Dr.JEEJA.S, M.D.Hom (Repertory), M.D.Hom (HOMOEOPATHIC PHYLOSOPHY)
Tutor, Department of Organon of Medicine, Govt.Homoeopathic Medical College, Thiruvananthapuram, Kerala.
The cerebellum is involved in the coordination of voluntary motor movement, balance and equilibrium and muscle tone. It is located just above the brain stem and toward the back of the brain. It is relatively well protected from trauma compared to the frontal and temporal lobes and brain stem.
Cerebellar injury results in movements that are slow and uncoordinated. Individuals with cerebellar lesions tend to sway and stagger when walking.
Damage to the cerebellum can lead to: 1) loss of coordination of motor movement (asynergia), 2) the inability to judge distance and when to stop (dysmetria), 3) the inability to perform rapid alternating movements (adiadochokinesia), 4) movement tremors (intention tremor), 5) staggering, wide based walking (ataxic gait), 6) tendency toward falling, 7) weak muscles (hypotonia), 8) slurred speech (ataxic dysarthria), and 9) abnormal eye movements (nystagmus).
- It is important for coordinating voluntary movements (e.g. walking, posture, speech) and for learning motor (skilled) behaviors.
- The cerebellum, like the cerebrum, has a cortex or outer covering of gray matter. The types/names of neurons and layers in the two cortices differ
One of the most impressive parts of the human brain, named the cerebellum, has been underestimated for centuries. Located at the lower back of the brain, it is a fist-sized structure whose function is now being reappraised. Formerly this structure was thought to have only a motor function, which it performed by helping other motor regions of the brain to do their work effectively. But during the past decade a broader view of its function has emerged as a result of new research, and now the cerebellum is regarded as a structure that can help not only motor but also nonmotor regions to do their work effectively. In fact, the cerebellum has been compared to a powerful computer, capable of making contributions both to the motor dexterity and to the mental dexterity of humans, both of which are required for the emergence of fluent human language.
This powerful mechanism at the bottom of the brain, which every person inherits as a birthright, is immature at birth but develops through childhood and adolescence, reaching its full structural growth by the 15th to 20th year of life. Judged by what it contains and by its external connections, the human cerebellum is an enormously impressive mechanism. First of all, it contains more nerve cells (neurons) than all the rest of the brain combined. Second, it is a more rapidly acting mechanism than any other part of the brain, and therefore it can process quickly whatever information it receives from other parts of the brain. Third, it receives an enormous amount of information from the highest level of the human brain (the cerebral cortex), which is connected to the human cerebellum by approximately 40 million nerve fibers. To appreciate what a torrent of information these 40 million fibers can send down from the cerebral cortex to the cerebellum, a comparison can be made with the optic fibers in the human brain. The optic tract contains approximately one million nerve fibers, which transmit to the brain the visual information that a human receives via the eyes. Forty times that much information can be sent from the cerebral cortex down to the cerebellum, including information from sensory areas of the cerebral cortex, from motor areas, from cognitive areas, from language areas, and even from areas involved in emotional functions.
. Functions of the Cerebellum
[Given that the cerebellum seems well organized to convey complex information to many other regions of the brain, where does it actually send this information? Each module of the cerebellum seems to be uniquely connected, both through its input and output connections, with different regions of the brain. Modules in the middle of the cerebellum (in the medial part) receive different input and send information to different output targets than do the modules in the lateral part of the cerebellum. Despite such differences in input and output, however, the circuitry within each module seems to be similar to that in every other module. For this reason, the basic processing that every module can perform on the incoming information would seem to be similar, no matter whether this incoming information represents motor, sensory, cognitive, linguistic, or any other kind of information.
Although many of these theories are considered controversial at present, it seems possible that each of them may be at least partially correct and that the present controversies can therefore be reconciled in the future. The present proposals encompass not only the traditional view that the cerebellum is involved in skilled motor performance but also the broader view that it is involved in skilled mental performance, and is also involved in various sensory functions including sensory acquisition, discrimination, tracking and prediction. A recent theory that is broad enough to encompass all of these motor, mental, and sensory functions has proposed that the cerebellum does the following basic processing: It makes predictions (based on prior experience or learning) about the internal conditions that are needed to perform a sequence of tasks in other regions of the brain, and it sets up such internal conditions in those regions automatically, thus preparing those regions for the optimal performance of the tasks. By doing this, the powerful and versatile computing capabilities of the cerebellum would be used for providing automatic help to various other regions of the brain, helping them to do their work better.
The Advantages of Automation
Experimental evidence has shown that the cerebellum is involved in the process by which novel motor tasks can, after some practice, be performed automatically. Through such automation, the performance can be improved: Sequences of movements can be made with greater speed, greater accuracy, and less effort. The cerebellum also is known to be involved in the mental rehearsal of motor tasks, which also can improve performance and make it more skilled.
