Cerebellum PDF

UNIT XI The Nervous System: C. Motor and Integrative Neurophysiology Dentate nucleus Cerebellothalamocortical Molecular tract...

UNIT XI The Nervous System: C. Motor and Integrative Neurophysiology Dentate nucleus Cerebellothalamocortical Molecular tract layer Reticulum of Cortex mesencephalon Purkinje Purkinje Inhibitory cell layer cell input from To thalamus Climbing Purkinje Granule fiber cells cell layer Red nucleus Deep Granule nuclear cells Deep cell nuclei Superior cerebellar Mossy Excitatory peduncle fiber afferent inputs from Input Fastigial nucleus Input (all other afferents) climbing (inferior fibers and Fastigioreticular tract olive) mossy fibers Paleocerebellum Output Figure 57-­7 Deep nuclear cells receive excitatory and inhibitory in- puts. The left side of this figure shows the basic neuronal circuit of Figure 57-­6 Principal efferent tracts from the cerebellum. the cerebellum, with excitatory neurons shown in red and the Purkin- je cell (an inhibitory neuron) shown in black. To the right is shown the physical relationship of the deep cerebellar nuclei to the cerebellar Output Signals From the Cerebellum cortex with its three layers. Deep Cerebellar Nuclei and the Efferent Pathways. Lo- cated deep in the cerebellar mass on each side are three deep cerebellar nuclei—the dentate, interposed, and fas- portion of the brain stem. This complex circuit mainly tigial. (The vestibular nuclei in the medulla also function helps coordinate the reciprocal contractions of agonist in some respects as if they were deep cerebellar nuclei and antagonist muscles in the peripheral portions of the because of their direct connections with the cortex of the limbs, especially in the hands, fingers, and thumbs. flocculonodular lobe.) All the deep cerebellar nuclei receive 3. A pathway that begins in the cerebellar cortex of the lat- signals from two sources: (1) the cerebellar cortex and (2) eral zone of the cerebellar hemisphere and then passes the deep sensory afferent tracts to the cerebellum. to the dentate nucleus, next to the ventrolateral and Each time an input signal arrives in the cerebellum, it ventroanterior nuclei of the thalamus, and, finally, to the divides and goes in two directions: (1) directly to one of cerebral cortex. This pathway plays an important role in the cerebellar deep nuclei and (2) to a corresponding area helping coordinate sequential motor activities initiated of the cerebellar cortex overlying the deep nucleus. Then, by the cerebral cortex. a fraction of a second later, the cerebellar cortex relays an inhibitory output signal to the deep nucleus. Thus, all in- put signals that enter the cerebellum eventually end in the FUNCTIONAL UNIT OF THE CEREBELLAR deep nuclei in the form of initial excitatory signals followed CORTEX—THE PURKINJE AND DEEP a fraction of a second later by inhibitory signals. From the NUCLEAR CELLS deep nuclei, output signals leave the cerebellum and are The cerebellum has about 30 million nearly identical func- distributed to other parts of the brain. tional units, one of which is shown to the left in Figure The general plan of the major efferent pathways leading out of the cerebellum is shown in Figure 57-­6 and consists 57-­7. This functional unit centers on a single, very large of the following pathways: Purkinje cell and on a corresponding deep nuclear cell. 1. A pathway that originates in the midline structures of To the top and right in Figure 57-­7, the three major the cerebellum (the vermis) and then passes through the layers of the cerebellar cortex are shown: the molecular fastigial nuclei into the medullary and pontile regions layer, Purkinje cell layer, and granule cell layer. Beneath of the brain stem. This circuit functions in close asso- these cortical layers, in the center of the cerebellar mass, ciation with the equilibrium apparatus and brain stem are the deep cerebellar nuclei that send output signals to vestibular nuclei to control equilibrium, as well as in as- other parts of the nervous system. sociation with the reticular formation of the brain stem to control the postural attitudes of the body. It was dis- Neuronal Circuit of the Functional Unit. Also shown cussed in detail in Chapter 56 in relation to equilibrium. in the left half of Figure 57-­7 is the neuronal circuit of 2. A pathway that originates in (1) the intermediate zone of the cerebellar hemisphere and then passes through the functional unit, which is repeated with little varia- (2) the interposed nucleus to (3) the ventrolateral and tion 30 million times in the cerebellum. The output from ventroanterior nuclei of the thalamus and then to (4) the functional unit is from a deep nuclear cell. This cell the cerebral cortex to (5) several midline structures of is continually under both excitatory