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Cerebellum and Basal Ganglia Contributions to Overall Motor Control
In addition to areas in the cerebral cortex that stimulate muscle contraction, two other brain structures are essential for normal motor function—the cerebellum and the basal ganglia . Neither of these structures can control muscle function by itself. Instead, these structures always function in association with other systems of motor control.
The cerebellum plays major roles in timing of motor activities and in rapid, smooth progression from one muscle movement to the next. It also helps control the intensity of muscle contraction when the muscle load changes and controls the necessary instantaneous interplay between agonist and antagonist muscle groups.
The basal ganglia help plan and control complex patterns of muscle movement. They control relative intensities of the separate movements, directions of movements, and sequencing of multiple successive and parallel movements to achieve specific complicated motor goals. This chapter explains the basic functions of the cerebellum and basal ganglia and discusses the overall brain mechanisms for achieving intricate coordination of total motor activity.
The Cerebellum and its Motor Functions
The cerebellum, illustrated in Figures 57-1 and 57-2 , has long been called a silent area of the brain, principally because electrical excitation of the cerebellum does not cause any conscious sensation and rarely causes any motor movement. Removal of the cerebellum, however, causes body movements to become highly abnormal. The cerebellum is especially vital during rapid muscular activities such as running, typing, playing the piano, and even talking. Loss of this area of the brain can cause almost total lack of coordination of these activities, even though its loss does not cause paralysis of any muscles.
How can the cerebellum be so important when it has no direct ability to cause muscle contraction? The answer is that it helps sequence and monitor motor activities and makes corrective adjustments while activities are being executed so that they will conform to the motor signals directed by the cerebral motor cortex and other parts of the brain.
The cerebellum receives continuously updated information about the desired sequence of muscle contractions from the brain motor control areas; it also receives continuous sensory information from the peripheral parts of the body, giving sequential changes in the status of each part of the body—its position, rate of movement, forces acting on it, and so forth. The cerebellum then compares the actual movements as depicted by the peripheral sensory feedback information with the movements intended by the motor system. If the two do not compare favorably, then instantaneous subconscious corrective signals are transmitted back into the motor system to increase or decrease the levels of activation of specific muscles.
The cerebellum also aids the cerebral cortex in planning the next sequential movement a fraction of a second in advance while the current movement is still being executed, thus helping the person to progress smoothly from one movement to the next. Also, it learns by its mistakes. If a movement does not occur exactly as intended, the cerebellar circuit learns to make a stronger or weaker movement the next time. To make this adjustment, changes occur in the excitability of appropriate cerebellar neurons, thus bringing subsequent muscle contractions into better correspondence with the intended movements.
Anatomical and Functional Areas of the Cerebellum
Anatomically, the cerebellum is divided into three lobes by two deep fissures, as shown in Figures 57-1 and 57-2 : (1) the anterior lobe, (2) the posterior lobe, and (3) the flocculonodular lobe. The flocculonodular lobe is the oldest portion of the cerebellum; it developed along with (and functions with) the vestibular system in controlling body equilibrium, as discussed in Chapter 56 .
Longitudinal Functional Divisions of the Anterior and Posterior Lobes
From a functional point of view, the anterior and posterior lobes are organized not by lobes but along the longitudinal axis, as demonstrated in Figure 57-2 , which shows a posterior view of the human cerebellum after the lower end of the posterior cerebellum has been rolled downward from its normally hidden position. Note, down the center of the cerebellum, a narrow band called the vermis, which is separated from the remainder of the cerebellum by shallow grooves. Most cerebellar control functions for muscle movements of the axial body , neck , shoulders , and hips are located in this area.
To each side of the vermis is a large, laterally protruding cerebellar hemisphere; each of these hemispheres is divided into an intermediate zone and a lateral zone. The intermediate zone of the hemisphere is concerned with controlling muscle contractions in the distal portions of the upper and lower limbs, especially the hands, fingers, feet, and toes. The lateral zone of the hemisphere operates at a much more remote level because this area joins with the cerebral cortex in the overall planning of sequential motor movements. Without this lateral zone, most discrete motor activities of the body lose their appropriate timing and sequencing and therefore become uncoordinated, as we discuss more fully later in this chapter.
Topographical Representation of the Body in the Vermis and Intermediate Zones
In the same manner that the cerebral sensory cortex, motor cortex, basal ganglia, red nuclei, and reticular formation all have topographical representations of the different parts of the body, so does the vermis and intermediate zones of the cerebellum. Figure 57-3 shows two such representations. Note that the axial portions of the body lie in the vermis part of the cerebellum, whereas the limbs and facial regions lie in the intermediate zones. These topographical representations receive afferent nerve signals from all the respective parts of the body, as well as from corresponding topographical motor areas in the cerebral cortex and brain stem. In turn, they send motor signals back to the same respective topographical areas of the cerebral motor cortex, as well as to topographical areas of the red nucleus and reticular formation in the brain stem.
