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Our movements feature at least two categories of action. First we decide on a goal and choose how to achieve it; then we make moves to reach the goal. As we move, we correct movement errors that direct us away from our goal. What are the neural bases of these choices? What neural bases enable the motor system to produce our movements?

The major components of our motor system are the cerebrum (forebrain), the brainstem, and the spinal cord. The cerebrum contributes to our conscious control of movement, while the brainstem and spinal cord direct our movements. In the face of impaired brainstem or spinal cord function, the forebrain can imagine movement but can no longer produce it. Neuroprosthetic devices, described in Research Focus 11-1, can replace the movement production function provided by the brainstem and spinal cord and restore forebrain control.

Getting from decisions to movements involves most of our nervous system. Figure 11-1 illustrates the stepwise sequence your CNS performs in the seemingly simple act of directing your hand to pick up a cup. Once you decide to pick it up, you visually inspect it to determine what part to grasp. The visual cortex relays this information through the cortical somatosensory regions to the motor regions that plan and initiate the movement. Only then does the brain send instructions to the spinal cord segments that control your arm and hand muscles.

As you grasp the cup’s handle, information from sensory receptors in your fingers aids in adjusting your grip and sends information back through the spinal cord to the somatosensory cortex and from there to the motor cortex to confirm that the act is complete—you are holding the cup. Other brain regions also participate in controlling the movement. The subcortical basal ganglia help to produce the appropriate amount of force for grasping the cup handle, while in the brainstem, the cerebellum helps to regulate the movement‘s timing and accuracy.

Clearly, the movement involved in picking up a cup involves widespread CNS regions. The action also requires that higher-function regions, including those that participate in deciding to pick up the cup, work through and influence the actions of lower functional areas. Remarkably, once you have made the decision to pick up the cup, most of the movement happens automatically, without conscious control. Few of us can describe the sequence of actions or produce an accurate pantomime of the movements we make to actually grasp a cup, although we have performed the action thousands of times (Davarpanah et al., 2015). To understand how these CNS regions work together to produce decisions and movements, we now consider the major components of the hierarchy, starting with the forebrain.

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Complex movements consist of many acts. Consider playing basketball. At every moment, players must make decisions and perform actions. Dribble, pass, and shoot are different movement categories, and each can be performed in many ways. Skilled players choose among the categories effortlessly and blend them together seemingly without thought.

An early explanation for control of such complex movements centered on feedback: after we act, we wait for feedback about how well the action has succeeded, then make the next movement. The pioneering neuroscience researcher Karl Lashley (1951), in the article “The Problem of Serial Order in Behavior,” found fault with this explanation.

Lashley realized that we perform skilled movements too quickly to rely on feedback about one movement before shaping the next. The time required to receive feedback about the first movement combined with the time needed to develop a plan for the subsequent movement and send a corresponding message to muscles is simply too long for effective action. Lashley argued that movements must be performed as motor sequences, with the next sequence held in readiness while the ongoing one is under way.

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All complex behaviors, including speaking, piano playing, and playing basketball, require selecting and executing multiple movements as sequences. Most of our motor learning is mastering sequences of action. As one sequence is being executed, the next sequence is being prepared to follow the first smoothly. The act of speaking illustrates this activity. When people use complex rather than simple word sequences, they are more likely to pause and make umm and ahh sounds, suggesting that it is taking them more time than usual to organize their speech sequences.

Initiating a Motor Sequence

The frontal lobes are responsible for planning and initiating motor sequences. Each frontal lobe is divided into several hierarchically arranged motor regions, including the three illustrated in Figure 11-2. From front to back, they are prefrontal cortex, premotor cortex, and primary motor cortex.

PREFRONTAL CORTEX Atop the hierarchy, the prefrontal cortex (PFC) plans our behavior. Deciding to get up at a certain hour to arrive at work on time, to stop at the library to return a book, even whether a behavior is right or wrong and whether it should be performed at all are examples. Humans with PFC injury often break social and legal rules not because they do not know the rules or the consequences of breaking them but because their decision making is faulty. The PFC does not specify the precise movements to be made. It simply specifies the goal.

PREMOTOR CORTEX To bring a plan to completion, the prefrontal cortex sends instructions to the premotor cortex, which produces movements coordinating many body parts. If the premotor cortex is damaged, sequences cannot be coordinated and goals cannot be accomplished. For example, the monkey on the right in Figure 11-3 has a lesion in the dorsal premotor cortex. The monkey has been given the task of extracting a piece of food wedged in a hole in a table (Brinkman, 1984).