and inhibitory influ- the thalamus and then to (6) the basal ganglia and (7) ences. The excitatory influences arise from direct connec- the red nucleus and reticular formation of the upper tions with afferent fibers that enter the cerebellum from 714 https://ebook2book.ir/  Chapter 57 Cerebellum and Basal Ganglia Contributions to Overall Motor Control the brain or the periphery. The inhibitory influence arises clear cells by both the climbing and the mossy fibers ex- entirely from the Purkinje cell in the cortex of the cerebel- cites them. By contrast, signals arriving from the Purkinje lum. cells inhibit them. Normally, the balance between these The afferent inputs to the cerebellum are mainly of two effects is slightly in favor of excitation so that under two types, one called the climbing fiber type and the other quiet conditions, output from the deep nuclear cell re- mains relatively constant at a moderate level of continu- UNIT XI called the mossy fiber type. The climbing fibers all originate from the inferior olives ous stimulation. of the medulla. There is one climbing fiber for about 5 to In execution of a rapid motor movement, the initiat- 10 Purkinje cells. After sending branches to several deep ing signal from the cerebral motor cortex or brain stem at nuclear cells, the climbing fiber continues all the way to the first greatly increases deep nuclear cell excitation. Then, outer layers of the cerebellar cortex, where it makes about another few milliseconds later, feedback inhibitory sig- 300 synapses with the soma and dendrites of each Purkinje nals from the Purkinje cell circuit arrive. In this way, there cell. This climbing fiber is distinguished by the fact that a is first a rapid excitatory signal sent by the deep nuclear single impulse in it will always cause a single, prolonged (up cells into the motor output pathway to enhance the motor to 1 second), peculiar type of action potential in each Pur- movement, followed within another small fraction of kinje cell with which it connects, beginning with a strong a second by an inhibitory signal. This inhibitory signal spike and followed by a trail of weakening secondary spikes. resembles a “delay line” negative feedback signal of the This action potential is called the complex spike. type that is effective in providing damping. That is, when The mossy fibers are all the other fibers that enter the motor system is excited, a negative feedback signal the cerebellum from multiple sources—the higher occurs after a short delay to stop the muscle movement brain, brain stem, and spinal cord. These fibers also from overshooting its mark. Otherwise, oscillation of the send collaterals to excite the deep nuclear cells. They movement would occur. then proceed to the granule cell layer of the cortex, where they also synapse with hundreds to thousands of Basket Cells and Stellate Cells Cause Lateral Inhibi- granule cells. In turn, the granule cells send extremely tion of Purkinje Cells in the Cerebellum. In addition small axons, less than 1 micrometer in diameter, up to to the deep nuclear cells, granule cells, and Purkinje cells, the molecular layer on the outer surface of the cerebel- two other types of neurons are located in the cerebel- lar cortex. Here the axons divide into two branches lum—basket cells and stellate cells, which are inhibitory that extend 1 to 2 millimeters in each direction paral- cells with short axons. Both the basket cells and the stel- lel to the folia. Many millions of these parallel nerve late cells are located in the molecular layer of the cerebel- fibers exist because there are some 500 to 1000 granule lar cortex, lying among and stimulated by the small par- cells for every 1 Purkinje cell. It is into this molecular allel fibers. These cells in turn send their axons at right layer that the dendrites of the Purkinje cells project and angles across the parallel fibers and cause lateral inhibi- 80,000 to 200,000 of the parallel fibers synapse with tion of adjacent Purkinje cells, thus sharpening the sig- each Purkinje cell. nal in the same manner that lateral inhibition sharpens The mossy fiber input to the Purkinje cell is quite dif- contrast of signals in many other neuronal circuits of the ferent from the climbing fiber input because the synaptic nervous system. connections are weak, so large numbers of mossy fibers Turn-­On/Turn-­Off and Turn-­Off/Turn-­On must be stimulated simultaneously to excite the Purkinje Output Signals From the Cerebellum cell. Furthermore, activation usually takes the form of a much weaker, short-­duration Purkinje cell action poten- The typical function of the cerebellum is to help provide tial called a simple spike, rather than the prolonged com- rapid turn-­on signals for the agonist muscles and simulta- plex action potential