Sections include: movement and the brain, movement disorders, keeping the body moving
Reaching for a pencil, grasping a doorknob, skiing, and tightrope walking—to name but a few physical actions—all involve well-coordinated movements made with well-balanced postures. In fact, whenever we move the three basic functions of movement, balance, and coordination work in concert to produce graceful, purposeful motions of body parts. This is actually quite a feat, because moving is a complex process.
We hardly ever contract just a single muscle; practically all of our body motions involve several muscles working in sequence or at once. For example, walking is produced by contracting all the muscles of the legs in different intensities and at different times. Similarly, reaching movements are produced by contracting all the muscles of the arm, while grasping and manipulating objects require contracting many muscles of the forearm, hand, and fingers. In all of these cases, the result is a well-coordinated movement—that is, a movement of a body part that actually consists of many movements of joints, occurring in proper sequence and of appropriate extent, such that the resulting motion is smooth, straight, and directed to the object of interest. This is the essence of coordination, which applies equally well to muscles, joints, and whole body parts. Coordination is the essence of motor skill.
Superimposed on coordinated movements is our sense of body balance, which helps us keep our posture against gravity. In fact, the presence of gravity calls for balance. In outer space, there is no gravity and therefore no need to balance; bodies just float. Compared with other animals, humans face the especially difficult challenge of balancing on just two feet with a narrow base. Yet it is common for us not only to stand upright easily and apparently effortlessly, but also to perform many other actions—walking, reaching, dancing, and chewing gum—while keeping our balance.
We are hardly aware of how we maintain balance and coordinate our movements except when we learn new tasks. But it takes time and effort for a baby to learn to stand upright, to reach, and to manipulate objects. We usually forget the frustration and false steps of our early years. But later in life, we might learn the steps of riding a bicycle, driving a car, using a computer or other complex machine, and, for a lucky few, walking a tightrope! For the first time in years, we are forced to think about how we are moving and to train ourselves to move and balance in new ways. It is no wonder that learning these tasks can make us feel as helpless as children.
We learn most of our basic motor skills during infancy and childhood and acquire different and more refined motor skills throughout adult life. Highly coordinated movements develop gradually from simpler components. For example, infants’ reaching movements are initially composed of small, sequential, poorly coordinated movements. These smaller movements gradually disappear and are replaced by larger movements that ultimately lead to smooth, single- component reaching movements several months after birth. Similarly, our initial unsteady toddling leads gradually to well-coordinated locomotion, and the same holds for other motor skills (manipulating objects, coloring within the lines, dressing, tying shoelaces, and so on). Most motor skills require good body balance, which develops gradually during the first year of life. Parents recognize the importance of balance when they applaud their babies’ first steps. The most important factors in uniting movement, balance, and coordination into a tightly linked whole are practice, practice, and practice.
Movement and the Brain
Two important breakthroughs in understanding how our brains control movement occurred in the second half of the nineteenth century. First, John Hughlings Jackson in England made astute observations on how epileptics suffered seizures that spread in patterns through their bodies. He inferred that there must be a corresponding, orderly pattern in the brain representing movements of those various body parts. Then Fritsch and Hitzig in Germany were able to evoke movements in dogs’ body parts by electrically stimulating the motor cortex area of their brains. Later, neurosurgeons evoked similar movements in humans who were awake on the operating table.
Scientists gradually developed valuable insights into the brain mechanisms behind coordination and balance, and specifically into the role of the cerebellum in these functions, from observing humans in clinics and performing experiments with animals. There is still much to learn; research teams are actively investigating and debating the neural mechanisms underlying voluntary movement, coordination, and balance.
It is no exaggeration to say that most of our brain is geared toward action. Vast areas in the cerebral cortex, the basal ganglia, the cerebellum, the brain stem, and the spinal cord are intimately connected and cooperate in initiating, producing, and controlling coordinating movements and maintaining body balance. Although damage to a specific area usually results in a distinguishable motor disorder, practically all of these areas interact in controlling motor function.