Note that the large lateral portions of the cerebellar hemispheres do not have topographical representations of the body. These areas of the cerebellum receive input signals almost exclusively from the cerebral cortex, especially the premotor areas of the frontal cortex, and from the somatosensory and other sensory association areas of the parietal cortex. This connectivity with the cerebral cortex allows the lateral portions of the cerebellar hemispheres to play important roles in planning and coordinating the body’s rapid sequential muscular activities that occur one after another within fractions of a second.
Neuronal Circuit of the Cerebellum
The human cerebellar cortex is actually a large folded sheet, about 17 centimeters wide by 120 centimeters long, with the folds lying crosswise, as shown in Figures 57-2 and 57-3 . Each fold is called a folium. Lying deep beneath the folded mass of cerebellar cortex are deep cerebellar nuclei.
Input Pathways to the Cerebellum
Afferent Pathways From Other Parts of the Brain
The basic input pathways to the cerebellum are shown in Figure 57-4 . An extensive and important afferent pathway is the corticopontocerebellar pathway, which originates in the cerebral motor and premotor cortices and also in the cerebral somatosensory cortex. It passes by way of the pontile nuclei and pontocerebellar tracts mainly to the lateral divisions of the cerebellar hemispheres on the opposite side of the brain from the cerebral areas.
In addition, important afferent tracts originate in each side of the brain stem. These tracts include the following: (1) an extensive olivocerebellar tract, which passes from the inferior olive to all parts of the cerebellum and is excited in the olive by fibers from the cerebral motor cortex, basal ganglia, widespread areas of the reticular formation, and spinal cord; (2) vestibulocerebellar fibers, some of which originate in the vestibular apparatus itself and others from the brain stem vestibular nuclei, with almost all of these fibers terminating in the flocculonodular lobe and fastigial nucleus of the cerebellum; and (3) reticulocerebellar fibers, which originate in different portions of the brain stem reticular formation and terminate in the midline cerebellar areas (mainly in the vermis).
Afferent Pathways From the Periphery.
The cerebellum also receives important sensory signals directly from the peripheral parts of the body, mainly through four tracts on each side, two of which are located dorsally in the cord and two ventrally. The two most important of these tracts are shown in Figure 57-5 , the dorsal spinocerebellar tract and the ventral spinocerebellar tract. The dorsal tract enters the cerebellum through the inferior cerebellar peduncle and terminates in the vermis and intermediate zones of the cerebellum on the same side as its origin. The ventral tract enters the cerebellum through the superior cerebellar peduncle, but it terminates in both sides of the cerebellum.
The signals transmitted in the dorsal spinocerebellar tracts come mainly from the muscle spindles and to a lesser extent from other somatic receptors throughout the body, such as Golgi tendon organs, large tactile receptors of the skin, and joint receptors. All these signals apprise the cerebellum of the momentary status of (1) muscle contraction, (2) degree of tension on the muscle tendons, (3) positions and rates of movement of the parts of the body, and (4) forces acting on the surfaces of the body.
The ventral spinocerebellar tracts receive much less information from the peripheral receptors. Instead, they are excited mainly by motor signals arriving in the anterior horns of the spinal cord from (1) the brain through the corticospinal and rubrospinal tracts and (2) the internal motor pattern generators in the cord itself. Thus, this ventral fiber pathway tells the cerebellum which motor signals have arrived at the anterior horns; this feedback is called the efference copy of the anterior horn motor drive.
The spinocerebellar pathways can transmit impulses at velocities up to 120 m/sec, which is the most rapid conduction in any pathway in the central nervous system. This speed is important for instantaneous apprisal of the cerebellum of changes in peripheral muscle actions.
In addition to signals from the spinocerebellar tracts, signals are transmitted into the cerebellum from the body periphery through the spinal dorsal columns to the dorsal column nuclei of the medulla and are then relayed to the cerebellum. Likewise, signals are transmitted up the spinal cord through the spinoreticular pathway to the reticular formation of the brain stem and also through the spino-olivary pathway to the inferior olivary nucleus. Signals are then relayed from both of these areas to the cerebellum. Thus, the cerebellum continually collects information about the movements and positions of all parts of the body even though it is operating at a subconscious level.
