The Cerebellum (Part 3, Chapter 5) Neuroscience Online: An Electronic Textbook for Neuroscience | Department of Neurobiology and Anatomy (2023)

5.1Summary: functions of the cerebellum

HecerebellumThe “small brain” is a structure located in the posterior part of the brain, below the occipital and temporal lobes of the cerebral cortex (fig. 5.1). Although the cerebellum makes up about 10% of the brain's volume, it contains more than 50% of the total number of neurons in the brain. Historically, the cerebellum has been considered a motor structure because damage to the cerebellum leads to impaired motor control and posture, and because most cerebellar output goes to parts of the motor system. Motor commands do not start in the cerebellum; rather, the cerebellum modifies motor commands from the descending pathways to make movements more adaptive and precise. The cerebellum participates in the following functions:

Maintain balance and posture.The cerebellum plays an important role in regulating posture to maintain balance. With inputs from vestibular receptors and proprioceptors, it modulates the commands given to motor neurons to compensate for changes in body position or changes in muscle load. Patients with cerebellar damage suffer from balance disorders and often develop stereotyped postural strategies to compensate for this problem (eg, broad-based stance).

Coordination of voluntary movements.Most movements consist of many different muscle groups working together in a coordinated manner over time. One of the primary functions of the cerebellum is to coordinate the timing and force of these different muscle groups to produce fluid movements of the limbs or body.

motor learning.The cerebellum is important for motor learning. The cerebellum plays an important role in adapting and fine-tuning motor programs to perform precise movements through trial and error (eg, learning to hit a baseball).

Cognitive functions.While the cerebellum is best understood for its involvement in motor control, it is also involved in some cognitive functions such as language. Thus, like the basal ganglia, the cerebellum has historically been considered part of the motor system, but its functions extend beyond motor control in ways that are not yet well understood.

5.2cerebellar anatomy

The cerebellum consists of two main parts (Fig. 5.2A). The deep cerebellar nuclei (or cerebellar nuclei) are the only exit structures from the cerebellum. These nuclei are surrounded by a very intricate layer of tissue called the cerebellar cortex, which contains almost all of the neurons in the cerebellum. A section of the cerebellum reveals the intricate pattern of folds and fissures that characterize the cerebellar cortex (Fig. 5.3). Like the cerebral cortex, the cerebellar gyri are repetitive in individuals and have been identified and named. We will only consider some of the larger divisions of the cerebellar cortex.

Divisions of the cerebellum.Two major medial-lateral fissures divide the cerebellar cortex into three main parts (Fig. 5.2B and Fig. 5.3). Hefisura posterolateralcoordinatedflocculent nodular lobefrom the cerebellar body, and the primary fissure separates the cerebellar body into: aposterior lobeianterior lobe(Figure 5.4). The cerebellum is also divided sagittally into three zones running from medial to lateral (Fig. 5.4). Helost(from the Latin word for worm) lies in the midsagittal plane of the cerebellum. Directly on the side of the worm is located.intermediate zone. At last,lateral hemispheresthey are located laterally to the intermediate zone (there are no clear morphological boundaries between the intermediate zone and the lateral hemisphere, which would be visible from a large specimen).

Cerebellar nuclei.All signals from the cerebellum come fromdeep cerebellar nuclei. Therefore, damage to the cerebellar nuclei has the same effect as complete damage to the entire cerebellum. It is important to understand the inputs, outputs, and anatomic relationships between the various cerebellar nuclei and subunits (Figure 5.5).

1. Hefastigion nucleusIt is the most medial of the cerebellar nuclei. It receives information from the vermis and from cerebellar afferent fibers that carry vestibular, somatosensory, auditory, and proximal visual information. It projects to the vestibular nuclei and the reticular formation.
2. Heinserted nucleithey include the embolic nucleus and the globular nucleus. They lie lateral to the nucleus of the fastigium. They receive a signal from the intermediate zone and from cerebellar afferent fibers that carry spinal, proximal somatosensory, auditory, and visual information. They project to the opposite side.red testicle(origenrubrospinal cord).
   
