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Chapter 5: Cerebellum

James Knierim, Ph.D., Department of Neuroscience, The Johns Hopkins University

Last Review 20 Oct 2020

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5.1 Overview: Functions of the Cerebellum

Figure 5.1 Cerebellum

The cerebellum (“little brain”) is a structure that is located at the back of the brain, underlying the occipital and temporal lobes of the cerebral cortex (Figure 5.1). Although the cerebellum accounts for approximately 10% of the brain’s volume, it contains over 50% of the total number of neurons in the brain. Historically, the cerebellum has been considered a motor structure, because cerebellar damage leads to impairments in motor control and posture and because the majority of the cerebellum’s outputs are to parts of the motor system. Motor commands are not initiated in the cerebellum; rather, the cerebellum modifies the motor commands of the descending pathways to make movements more adaptive and accurate. The cerebellum is involved in the following functions:

Maintenance of balance and posture. The cerebellum is important for making postural adjustments in order to maintain balance. Through its input from vestibular receptors and proprioceptors, it modulates commands to motor neurons to compensate for shifts in body position or changes in load upon muscles. Patients with cerebellar damage suffer from balance disorders, and they often develop stereotyped postural strategies to compensate for this problem (e.g., a wide-based stance).

Coordination of voluntary movements. Most movements are composed of a number of different muscle groups acting together in a temporally coordinated fashion. One major function of the cerebellum is to coordinate the timing and force of these different muscle groups to produce fluid limb or body movements.

Motor learning. The cerebellum is important for motor learning. The cerebellum plays a major role in adapting and fine-tuning motor programs to make accurate movements through a trial-and-error process (e.g., learning to hit a baseball).

Cognitive functions. Although the cerebellum is most understood in terms of its contributions to motor control, it is also involved in certain cognitive functions, such as language. Thus, like the basal ganglia, the cerebellum is historically considered as part of the motor system, but its functions extend beyond motor control in ways that are not yet well understood.

5.2 Cerebellar Gross Anatomy

The cerebellum consists of two major parts (Figure 5.2A). The cerebellar deep nuclei (or cerebellar nuclei) are the sole output structures of the cerebellum. These nuclei are encased by a highly convoluted sheet of tissue called the cerebellar cortex, which contains almost all of the neurons in the cerebellum. A cross-section through the cerebellum reveals the intricate pattern of folds and fissures that characterize the cerebellar cortex (Figure 5.3). Like the cerebral cortex, cerebellar gyri are reproducible across individuals and have been identified and named. We will only be concerned with some of the larger divisions of the cerebellar cortex.

	Figure 5.2 (A) Cerebellar deep nuclei and cerebellar cortex in an idealized brain section. (B) External morphology of the cerebellum.
	Figure 5.3 Midsagittal cross-section of cerebellum showing the three primary lobes of the cerebellum.

Divisions of the cerebellum. Two major fissures running mediolaterally divide the cerebellar cortex into three primary subdivisions (Figure 5.2B and Figure 5.3). The posterolateral fissure separates the flocculonodular lobe from the corpus cerebelli, and the primary fissure separates the corpus cerebelli into a posterior lobe and an anterior lobe (Figure 5.4). The cerebellum is also divided sagittally into three zones that run from medial to lateral (Fig. 5.4). The vermis (from the Latin word for worm) is located along the midsagittal plane of the cerebellum. Directly lateral to the vermis is the intermediate zone. Finally, the lateral hemispheres are located lateral to the intermediate zone (there are no clear morphological borders between the intermediate zone and the lateral hemisphere that are visible from a gross specimen).

Figure 5.4 Divisions of cerebellum. Click PLAY to see schematic “unfolding” of cerebellum.

Cerebellar nuclei. All outputs from the cerebellum originate from the cerebellar deep nuclei. Thus, a lesion to the cerebellar nuclei has the same effect as a complete lesion of the entire cerebellum. It is important to know the inputs, outputs, and anatomical relationships between the different cerebellar nuclei and the subdivisions of the cerebellum (Figure 5.5).

Figure 5.5 Input and output pathways of the cerebellum. Click on the names of each cerebellum functional subdivision (cerebrocerebellum, spinocerebellum, and vestibulocerebellum) to view each pathway in isolation. The cerebellar deep nuclei are the sole outputs of the cerebellum.