caused by climbing fiber input. neous reciprocal turn-­off signals for the antagonist mus- cles at the onset of a movement. Then, on approaching Purkinje Cells and Deep Nuclear Cells Fire Continu- termination of the movement, the cerebellum is mainly ously Under Normal Resting Conditions. One charac- responsible for timing and executing the turn-­off signals teristic of both Purkinje cells and deep nuclear cells is that to the agonists and the turn-­on signals to the antagonists. normally both of them fire continuously; the Purkinje cell Although the exact details are not fully known, one can fires at about 50 to 100 action potentials per second, and speculate from the basic cerebellar circuit of Figure 57-­7 the deep nuclear cells fire at much higher rates. Further- how this process might work, as follows. more, the output activity of both these cells can be modu- Let us suppose that the turn-­on/turn-­off pattern of lated upward or downward. agonist/antagonist contraction at the onset of move- ment begins with signals from the cerebral cortex. These Balance Between Excitation and Inhibition at the signals pass through noncerebellar brain stem and cord Deep Cerebellar Nuclei. Referring again to the circuit of pathways directly to the agonist muscle to begin the initial Figure 57-­7, note that direct stimulation of the deep nu- contraction. 715 https://ebook2book.ir/ UNIT XI The Nervous System: C. Motor and Integrative Neurophysiology At the same time, parallel signals are sent by way These mechanisms are still partly speculation. They of the pontile mossy fibers into the cerebellum. One are presented here to illustrate ways by which the cerebel- branch of each mossy fiber goes directly to deep lum could cause exaggerated turn-­on and turn-­off signals, nuclear cells in the dentate or other deep cerebellar thus controlling the agonist and antagonist muscles, as nuclei, which instantly sends an excitatory signal back well as the timing. into the cerebral corticospinal motor system, either by The Purkinje Cells “Learn” to Correct way of return signals through the thalamus to the cere- Motor Errors—Role of the Climbing Fibers bral cortex or by way of neuronal circuitry in the brain stem, to support the muscle contraction signal that had The degree to which the cerebellum supports onset and already been begun by the cerebral cortex. As a con- offset of muscle contractions, as well as timing of con- sequence, the turn-­on signal, after a few milliseconds, tractions, must be learned by the cerebellum. Typically, becomes even more powerful than it was at the start when a person first performs a new motor act, the degree because it becomes the sum of both the cortical and of motor enhancement by the cerebellum at the onset of the cerebellar signals. This effect is the normal effect contraction, the degree of inhibition at the end of contrac- when the cerebellum is intact, but in the absence of tion, and the timing of these are almost always incorrect the cerebellum, the secondary extra supportive signal for precise movements. However, after the act has been is missing. This cerebellar support makes the turn-­on performed many times, the individual events become muscle contraction much stronger than it would be if progressively more precise, sometimes requiring only a the cerebellum did not exist. few movements before the desired result is achieved, but Now, what causes the turn-­off signal for the agonist at other times requiring hundreds of movements. muscles at the termination of the movement? Remem- How do these adjustments come about? The exact ber that all mossy fibers have a second branch that answer is not known, although it is known that sensitivity transmits signals by way of the granule cells to the cer- levels of cerebellar circuits progressively adapt during the ebellar cortex and, eventually, by way of “parallel” fibers, training process, especially the sensitivity of the Purkinje to the Purkinje cells. The Purkinje cells in turn inhibit cells to respond to the granule cell excitation. Further- the deep nuclear cells. This pathway passes through more, this sensitivity change is brought about by signals some of the smallest, slowest-­conducting nerve fibers from the climbing fibers entering the cerebellum from the in the nervous system—that is, the parallel fibers of the inferior olivary complex. cerebellar cortical molecular layer, which have diam- Under resting conditions, the climbing fibers fire about eters of only a fraction of a millimeter. Also, the sig- once per second, but they cause extreme depolarization nals from these fibers are weak, so they require a finite of the entire dendritic tree of the Purkinje cell, lasting for period to build up enough excitation in the dendrites up to 1 second, each time they fire. During