Cortical and Brain Stem Control of Motor Function

Most “voluntary” movements initiated by the cerebral cortex are achieved when the cortex activates “patterns” of function stored in lower brain areas—the cord, brain stem, basal ganglia, and cerebellum. These lower centers, in turn, send specific control signals to the muscles.

For a few types of movements, however, the cortex has almost a direct pathway to the anterior motor neurons of the cord, bypassing some motor centers on the way. This is especially true for control of the fine dexterous movements of the fingers and hands. This chapter and Chapter 57 explain the interplay among the different motor areas of the brain and spinal cord to provide overall synthesis of voluntary motor function.

Motor Cortex and Corticospinal Tract

Figure 56-1 shows the functional areas of the cerebral cortex. Anterior to the central cortical sulcus, occupying approximately the posterior one third of the frontal lobes, is the motor cortex. Posterior to the central sulcus is the somatosensory cortex (an area discussed in detail in earlier chapters), which feeds the motor cortex many of the signals that initiate motor activities.

Figure 56-1, Motor and somatosensory functional areas of the cerebral cortex. The numbers 4, 5, 6, and 7 are Brodmann’s cortical areas, as explained in Chapter 48 .

The motor cortex is divided into three subareas, each of which has its own topographical representation of muscle groups and specific motor functions: (1) the primary motor cortex; (2) the premotor area; and (3) the supplementary motor area.

Primary Motor Cortex

The primary motor cortex, shown in Figure 56-1 , lies in the first convolution of the frontal lobes anterior to the central sulcus. It begins laterally in the sylvian fissure, spreads superiorly to the uppermost portion of the brain, and then dips deep into the longitudinal fissure. (This area is the same as area 4 in Brodmann’s classification of the brain cortical areas, shown in Figure 48-5 .)

Figure 56-1 lists the approximate topographical representations of the different muscle areas of the body in the primary motor cortex, beginning with the face and mouth region near the sylvian fissure; the arm and hand area, in the midportion of the primary motor cortex; the trunk, near the apex of the brain; and the leg and foot areas, in the part of the primary motor cortex that dips into the longitudinal fissure. This topographical organization is demonstrated even more graphically in Figure 56-2 , which shows the degrees of representation of the different muscle areas as mapped by Penfield and Rasmussen. This mapping was done by electrically stimulating the different areas of the motor cortex in human beings were undergoing neurosurgery. Note that more than half of the entire primary motor cortex is concerned with controlling the muscles of the hands and the muscles of speech. Point stimulation in these hand and speech motor areas on rare occasion causes contraction of a single muscle, but most often, stimulation contracts a group of muscles. To express this in another way, excitation of a single motor cortex neuron usually excites a specific movement rather than one specific muscle. To do this, it excites a “pattern” of separate muscles, each of which contributes its own direction and strength of muscle movement.

Figure 56-2, Degree of representation of the different muscles of the body in the motor cortex.

Premotor Area

The premotor area, also shown in Figure 56-1 , lies 1 to 3 centimeters anterior to the primary motor cortex. It extends inferiorly into the sylvian fissure and superiorly into the longitudinal fissure, where it abuts the supplementary motor area, which has functions similar to those of the premotor area. The topographical organization of the premotor cortex is roughly the same as that of the primary motor cortex, with the mouth and face areas located most laterally; as one moves upward, the hand, arm, trunk, and leg areas are encountered.

Nerve signals generated in the premotor area cause much more complex “patterns” of movement than the discrete patterns generated in the primary motor cortex. For example, the pattern may be to position the shoulders and arms so that the hands are properly oriented to perform specific tasks. To achieve these results, the most anterior part of the premotor area first develops a “motor image” of the total muscle movement that is to be performed. Then, in the posterior premotor cortex, this image excites each successive pattern of muscle activity required to achieve the image. This posterior part of the premotor cortex sends its signals either directly to the primary motor cortex to excite specific muscles or, often, by way of the basal ganglia and thalamus back to the primary motor cortex.

A special class of neurons called mirror neurons becomes active when a person performs a specific motor task or when he or she observes the same task performed by others. Thus, the activity of these neurons “mirrors” the behavior of another person as though the observer was performing the specific motor task. Brain imaging studies indicate that these neurons transform sensory representations of acts that are heard or seen into motor representations of these acts. Many neurophysiologists believe that these mirror neurons may be important for understanding the actions of other people and for learning new skills by imitation. Thus, the premotor cortex, basal ganglia, thalamus, and primary motor cortex constitute a complex overall system for the control of complex patterns of coordinated muscle activity.

Supplementary Motor Area

The supplementary motor area has yet another topographical organization for the control of motor function. It lies mainly in the longitudinal fissure but extends a few centimeters onto the superior frontal cortex. Contractions elicited by stimulating this area are often bilateral rather than unilateral. For example, stimulation frequently leads to bilateral grasping movements of both hands simultaneously; these movements are perhaps rudiments of the hand functions required for climbing. In general, this area functions in concert with the premotor area to provide body-wide attitudinal movements, fixation movements of the different segments of the body, positional movements of the head and eyes, and so forth, as background for the finer motor control of the arms and hands by the premotor area and primary motor cortex.