Here’s how this system works when you set out to perform a particular movement, such as walking across a room. To begin with, the nerve cells in your body are always sending your brain information about your position: standing or sitting, with your weight on one foot or distributed across both, and so on. Based on that information, your brain sends the appropriate messages to your muscles to step forward with one leg. These signals descend through the spinal cord and out the nerves to the proper muscles. Some muscles cause joints to flex, some cause them to extend, all in coordinated fashion. One leg moves while the other adjusts to maintain your balance.
Your spinal cord is not simply a conduit for these signals but a highly complex structure that organizes signals from the body’s periphery (muscles, skin, joints) and center (brain). While spinal motor functions move your body, spinal sensory functions mediate not only the common sensations, such as touch, pain, temperature, and position, but also such powerful sensations as ticklishness, itchiness, bladder and bowel urgency, and muscle weariness. Autonomic functions control most of our internal organs, including bladder and bowel contractions, digestion, heart rate, breathing, and blood pressure.
The spinal cord is shorter than most people think. It starts at the base of the skull and stops a few inches below the lowest rib, typically a distance of about 20 inches in an average-size adult. The cord is encased in a tough membranous sheath called the dura, which also holds the cerebrospinal fluid that bathes the cord. The spinal cord lies in the middle of a marvelous articulated bony structure called the spine, which has 8 neck or cervical segments, 12 chest or thoracic segments, 5 lumbar segments, and 5 tail or sacral segments.
The spinal cord is connected to the body through nerve roots. Two pairs of these roots emerge from the spinal cord at each segmental level and run through openings in the spinal column between vertebral segments. Nerve fibers, or axons, that go from the spinal cord to muscles originate from motoneurons situated in the spinal cord and emerge from the cord in the roots toward the front. Axons carrying sensation to the spinal cord all come from a collection of nerve cells or ganglia adjacent to the spinal cord.
The neurons in the dorsal (toward the back) root ganglia are by far the largest cells known. These neurons send axons out to the body and to the spinal cord. The peripheral end of the neuron may go to the toes. The other end enters the cord and goes all the way to the brain stem. Depending on your height, the neurons may be several feet long. Large axons carry position and touch sensations while smaller fibers carry pain and temperature sensations.
About 20 million nerve fibers, or axons, are packed inside the human spinal cord. Some axons go from one part of the spinal cord to another while others connect the brain to the cord and vice versa. The spinal cord also contains many nerve cells that connect to muscles, called motoneurons (see figure above). Other spinal neurons receive signals from the brain or sensory signals from the body and then relay these signals to other neurons in the spinal cord or brain. These spinal neurons mediate reflexes and even complex behaviors such as walking. In addition, they filter incoming sensory signals and differentiate between normal and painful sensations. In fact, the spinal cord can support walking without the brain’s involvement past the initial impulse to walk.
One major source of information sent to the spinal motor centers is our antigravity system: the brain stem and those muscles in the neck, around the spine, and in the legs (the gastrocnemius muscles) that resist the downward pull of gravity. We depend on this system to maintain our upright posture. The antigravity nuclei in the brain stem mediate the reflexes that adjust our body position or muscle contraction slightly so that we do not fall. The cerebellum is heavily involved in this function; indeed, it is a cardinal sign of damage to the part of the cerebellum that controls balance when a person cannot stand upright, especially with feet close together and eyes closed.
Traditionally, we regard the motor cortex as the key structure in initiating movement. Our elaborate use of hands and fingers to manipulate objects seems to depend clearly on intact motor cortical function. Following a stroke affecting the motor cortex or the signals from it, a person’s hands usually remain clumsy after most other functions have recovered. At one point, neuroscientists thought that specific areas of the motor cortex controlled the contraction and relaxation of different muscles, just as specific areas of the sensory cortex organized the signals arriving from corresponding parts of the body. More recent research has shown that tightly interconnected neurons extend down in columns from the top layer of the motor cortex. Each column seems to control a group of related muscles.
Furthermore, many other areas of the brain are also involved in starting movements. The cellular mechanisms that promote motor function have been studied in detail by recording the impulse activity of single neurons in monkeys as they perform various motor tasks. Some clear results have emerged from such studies. First, many motor areas in the brain are activated concurrently when a movement is initiated, including the motor and premotor cortex, the cerebellum, and the basal ganglia. Therefore, generating a movement involves many competing areas, not just the motor cortex.