1. The fastigial nucleus is the most medially located of the cerebellar nuclei. It receives input from the vermis and from cerebellar afferents that carry vestibular, proximal somatosensory, auditory, and visual information. It projects to the vestibular nuclei and the reticular formation.
2. The interposed nuclei comprise the emboliform nucleus and the globose nucleus. They are situated lateral to the fastigial nucleus. They receive input from the intermediate zone and from cerebellar afferents that carry spinal, proximal somatosensory, auditory, and visual information. They project to the contralateral red nucleus (the origin of the rubrospinal tract).
   
3. The dentate nucleus is the largest of the cerebellar nuclei, located lateral to the interposed nuclei. It receives input from the lateral hemisphere and from cerebellar afferents that carry information from the cerebral cortex (via the pontine nuclei). It projects to the contralateral red nucleus and the ventrolateral (VL) thalamic nucleus.
   
4. The vestibular nuclei are located outside the cerebellum, in the medulla. Hence, they are not strictly cerebellar nuclei, but they are considered to be functionally equivalent to the cerebellar nuclei because their connectivity patterns are identical to the cerebellar nuclei. The vestibular nuclei receive input from the flocculonodular lobe and from the vestibular labyrinth. They project to various motor nuclei and originate the vestibulospinal tracts.

In addition to these inputs, all cerebellar nuclei and all regions of cerebellum get special inputs from the inferior olive of the medulla (discussed below).

It is convenient to remember that the anatomical locations of the cerebellar nuclei correspond to the cerebellar cortex regions from which they receive input. Thus, the medially located fastigial nucleus receives input from the medially located vermis; the slightly lateral interposed nuclei receive input from the slightly lateral intermediate zone; and the most lateral dentate nucleus receives input from the lateral hemispheres.

Cerebellar peduncles. Three fiber bundles carry the input and output of the cerebellum.

1. The inferior cerebellar peduncle (also called the restiform body) primarily contains afferent fibers from the medulla, as well as efferents to the vestibular nuclei.
   
2. The middle cerebellar peduncle (also called the brachium pontis) primarily contains afferents from the pontine nuclei.
   
3. The superior cerebellar peduncle (also called the brachium conjunctivum) primarily contains efferent fibers from the cerebellar nuclei, as well as some afferents from the spinocerebellar tract.
   

Thus, the inputs to the cerebellum are conveyed primarily through the inferior and middle cerebellar peduncles, whereas the outputs are conveyed primarily through the superior cerebellar peduncle. The inputs arise from the ipsilateral side of the body, and the outputs also go to the ipsilateral side of the body. Note that this is true even for the outputs to the contralateral red nucleus. Recall from the chapter on descending motor pathways that the rubrospinal tract immediately crosses the midline after the fibers leave the red nucleus. Thus, cerebellar output to the red nucleus affects the ipsilateral side of the body by a double-crossed pathway. Unlike the cerebral cortex, the cerebellum receives input from, and controls output to, the ipsilateral side of the body, and damage to the cerebellum therefore results in deficits to the ipsilateral side of the body.

5.3 Functional Subdivisions of the Cerebellum

The anatomical subdivisions described above correspond to three major functional subdivisions of the cerebellum.

Vestibulocerebellum. The vestibulocerebellum comprises the flocculonodular lobe and its connections with the lateral vestibular nuclei. Phylogenetically, the vestibulocerebellum is the oldest part of the cerebellum. As its name implies, it is involved in vestibular reflexes (such as the vestibuloocular reflex; see below) and in postural maintenance.

Spinocerebellum. The spinocerebellum comprises the vermis and the intermediate zones of the cerebellar cortex, as well as the fastigial and interposed nuclei. As its name implies, it receives major inputs from the spinocerebellar tract. Its output projects to rubrospinal, vestibulospinal, and reticulospinal tracts. It is involved in the integration of sensory input with motor commands to produce adaptive motor coordination.

Cerebrocerebellum. The cerebrocerebellum is the largest functional subdivision of the human cerebellum, comprising the lateral hemispheres and the dentate nuclei. Its name derives from its extensive connections with the cerebral cortex, via the pontine nuclei (afferents) and the VL thalamus (efferents). It is involved in the planning and timing of movements. In addition, the cerebrocerebellum is involved in the cognitive functions of the cerebellum.