Output Signals From the Cerebellum
Deep Cerebellar Nuclei and the Efferent Pathways
Located deep in the cerebellar mass on each side are three deep cerebellar nuclei— the dentate, interposed, and fastigial. (The vestibular nuclei in the medulla also function in some respects as if they were deep cerebellar nuclei because of their direct connections with the cortex of the flocculonodular lobe.) All the deep cerebellar nuclei receive signals from two sources: (1) the cerebellar cortex and (2) the deep sensory afferent tracts to the cerebellum.
Each time an input signal arrives in the cerebellum, it divides and goes in two directions: (1) directly to one of the cerebellar deep nuclei and (2) to a corresponding area of the cerebellar cortex overlying the deep nucleus. Then, a fraction of a second later, the cerebellar cortex relays an inhibitory output signal to the deep nucleus. Thus, all input signals that enter the cerebellum eventually end in the deep nuclei in the form of initial excitatory signals followed a fraction of a second later by inhibitory signals. From the deep nuclei, output signals leave the cerebellum and are distributed to other parts of the brain.
The general plan of the major efferent pathways leading out of the cerebellum is shown in Figure 57-6 and consists of the following pathways:
- 1.
A pathway that originates in the midline structures of the cerebellum (the vermis ) and then passes through the fastigial nuclei into the medullary and pontile regions of the brain stem. This circuit functions in close association with the equilibrium apparatus and brain stem vestibular nuclei to control equilibrium, as well as in association with the reticular formation of the brain stem to control the postural attitudes of the body. It was discussed in detail in Chapter 56 in relation to equilibrium.
- 2.
A pathway that originates in (1) the intermediate zone of the cerebellar hemisphere and then passes through (2) the interposed nucleus to (3) the ventrolateral and ventroanterior nuclei of the thalamus and then to (4) the cerebral cortex to (5) several midline structures of the thalamus and then to (6) the basal ganglia and (7) the red nucleus and reticular formation of the upper portion of the brain stem. This complex circuit mainly helps coordinate the reciprocal contractions of agonist and antagonist muscles in the peripheral portions of the limbs, especially in the hands, fingers, and thumbs.
- 3.
A pathway that begins in the cerebellar cortex of the lateral zone of the cerebellar hemisphere and then passes to the dentate nucleus, next to the ventrolateral and ventroanterior nuclei of the thalamus, and, finally, to the cerebral cortex. This pathway plays an important role in helping coordinate sequential motor activities initiated by the cerebral cortex.
Functional Unit of the Cerebellar Cortex—the Purkinje and Deep Nuclear Cells
The cerebellum has about 30 million nearly identical functional units, one of which is shown to the left in Figure 57-7 . This functional unit centers on a single, very large Purkinje cell and on a corresponding deep nuclear cell.
To the top and right in Figure 57-7 , the three major layers of the cerebellar cortex are shown: the molecular layer, Purkinje cell layer, and granule cell layer. Beneath these cortical layers, in the center of the cerebellar mass, are the deep cerebellar nuclei that send output signals to other parts of the nervous system.
Neuronal Circuit of the Functional Unit
Also shown in the left half of Figure 57-7 is the neuronal circuit of the functional unit, which is repeated with little variation 30 million times in the cerebellum. The output from the functional unit is from a deep nuclear cell. This cell is continually under both excitatory and inhibitory influences. The excitatory influences arise from direct connections with afferent fibers that enter the cerebellum from the brain or the periphery. The inhibitory influence arises entirely from the Purkinje cell in the cortex of the cerebellum.
The afferent inputs to the cerebellum are mainly of two types, one called the climbing fiber type and the other called the mossy fiber type.
The climbing fibers all originate from the inferior olives of the medulla. There is one climbing fiber for about 5 to 10 Purkinje cells. After sending branches to several deep nuclear cells, the climbing fiber continues all the way to the outer layers of the cerebellar cortex, where it makes about 300 synapses with the soma and dendrites of each Purkinje cell. This climbing fiber is distinguished by the fact that a single impulse in it will always cause a single, prolonged (up to 1 second), peculiar type of action potential in each Purkinje cell with which it connects, beginning with a strong spike and followed by a trail of weakening secondary spikes. This action potential is called the complex spike.