this time, the of the Purkinje cell to excite it. However, once the Pur- Purkinje cell fires with one initial strong output spike, fol- kinje cell is excited, it sends a strong inhibitory signal to lowed by a series of diminishing spikes. When a person the same deep nuclear cell that had originally turned on performs a new movement for the first time, feedback sig- the movement. Therefore, this signal helps turn off the nals from the muscle and joint proprioceptors will usually movement after a short time. denote to the cerebellum how much the actual movement Thus, one can see how the complete cerebellar circuit fails to match the intended movement, and the climbing could cause a rapid turn-­on agonist muscle contraction fiber signals alter the long-­term sensitivity of the Purkinje at the beginning of a movement and yet also cause a pre- cells in some way. Over a period, this change in sensitivity, cisely timed turn-­off of the same agonist contraction after along with other possible “learning” functions of the cer- a given period. ebellum, is believed to make the timing and other aspects Now, let us speculate on the circuit for the antago- of cerebellar control of movements approach perfection. nist muscles. Most important, remember that there are When this state has been achieved, the climbing fibers no reciprocal agonist-­antagonist circuits throughout the spi- longer need to send “error” signals to the cerebellum to nal cord for virtually every movement that the cord can cause further change. initiate. Therefore, these circuits are part of the basis for antagonist turn-­off at the onset of movement and then FUNCTION OF THE CEREBELLUM IN turn-­on at termination of movement, mirroring whatever OVERALL MOTOR CONTROL occurs in the agonist muscles. But also remember that The nervous system uses the cerebellum to coordinate the cerebellum contains several other types of inhibitory motor control functions at three levels: cells besides Purkinje cells. The functions of some of these 1. The vestibulocerebellum. This level consists princi- cells are still to be determined; they, too, could play roles pally of the small flocculonodular cerebellar lobes in the initial inhibition of the antagonist muscles at onset that lie under the posterior cerebellum and adjacent of a movement and subsequent excitation at the end of a portions of the vermis. It provides neural circuits movement. for most of the body’s equilibrium movements. 716 https://ebook2book.ir/  Chapter 57 Cerebellum and Basal Ganglia Contributions to Overall Motor Control 2. The spinocerebellum. This level consists of most of Motor cortex the vermis of the posterior and anterior cerebellum plus the adjacent intermediate zones on both sides of the vermis. It provides the circuitry for coordi- nating mainly movements of the distal portions of UNIT XI the limbs, especially the hands and fingers. 3. The cerebrocerebellum. This level consists of the Red nucleus Thalamus large lateral zones of the cerebellar hemispheres, lateral to the intermediate zones. It receives virtu- ally all its input from the cerebral motor cortex and adjacent premotor and somatosensory cortices of Intermediate zone of Mesencephalon, the cerebrum. It transmits its output information in cerebellum pons, and medulla the upward direction back to the brain, function- ing in a feedback manner with the cerebral cortical sensorimotor system to plan sequential voluntary body and limb movements. These movements are Corticospinal tract planned as much as tenths of a second in advance Spinocerebellar of the actual movements. This process is called de- tract Reticulospinal velopment of “motor imagery” of movements to be and rubrospinal performed. tracts The Vestibulocerebellum Functions in Muscles Association With the Brain Stem and Spinal Cord to Control Equilibrium and Postural Movements Figure 57-­8 Cerebral and cerebellar control of voluntary move- ments, involving especially the intermediate zone of the cerebellum. The vestibulocerebellum originated phylogenetically at about the same time that the vestibular apparatus in the inner ear developed. Furthermore, as discussed in Chap- performed rapidly? The answer is that the signals from the ter 56, loss of the flocculonodular lobes and adjacent por- periphery tell the brain how rapidly and in which direc- tions of the vermis of the cerebellum, which constitute the tions the body parts are moving. It is then the function vestibulocerebellum, causes extreme disturbance of equi- of the vestibulocerebellum to calculate in advance from librium and postural movements. these rates and directions where the different parts will In people with vestibulocerebellar dysfunction, equi- be during the next few milliseconds. The results of these librium is far more disturbed during