5.4 Histology and Connectivity of Cerebellar Cortex

The cerebellar cortex is divided into three layers (Figure 5.6). The innermost layer, the granule cell layer, is made of 5 x 10¹⁰ small, tightly packed granule cells. The middle layer, the Purkinje cell layer, is only 1-cell thick. The outer layer, the molecular layer, is made of the axons of granule cells and the dendrites of Purkinje cells, as well as a few other cell types. The Purkinje cell layer forms the border between the granule and molecular layers.

Figure 5.6 Cerebellar circuitry. This basic pattern is repeated throughout all regions of the cerebellum.

Granule cells. Granule cells are very small, densely packed neurons that account for the huge majority of neurons in the cerebellum. Indeed, cerebellar granule cells account for more than half of the neurons in the entire brain. These cells receive input from mossy fibers and project to the Purkinje cells.

Figure 5.7 Front view of Purkinje cell. Click PLAY to see side view of the Purkinje cell. This view shows that the cell is virtually flat in this dimension. Note the parallel fibers of the granule cells that run perpendicularly to the Purkinje cell.

Purkinje cells. The Purkinje cell is one of the most striking cell types in the mammalian brain. Its apical dendrites form a large fan of finely branched processes (Figure 5.7). Remarkably, this dendritic tree is almost two-dimensional; looked at from the side, the dendritic tree is flat (click PLAY on Figure 5.7). Moreover, all Purkinje cells are oriented in parallel. This arrangement has important functional considerations, as we shall see below.
Other cell types. In addition to the major cell types (granule cells and Purkinje cells), the cerebellar cortex also contains various interneuron types, including the Golgi cell, the basket cell, and the stellate cell.

Connectivity. The cerebellar cortex has a relatively simple, stereotyped connectivity pattern that is identical throughout the whole structure. Figure 6 illustrates a simplified diagram of the connectivity of the cerebellum. Cerebellar input can be divided into two distinct classes.

1. Mossy fibers originate in the pontine nuclei, the spinal cord, the brainstem reticular formation, and the vestibular nuclei, and they make excitatory projections onto the cerebellar nuclei and onto granule cells in the cerebellar cortex. They are called mossy fibers because of the tufted appearance of their synaptic contacts with granule cells. There is a large degree of divergence in the mossy fiber-granule cell connection, as each mossy fiber innervates hundreds of granule cells. The granule cells send axons up toward the cortical surface. Each axon bifurcates in the molecular layer, sending a collateral in opposite directions. These fibers, called parallel fibers, run parallel to the folds of the cerebellar cortex, where they make excitatory synapses with Purkinje cells along the way (Figure 5.7, rotated view after PLAY). The two-dimensional arbors of the Purkinje cell dendrites are oriented perpendicular to the parallel fibers. Thus, the arrangement of Purkinje cells and parallel fibers resembles telephone lines running between telephone poles. Each parallel fiber makes contact with hundreds of Purkinje cells; because of the high degree of divergence of the mossy fiber-granule cell synapses, the firing of each Purkinje cell can be influenced (disynaptically) by thousands of mossy fibers.
   
2. Climbing fibers originate exclusively in the inferior olive and make excitatory projections onto the cerebellar nuclei and onto the Purkinje cells of the cerebellar cortex. They are called climbing fibers because their axons climb and wrap around the dendrites of the Purkinje cell like a climbing vine. Each Purkinje cell receives a single, extremely powerful input from a single climbing fiber. In contrast to mossy fibers and parallel fibers, each climbing fiber contacts only 10 Purkinje cells on average, making ~300 synapses with each Purkinje cell. Thus, the climbing fiber is a restricted, but extremely powerful, excitatory input onto Purkinje cells.

The Purkinje cell is the sole source of output from the cerebellar cortex. It is important to note that Purkinje cells make inhibitory connections onto the cerebellar nuclei. (Note the distinction between the Purkinje cells, which constitute the sole output of the cerebellar cortex, and the cerebellar nuclei, which constitute the sole output of the entire cerebellum.) Almost all of the spikes generated by the Purkinje cell are caused by its parallel-fiber inputs. These inputs cause the Purkinje cell to fire at a high resting rate (~70 spikes/sec), tonically inhibiting its cerebellar nucleus targets. The powerful inputs from climbing fibers occur less frequently (~1 spike/sec); thus, they have a minor influence on the overall firing rate of the Purkinje cell. The Purkinje cell spikes that are generated by climbing fibers are calcium-spikes, however, which allow the climbing fibers to initiate a number of calcium-dependent changes in the Purkinje cell. As described below, one important change appears to be a long-lasting change in the strength of the parallel-fiber inputs to the Purkinje cell.