The mossy fibers are all the other fibers that enter the cerebellum from multiple sources—the higher brain, brain stem, and spinal cord. These fibers also send collaterals to excite the deep nuclear cells. They then proceed to the granule cell layer of the cortex, where they also synapse with hundreds to thousands of granule cells. In turn, the granule cells send extremely small axons, less than 1 micrometer in diameter, up to the molecular layer on the outer surface of the cerebellar cortex. Here the axons divide into two branches that extend 1 to 2 millimeters in each direction parallel to the folia. Many millions of these parallel nerve fibers exist because there are some 500 to 1000 granule cells for every 1 Purkinje cell. It is into this molecular layer that the dendrites of the Purkinje cells project and 80,000 to 200,000 of the parallel fibers synapse with each Purkinje cell.
The mossy fiber input to the Purkinje cell is quite different from the climbing fiber input because the synaptic connections are weak, so large numbers of mossy fibers must be stimulated simultaneously to excite the Purkinje cell. Furthermore, activation usually takes the form of a much weaker, short-duration Purkinje cell action potential called a simple spike, rather than the prolonged complex action potential caused by climbing fiber input.
Purkinje Cells and Deep Nuclear Cells Fire Continuously Under Normal Resting Conditions
One characteristic of both Purkinje cells and deep nuclear cells is that normally both of them fire continuously; the Purkinje cell fires at about 50 to 100 action potentials per second, and the deep nuclear cells fire at much higher rates. Furthermore, the output activity of both these cells can be modulated upward or downward.
Balance Between Excitation and Inhibition at the Deep Cerebellar Nuclei
Referring again to the circuit of Figure 57-7 , note that direct stimulation of the deep nuclear cells by both the climbing and the mossy fibers excites them. By contrast, signals arriving from the Purkinje cells inhibit them. Normally, the balance between these two effects is slightly in favor of excitation so that under quiet conditions, output from the deep nuclear cell remains relatively constant at a moderate level of continuous stimulation.
In execution of a rapid motor movement, the initiating signal from the cerebral motor cortex or brain stem at first greatly increases deep nuclear cell excitation. Then, another few milliseconds later, feedback inhibitory signals from the Purkinje cell circuit arrive. In this way, there is first a rapid excitatory signal sent by the deep nuclear cells into the motor output pathway to enhance the motor movement, followed within another small fraction of a second by an inhibitory signal. This inhibitory signal resembles a “delay line” negative feedback signal of the type that is effective in providing damping. That is, when the motor system is excited, a negative feedback signal occurs after a short delay to stop the muscle movement from overshooting its mark. Otherwise, oscillation of the movement would occur.
Basket Cells and Stellate Cells Cause Lateral Inhibition of Purkinje Cells in the Cerebellum
In addition to the deep nuclear cells, granule cells, and Purkinje cells, two other types of neurons are located in the cerebellum— basket cells and stellate cells , which are inhibitory cells with short axons. Both the basket cells and the stellate cells are located in the molecular layer of the cerebellar cortex, lying among and stimulated by the small parallel fibers. These cells in turn send their axons at right angles across the parallel fibers and cause lateral inhibition of adjacent Purkinje cells, thus sharpening the signal in the same manner that lateral inhibition sharpens contrast of signals in many other neuronal circuits of the nervous system.
Turn-On/Turn-Off and Turn-Off/Turn-On Output Signals From the Cerebellum
The typical function of the cerebellum is to help provide rapid turn-on signals for the agonist muscles and simultaneous reciprocal turn-off signals for the antagonist muscles at the onset of a movement. Then, on approaching termination of the movement, the cerebellum is mainly responsible for timing and executing the turn-off signals to the agonists and the turn-on signals to the antagonists. Although the exact details are not fully known, one can speculate from the basic cerebellar circuit of Figure 57-7 how this process might work, as follows.
Let us suppose that the turn-on/turn-off pattern of agonist/antagonist contraction at the onset of movement begins with signals from the cerebral cortex. These signals pass through noncerebellar brain stem and cord pathways directly to the agonist muscle to begin the initial contraction.
At the same time, parallel signals are sent by way of the pontile mossy fibers into the cerebellum. One branch of each mossy fiber goes directly to deep nuclear cells in the dentate or other deep cerebellar nuclei, which instantly sends an excitatory signal back into the cerebral corticospinal motor system, either by way of return signals through the thalamus to the cerebral cortex or by way of neuronal circuitry in the brain stem, to support the muscle contraction signal that had already been begun by the cerebral cortex. As a consequence, the turn-on signal, after a few milliseconds, becomes even more powerful than it was at the start because it becomes the sum of both the cortical and the cerebellar signals. This effect is the normal effect when the cerebellum is intact, but in the absence of the cerebellum, the secondary extra supportive signal is missing. This cerebellar support makes the turn-on muscle contraction much stronger than it would be if the cerebellum did not exist.