Furthermore, these studies have found that within a particular area individual cells can be more or less active during various movements (for example, moving an arm left, moving an arm right, and so on). Those movements therefore depend on the concurrent activation of groups of cells. Which cells in an area are activated determines the direction, speed, force, and other aspects of the movement. Finally, cell activity in various motor areas is almost constantly modulated by other factors, such as sensory stimuli in the environment and attention. Positronemission tomography (PET) and functional magnetic resonance imaging (fMRI) studies of human brains have corroborated these findings and provided valuable information on how different areas are involved in motor function. This field is expanding quickly, and more discoveries will doubtless be forthcoming.
Movement Disorders
Losing too many neurons from the motor structures results in obvious disease. For example, severe alcoholism can lead to atrophy of the anterior lobe of the cerebellum and difficulty walking. Parkinson’s disease involves loss of dopamine-producing neurons in the substantia nigra; it creates a variety of movement abnormalities such as akinesia (difficulty in initiating movement), rigidity, and tremor. Widespread cortical atrophy and a variety of motor defects characterize Alzheimer’s disease.
The most obvious motor deficits result from serious brain damage and are hard to miss. For example, strokes often result in paralysis and spasticity of the arm and leg on one side (hemiplegia) due to damage of the internal capsule— the tract connecting the motor cortical outflow to the brain stem, cerebellum, and spinal cord. Lesions in certain areas in the parietal or frontal cortex can affect motor functions more insidiously. Such lesions frequently result in apraxias of various kinds, meaning difficulty in performing motor skills a person has already learned (for example, how to dress, strike a match, and so on), copying shapes from a template or from memory, assembling objects from component parts, or imitating common gestures. Such difficulties will not become apparent until the person is asked to perform these actions.
Disorders of coordination and balance are more commonly the result of cerebellar damage. These diseases can interfere with the fine-tuning of muscular movement and result in coarse, uncoordinated movement. This type of condition is called ataxia and is easily seen in a person’s jerky to-and-fro motion of the trunk and unsteady gait.
A very different class of motor abnormality comprises involuntary movements that a person cannot stop. These include:
■ rather innocuous tics
■ slow, writhing, well-coordinated movements of the arm (choreo-athetosis)
■ rhythmic movements of the fingers (resting tremor)
■ involuntary movements of the face and mouth (tardive dyskinesia)
■ wildly violent, throwing arm movements (hemiballismus)
Characteristically, almost all of these abnormal movements are the result of problems in the normal functioning of the basal ganglia. Some can be overlooked, while others intrude on people’s lives.
Finally another motor disorder, called dystonia, shows up as a continuous contraction of certain muscle groups, resulting in steady, strange, abnormal postures (for example, torticollis). These disorders, unlike those discussed in the previous paragraph, can hardly be missed. They are always present, and are very difficult to treat or alleviate.
In all cases of motor dysfunction, a person should consult with a neurologist about treatment.
Keeping the Body Moving
No two people are exactly alike, and that truism certainly applies to our movements. Indeed, how we move is an essential part of our individuality. That said, movement, coordination, and balance are very similar in men and women, as shown by how both sexes can excel in all kinds of sports. Women and men also share a common fate in that their motor systems change similarly with age. We may become wiser as we get older, but our motor skills tend to deteriorate. Movements take longer to start, they are slower and not as well coordinated, and keeping our balance becomes harder. These are manifestations of the cumulative loss of neurons with age.
To keep the motor system in good shape, it is important to live a healthy life. For example, a balanced, low-fat diet and regular exercise help reduce the risk of atherosclerosis and therefore the risk of strokes. People with high cholesterol, high triglycerides, or various lipid disorders should receive appropriate treatment for these problems. No one who wants to keep his or her brain healthy should smoke.
Exercise can add healthy and active years to your life. It is never too late to start exercising, and even small improvements in physical fitness can significantly raise the quality of your life. Resistance training is important because it is the only form of exercise that can slow and even reverse the decline in muscle mass, bone density, and strength. Adding workouts that focus on speed and agility are also useful, while exercises that increase flexibility can help reduce the stiffness and loss of balance that accompanies aging. Just as practice in our early years is the best way to make our movements smoother and more coordinated, practice helps us maintain those skills as long as possible.
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