Some Specialized Areas of Motor Control Found in the Human Motor Cortex

A few highly specialized motor regions of the human cerebral cortex (shown in Figure 56-3 ) control specific motor functions. These regions have been localized either by electrical stimulation or by noting the loss of motor function when destructive lesions occur in specific cortical areas. Some of the more important regions are described in the following sections.

Figure 56-3, Representation of the different muscles of the body in the motor cortex and location of other cortical areas responsible for specific types of motor movements.

Broca’s Area (Motor Speech Area)

Figure 56-3 shows a premotor area labeled “word formation” lying immediately anterior to the primary motor cortex and immediately above the sylvian fissure. This region is called Broca’s area. Damage to it does not prevent a person from vocalizing but makes it impossible for the person to speak whole words rather than uncoordinated utterances or an occasional simple word such as “no” or “yes.” A closely associated cortical area also causes appropriate respiratory function, so respiratory activation of the vocal cords can occur simultaneously with the movements of the mouth and tongue during speech. Thus, the premotor neuronal activities related to speech are highly complex.

“Voluntary” Eye Movement Field

In the premotor area immediately above Broca’s area is a locus for controlling voluntary eye movements. Damage to this area prevents a person from voluntarily moving the eyes toward different objects. Instead, the eyes tend to lock involuntarily onto specific objects, an effect controlled by signals from the occipital visual cortex, as explained in Chapter 52 . This frontal area also controls eyelid movements such as blinking.

Head Rotation Area

Slightly higher in the motor association area, electrical stimulation elicits head rotation. This area is closely associated with the eye movement field; it directs the head toward different objects.

Area for Hand Skills

In the premotor area immediately anterior to the primary motor cortex for the hands and fingers is a region that is important for “hand skills.” That is, when tumors or other lesions cause destruction in this area, hand movements become uncoordinated and nonpurposeful, a condition called motor apraxia.

Transmission of Signals From the Motor Cortex to the Muscles

Motor signals are transmitted directly from the cortex to the spinal cord through the corticospinal tract and indirectly through multiple accessory pathways that involve the basal ganglia, cerebellum, and various nuclei of the brain stem. In general, the direct pathways are concerned with discrete and detailed movements, especially of the distal segments of the limbs, particularly the hands and fingers.

Corticospinal (Pyramidal) Tract

The most important output pathway from the motor cortex is the corticospinal tract, also called the pyramidal tract, shown in Figure 56-4 . The corticospinal tract originates about 30% from the primary motor cortex, 30% from the premotor and supplementary motor areas, and 40% from the somatosensory areas posterior to the central sulcus.

After leaving the cortex, it passes through the posterior limb of the internal capsule (between the caudate nucleus and the putamen of the basal ganglia) and then downward through the brain stem, forming the pyramids of the medulla. Most of the pyramidal fibers then cross in the lower medulla to the opposite side and descend into the lateral corticospinal tracts of the cord, finally terminating principally on the interneurons in the intermediate regions of the cord gray matter. A few terminate on sensory relay neurons in the dorsal horn, and a very few terminate directly on the anterior motor neurons that cause muscle contraction.

A few of the fibers do not cross to the opposite side in the medulla but pass ipsilaterally down the cord in the ventral corticospinal tracts. Many, if not most, of these fibers eventually cross to the opposite side of the cord either in the neck or in the upper thoracic region. These fibers may be concerned with control of bilateral postural movements by the supplementary motor cortex.

The most impressive fibers in the pyramidal tract are a population of large myelinated fibers with a mean diameter of 16 micrometers. These fibers originate from giant pyramidal cells, called Betz cells, that are found only in the primary motor cortex. The Betz cells are about 60 micrometers in diameter, and their fibers transmit nerve impulses to the spinal cord at a velocity of about 70 m/sec, the most rapid rate of transmission of any signals from the brain to the cord. There are about 34,000 of these large Betz cell fibers in each corticospinal tract. The total number of fibers in each corticospinal tract is more than 1 million, so these large fibers represent only 3% of the total. The other 97% are mainly fibers smaller than 4 micrometers in diameter that conduct background tonic signals to the motor areas of the cord.