Because the cerebellum is connected to regions of the brain that perform not only motor but also mental and sensory tasks, it can automatize not only motor but also mental and sensory skills in the human brain. As with motor skills, several advantages accrue from learning to perform the other skills automatically, without conscious attention to detail.
The skills involved in human communication, for example, require both motor and mental activity: the motor activity of speech or gesture, and the mental activity that formulates what is to be said. In the course of learning these skills, an individual’s performance can be improved incrementally through practice so that the skills eventually can be performed without conscious attention to detail. For example, in recalling words stored in the memory, the activity can be performed without conscious attention to the details of how the words are selected by the brain during the retrieval process.
To the extent that an individual can perform some mental activities without conscious attention to detail, the conscious part of the brain is freed to attend to other mental activities, thus enlarging its cognitive scope. Such enlargement of human capabilities is attributable in no small part to the enlarged human cerebellum and its contribution to the automation of mental activities, which appears to have been a prerequisite for the emergence of human language. Because such language confers a unique and inestimable advantage on humans, the cerebellum can be regarded as an underestimated treasure submerged at the bottom of the brain.
The Cerebellum Functional Anatomy
The Cerebellum lies on the posterior aspect of the BrainStem, attached to it by three pairs of Cerebellar Peduncles that contain both Afferent and Efferent Nerve fibers. A centrally placed Vermis supports the two laterally placed Cerebellar Hemispheres. Small folds abound in all parts of the Cerebellum and are known as Folia. These Folia let the Convolutions of the Cerebellum provide a vastly increased surface area for placement of Neurons. An outer covering of Gray Matter, the Cerebellar Cortex, overlies the Medullary Body of White Matter. Deep Nuclei lie more or less centrally within the organ.
The Cerebellum is Composed of Three Portions:
- The VestibuloCerebellum or archeo cerebellum
- is the phylogenetically oldest part of the organ and anatomically corresponds to the Flocculus and Nodule. This portion is present in all Vertebrates.
As the name Vestibulo suggests, this portion of the Cerebellum has Afferent and Efferent connections mainly with the Vestibular Apparatus of the Inner Ear (Semicircular Canals and Maculae).
The information conveyed is related to changes in head position, acceleration, decelerration and angular movements. Recently, fibers from the Retina and from the parts of the Brain concerned with Eye movements have been shown to terminate in the VestibuloCerebellum.
- The SpinoCerebellum or palleo cerebellum corresponds anatomically to the Vermis and receives general Sensory (Touch, Pressure, Thermal) and Proprioceptive impulses chiefly from the Ascending Pathways of the Spinal Cord.
These types of impulses give information concerning the Rate, Force, and Direction of movements as detected by Skin, Muscle, and Tendon Receptors. This aspect of movement is sometimes called Performance.
- The PontoCerebellum or neo cerebellum corresponds anatomically to most of the Anterior and Posterior Lobes of the organ and is the largest part of the Cerebellum in animals capable of skilled or complex types of movement.
The major input to this part of the Cerebellum is via PontoCerebellar fibers originating from Nuclei in the Pons that, in turn, have received impulses from the Motor regions of the Cerebral Cortex. This part of the system conveys Cortical Intent.
D. The cerebellum:
The cerebellum is involved in the coordination of movement. A simple way to look at its purpose is that it compares what you thought you were going to do (according to motor cortex) with what is actually happening down in the limbs (according to proprioceptive feedback), and corrects the movement if there is a problem. The cerebellum is also partly responsible for motor learning, such as riding a bicycle. Unlike the cerebrum, which works entirely on a contralateral basis, the cerebellum works ipsilaterally.
The cerebellum (“little brain”) has convolutions similar to those of cerebral cortex, only the folds are much smaller. Like the cerebrum, the cerebellum has an outer cortex, an inner white matter, and deep nuclei below the white matter.
Cat cerebellum, sagittal section
|Single folium, enlarged|
If we enlarge a single fold of cerebellum, or a folium, we can begin to see the organization of cell types. The outermost layer of the cortex is called the molecular layer, and is nearly cell-free. Instead it is occupied mostly by axons and dendrites. The layer below that is a monolayer of large cells called Purkinje cells, central players in the circuitry of the cerebellum. Below the Purkinje cells is a dense layer of tiny neurons called granule cells. Finally, in the center of each folium is the white matter, all of the axons traveling into and out of the folia.
These cell types are hooked together in stereotypical ways throughout the cerebellum.
|However, the individual parallel fibers are not a strong drive to the Purkinje cells. The Purkinje cell dendrites fan out within a plane, like the splayed fingers of one hand. If you were to turn a Purkinje cell to the side, it would have almost no width at all. The parallel fibers run perpendicular to the Purkinje cells, so that they only make contact once as they pass through the dendrites.|
Although each parallel fiber touches each Purkinje cell only once, the thousands of parallel fibers working together can drive the Purkinje cells to fire like mad.
