluntary movement; however, major differences in their routes and destinations suggest that they have different functions. This difference was first demonstrated in two experiments on monkeys that were reported by Lawrence and Kuypers in 1968.
Evolutionary Perspective
In their first experiment, Lawrence and Kuypers (1968a) transected (cut through) the left and right dorsolateral corticospinal tracts of their subjects in the medullary pyramids, just above the decussation of the tracts. Following surgery, these monkeys could stand, walk, and climb quite normally; however, their ability to use their limbs for other activities was impaired. For example, their reaching movements were weak and poorly directed, particularly in the first few days following the surgery. Although there was substantial improvement in the monkeys’ reaching ability over the ensuing weeks, two other deficits remained unabated. First, the monkeys never regained the ability to move their fingers independently of one another; when they picked up pieces of food, they did so by using all of their fingers as a unit, as if they were glued together. And second, they never regained the ability to release objects from their grasp; as a result, once they picked up a piece of food, they often had to root for it in their hand like a pig rooting for truffles in the ground. In view of this latter problem, it is remarkable that they had no difficulty releasing their grasp on the bars of their cage when they were climbing. This point is important because it shows that the same response performed in different contexts can be controlled by different parts of the central nervous system. The point is underlined by the finding that some patients can stretch otherwise paralyzed limbs when they yawn (Provine, 2005).
In their second experiment, Lawrence and Kuypers (1968b) made additional transections in the monkeys whose dorsolateral corticospinal tracts had already been transected in the first experiment. The dorsolateral corticorubrospinal tract was transected in one group of these monkeys. The monkeys could stand, walk, and climb after this second transection; but when they were sitting, their arms hung limply by their sides (remember that monkeys normally use their arms for standing and walking). In those few instances in which the monkeys did use an arm for reaching, they used it like a rubber-handled rake—throwing it out from the shoulder and using it to draw small objects of interest back along the floor.
FIGURE 8.9 The two divisions of the ventromedial motor pathway: the ventromedial corticospinal tract and the ventromedial cortico-brainstem-spinal tract. The projections from only one hemisphere are shown.
The other group of monkeys in the second experiment had both of their ventromedial tracts transected. In contrast to the first group, these subjects had severe postural abnormalities: They had great difficulty walking or sitting. If they did manage to sit or stand without clinging to the bars of their cages, the slightest disturbance, such as a loud noise, frequently made them fall. Although they had some use of their arms, the additional transection of the two ventromedial tracts eliminated their ability to control their shoulders. When they fed, they did so with elbow and whole-hand movements while their upper arms hung limply by their sides.
What do these experiments tell us about the roles of the various descending sensorimotor tracts in the control of primate movement? They suggest that the two ventromedial tracts are involved in the control of posture and whole-body movements (e.g., walking and climbing) and that they can exert control over the limb movements involved in such activities. In contrast, both dorsolateral tracts—the corticospinal tract and the corticorubrospinal tract—control the movements of the limbs. This redundancy was presumably the basis for the good recovery of limb movement after the initial lesions of the corticospinal dorsolateral tract. However, only the corticospinal division of the dorsolateral system is capable of mediating independent movements of the digits.
8.7 Sensorimotor Spinal Circuits
We have descended the sensorimotor hierarchy to its lowest level: the spinal circuits and the muscles they control. Psychologists, including me, tend to be brain-oriented, and they often think of the spinal cord motor circuits as mere cables that carry instructions from the brain to the muscles. If you think this way, you will be surprised: The motor circuits of the spinal cord show considerable complexity in their functioning, independent of signals from the brain (see Grillner & Jessell, 2009). Again, the business metaphor helps put this in perspective: Can the office workers (spinal circuits) and workers (muscles) of a company function effectively when all of the executives and branch managers are at a convention in Hawaii? Of course they can—and the sensorimotor spinal circuits are also capable of independent functioning.
Muscles
Motor units are the smallest units of motor activity. Each motor unit comprises a single motor neuron and all of the individual skeletal muscle fibers that it innervates (see Figure 8.10). When the motor neuron fires, all the muscle fibers of its unit contract together. Motor units differ appreciably in the number of muscle fibers they contain; the units with the fewest fibers—those of the fingers and face—permit the highest degree of selective motor control.
A skeletal muscle comprises hundreds of thousands of threadlike muscle fibers bound together in a tough membrane and attached to a bone by a tendon. Acetylcholine, which is released by motor neurons at neuromuscular junctions , activates the motor end-plate on each muscle fiber and causes the fiber to contract. Contraction is the only method that muscles have for generating force, thus any muscle can generate force in only one direction. All of the motor neurons that innervate the fibers of a single muscle are called its motor pool .
Watch
Muscle Contraction www.mypsychlab.com
Although it is an oversimplification (see Gollnick & Hodgson, 1986), skeletal muscle fibers are often considered to be of two basic types: fast and slow. Fast muscle fibers, as you might guess, are those that contract and relax quickly. Although they are capable of generating great force, they fatigue quickly because they are poorly vascularized (have few blood vessels, which gives them a pale color). In contrast, slow muscle fibers, although slower and weaker, are capable of more sustained contraction because they are more richly vascularized (and hence much redder). Each muscle has both fast and slow fibers—the fast muscle fibers participate in quick movements such as jumping, whereas the slow muscle fibers participate in gradual movements such as walking. Because each muscle can apply force in only one direction, joints that move in more than one direction must be controlled by more than one muscle. Many skeletal muscles belong unambiguously to one of two categories: flexors or extensors. Flexors act to bend or flex a joint, and extensors act to straighten or extend it. Figure 8.11 on page 208 illustrates the biceps and triceps—the flexor and extensor, respectively, of the elbow joint. Any two muscles whose contraction produces the same movement, be it flexion or extension, are said to be synergistic muscles ; those that act in opposition, like the biceps and the triceps, are said to be antagonistic muscles .
To understand how muscles work, it is important to realize that they are elastic, rather than inflexible and cablelike. If you think of an increase in muscle tension as being analogous to an increase in the tension of an elastic band joining two bones, you will appreciate that muscle contraction can be of two types. Activation of a muscle can increase the tension that it exerts on two bones without shortening and pulling them together; this is termed isometric contraction . Or it can shorten and pull them together; this is termed dynamic contraction . The tension in a muscle can be increased by increasing the number of neurons in its motor pool that are firing, by increasing the firing rates of those that are already firing, or more commonly by a combination of these two changes.
FIGURE 8.10 An electron micrograph of a motor unit: a motor neuron (pink) and the muscle fibers that it innervates.
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