performance of rapid calculations are the key to the brain’s progression to the motions than during inactivity, especially when these next sequential movement. movements involve changes in direction of movement Thus, during control of equilibrium, it is presumed and stimulate the semicircular ducts. This phenomenon that information from both the body periphery and the suggests that the vestibulocerebellum is important in con- vestibular apparatus is used in a typical feedback con- trolling balance between agonist and antagonist muscle trol circuit to provide anticipatory correction of postural contractions of the spine, hips, and shoulders during motor signals necessary for maintaining equilibrium even rapid changes in body positions as required by the ves- during extremely rapid motion, including rapidly chang- tibular apparatus. ing directions of motion. One of the major problems in controlling balance is the amount of time required to transmit position sig- Spinocerebellum—Feedback Control nals and velocity of movement signals from the differ- of Distal Limb Movements via the ent parts of the body to the brain. Even when the most Intermediate Cerebellar Cortex and the rapidly conducting sensory pathways are used, up to 120 Interposed Nucleus m/sec in the spinocerebellar afferent tracts, the delay for As shown in Figure 57-­8, the intermediate zone of each transmission from the feet to the brain is still 15 to 20 cerebellar hemisphere receives two types of information milliseconds. The feet of a person running rapidly can when a movement is performed: (1) information from the move as much as 10 inches during that time. Therefore, cerebral motor cortex and from the midbrain red nucleus, it is never possible for return signals from the peripheral telling the cerebellum the intended sequential plan of move- parts of the body to reach the brain at the same time that ment for the next few fractions of a second; and (2) feed- the movements actually occur. How, then, is it possible back information from the peripheral parts of the body, for the brain to know when to stop a movement and to especially from the distal proprioceptors of the limbs, tell- perform the next sequential act when the movements are ing the cerebellum what actual movements result. 717 https://ebook2book.ir/ UNIT XI The Nervous System: C. Motor and Integrative Neurophysiology After the intermediate zone of the cerebellum has into the mechanisms. For motor control by the nervous compared the intended movements with the actual move- system, the cerebellum provides most of this damping ments, the deep nuclear cells of the interposed nucleus function. send corrective output signals (1) back to the cerebral motor cortex through relay nuclei in the thalamus and (2) Cerebellar Control of Ballistic Movements. Most rapid to the magnocellular portion (the lower portion) of the movements of the body, such as the movements of the red nucleus that gives rise to the rubrospinal tract. The fingers in typing, occur so rapidly that it is not possible to rubrospinal tract in turn joins the corticospinal tract in receive feedback information either from the periphery innervating the lateralmost motor neurons in the ante- to the cerebellum or from the cerebellum back to the mo- rior horns of the spinal cord gray matter, the neurons that tor cortex before the movements are over. These move- control the distal parts of the limbs, particularly the hands ments are called ballistic movements, meaning that the and fingers. entire movement is preplanned and set into motion to go This part of the cerebellar motor control system pro- a specific distance and then to stop. Another important vides smooth, coordinated movements of the agonist example is the saccadic movements of the eyes, in which and antagonist muscles of the distal limbs for performing the eyes jump from one position to the next when read- acute purposeful patterned movements. The cerebellum ing or when looking at successive points along a road as a seems to compare the “intentions” of the higher levels of person is moving in a car. the motor control system, as transmitted to the interme- Three major changes occur in these ballistic move- diate cerebellar zone through the corticopontocerebellar ments when the cerebellum is removed: (1) the move- tract, with the “performance” by the respective parts of ments are slow to develop and do not have the extra onset the body, as transmitted back to the cerebellum from the surge that the cerebellum usually provides; (2) the force periphery. In fact, the ventral spinocerebellar tract even developed is weak; and (3) the movements are slow to transmits back to the cerebellum an “efference” copy of the turn off, usually allowing the movement to go well beyond actual motor control signals that reach