The monkey simply pushes the food with a finger. The food drops to the floor and is lost. The monkey has to catch the food by holding a palm beneath the hole as the food is being pushed out. But this brain-injured animal is unable to make the two complementary movements together. It can push the food with a finger and extend an open palm, but it cannot coordinate the action of its two hands, as the healthy monkey on the left can.

PRIMARY MOTOR CORTEX The primary motor cortex (area M1) specializes in producing focal skilled movements, such as those involving our arms, hands, and mouth. To understand its role, consider the rich array of movements we can use to grasp objects. We can hold them in one hand, with two hands, or between a hand and another part of the body, for example.

In using the precision pincer grip (Figure 11-4A), we hold an object between the thumb and index finger. We can perform many precision grips using the thumb and other fingers in opposition. The pincer grip not only allows us to pick up small objects easily but also allows us to use them with considerable skill. In contrast, when using a power grip (Figure 11-4B), we hold an object with all our fingers and with more power but fewer skilled options.

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People with damage to M1 have difficulty performing reaching movements and shaping their fingers correctly to perform various hand grasps. They also have difficulty performing many skilled hand, arm, and trunk movements, bringing the hand to the mouth to feed themselves, and in talking (Lang & Schieber, 2004).

The frontal lobe regions in each hemisphere that plan, coordinate, and execute different kinds of movements are hierarchically related. After the prefrontal cortex formulates a plan of action, it instructs the premotor cortex to organize the appropriate movement sequence, which the primary motor cortex executes. This hierarchy is supported by findings from studies of cerebral blood flow, which serves as an indicator of neural activity. Figure 11-5 shows the brain regions that were active as the participants in one such study performed various tasks (Roland, 1993).

As the participants use a finger to push a lever, increased blood flow is limited to the primary somatosensory and primary motor cortex (Figure 11-5A). As the participants execute a sequence of finger movements, blood flow also increases in the premotor cortex (Figure 11-5B). And as the participants use a finger to trace their way through a maze, a task that requires coordinated movements in relation to a goal, blood flow increases in the prefrontal cortex as well (Figure 11-5C). Notice that as the participants were performing these tasks, relative blood flow increased most in the regions taking part in the required movements rather than throughout the frontal lobe.

In a series of studies, the Swiss neuroscientist Walter Hess (1957) found that the brainstem controls species-typical behaviors, actions displayed by every member of a species—the pecking of a robin, the hissing of a cat, or the breaching of a whale. Species-typical movements, then, are mainly innate and organized by brainstem neurons. Hess developed the technique of implanting electrodes into the brains of cats and other animals and cementing them in place. These electrodes could then be attached to stimulating leads in the freely moving animal without causing it much discomfort.

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By stimulating the brainstem, Hess elicited the innate movements that the animal might be expected to make. A resting cat could be induced to suddenly leap up with an arched back and erect hair as though frightened, for example, by an approaching dog. The elicited movements began abruptly when the stimulating current was turned on and ended equally abruptly when it was turned off. An animal performed such species-typical behaviors in a subdued manner when the stimulating current was low and displayed increased vigor as the stimulating current was turned up.

The actions varied, depending on the brainstem site that was stimulated. Stimulating some sites produced head turning; others, walking or running; and still others elicited displays of aggression or fear. The animal’s response to a particular stimulus could be modified accordingly. For instance, when shown a stuffed toy, a cat responded to electrical stimulation of some brainstem sites by stalking it and to stimulation of other sites by showing fear and withdrawing.

Hess’s experiments have been confirmed and expanded on by other researchers using many animal species. Experiment 11-1 shows the effects of brainstem stimulation on a chicken under various conditions (von Holst, 1973). Notice the effect of context: how the neural site stimulated interacts both with the object presented and with the stimulation’s duration.

With stimulation at a certain site alone, the chicken displays only restless behavior. When a fist is displayed, the same stimulation elicits slightly threatening behavior. When the display switches from a fist to a stuffed polecat, the chicken responds with vigorous threats. Finally, with continued stimulation in the presence of the polecat, the chicken flees, screeching.

Such experiments show that producing complex patterns of adaptive behavior is an important brainstem function. These adaptive patterns include movements used in eating and drinking and in sexual behavior. Animals can be induced to display survival-related behaviors when certain brainstem areas are stimulated. An animal can even be induced to eat nonfood objects, such as chips of wood, if the part of the brainstem that triggers chewing is stimulated.

