Cerebellar Anatomy and Motor Control Mechanisms

 

CEREBELLUM:

Gross Appearance:

The tentorium cerebelli covers the cerebellum superiorly and is located in the posterior cerebral fossa. It is the biggest portion of the hindbrain and is located behind the medulla oblongata, the pons, and the fourth ventricle. The cerebellum has a slightly oval form with a restricted middle section. It is made up of two hemispheres of the cerebellum connected by a thin median vermis. The superior, middle, and inferior cerebellar peduncles are three symmetrical bundles of nerve fibers that connect the cerebellum to the posterior part of the brainstem.

The same side of the body is controlled by each cerebellar hemisphere, and although the cerebellum lacks a direct connection to lower motor neurons, it nevertheless maintains control over those motions through the cerebral cortex and brainstem.

The anterior, middle, and flocculo-nodular lobes are the three primary lobes that make up the cerebellum. The major fissure, a broad V-shaped fissure that divides the anterior lobe from the middle lobe, is visible on the superior surface of the cerebellum. The largest region of the cerebellum, the middle lobe (also known as the posterior lobe), is located between the primary and uvulonodular fissures. Positioned behind the uvulonodular fissure is the flocculonodular lobe. There is no morphologic or functional relevance to the deep horizontal fissure that divides the superior and inferior surfaces of the cerebellum along its edge.

Structures:

The cerebellum is made up of inner white matter and an outside layer of gray matter known as the cortex. The intracerebellar nuclei are three masses of gray matter embedded in the white matter of each hemisphere.

Cerebellar Cortex:

You can think of the cerebellar cortex as a big sheet having folds in the transverse or coronal plane. Every fold, or folium, has a core of white matter with gray matter covering it on the outside.

The folia are divided at right angles by a portion that passes through the cerebellum parallel to the median plane. The cut surface, known as the arbor vitae, has a branching look.

The cortex's gray matter has a consistent organization throughout. It can be separated into three layers: the granular layer is an interior layer,

· Purkinje cell layer is the intermediate layer,

· Molecular layer is the outer layer. 

Molecular Layer:

The outer stellate cell and the inner basket cell are the two types of neurons found in the molecular layer. These neurons are dispersed along several thin axons that run parallel to the folia's long axis, as well as dendritic arborizations. In the spaces between these structures are neuroglial cells.

Purkinje Cell Layer:

The massive Golgi type I neurons are called Purkinje cells. They are stacked in a single layer and have a flask-like form. The dendrites of these cells are observed to enter the molecular layer on a plane transverse to the folium, where they undergo abundant branching. Short, thick dendritic spines cover the ensuing branches, which are smooth save for the major and secondary branches. It has been demonstrated that the parallel fibers originating from the granule cell axons and the spines make synaptic connections.

Exploring the Path of the Purkinje Cell Axon:

The axon emerges at the base of the Purkinje cell and travels through the granular layer before entering the white matter. The axon receives a myelin coating upon entering the white matter and ends by synapsing with cells of one of the intracerebellar nuclei. The Purkinje axon's collateral branches form synaptic connections with the granular layer's stellate cells and basket dendrites in nearby folia or farther away. A small number of the axons from Purkinje cells terminate directly in the brainstem's vestibular nuclei.

Granular Layer:

The granular layer is full of tiny cells with sparse cytoplasm and heavily stained nuclei. Four or five dendrites grow from each cell; these end segments resemble claws and make synaptic contact with mossy fiber input. Each granule cell's axon enters the molecular layer, where it bifurcates at a junction, with the branches extending parallel to the cerebellar folium's long axis. The Purkinje cells' dendritic processes are at a right angle to these fibers, which are referred to as parallel fibers. The spinous processes of the Purkinje cell dendrites are synaptically contacted by the majority of parallel fibers. There are neuroglial cells all over this layer. Golgi cells are dispersed throughout the granular layer. Their axons terminate by dividing into branches that synapse with the granular cell dendrites, and their dendrites ramify in the molecular layer.

Functional Areas:

The three functional areas of the cerebellar cortex have been identified through the use of positron emission tomography in experiments and clinical observations by neurologists and neurosurgeons.
The movements of the body's long axis, which includes the neck, shoulders, thorax, belly, and hips, are influenced by the cortex of the vermis. The cerebellar hemisphere's "intermediate zone" is located directly lateral to the vermis. It has been demonstrated that the muscles of the distal limbs, particularly the hands and feet, are under the control of this area. Each cerebellar hemisphere's lateral zone is thought to be involved in the conscious evaluation of movement faults as well as the planning of the body's successive motions.