the anterior motor the intended mark. Therefore, in the absence of the cer- neurons, and this information is also integrated with the ebellar circuit, the motor cortex has to think extra hard to signals arriving from the muscle spindles and other pro- turn ballistic movements on and off. Thus, the automa- prioceptor sensory organs, transmitted principally in the tism of ballistic movements is lost. dorsal spinocerebellar tract. Similar comparator signals Considering once again the circuitry of the cerebellum, also go to the inferior olivary complex; if the signals do one sees that it is beautifully organized to perform this not compare favorably, the olivary–Purkinje cell system, biphasic, first excitatory and then delayed inhibitory func- along with possibly other cerebellar learning mecha- tion that is required for preplanned rapid ballistic move- nisms, eventually corrects the motions until they perform ments. Also, the built-­in timing circuits of the cerebellar the desired function. cortex are fundamental to this particular ability of the cerebellum. Function of the Cerebellum to Prevent Overshoot and to “Damp” Movements. Almost all movements of the Cerebrocerebellum—Function of the body are “pendular.” For example, when an arm is moved, Large Lateral Zone of the Cerebellar momentum develops, and the momentum must be over- Hemisphere to Plan, Sequence, and Time come before the movement can be stopped. Because of Complex Movements momentum, all pendular movements have a tendency to In humans the lateral zones of the two cerebellar hemi- overshoot. If overshooting occurs in a person whose cer- spheres are highly developed and greatly enlarged. This ebellum has been destroyed, the conscious centers of the characteristic goes along with human abilities to plan and cerebrum eventually recognize this error and initiate a perform intricate sequential patterns of movement, espe- movement in the reverse direction to attempt to bring the cially with the hands and fingers, and to speak. Yet, the arm to its intended position. However, the arm, by virtue large lateral zones of the cerebellar hemispheres have no of its momentum, overshoots once more in the opposite direct input of information from the peripheral parts of direction, and appropriate corrective signals must again the body. In addition, almost all communication between be instituted. Thus, the arm oscillates back and forth past these lateral cerebellar areas and the cerebral cortex is not its intended point for several cycles before it finally fixes with the primary cerebral motor cortex but instead with on its mark. This effect is called an action tremor or inten- the premotor area and primary and association somato- tion tremor. sensory areas. If the cerebellum is intact, appropriate learned, sub- Even so, destruction of the lateral zones of the cerebel- conscious signals stop the movement precisely at the lar hemispheres, along with their deep nuclei, the dentate intended point, thereby preventing the overshoot and the nuclei, can lead to extreme incoordination of complex tremor. This activity is the basic characteristic of a damp- purposeful movements of the hands, fingers, and feet and ing system. All control systems regulating pendular ele- of the speech apparatus. This condition has been difficult ments that have inertia must have damping circuits built to understand because there is no direct communication 718 https://ebook2book.ir/  Chapter 57 Cerebellum and Basal Ganglia Contributions to Overall Motor Control between this part of the cerebellum and the primary but both of these require cerebellar participation. As an motor cortex. However, experimental studies suggest that example, a person can predict from the changing visual these portions of the cerebellum are concerned with two scene how rapidly he or she is approaching an object. A other important but indirect aspects of motor control: (1) striking experiment that demonstrates the importance of planning of sequential movements and (2) “timing” of the the cerebellum in this ability is the effects of removing the large lateral portions of the cerebellum in monkeys. Such UNIT XI sequential movements. a monkey occasionally charges into the wall of a corridor Planning of Sequential Movements. The planning of because it is unable to predict when it will reach the wall. sequential movements requires that the lateral zones of It is quite possible that the cerebellum provides a “time the hemispheres communicate with both the premotor base,” perhaps using time-­delay circuits, against which and sensory portions of the cerebral cortex, and it re- signals from other parts of the central nervous system can quires two-­way communication between these cerebral be compared. It is often stated that the cerebellum is par- cortex areas