The motor systems of the brain and spinal cord work together to control body movements. These systems must fulfill diverse tasks because the functions of the muscles of the body differ markedly. Consider, for instance, the fine control required of hand muscles when grasping a china cup, in contrast to the gross strength required of back and leg muscles in lifting a box full of books. The muscles that move the eyes have an entirely different set of tasks, such as positioning the eyes to capture information from the visual world. The principal function of facial muscles is not movement but rather creating facial expressions as well as assisting in speech articulation. These jobs are so varied that it is not surprising that the motor systems have many specific components devoted to the control of different motor functions.

The motor systems are clinically very important because damage, depending on the location and severity, produces wide-ranging impairments in strength, volition, and coordination. Profound weakness or paralysis after a stroke or a traumatic spinal cord injury limits an injured person’s capacity for independence. Disordered coordination can make control of simple daily activities, such as tying a knot or eating, impossible, requiring assistance by a care giver. Defective eye movement control can impair cognitive functions, such as a reduced capacity to read text or discern the locations of objects.

The first three motor system chapters examine the components that connect higher control centers with motor neurons; these motor systems are essential for contracting muscle. This chapter focuses on the neuroanatomy of descending spinal cord pathways and spinal cord circuits for limb control and posture. Next, the pathways that control facial and other head muscles are discussed in Chapter 11. Because eye movement and balance share many neural circuits, and a strong interrelationship with the vestibular system, these topics are covered jointly in Chapter 12. The last two motor system chapters focus on important behavioral integrative centers: the cerebellum (Chapter 13) and basal ganglia (Chapter 14).

Diverse Central Nervous System Structures Comprise the Motor Systems

Four separate components of the central nervous system together control skeletal muscles of the limbs and trunk muscles for posture (Figure 10–2): (1) descending motor pathways, together with their associated origins in the cerebral cortex and brain stem, (2) spinal motor circuits, including motor neurons and interneurons, (3) basal ganglia (yellow), and (4) the cerebellum.

The regions of the cerebral cortex and brain stem that contribute to the descending motor pathways are organized much like the ascending sensory pathways but in reverse: from the cerebral cortex or brain stem to the muscles of our limbs and trunk. The brain stem motor pathways engage in relatively automatic control, such as rapid postural adjustments and in-flight correction of misdirected movements. By contrast, the cortical motor pathways participate in more refined, flexible, and adaptive control, such as reaching to objects, grasping, and tool use.

Motor neurons and interneurons comprise the spinal motor circuits, the second component of the limb and posture control systems. The motor pathways synapse directly on motor neurons as well as on interneurons that in turn synapse on motor neurons (Figure 10–2). For muscles of the limbs and trunk, motor neurons and most interneurons are found in the ventral horn and intermediate zone of the spinal cord (Figure 10–3). The intermediate zone corresponds primarily to the spinal gray matter lateral to the central canal. It is sometimes included within the ventral horn. For muscles of the head, including facial muscles, the motor neurons and interneurons are located in the cranial nerve motor nuclei and the reticular formation (see Chapter 11). The spinal motor circuits are not only the target of the descending motor pathways but also can function relatively independently through their reflexive and intrinsic motor actions. The simplest reflex is the monosynaptic stretch reflex (see Figure 2–6). More complex, polysynaptic, reflexes produce automatic limb withdrawal from a noxious stimulus as well as postural reflexes. Stepping patterns are organized by spinal circuits within the lumbar and sacral segments. When motor pathways synapse directly on motor neurons, they are able to activate individual muscles and even groups of muscle fibers within a muscle. This is because a single motor neuron connects to a limited set of fibers within one muscle, termed the motor unit. By connecting to interneurons in spinal circuits, a motor pathway is able to regulate motor reflex actions. For example, if you grasp an unexpectedly hot teacup that is a family heirloom, you may not want to reflexively release your grip, for risk of breaking the cup. Activating muscles through interneurons, rather than more directly through motor neurons, may also enable a motor pathway to select a group of muscles that have a complex behavioral action, such as stepping.

The third and fourth components of the motor systems, the cerebellum and the basal ganglia (Figure 10–2; see Chapters 13 and 14), do not contain neurons that project directly to spinal motor circuits. Nevertheless, these structures have a powerful regulatory influence over motor behavior. They act indirectly in controlling motor behavior through their effects on the descending brain stem pathways and, via the thalamus, cortical pathways (Figure 10–2). The specific contributions of the cerebellum and basal ganglia to movement control are surprisingly elusive despite centuries of study. The cerebellum is part of a set of neural circuits for ensuring that movements are precise and accurate. However, this description only captures a small part of cerebellar function; the cerebellum has additional nonmotor functions. The specific contributions of the basal ganglia to normal motor action are largely unknown. However, we are well aware of how movements become disordered when the basal ganglia are damaged. For example, in patients with Parkinson disease, a neurodegenerative disease that primarily affects the basal ganglia, movements are slow or fail to be initiated and patients have significant tremor. Like the cerebellum, the basal ganglia have many nonmotor functions.