3. Hedentate coreIt is the largest of the cerebellar nuclei, located lateral to the intermediate nuclei. It receives a signal from the lateral hemisphere and from cerebellar afferents, which carry information from the cerebral cortex (via the pons nuclei). Design the opposite side.red testicleiVentrolateral thalamic nucleus (VL)..
   
4. Hevestibular nucleiThey are located outside the cerebellum, in the medulla. Thus, they are not strictly cerebellar nuclei, but are considered functionally equivalent to cerebellar nuclei because their connection patterns are identical to those of cerebellar nuclei. The vestibular nuclei receive a signal from the flocculent nodular lobe and the vestibular labyrinth. They project to various motor nuclei and initiate the vestibulospinal tracts.

In addition to these stimuli, all cerebellar nuclei and all areas of the cerebellum receive special stimuli from the inferior olive medulla (discussed later).

It is worth remembering that the anatomical location of the cerebellar nuclei corresponds to the areas of the cerebellar cortex from which they receive information. Thus, the centrally located frontal nucleus receives the signal from the centrally located worm; slightly lateral located nuclei receive input from a slightly lateral intermediate zone; and the more lateral dentate nucleus receives a signal from the lateral hemispheres.

cerebellar peduncles.Three bundles of fibers carry the input and output of the cerebellum.

1. Heinferior branch of the cerebellum(so-calledrestiform body) contains mainly afferent fibers from the medulla, as well as efferents to the vestibular nuclei.
   
2. Hemiddle cerebellar peduncle(so-calledbridge arm) contains mainly inputs from the pontine nuclei.
   
3. Hesuperior cerebellar branch(so-calledconjunctival arm) contains mainly efferent fibers from the cerebellar nuclei, as well as some afferent fibers from the spinocerebellar tract.
   

Thus, inputs to the cerebellum are primarily carried by the inferior and middle cerebellar peduncles, while outputs are primarily carried by the superior cerebellar peduncles. The inputs come from the same side of the body and the outputs also go to the same side of the body. Note that this applies even to departures to the opposite red core. remember ofchapter on descending motor pathwaysThatrubrospinal cordimmediately crosses the midline after exiting the fibersred testicle. Thus, the output from the cerebellum to the red nucleus affects the same side of the body through a doubly crossed pathway. Unlike the cerebral cortex, the cerebellum receives and controls signals from the same side of the body, and therefore damage to the cerebellum causes deficits on the same side of the body.

5.3Functional divisions of the cerebellum.

The anatomical divisions described above correspond to the three main functional divisions of the cerebellum.

vestibulocerebellosThe vestibule of the cerebellum is formed byflocculent nodular lobeand its connections withlateral vestibular nuclei. From a phylogenetic point of view, the cerebellar vestibule is the oldest part of the cerebellum. As the name suggests, it is involved in vestibular reflexes (such as the vestibulo-ocular reflex; see below) and postural maintenance.

spinocerebellumThe cerebellar medulla includes:lostiintermediate zonescerebellar cortex as wellmailIinserted testicles. As the name suggests, it receives its main inputs from the spinocerebellar tract. Its results are directed at the rubrospinal, vestibulospinal, and reticulospinal tracts. Participates in the integration of sensory stimuli with motor commands to produce adaptive motor coordination.

CerebellumThe cerebellum is the largest functional unit of the human cerebellum and comprises:side hemispheresisawed testicles. Its name is derived from the extensive connections to the cerebral cortex, via the nuclei of the pons (afferent) and the VL thalamus (efferent). Participates in the planning and synchronization of movements. In addition, the cerebellum is involved in the cognitive functions of the cerebellum.

5.4Histology and connectivity of the cerebellar cortex.

The cerebellar cortex is divided into three layers (Fig. 5.6). The innermost layer, the granule cell layer, consists of 5 x 10¹⁰Small and very compact granular cells. The middle layer, the so-calledPurkinje she was wholethe layer is only 1 cell thick. The outer layer, the molecular layer, is made up of granule cell axons and Purkinje cell dendrites, as well as various other cell types. The Purkinje cell layer is the boundary between the granular and molecular layers.

granule cells.Granule cells are very small, densely packed neurons that make up the vast majority of neurons in the cerebellum. In fact, cerebellar granule cells make up more than half of the neurons in the entire brain. These cells receive the signal from the mossy fibers and send it to the Purkinje cells.