The second main type of input to the folium is the climbing fiber. The climbing fibers go straight to the Purkinje cell layer and snake up the Purkinje dendrites, like ivy climbing a trellis. Each climbing fiber associates with only one Purkinje cell, but when the climbing fiber fires, it provokes a large response in the Purkinje cell.
|The Purkinje cell (left) compares and processes the varying inputs it gets, and finally sends its own axons out through the white matter and down to the deep nuclei. Although the inhibitory Purkinje cells are the main output of the cerebellar cortex, the output from the cerebellum as a whole comes from the deep nuclei. The three deep nuclei are responsible for sending excitatory output back to the thalamus, as well as to postural and vestibular centers.|
|There are a few other cell types in cerebellar cortex, which can all be lumped into the category of inhibitory interneuron. The Golgi cell is found among the granule cells. The stellate and basket cells live in the molecular layer. The basket cell (right) drops axon branches down into the Purkinje cell layer where the branches wrap around the cell bodies like baskets.|
|The Cerebellum lies on the posterior aspect of the BrainStem, attached to it by three pairs of Cerebellar Peduncles that contain both Afferent and Efferent Nerve fibers. A centrally placed Vermis supports the two laterally placed Cerebellar Hemispheres. Small folds abound in all parts of the Cerebellum and are known as Folia.
These Folia let the Convolutions of the Cerebellum provide a vastly increased surface area for placement of Neurons. An outer covering of Gray Matter, the Cerebellar Cortex, overlies the Medullary Body of White Matter. Deep Nuclei lie more or less centrally within the organ.
Cerebellar Cortex Cells
The Cerebellar Cortex contains the cells and circuitry that enable the organ to carry out its functions, and the structure is the same in all parts of the organ. The Inner or Deepest layer of the Cortex, called the Granule Layer, consists of many closely packed Granule Cells and cells called Golgi Cells.
The middle layer of the Cortex consists of a single row of large Purkinje Cells associated with Basket Cells. The Purkinje Dendrites ramify in the Outer Molecular Layer and are associated with Stellate Cells.
Input to these cells is provided by Mossy and Climbing Fibers. Mossy Fibers synapse with Golgi or Granule Cells (and also send branches to the Deep Nuclei), while Climbing Fibers reach all Five basic cell types (Granule, Golgi, Purkinje, Basket, and Stellate) and the Deep Nuclei.
This input is all excitatory. The Deep Nuclei receive impulses from the Mossy and Climbing Fibers and from the Purkinje cells. The Deep Nuclei generate the Output of the Cerebellum to all aspects of Motor Activity and the Impulses are apparently All Excitatory to the cells affected.
The Purkinje Cells, on the other hand, are invariably Inhibitory to the Nuclei they affect.
The net effect of this circuitry is summarized as:
The Cerebellum consists of:
The Major Internal Cerebellar Nuclei
1. Fastigial Nucleus
2. Dentate Nucleus
3. Interpositus Nucleus, is divided into:
Most Purkinje Cells in any of the three functional Cerebellar zones (Vermis, Intermediate Hemisphere, & Lateral Hemisphere) project to the same Internal Cerebellar Nucleus.
Projections from the Cerebellar Nuclei terminate in specific loci and, therefore, modulate specific aspects of motor function:
- Fastigial Nucleus to the Lateral Vestibular and Reticular Nuclei (for balance and posture)
- Interpositus Nucleus to the Red Nucleus and the Ventrolateral Nucleus of the Thalamus (for posture, gait, and coarse movements)
- Dentate Nucleus to the VentroLateral Nucleus of the Thalamus (for skilled movements of hands and fingers)
*Internal Nuclei are excitatory on muscle tone & motor activity*Cerebellum Functions In Movement Control
The Cerebellum Contributes To Voluntary Movements:
- It correlates incoming muscle and other sensory information
- It computes the most effective deployment of muscular effort necessary to accomplish a required task
- It composes the necessary outgoing commands to the Spinal motor Neurons and the Motor Cortex
- Some zones and associated internal nuclei show electrical activity only after the onset of movement (e.g. the Interpositus Nucleus)
- Their purpose is thought to be compensation on the basis of Sensory Feedback
Other zones show electrical activity before the onset of movement (e.g. the Dentate Nucleus). They probably participate in the generation of motor sequences.
Their particular function is control of the relative timing of Agonist and Antagonist Alpha Motor Neuron activity to effect a smooth pattern of limb Acceleration, Deceleration, Stop, and Acceleration in the Opposite Direction.
Mild Cerebellar Dysfunction results in inability to judge the range of limb movements without watching them.