Intracerebellar Nuclei:

On either side of the midline, the cerebellum's white matter contains four embedded masses of gray matter. These nuclei are the globose, the dentate, the emboliform, and the fastigial, arranged from lateral to medial.
The dentate nucleus, which resembles a crumpled bag with its entrance facing medially, is the biggest cerebellar nucleus. White matter composed of efferent fibers that exit the nucleus through the aperture and make up a significant portion of the superior cerebellar peduncle fills the inside of the bag. The ovoid, partially-covered hilus of the dentate nucleus is medial to the oval, emboliform nucleus. One or more rounded cell groups that are positioned medial to the emboliform nucleus make up the globose nucleus. Larger than the globose nucleus, the fastigial nucleus is located close to the fourth ventricle's roof and the vermis' midline. The simple branching dendrites of big, multipolar neurons make up the intracerebellar nuclei. In the superior and inferior cerebellar peduncles, the axons form the cerebellar outflow.

White Matter:

The phrase arbor vitae refers to the modest quantity of white matter in the vermis that resembles the trunk and branches of a tree. White matter makes up a significant portion of each cerebellar hemisphere.
The three types of fibers that comprise the white matter are efferent, afferent, and intrinsic.
The cerebellum's intrinsic fibers link its many sections rather than departing from it. Some link the vermis and cerebellar cortex folia on the same side, whereas others link the two cerebellar hemispheres.
The majority of the white matter is made up of afferent fibers, which travel to the cerebellar cortex. The inferior and middle cerebellar peduncles are the primary entrance points for them into the cerebellum. The axons of the Purkinje cells in the cerebellar cortex give rise to the efferent fibers, which are the cerebellum's output. The vast majority of the axons of Purkinje cells connect and pass through the neurons of the fastigial, globose, emboliform, and dentate cerebellar nuclei.

Synaptic Patterns of Purkinje Cell Axons:

The neurons' axons then exit the cerebellum. A small number of Purkinje cell axons in the vermis and flocculonodular lobe avoid the cerebellar nuclei and exit the cerebellum without forming synapses.
The superior cerebellar peduncle is the exit point for fibers from the dentate, emboliform, and globose nuclei that exit the cerebellum. Fibers departing from the inferior cerebellar peduncle originate in the fastigial nucleus.

Cerebellar Cortical Mechanism:

Numerous cytologic and physiological studies have led to the identification of certain fundamental processes that the cerebellar cortex is thought to be involved in. The two primary pathways of input to the cortex are the mossy and ascending fibers, which excite the Purkinje cells.
The olivocerebellar tracts' terminal fibers are known as the climbing fibers. They climb across the layers of the cortex like a vine on a tree, which is how they got their name. They repeatedly divide to reach the molecular layer after passing through the cortex's granular layer. Every climbing fiber encircles a Purkinje cell dendrite and forms several synaptic contacts with it. Only one climbing fiber synaptically contacts a single Purkinje neuron.

Insights into Inhibition and Motor Control:

On the other hand, one climbing fiber contacts one to ten Purkinje neurons. Each climbing fiber has a few side branches that splice with the stellate cells and basket cells. All other cerebellar afferent pathways terminate with the mossy fibers. Their excitatory effect is significantly more diffuse and they have several branches. Through the granule cells, a single mossy fiber can activate thousands of Purkinje cells. Then, what role do the Golgi, stellate, and basket cells the remaining cells in the cerebellar cortex play? Microelectrode-based neurophysiologic study would suggest that they function as inhibitory interneurons.

They probably affect the level of Purkinje cell excitation resulting from the ascending and mossy fiber input in addition to restricting the area of cortex that is activated. This is how the Purkinje cells transfer varying inhibitory signals to the intracerebellar nuclei, which then influence muscle contraction via the brainstem and cerebral cortex's motor control regions. Consequently, the cerebellar cortex is a functional unit with Purkinje cells at its core.

Cerebellar Affferent fibres:

The medulla oblongata, pons, cerebral cortex, and spinal cord send important afferent routes to the cerebellum.

Brain Cortex-Derived Cerebellar Afferent Fibers:

The three paths that the cerebral cortex uses to transmit information to the cerebellum are 

• The corticopontocerebellar pathway, 
• The cerebro-olivocerebellar pathway, and 
• The cerebroreticulocerebellar pathway.