Many Cortical Regions Are Recruited Into Action During Visually Guided Movements

Myriad areas of the cerebral cortex provide the motor systems with information essential for making accurate movements. For example, during visually guided movements—such as reaching to grasp a cup—the process of translating thoughts and sensations into action begins with the initial decision to move. This process is dependent on the cortical limbic and prefrontal association areas (Figure 10–2; brain inset upper left), which are involved in emotions, motivation, and cognition. The magnocellular visual system processes visual information about the locations of objects for guiding movements. It provides the major visual input to the “where” or “action” pathway (Figure 10–2, inset; see Figures 7–15 and 7–16). The magnocellular visual system projects to the posterior parietal cortex, an area important for identifying the location of salient objects in the environment and for directing our attention to these objects (Figure 10–2). Visual information next is distributed to certain premotor areas of the frontal lobe, where the plan of action to reach for the cup is formed, a plan that specifies the path the hand takes to reach the object and prepares the hand for grasping or manipulation once contact occurs. The next step in translating into action the decision to reach is directing muscles to contract. This step primarily involves the corticospinal tract, which originates from both the premotor areas and the primary motor cortex. The corticospinal tract transmits control signals to motor neurons and to interneurons (Figure 10–2). The cortical motor pathways also recruit brain stem pathways for coordinating voluntary movements with postural adjustments (Figure 10–2), such as maintaining balance when lifting a heavy object.

Descending pathways serve a diversity of functions. We recognize their principal function in controlling movements because when they become damaged, people become weak or paralyzed. However, in addition to movement control, the descending pathways regulate somatic sensory processing and the autonomic nervous system. Whereas the motor control functions are mediated by synaptic connections with motor neurons and interneurons, the somatic sensory functions are associated with connections on dorsal horn neurons and somatic sensory relay nuclei in the brain stem. We have already learned about one somatic sensory regulatory function: the pain suppression function of the raphespinal pathway (see Chapter 5). The descending pathways synapse on preganglionic autonomic neurons in the brain stem and spinal cord to regulate autonomic nervous system functions. The autonomic control pathways are examined in Chapter 15 with the autonomic nervous system. Finally, the descending pathways also can influence plasticity of spinal circuits during motor learning and have important trophic functions during development. The pathways from the cerebral cortex serve all of these functions. It is not known if the same applies to all of the brain stem pathways. Here we will focus on the motor control functions of the descending pathways.

Multiple Parallel Motor Control Pathways Originate From the Cortex and Brain Stem

Seven major descending motor control pathways terminate in motor centers of the brain stem and spinal cord (Table 10–1). Three of these pathways originate in layer V of the cerebral cortex, primarily in the frontal lobe: (1) the lateral corticospinal tract, (2) the ventral (or anterior) corticospinal tract, and (3) the corticobulbar tract. The corticobulbar tract terminates primarily in cranial motor nuclei in the pons and medulla and is the cranial equivalent of the corticospinal tracts; it will be considered in Chapter 11. Collectively, these cortical pathways participate in controlling the most adaptive and flexible movements, such as finger control during tool use, regulating posture during limb movements, and during speech. The remaining four pathways originate from brain stem nuclei: (4) the rubrospinal tract, involved in automatic limb control; (5) the reticulospinal tracts, which participate in automatic control of proximal muscles and locomotion; (6) the tectospinal tract, involved in coordinating head movements with eye movements; and (7) the vestibulospinal tracts, critical for maintaining balance. There are also neurotransmitter-specific descending pathways (see Chapter 2) that originate from the serotonergic raphe nuclei, noradrenergic locus ceruleus, and dopaminergic neurons of the midbrain and diencephalon, all of which have extensive spinal cord terminations. In addition to pain regulation, the actions of these descending pathways can rapidly regulate both the strength of muscle contraction and reflexes.

Three Rules Govern the Logic of the Organization of the Descending Motor Pathways

How are the actions of the myriad descending motor pathways coordinated during movement? What is the logic of their organization? By taking a combined clinical, anatomical, and physiological perspective, three principles governing the organization of the motor pathways emerge.

The functional organization of the descending pathways parallels the somatotopic organization of the motor nuclei in the ventral horn

The motor neurons innervating limb muscles, and the interneurons from which they receive input, are located in the lateral ventral horn and intermediate zone. In contrast, motor neurons innervating axial and girdle muscles (ie, neck and shoulder muscles), and their associated interneurons, are located in the medial ventral horn and intermediate zone. The mediolateral somatotopic organization of the intermediate zone and ventral horn is easy to remember because it mimics the form of the body (Figure 10–3). This mediolateral somatotopic organization also applies to the location of the descending motor pathways in the spinal cord white matter (discussed in detail below). The pathways that descend in the lateral portion of the spinal cord white matter control limb muscles. In contrast, the pathways that descend in the medial portion of the white matter control axial and girdle muscles.