5.5 Damage to Cerebellum Produces Movement Disorders

Much of what is known about cerebellar function comes from studies of patients with cerebellar damage. In general, such patients display uncoordinated voluntary movements and problems maintaining balance and posture. The following are some symptoms of cerebellar damage (we will discuss more symptoms in the next chapter):

1. Decomposition of movement. Most of our movements involve the coordinated activity of many muscle groups and different joints to produce a smooth trajectory of the body part through space. Patients with cerebellar dysfunction are unable to produce these coordinated, smooth movements. Instead, they often break the movements down into their component parts in order to execute the desired trajectory. For example, touching one’s finger to one’s nose requires the coordinated activity of shoulder, elbow, and wrist joints. Cerebellar patients must first perform the shoulder movement, then the elbow movement, and finally the wrist movement in sequence, rather than as one, uniform motion.
   
2. Intention tremor. When making a movement to a target, cerebellar patients often produce an involuntary tremor that increases as they approach closer to the target. For example, if reaching for a cup, the hand starts out in a direct line toward the cup; as it gets closer, however, the hand begins to move back and forth as it attempts to make contact with the cup.
   
3. Dysdiadochokinesia. Patients have difficulty performing rapidly alternating movements, such as hitting a surface rapidly and repeatedly with the palm and back of the hand.
4. Deficits in motor learning. Experimental studies have demonstrated that cerebellar damage causes deficits in motor learning in both human patients and experimental animals. One prominent experimental model is the vestibuloocular reflex (VOR). This reflex allows us to maintain gaze on an object when the head is rotated (Figure 5.8). Vestibular signals detect the head movement, and send signals through the cerebellum to the eye muscles to precisely counter the head rotation and maintain a stable center of gaze. The motor commands to the eyes must be calibrated precisely with experience, and this calibration appears to be the job of the cerebellum. Experiments have been performed in which subjects wore prisms that magnified the visual image. When the subjects’ heads were moved, the VOR caused the visual image to shift on the retina rather than remaining stable. Over days, however, the VOR slowly adjusted, such that the proper compensatory eye movements were made to keep the retinal image stable when the head was rotated. In experimental animals, lesions to the cerebellum prevent this adjustment of the VOR.
   

Figure 5.8 Vestibuloocular reflex (VOR) and cerebellar learning. Click PLAY to begin demonstration. Under normal conditions, when a human or animal subject rotates the head back and forth, the eyes rotate in an equal and opposite direction in order to keep the image stable on the retina. The vestibular system provides the input regarding the head movement, and the motor system has to learn the precise output commands in order to keep the image stable. When magnifying glasses are placed on the animal, the eyes do not move fast enough to compensate for the increased speed of movement of the magnified image, and thus the image moves along the retina (termed “retinal slip”) in the direction opposite to the movement of the head. Over time, however, the motor system learns to move the eyes faster (e.g., the gain of the eye movement command is increased), and the image becomes stable again. When the goggles are removed, the eyes now move too quickly, causing retinal slip in the same direction as head movement. With time, the system will learn to calibrate the VOR again. Patients and experimental animals with damage to the vestibulocerebellum are not able to adapt their VOR to the addition and removal of the goggles, demonstrating the role of the cerebellum in this form of motor learning.

A second example of cerebellum-dependent motor learning involves the execution of accurate, coordinated movements. Subjects wore prism goggles that shifted the visual image to the right, and they were asked to then throw balls at a target on the wall. Because of the prisms, the accuracy of the subjects was initially quite low, as the balls consistently hit to the left of the target. With repeated practice, however, the subjects became more and more accurate at hitting the target. When the goggles were removed, the subject now began to throw the balls to the right of the target, because their motor programs had been recalibrated to use the shifted visual input. Over time, once again, they gradually increased their accuracy. Patients with cerebellar damage never learned to compensate for the prism, as their balls always landed to the left of the target when the goggles were worn. When the goggles were removed, they were immediately accurate at hitting the target, because they never made compensations for the earlier prism trials.

A third example involves the Pavlovian classical conditioning of the eye blink reflex. In this task, a neutral stimulus (such as a tone) is paired with a noxious stimulus (such as a puff of air to the eye) that causes a reflexive eye blink. Over time, experimental animals will learn to close their eye when the tone occurs, in anticipation of the air puff. This learned eyelid closure is remarkably well-timed to peak at the expected time of the puff. Animals with cerebellar damage do not learn to produce the eyelid closure in response to the tone.