with corresponding areas of the basal ganglia. ticularly helpful in interpreting rapidly changing spatio- It seems that the “plan” of sequential movements actually temporal relations in sensory information. begins in the sensory and premotor areas of the cerebral cortex, and from there the plan is transmitted to the later- Clinical Abnormalities of the Cerebellum al zones of the cerebellar hemispheres. Then, amid much Destruction of small portions of the lateral cerebellar cortex two-­way traffic between the cerebellum and the cerebral seldom causes detectable abnormalities in motor function. cortex, appropriate motor signals provide transition from In fact, several months after as much as one-half of the lat- one sequence of movements to the next. eral cerebellar cortex on one side of the brain has been re- An interesting observation that supports this view is that moved, if the deep cerebellar nuclei are not removed along many neurons in the cerebellar dentate nuclei display the with the cortex, the motor functions of the animal appear activity pattern for the sequential movement that is yet to to be almost normal as long as the animal performs all come while the present movement is still occurring. Thus, movements slowly. Thus, the remaining portions of the mo- the lateral cerebellar zones appear to be involved not with tor control system are capable of compensating to a great what movement is happening at a given moment but with extent for loss of parts of the cerebellum. what will be happening during the next sequential move- To cause serious and continuing dysfunction of the cerebellum, the cerebellar lesion usually must involve one ment a fraction of a second or perhaps even seconds later. or more of the deep cerebellar nuclei—the dentate, inter- To summarize, one of the most important features posed, or fastigial nuclei. of normal motor function is one’s ability to progress Dysmetria and Ataxia smoothly from one movement to the next in orderly suc- cession. In the absence of the large lateral zones of the cer- Two of the most important symptoms of cerebellar disease ebellar hemispheres, this capability is seriously disturbed are dysmetria and ataxia. In the absence of the cerebellum, for rapid movements. the subconscious motor control system cannot predict how far movements will go. Therefore, the movements ordinarily overshoot their intended mark; then, the conscious portion of Timing Function for Sequential Movements. Another the brain overcompensates in the opposite direction for the important function of the lateral zones of the cerebellar succeeding compensatory movement. This effect is called dys- hemispheres is to provide appropriate timing for each metria, and it results in uncoordinated movements that are succeeding movement. In the absence of these cerebellar called ataxia. Dysmetria and ataxia can also result from le- zones, one loses the subconscious ability to predict how sions in the spinocerebellar tracts because feedback informa- far the different parts of the body will move in a given tion from the moving parts of the body to the cerebellum is time. Without this timing capability, the person becomes essential for cerebellar timing of movement termination. unable to determine when the next sequential movement Past Pointing needs to begin. As a result, the succeeding movement Past pointing means that in the absence of the cerebellum, may begin too early or, more likely, too late. Therefore, le- a person ordinarily moves the hand or some other moving sions in the lateral zones of the cerebellum cause complex part of the body considerably beyond the point of inten- movements (e.g., those required for writing, running, or tion. This movement results from the fact that normally the even talking) to become incoordinate and lacking ability cerebellum initiates most of the motor signal that turns off to progress in orderly sequence from one movement to a movement after it is begun; if the cerebellum is not avail- the next. Such cerebellar lesions are said to cause failure able to initiate this motor signal, the movement ordinarily of smooth progression of movements. goes beyond the intended mark. Therefore, past pointing is actually a manifestation of dysmetria. Extramotor Predictive Functions of the Cerebrocer- Failure of Progression ebellum. The cerebrocerebellum (the large lateral lobes) Dysdiadochokinesia—Inability to Perform Rapid Alter- also helps to “time” events other than movements of the nating Movements. When the motor control system fails to body. For example, the rates of progression of both audi- predict where the different parts of the body will be at a giv- tory and visual phenomena can be predicted by the brain, en time, it “loses” perception of the parts during rapid motor 719 https://ebook2book.ir/