Movements are controlled by direct cortical projections to the spinal cord and indirect cortical projections via brain stem nuclei

The lateral and ventral corticospinal tracts can control spinal circuits through their direct spinal projections. In addition, the cortical motor regions project to brain stem nuclei that give rise to motor pathways: the red nucleus, superior colliculus, reticular formation, and vestibular nuclei. This is shown in Figure 10–2 (center) as an arrow connecting the cortical pathway with the brain stem. Thus, the cerebral cortex can also influence movements through indirect brain stem connections. For example, the two components of the corticoreticulo-spinal pathway are the cortical projection to particular nuclei of the reticular formation and then the reticulospinal tract’s projection to the spinal cord. Considering all combinations of pathways, the projection neurons of the cerebral cortex constitute the highest level in the hierarchy, the brain stem projection neurons comprise the next lower level, and spinal interneurons and the motor neurons are the two lowest levels. Study of patients with motor pathway damage suggests that direct spinal connections mediate more precise joint control, such as our ability to move one finger independent of the others (termed fractionated control), whereas the indirect connections serve coarser control functions, such as moving all fingers together during a powerful grasp. Can brain stem motor pathways work independent of the cortical pathways? The answer is probably yes, based on laboratory animal studies. Both the cerebellum and basal ganglia have direct connections with brain stem motor pathways (Figure 10–2), which could influence brain stem motor pathway function without cortex involvement. A challenge for promoting recovery of motor function after cortical damage, such as a stroke, is to foster a more effective independent brain stem control.

There are monosynaptic and polysynaptic connections between the motor pathways and motor neurons

Typically the axon of a descending projection neuron, in addition to making monosynaptic connections with motoneurons, makes monosynaptic connections with spinal cord interneurons (Figure 10–2, bottom). Depending on the particular interneuron type (see Figure 10–16A), the interneurons have different functions. Some interneurons receive input from somatic sensory receptors for the reflex control of movement. For example, particular interneurons receive input from nociceptors and mediate limb withdrawal reflexes in response to painful stimuli, such as when you jerk your hand away from a hot stove. Other interneurons coordinate the left-right limb motor actions during walking, while others still are important for upper limb-lower limb coordination.

Two Laterally Descending Pathways Control Limb Muscles

The lateral corticospinal tract and the rubrospinal tract (Table 10–1) are the two lateral motor pathways. The neurons that give rise to these pathways have a somatotopic organization. Moreover, the lateral corticospinal and rubrospinal tracts control muscles on the contralateral side of the body. The lateral corticospinal tract is the principal motor control pathway in humans. A lesion of this pathway anywhere along its path to the motor neuron, such as after a stroke in the subcortical white matter, produces devastating and persistent limb use impairments. Box 10–1 discusses the major changes in motor and reflex function after damage to the corticospinal system. There is a loss of voluntary control and weakness, termed paresis, that can make standing impossible without assistance. There is also a loss of the ability to move one finger independent of the others, which is termed fractionation. Manual dexterity depends on fractionation, without which hand movements are clumsy and imprecise. Loss of corticospinal tract control after stroke is the most common cause of paralysis.

The major site of origin of the lateral corticospinal tract is the primary motor cortex (Figure 10–4A, inset), although axons also originate from the premotor cortical regions (supplementary motor area, cingulate motor area, and premotor cortex) and the somatic sensory cortical areas. Descending axons in the tract that originate in the primary motor cortex course within the cerebral hemisphere in the posterior limb of the internal capsule and, in the midbrain, in the basis pedunculi (Figure 10–4A). Next on its descending course, the tract disappears beneath the ventral surface of the pons only to reappear on the ventral surface of the medulla as the pyramid. At the junction of the spinal cord and medulla, most of the axons decussate (pyramidal decussation) and descend in the dorsolateral portion of the lateral column of the spinal cord white matter; hence the name lateral corticospinal tract (see Figure 10–6). This pathway terminates primarily in the lateral portions of the intermediate zone and ventral horn of the cervical and lumbosacral cord, the locations of neurons that control the arm and leg. There is a contingent of axons that terminate on the ipsilateral side. Some of these axons descend ipsilaterally; many descend contralaterally and re-cross the midline within the gray matter in lamina 10. The function of these ipsilateral axons is not well understood. After unilateral damage to the lateral corticospinal tract, the ipsilaterally terminating axons may contribute to recovery of some motor functions.