5.6 Cerebellum and Control Systems

What do the various symptoms of cerebellar damage have in common that reveal the function of the cerebellum? A number of different theories have been proposed. Recall the discussion in Chapter 1 of the ubiquitous use of sensory information in motor control. The cerebellum receives extensive sensory input, and it appears to use this input to guide movements in both a feedback and feedforward control manner.

Feedback control systems

In a feedback controller, a desired output is compared continuously with the actual output, and adjustments are made during the execution of the movement until the actual movement matches the desired movement. A common example of a feedback control system is the thermostat in your home (Figure 5.9).

Figure 5.9 A feedback control system, such as the thermostat in your home, is sufficient for slow movements, such as posture. The myotatic reflex is an example of a feedback control system in the spinal cord.

The thermostat is set to a desired temperature (e.g., 72°), and a thermometer measures the current temperature in the room. If the thermostat (the comparator) detects that the room is cooler than the desired temperature, it sends an error signal that turns on the furnace. If the comparator detects that the room is warmer than the desired setting, its sends an error signal that turns on the air conditioner.

Feedback control systems can produce very accurate outputs; however, in general they are slow. In order to change the output, the effector must wait until information is transmitted from the sensor to the comparator and then to the effector. At this point, another comparison is made, and the process continues. Consider further the thermostat example. If the temperature reads 5° cooler than desired, the thermostat can instruct the furnace to turn on at a moderate heat. It reads the new room temperature, and, if it is still too cool, it instructs the furnace to deliver more heat, and so on. Although this will eventually produce an accurate room temperature at the desired point, it takes a number of cycles to reach that point. One possible solution for quicker results would be to turn an enormous furnace on full-blast, such that is heats the room very quickly. This solution, however, can generate another problem. It will tend to cause the system to oscillate if the feedback pathways are slow. For example, assume that the furnace can heat the room at the rate of 5° per second, but that it takes 2 seconds for the thermometer to adjust to the new temperature, and for the new error signal to turn the furnace off. In those 2 seconds, the furnace has heated the room up 10°, and now it is too warm. So the error signal turns on the air conditioner, and it cools the room at 5°/sec. Of course, it also takes 2 sec to receive the feedback, and by the time it is told to shut off, it has cooled the room by 10°. You can see what happens: the system will be sent into an endless oscillation of being 5° too hot and 5° too cold. In order for a feedback system to work well, the transmission time of sensory information through the comparator to the effector must be rapid compared to the time of the action.

Feedback control systems work well only when the sensory feedback about the actual output is fast relative to the actual output. If the actual output is faster than the sensor’s ability to provide feedback, then the system will tend to oscillate between overshooting and undershooting the desired output. Thus, a feedback controller is useful for slow movements, like postural adjustments. The role of the myotatic reflex in posture maintenance is an example of a feedback controller in the spinal cord, and the cerebellum plays a role in coordinating these postural adjustments. Feedback control is not effective for most of the fast movements we make routinely (such as an eye movement or reaching out for a cup). For these movements, a feedforward controller is needed.

Feed forward control systems

In a feedforward control system, when a desired output is sent to the controller, the controller evaluates sensory information about the environment and about the system itself before the output commands are generated. It uses the sensory information to program the best set of instructions to accomplish the desired output. However, in a pure feedforward system, once the commands are sent, there is no way to alter them (i.e., there is no feedback loop). The advantage of a feedforward system is that it can produce the precise set of commands for the effector without needing to constantly check the output and make corrections during the movement itself. The main disadvantage, however, is that the feedforward controller requires a period of trial-and-error learning before it can function properly. In most biological systems, it is hard (perhaps impossible) to pre-program all of the possible sensory conditions that the controller may encounter during the life of the organism. Furthermore, the environment and conditions under which actions are made are constantly changing, and the feedforward controller must be able to adapt its output commands to account for these changes.

 

Figure 5.10 A feedforward control system is needed for fast movements, because a feedback system is too slow.