Purkinje cells.The Purkinje cell is one of the most amazing cell types in the mammalian brain. Their apical dendrites form a large fan of finely branched outgrowths (Fig. 5.7). Interestingly, this dendritic tree is almost two-dimensional; Viewed from the side, the dendritic tree is flat (click PLAY in Figure 5.7). Also, all Purkinje cells are oriented in parallel. This arrangement has important functional considerations, as we will see below.
Other types of cells. In addition to the major cell types (granule cells and Purkinje cells), the cerebellar cortex also contains several types of interneurons, including the Golgi cell, basket cell, and stellate cell.

Communication.The cerebellar cortex has a relatively simple stereotyped pattern of connections that is identical throughout the structure. Figure 6 illustrates a simplified diagram of the connections of the cerebellum. The inputs of the cerebellum can be divided into two distinct classes.

1. mossy fibersFromo'clock testiclesthe spinal cord, the reticular formation of the brain stem, and the vestibular nuclei, and produce excitatory projections to the cerebellar nuclei and granule cells of the cerebellar cortex. They are called mossy fibers because of the bushy appearance of their synaptic contacts with the granule cells. There is a high degree of divergence in the connection of mossy fibers to granule cells, as each mossy fiber innervates hundreds of granule cells. Granule cells send axons toward the cortical surface. Each axon bifurcates at the molecular layer, sending protection in opposite directions. These fibers, calledparallel fibrasThey run parallel to the folds of the cerebellar cortex, where they form excitatory synapses withPurkinje cellsalong the path (Figure 5.7, rotated view of RUN). The two-dimensional stalks 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 is in contact with hundreds of Purkinje cells; Due to the high degree of divergence of mossy and granule cell synapses, the activation of each Purkinje cell can be affected (desynaptically) by thousands of mossy fibers.
   
2. climbing fibersonly comes fromworse Oiland make excitatory projections on the cerebellar nuclei and onPurkinje cellscerebellar cortex. They are called climbing fibers because their axons climb and wrap around the dendrites of Purkinje cells like a vine. Each Purkinje cell receives a single, extremely powerful input from a single climbing fiber. Unlike mossy and parallel fibers, each climbing fiber comes into contact with only 10 Purkinje cells on average, forming about 300 synapses with each Purkinje cell. Thus, the climbing fiber provides a limited but extremely strong excitatory stimulus to the Purkinje cells.

The Purkinje cell is the sole source of production for the cerebellar cortex.It should be noted that Purkinje cells form inhibitory connections with the cerebellar nuclei. (Note the difference between the Purkinje cells, which are the only output from the cerebellar cortex, and the cerebellar nuclei, which are the only output from the entire cerebellum.) Almost all the impulses generated by a Purkinje cell are due to its parallel fibers. tickets. Input causes the Purkinje cell to fire at a high resting rate (~70 spikes per second), tonically inhibiting targets in the cerebellar nucleus. Strong signals from climbing fibers are less frequent (~1 hop/s); therefore, they have little effect on the overall activation rate of the Purkinje cell. However, the Purkinje cell spikes generated by the climbing fibers are calcium spikes that allow the climbing fibers to initiate a series of calcium-dependent changes in the Purkinje cell. As described below, one important change appears to be the long-term power shift of parallel fiber inputs to the Purkinje cell.

5.5Damage to the cerebellum causes movement disorders.

Most of the knowledge about cerebellar function comes from studies of patients with cerebellar damage. In general, these patients show uncoordinated voluntary movements and problems with balance and posture. Some of the symptoms of cerebellar damage are listed below (more symptoms are discussed inNext chapter):

1. Traffic distribution.Most of our movements involve the coordinated activity of many muscle groups and various joints to produce a fluid path of body parts in space. Patients with cerebellar dysfunction are unable to perform these fluid, coordinated movements. Instead, they often break movements into parts to execute the desired trajectory. For example, touching your nose with your finger requires coordinated activity at the shoulder, elbow, and wrist joints. Cerebellar patients must move the shoulder first, then the elbow, and finally the wrist, not in a single smooth motion.
   