Severe Cerebellar Dysfunction results in inability to perform limb movements smoothly and efficiently even while watching them.
Internal Cerebellar Organization
A – Incoming tracts
B – Purkinje cells
C – Granule cells
D – Internal Nuclei along with their outflow tracts
There are also three populations of local Inhibitory InterNeurons that modulate the function of Purkinje & Granule Cells:
- Basket cells
- Stellate cells
- Golgi cells
Input & Output Neurons
Simple Spike Discharges:
Incoming Mossy Fibers form excitatory Synapses with Granule Cells:
- Granule Cells are the major input cells
- Axons project toward the surface, bifurcate, and form a layer of parallel fibers in the direction of the Folia
- Each parallel fiber forms excitatory synapses with a sequence of several dozen Purkinje Cells at their flattened Dendritic bushes
- Purkinje Cells are the major Cortical Output Cells
Excitation of a sufficient number of parallel fibers contacting a given Purkinje Cell will cause that Cell to discharge trains of simple spikes that inhibit the muscles to which the Cell projects.
Complex Spike Discharges
Purkinje Cells also receive excitatory input from Climbing Fibers that originate mostly in the opposite Inferior Olive. (Each region of the Inferior Olive projects to a seperate longitudinal strip of Cerebellar Cortex).
Climbing Fibers synapse extensively with dendrites of Purkinje Cells, but not with the Cell Body. Climbing Fiber input to a Purkinje Cell produces in that Cell a large, prolonged complex spike discharge.
In addition to the obvious anatomic division of the Cerebellum into horizontal folds, there is a functional subdivision into three vertical strips:
1 – Vermis
2 – Intermediate Hemisphere
3 – Lateral Hemisphere
Different body parts are topographically mapped in these zones:
* Trunk and Head in the Vermis
* Limbs stretched out toward the Lateral Hemisphere.
Nearly all Purkinje cells in any one of the three strips project to the same internal Cerebellar Nucleus:
- Vermis to the Fastigial Nucleus
- Intermediate Hemisphere to the Interpositus Nucleus (Globose & Emboliform Nuclei)
- Lateral Hemisphere to the Dentate Nucleus
- ( Purkinje Cells in the Flocculonodular Lobe are an exception because they project directly to the Lateral Vestibular Nucleus in the MidBrain, where they participate in the control of Eye movements.)
Are excited by parallel fibers inhibit Purkinje Cells.
Are excited by parallel fibers The axons of each of these form baskets of inhibitory synapses around 20 or more Purkinje Cells.
Have dendritic trees that spread in all directions among the parallel fibers (unlike the flattened Purkinje dendrites, which are confined to a narrow longitudinal layer); are excited by input from parallel fibers and Mossy fibers; act to inhibit Granule Cells
E. Inputs and outputs of the cerebellum:
The cerebellum operates in 3’s: there are 3 highways leading in and out of the cerebellum, there are 3 main inputs, and there are 3 main outputs from 3 deep nuclei. They are:
The 3 highways are the peduncles, or “stalks”. There are 3 pairs: the inferior, middle, and superior peduncles.
The 3 inputs are: Mossy fibers from the spinocerebellar pathways, climbing fibers from the inferior olive, and more mossy fibers from the pons, which are carrying information from cerebral cortex. The mossy fibers from the spinal cord have come up ipsilaterally, so they do not need to cross. The fibers coming down from cerebral cortex, however, DO need to cross (remember the cerebrum is concerned with the opposite side of the body, unlike the cerebellum). These fibers synapse in the pons (hence the huge block of fibers in the cerebral peduncles labeled “corticopontine”), cross, and enter the cerebellum as mossy fibers.
The 3 deep nuclei are the fastigial, interposed, and dentate nuclei. The fastigial nucleus is primarily concerned with balance, and sends information mainly to vestibular and reticular nuclei. The dentate and interposed nuclei are concerned more with voluntary movement, and send axons mainly to thalamus and the red nucleus.
Clinical Signs of Cerebellar Dysfunction
With one exception, all cerebellar signs reflect inaccurate, inappropriate, or absent clock signals necessary for smooth, coordinated, synergistic movement. The following common signs of CBL dysfunction are due to inappropriate timing signals:
Another important sign of CBL dysfunction is hypotonia. This is due to a decreased firing frequency of the gamma motor neurons innervating the muscle spindles of the affected limb.
Depending on the distribution of CBL signs, the following two syndromes are often recognized:
Archicerebellar Syndrome: Incoordination of the muscles of equilibrium; dysequilibrium with wide-based gate; disturbances of stance and gait. Heel-to-toe walking is impaired.
Neocerebellar Syndrome: Incoordination of muscles for voluntary movements. Heel-to-shin movements and rapid alternating movements (diadochokinesis) are impaired.