Grooming illustrates how the brainstem coordinates complex action patterns (Kalueff et al., 2007). A grooming rat sits back on its haunches, licks its paws, wipes its nose with its paws, then wipes its paws across its face, and finally turns to lick the fur on its body. These movements are always performed in the same order, from the face to the shoulders and then toward the rear of the body. The next time you dry off after a shower or swimming, note the grooming sequence you use. Humans’ grooming sequence is very similar to the one rats use.

The brainstem is also important for maintaining posture, standing upright, coordinating limb movements, swimming and walking, grooming the fur, and making nests. The effects of damage to brainstem regions that organize many adaptive movements can be seen in the effects of locked-in syndrome, similar to those Scott Mackler experienced in connection with ALS (see Research Focus 11-1, Neuroprosthetics). The patient with locked-in syndrome is aware and awake but cannot move or communicate verbally because nearly all voluntary muscles except those of the eyes are completely paralyzed.

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The effects of brainstem damage on behavior can also be seen in cerebral palsy (CP), a disorder primarily of motor function: making voluntary movements becomes difficult, whereas many aspects of conscious behavior controlled by the cortex may remain intact. CP is often caused by brainstem trauma before or shortly after birth. As described in Clinical Focus 11-2, Cerebral Palsy, trauma leading to cerebral palsy can sometimes happen in early infancy as well. Advances in neuroprosthetic technology, touch-screen technology, and remotely controlled computers all signal important steps toward allowing people with physical disabilities to become more independent.

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The late actor Christopher Reeve, who portrayed Superman in three 1980s films, was thrown from a horse during a riding competition in 1995. Reeve’s spinal cord was severed near its upper end, at the C1–C2 level (see Spinal Segments and Dermatomes, page 359). The injury left Reeve’s brain intact and functioning and his remaining spinal cord intact and functioning too, but his brain and spinal cord were no longer connected.

Other than head movements and slight movement in his shoulders, Reeve’s body was completely paralyzed. He was even unable to breathe without assistance. A century ago such a severe injury would have been fatal, but modern and timely medical treatment allowed Reeve to survive for nearly a decade.

A cut high on the spinal cord, such as Christopher Reeve survived, entails paralysis and loss of sensation in the arms and legs, a condition called quadriplegia. If the cut is low, paraplegia results: paralysis and loss of sensation are confined to the legs and lower body, as described in Clinical Focus 11-3, Spinal Cord Injury, on page 365. Christopher Reeve and his late wife Dana founded the Christopher and Dana Reeve Foundation for spinal cord research. It is dedicated to improving the life and function of spinal cord–injured people and also to searching for cures for spinal cord injury.

Far from being a mere relay between the body and brain, the spinal cord contains complex motor programs. A spinal cord patient can walk on a conveyor belt if the body is supported. Indeed, Christopher Reeve was able to walk in a swimming pool, where his body was supported by water.

When a spinal cord patient’s leg is moved backward on a conveyor belt, causing the foot to lose support, the limb reflexively lifts off the belt and swings forward underneath the body. As the foot touches the surface of the belt again, tactile receptors initiate the reflex that causes the foot to push against the surface and support the body’s weight. In this way, several spinal reflexes work together to facilitate the complex movement of walking. Walking’s reflexive organization can even be obtained in a premature or newborn baby: when held upright, the baby will perform stepping movements.

Among the complex reflexes observed in other vertebrates is the scratch reflex. Here, an animal reflexively scratches a part of its body in response to a stimulus from the body surface. The complexity of the scratch reflex is revealed in the movement’s accuracy. Absent the brain’s direction, the tip of a limb, usually a hind limb in a quadruped, can be correctly directed to the irritated body part. Typically, itching is the sensation that elicits scratching; it is likely that the sensory receptors on the skin that produce itch evolved for detecting parasites and other foreign objects. We return to itching sensations in Section 11-4.

In humans and other animals with a severed spinal cord, spinal reflexes still function, even though the spinal cord is cut off from communication with the brain. As a result, the paralyzed limbs may display spontaneous movements or spasms. But the brain can no longer guide the timing of these movements. Consequently, reflexes related to bladder and bowel control may have to be artificially stimulated by caregivers.

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A Hierarchy of Movement Control

Before you continue, check your understanding.

Question 1

Question 2

Question 3

Question 4

Question 5

Answers appear in the Self Test section of the book.