Now, what causes the turn-off signal for the agonist muscles at the termination of the movement? Remember that all mossy fibers have a second branch that transmits signals by way of the granule cells to the cerebellar cortex and, eventually, by way of “parallel” fibers, to the Purkinje cells. The Purkinje cells in turn inhibit the deep nuclear cells. This pathway passes through some of the smallest, slowest-conducting nerve fibers in the nervous system—that is, the parallel fibers of the cerebellar cortical molecular layer, which have diameters of only a fraction of a millimeter. Also, the signals from these fibers are weak, so they require a finite period to build up enough excitation in the dendrites of the Purkinje cell to excite it. However, once the Purkinje cell is excited, it sends a strong inhibitory signal to the same deep nuclear cell that had originally turned on the movement. Therefore, this signal helps turn off the movement after a short time.
Thus, one can see how the complete cerebellar circuit could cause a rapid turn-on agonist muscle contraction at the beginning of a movement and yet also cause a precisely timed turn-off of the same agonist contraction after a given period.
Now, let us speculate on the circuit for the antagonist muscles. Most important, remember that there are reciprocal agonist-antagonist circuits throughout the spinal cord for virtually every movement that the cord can initiate. Therefore, these circuits are part of the basis for antagonist turn-off at the onset of movement and then turn-on at termination of movement, mirroring whatever occurs in the agonist muscles. But also remember that the cerebellum contains several other types of inhibitory cells besides Purkinje cells. The functions of some of these cells are still to be determined; they, too, could play roles in the initial inhibition of the antagonist muscles at onset of a movement and subsequent excitation at the end of a movement.
These mechanisms are still partly speculation. They are presented here to illustrate ways by which the cerebellum could cause exaggerated turn-on and turn-off signals, thus controlling the agonist and antagonist muscles, as well as the timing.
The Purkinje Cells “Learn” to Correct Motor Errors—Role of the Climbing Fibers
The degree to which the cerebellum supports onset and offset of muscle contractions, as well as timing of contractions, must be learned by the cerebellum. Typically, when a person first performs a new motor act, the degree of motor enhancement by the cerebellum at the onset of contraction, the degree of inhibition at the end of contraction, and the timing of these are almost always incorrect for precise movements. However, after the act has been performed many times, the individual events become progressively more precise, sometimes requiring only a few movements before the desired result is achieved, but at other times requiring hundreds of movements.
How do these adjustments come about? The exact answer is not known, although it is known that sensitivity levels of cerebellar circuits progressively adapt during the training process, especially the sensitivity of the Purkinje cells to respond to the granule cell excitation. Furthermore, this sensitivity change is brought about by signals from the climbing fibers entering the cerebellum from the inferior olivary complex.
Under resting conditions, the climbing fibers fire about once per second, but they cause extreme depolarization of the entire dendritic tree of the Purkinje cell, lasting for up to 1 second, each time they fire. During this time, the Purkinje cell fires with one initial strong output spike, followed by a series of diminishing spikes. When a person performs a new movement for the first time, feedback signals from the muscle and joint proprioceptors will usually denote to the cerebellum how much the actual movement fails to match the intended movement, and the climbing fiber signals alter the long-term sensitivity of the Purkinje cells in some way. Over a period, this change in sensitivity, along with other possible “learning” functions of the cerebellum, is believed to make the timing and other aspects of cerebellar control of movements approach perfection. When this state has been achieved, the climbing fibers no longer need to send “error” signals to the cerebellum to cause further change.
Function of the Cerebellum in Overall Motor Control
The nervous system uses the cerebellum to coordinate motor control functions at three levels:
- 1.
The vestibulocerebellum. This level consists principally of the small flocculonodular cerebellar lobes that lie under the posterior cerebellum and adjacent portions of the vermis. It provides neural circuits for most of the body’s equilibrium movements.
- 2.
The spinocerebellum. This level consists of most of the vermis of the posterior and anterior cerebellum plus the adjacent intermediate zones on both sides of the vermis. It provides the circuitry for coordinating mainly movements of the distal portions of the limbs, especially the hands and fingers.
- 3.
The cerebrocerebellum. This level consists of the large lateral zones of the cerebellar hemispheres, lateral to the intermediate zones. It receives virtually all its input from the cerebral motor cortex and adjacent premotor and somatosensory cortices of the cerebrum. It transmits its output information in the upward direction back to the brain, functioning in a feedback manner with the cerebral cortical sensorimotor system to plan sequential voluntary body and limb movements. These movements are planned as much as tenths of a second in advance of the actual movements. This process is called development of “motor imagery” of movements to be performed.
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