Other Fiber Pathways From the Motor Cortex

The motor cortex gives rise to large numbers of additional, mainly small fibers that go to deep regions in the cerebrum and brain stem, including the following:

1.
The axons from the giant Betz cells send short collaterals back to the cortex. These collaterals are believed to inhibit adjacent regions of the cortex when the Betz cells discharge, thereby “sharpening” the boundaries of the excitatory signal.
2.
A large number of fibers pass from the motor cortex into the caudate nucleus and putamen. From there, additional pathways extend into the brain stem and spinal cord, as discussed in the next chapter, mainly to control body postural muscle contractions.
3.
A moderate number of motor fibers pass to red nuclei of the midbrain . From these nuclei, additional fibers pass down the cord through the rubrospinal tract.
4.
A moderate number of motor fibers deviate into the reticular substance and vestibular nuclei of the brain stem; from there, signals go to the cord via reticulospinal and vestibulospinal tracts, and others go to the cerebellum via reticulocerebellar and vestibulocerebellar tracts.
5.
A tremendous number of motor fibers synapse in the pontile nuclei, which give rise to the pontocerebellar fibers, carrying signals into the cerebellar hemispheres.
6.
Collaterals also terminate in the inferior olivary nuclei, and from there, secondary olivocerebellar fibers transmit signals to multiple areas of the cerebellum.

Thus, the basal ganglia, brain stem, and cerebellum all receive strong motor signals from the corticospinal system every time a signal is transmitted down the spinal cord to cause a motor activity.

Incoming Sensory Fiber Pathways to the Motor Cortex

The functions of the motor cortex are controlled mainly by nerve signals from the somatosensory system but also, to some degree, from other sensory systems such as hearing and vision. Once the sensory information is received, the motor cortex operates in association with the basal ganglia and cerebellum to excite appropriate motor actions. The more important incoming fiber pathways to the motor cortex are the following:

1.
Subcortical fibers from adjacent regions of the cerebral cortex, especially from (a) the somatosensory areas of the parietal cortex, (b) the adjacent areas of the frontal cortex anterior to the motor cortex, and (c) the visual and auditory cortices.
2.
Subcortical fibers that arrive through the corpus callosum from the opposite cerebral hemisphere. These fibers connect corresponding areas of the cortices in the two sides of the brain.
3.
Somatosensory fibers that arrive directly from the ventrobasal complex of the thalamus. These fibers relay mainly cutaneous tactile signals and joint and muscle signals from the peripheral body.
4.
Tracts from the ventrolateral and ventroanterior nuclei of the thalamus, which in turn receive signals from the cerebellum and basal ganglia. These tracts provide signals that are necessary for coordination among the motor control functions of the motor cortex, basal ganglia, and cerebellum.
5.
Fibers from the intralaminar nuclei of the thalamus. These fibers control the general level of excitability of the motor cortex in the same way they control the general level of excitability of most other regions of the cerebral cortex.

The Red Nucleus Serves as an Alternative Pathway for Transmitting Cortical Signals to the Spinal Cord

The red nucleus, located in the mesencephalon, functions in close association with the corticospinal tract. As shown in Figure 56-5 , it receives a large number of direct fibers from the primary motor cortex through the corticorubral tract, as well as branching fibers from the corticospinal tract as it passes through the mesencephalon. These fibers synapse in the lower portion of the red nucleus, the magnocellular portion, which contains large neurons similar in size to the Betz cells in the motor cortex. These large neurons then give rise to the rubrospinal tract, which crosses to the opposite side in the lower brain stem and follows a course immediately adjacent and anterior to the corticospinal tract into the lateral columns of the spinal cord.

Figure 56-5, The corticorubrospinal pathway for motor control, also showing the relation of this pathway to the cerebellum.

The rubrospinal fibers terminate mostly on the interneurons of the intermediate areas of the cord gray matter, along with the corticospinal fibers, but some of the rubrospinal fibers terminate directly on anterior motor neurons, along with some corticospinal fibers. The red nucleus also has close connections with the cerebellum, similar to the connections between the motor cortex and the cerebellum.

The Corticorubrospinal System Is an Accessory Pathway for Transmitting Relatively Discrete Signals From the Motor Cortex to the Spinal Cord

The magnocellular portion of the red nucleus has a somatographic representation of all the muscles of the body, as does the motor cortex. Therefore, stimulation of a single point in this portion of the red nucleus causes contraction of either a single muscle or a small group of muscles. However, the fineness of representation of the different muscles is far less developed than in the motor cortex, especially in human beings, who have relatively small red nuclei.

The corticorubrospinal pathway serves as an accessory route for transmission of relatively discrete signals from the motor cortex to the spinal cord. When the corticospinal fibers are destroyed but the corticorubrospinal pathway is intact, discrete movements can still occur, except that the movements for fine control of the fingers and hands are considerably impaired. Wrist movements are still functional, which is not the case when the corticorubrospinal pathway is also blocked.

Therefore, the pathway through the red nucleus to the spinal cord is associated with the corticospinal system. Furthermore, the rubrospinal tract lies in the lateral columns of the spinal cord, along with the corticospinal tract, and terminates on the interneurons and motor neurons that control the more distal muscles of the limbs. Therefore, the corticospinal and rubrospinal tracts together are called the lateral motor system of the cord, in contradistinction to a vestibuloreticulospinal system, which lies mainly medially in the cord and is called the medial motor system of the cord, as discussed later in this chapter .

Excitation of the Spinal Cord Motor Control Areas by the Primary Motor Cortex and Red Nucleus

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