2. The tremor of intention.When making a movement towards a target, cerebellar patients often produce an involuntary tremor that increases the closer they get to the target. For example, if you're reaching for a cup, your palm starts in a straight line toward the cup; however, as it gets closer, the hand begins to move back and forth in an attempt to make contact with the cup.
   
3. Dysdiadochokinesia.Patients have difficulty performing rapidly changing movements, such as rapidly and repeatedly tapping the palm or back of the hand against a surface.
4. Deficits in motor learning.Experimental studies have shown that damage to the cerebellum causes motor learning deficits in both patients and experimental animals. One of the most important experimental models is the so-calledvestibulo-ocular reflex (VOR). This reflex allows us to keep our gaze fixed on an object when the head is turned (Figure 5.8). Vestibular signals detect head movement and send signals through the cerebellum to the eye muscles to precisely counteract head rotation and maintain a stable center of vision. Motor commands given to the eyes must be precisely calibrated by experience, and this calibration appears to be the task of the cerebellum. Experiments were carried out in which the subjects wore prisms that magnified the visible image. When the subjects' heads moved, the VOR caused the visual image on the retina to change instead of remaining stable. However, as the days passed, the VOR slowly adjusted so that adequate compensatory eye movements were made to maintain a stable image on the retina while the head was turned. In experimental animals, changes in the cerebellum prevent this regulation of the VOR.
   

A second example of cerebellar-dependent motor learning involves making precise and coordinated movements. Subjects wore prismatic glasses that shifted the image to the right and were then asked to throw balls at a target on a wall. Due to the prisms, the accuracy of the targets was initially quite low, as the bullets constantly hit the left side of the target. However, as the exercises were repeated, the subjects became increasingly accurate in hitting the target. After removing the glasses, the subject began to throw balls to the right of the target while their motor programs recalibrated to take advantage of the displaced visual stimuli. Over time, they gradually increased their accuracy again. Patients with cerebellar damage never learned to compensate for the prism because their bullets always landed to the left of the target when they were wearing glasses. After removing their glasses, they hit their mark immediately, as they never made up for previous attempts at the prism.

The third example refers to the classical conditioning of the blink reflex according to Pavlov. In this task, a neutral stimulus (such as a sound) is combined with a noxious stimulus (such as a puff of air into the eye) that causes the eye to blink reflexively. Over time, the test animals will learn to close their eyes when they hear a sound, waiting for a breath of air. This learned closure of the eyelids is very well timed to peak at the expected moment of the puff. Animals with cerebellar damage do not learn to close their eyelids in response to sound.

5.6Cerebellum and control systems.

What do the different symptoms of cerebellar damage that reveal cerebellar function have in common? Many different theories have been proposed. Recall the discussion inChapter 1the ubiquitous use of sensory information in motor control. The cerebellum receives extensive sensory inputs and appears to use them to direct movements in a controlled manner by both feedback and feedback.

Feedback control systems

In a feedback controller, the requested output is continually compared to the actual output, and adjustments are made as the motion is executed until the actual motion matches the desired motion. A typical example of a feedback control system is a home thermostat (Figure 5.9).

The thermostat sets the desired temperature (eg 72°) and the thermometer measures the current room temperature. If the thermostat (comparator) detects that the room is colder than the desired temperature, it sends an error signal that turns on the stove. If the comparator detects that the room is warmer than the desired setting, it will send an error signal and turn on the air conditioner.