Let us extend the thermostat example to see how a temperature controller operating as a feedforward system would work to raise the temperature of a room from 70° to 75°. The controller would use diverse sensory information about the environment before sending its command to the furnace (Figure 5.10). For example, it would read the current temperature, the current humidity level, the size of the room, the number of people in the room, and so forth. Based on this information, it would direct the furnace to turn on for a pre-set period of time, and that’s it. There would be no need to continually compare the current temperature with the desired setting, as the system has predetermined how long the furnace needs to be working in order to achieve the desired temperature. How did the controller obtain this information? A feedforward controller requires a large amount of experience in order to learn the appropriate actions needed for each set of environmental conditions. If on one trial it turns the furnace off too soon and the room does not reach the desired temperature, it adjusts its programming such that the next time it encounters the same environmental conditions, it turns the furnace on for a longer period of time. Through many such instances of trial and error learning, the feedforward system creates a “look-up table” that tells it how long the furnace needs to be active under the current conditions. The key distinction between a feedback and feedforward system is that the feedback system uses sensory information to generate an error signal during the control of a movement, whereas a feedforward system uses sensory information in advance of a movement. Any error signal about the final output is used by the feedforward system only to change its programming of future movements.

The cerebellum may be a feedforward control system

The cerebellar involvement in the VOR may be explained in terms of the learning requirements of a feedforward controller. When the head moves, a compensatory eye movement must be made to maintain a stable gaze. The cerebellum receives sensory input from the vestibular system informing it that the head is moving. It also receives input from eye muscle proprioceptors and other relevant sources of information about current conditions in order to make an accurate compensatory eye movement. It evaluates all of this advance sensory information and calculates the proper eye movement to exactly counterbalance the head movement. What if the eye movement does not match the head movement, however, and the visual image moves across the retina (such as in the experimental condition in which a prism was worn, or in a real-life situation in which an individual wears new prescription eyeglasses)? The retinal slip constitutes an error signal to tell the cerebellum that next time these conditions are met, adjust the eye movement to decrease the retinal slip. This trial and error sequence will be repeated until the movement is properly calibrated; moreover, these mechanisms will ensure that the movements stay calibrated.

As another example, the coordination of movements requires that muscle groups be activated in precise temporal sequence. Not only do the different joints need to be coordinated temporally, but even antagonist muscles that control the same joint need precise temporal coordination. For example, an extensor muscle needs to be activated to start a reaching movement, and the corresponding flexor muscle needs to be activated at the end of the movement to stop the movement appropriately. The precise timing of muscle contractions and the force necessary for each contraction varies with the amount of load placed on a muscle, as well as on the inherent properties of the muscle itself (e.g., elasticity). These variables are constantly changing throughout life, as one grows, gains/loses weights, and ages. Moreover, a similar movement will require different patterns of motor activity depending on the weight being born by the muscle (for example, if an extended hand is empty or holding a heavy weight). The cerebellum appears necessary for the proper timing and coordination of muscle groups, very likely through a trial-and-error learning mechanism discussed previously. Such a role helps explain the deficits seen in dysdiadochokinesia, in which patients cannot perform rapidly alternating sequences of movements.

It is believed that the mossy fiber inputs to the cerebellum convey the sensory information used to evaluate the overall sensory context of the movement. Mossy fibers are known to respond to sensory stimuli; they are also correlated with different movements (Figure 5.11). These fibers convey such information as: Where are the appropriate body parts (proprioceptors), what is the current load on the muscle (proprioceptors, somatosensory receptors, etc.), what other sensory information can predict a useful response (e.g., the tone in the eye blink conditioning), what are the desired movements (motor cortex). The error signal is believed to be conveyed by the climbing fiber inputs. Climbing fibers are known to be especially active when an unexpected event occurs, such as when a greater load than expected is placed on a muscle or when a toe is stubbed. Thus, the large divergence of input from the mossy fibers to the granule cells to the parallel fibers is believed to create complex representations of the entire sensory context at present and the desired motor output. When the desired output is not achieved, the climbing fibers signal this error and trigger a calcium spike in the Purkinje cell. The influx of calcium changes the connection strengths between parallel fibers and Purkinje cells, such that the next time the same behavioral context occurs, the motor output will be modified to more closely approximate the desired output.

	The cerebellum may act as a feedback control system for slow movements and a feedforward controller for fast movements. In its function as a feedforward controller, the mossy fibers may provide information regarding the desired output from motor cortex and the advance sensory information about the state of the worlds and the body. The climbing fibers may convey information about movement errors, which provides a teaching signal such that the cerebellum is more likely to produce the correct movement the next time the output is desired.
Figure 5.11
Figure 5.12

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