Feedback control systems can produce very accurate results; however, they are generally slow. To change the output signal, the effector must wait for the information to be transferred from the sensor to the comparator and then to the effector. At this point, another comparison is made and the process continues. Let us further consider the example of a thermostat. If the temperature is 5° below the desired temperature, the thermostat can order the stove to operate at a moderate temperature. It reads the new temperature in the room, and if it's still too cold, it tells the stove to provide more heat, and so on. While this will eventually lead to exact room temperature at the desired point, it takes several cycles to reach that point. A possible solution for faster results would be to turn a huge stove on full power so that it heats up the room very quickly. However, this solution can cause another problem. If the feedback paths are slow, it will cause the system to oscillate. For example, let's say the stove can heat the room at 5° per second, but it takes 2 seconds for the thermometer to adjust to the new temperature and a new error signal causes the stove to turn off. During these 2 seconds the stove has heated the room by 10° and now it is too hot. Therefore, the error signal turns on the air conditioner and cools the room at 5°/s. Of course it also takes 2 seconds to receive feedback, and by the time the shutdown command is given, the room has cooled 10°. You can see what happens: the system will oscillate endlessly when it is 5° hotter and 5° colder. For a feedback system to work well, the time it takes for the comparator to transmit sensory information to the effector must be fast compared to the execution time.

Feedback control systems only work well when the sensory feedback on the actual score is fast relative to the actual score. If the actual power output is faster than the sensor's ability to provide feedback, the system will tend to oscillate between above and below the requested power level. Therefore, a feedback controller is useful for slow movements like posture correction. The role of the stretch reflex in maintaining posture 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 regularly make (such as eye movements or reaching for a cup). A feedback controller is needed for these movements.

Feedback control systems

W.advanced control systemWhen the requested output is sent to the controller, the controller evaluates sensory information about the environment and the system itself before generating output commands. It uses sensory information to program the best set of instructions to achieve the desired result. However, in a pure feedback system, once the commands are sent, there is no way to change them (ie no feedback loop). The advantage of an advance system is that you can generate a precise set of commands to the effector without having to constantly check the output and make corrections during the movement itself. However, the main disadvantage is that a feedback controller requires a period of learning by trial and error before it can work properly. In most biological systems, it is difficult (perhaps impossible) to program all possible sensory conditions that a controller may encounter throughout the organism's lifetime. Also, the environment and conditions under which actions are taken are constantly changing, and a feedback controller must be able to adjust its output commands to accommodate these changes.

Let's expand on the thermostat example to see how an early temperature controller would work to increase the room temperature from 70° to 75°. The controller would use a variety of sensory information about the environment before sending a command to the oven (Figure 5.10). For example, it will read the current temperature, the current humidity level, the size of the room, the number of people in the room, etc. Based on this information, it will tell the stove to turn on for the set time and that's it. There would be no need to constantly compare the current temperature with the desired set point because the system predetermined how long the oven must run to reach the desired temperature. How did the administrator get this information? A feedback controller requires a lot of experience to learn the appropriate actions needed for each set of environmental conditions. If in an attempt to turn off the stove too soon and the room does not reach the desired temperature, it adjusts its programming so that the next time it encounters the same environmental conditions it turns on the stove longer. Through many of these trial and error learning cases, the predictive system creates a "look-up table" that indicates how long the furnace should be on under current conditions. The key difference between a feedback system and a feedback system is that a feedback system uses sensory information to generate an error signal during movement control whereas a feedback system uses sensory information before movement. Any error signal with respect to the final start signal is used by the anticipation system only to reschedule future moves.

The cerebellum may be an anticipatory control system.

The involvement of the cerebellum in the VOR can be explained in terms of the feedback controller's learning requirements. When the head moves, compensatory eye movements must be made to maintain a steady gaze. The cerebellum receives sensory input from the vestibular system, informing it of head movement. It also receives input from eye muscle proprioceptors and other relevant sources of information about current conditions to perform accurate compensatory eye movement. It evaluates all of this advanced sensory information and calculates the correct eye movement to accurately balance head movement. However, what if the eye movements do not match the head movements and the image moves across the retina (for example, in an experimental setting where a prism was used, or in a situation where real life where a person wears new prescription glasses?) )? Retinal shift is an error signal that tells the cerebellum that the next time these conditions are met, it should adjust eye movement to reduce retinal shift. This trial and error sequence will be repeated until the movement is correctly calibrated; Furthermore, these mechanisms will ensure that the movements remain calibrated.

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