The Cerebro-Olivocerebellar Cortex:

The cortico-olivary fibers originate from the nerve cells found in the cerebral cortex's frontal, parietal, temporal, and occipital lobes. They then descend via the internal capsule and corona radiata and terminate bilaterally on the inferior olivary nuclei. Through the inferior cerebellar peduncle, fibers originating from the inferior olivary nuclei cross the midline and reach the opposing cerebellar hemisphere. The climbing fibers in the cerebellar cortex are where these fibers come to an end.

Cerebroreticulocerebellar Pathway:

Neural cells from various regions of the cerebral cortex, especially the sensorimotor areas, give rise to the corticoreticular fibers. They descend to come to an end in the

• Three pathways provide information from the cerebral cortex to the cerebellum: The corticopontocerebellar pathway,

• Two pathways in particular are the cerebro-olivocerebellar and the cerebroreticulocerebellar.

Cerebellar Corticoponto Pathway:

The cerebral cortex's frontal, parietal, temporal, and occipital lobes are the source of the corticopontine fibers, which descend via the corona radiata and internal capsule before ending on the pontine nuclei. The transverse processes originate in the pontine nuclei.

Cerebroreticulocerebellar Pathway:

Neural cells from various regions of the cerebral cortex, especially the sensorimotor areas, give rise to the corticoreticular fibers. On one side, they descend to terminate in the reticular formation, and on the other, they terminate in the pons and medulla. The inferior and middle cerebellar peduncles on the same side of the cerebellar hemisphere receive the reticulocerebellar fibers, which are produced by the cells in the reticular formation. The regulation of voluntary movement is significantly impacted by the relationship between the cerebellum and the cerebrum. It is likely that information about the beginning of movement in the cerebral cortex is sent to the cerebellum, allowing for the monitoring of the movement and the making of necessary modifications to the muscle activity.

Cerebellar Afferent Fibers From the Spinal Cord:

Three pathways:
1.    The anterior spinocerebellar tract,
2.     The posterior spinocerebellar tract,
3.     The cuneocerebellar tract
are used by the spinal cord to transmit data from somatosensory receptors to the cerebellum.

Anterior Spinocerebellar Tract:

The neurons of the nucleus dorsalis (Clarke column), at the base of the posterior gray column, synapse with the axons that enter the spinal cord from the posterior root ganglion. Some of these neurons' axons climb as the anterior spinocerebellar tract in the lateral white commision on the same side, but the majority of their axons cross to the opposite side and ascend as the anterior spinocerebellar tract in the contralateral white column. The fibers become mossy fibers in the cerebellar cortex after exiting the cerebellum via the superior cerebellar peduncle. Additionally, collateral branches that terminate in the deep cerebellar nuclei release. It is believed that the fibers in the spinal cord that cross over to the other side also cross back within the cerebellum.

The fibers of the anterior spinocerebellar tract, which spans all spinal cord segments, carry information about muscles and joints from the muscle spindles, tendon organs, and joint receptors of the upper and lower limbs. This tract probably transports information from the skin and superficial fascia to the cerebellum.

Posterior Spinocerebellar Tract:

The posterior gray column is where the axons from the posterior root ganglion that enter the spinal cord end by synapsing on the neurons at the column's base. The nucleus dorsalis is the aggregate name for these neurons (Clarke column). These neurons' axons climb as the posterior spinocerebellar tract to the medulla oblongata after entering the posterolateral region of the lateral white column on the same side. Here, the tract ends as mossy fibers in the cerebellar cortex after entering the cerebellum through the inferior cerebellar peduncle. Additionally, there are collateral branches that terminate in the deep cerebellar nuclei. The tendon organs, joint receptors in the trunk and lower limbs, and muscle spindles all provide information about muscle joints to the posterior spinocerebellar tract.

Cuneocerebellar Tract:

The inferior cerebellar peduncle on the same side of the medulla oblongata is where these fibers enter the cerebellar hemisphere after emerging from the nucleus cuneatus. The fibers end up looking like mossy fibers. within the cortical cerebellum. Additionally, there are collateral branches that terminate in the deep cerebellar nuclei. The upper limb and upper region of the thorax's joint receptors, tendon organs, and muscle spindles all provide information about muscles to the cuneocerebellar tract.

Vestibular Nerve Afferent Fibers in the Cerebellum:

The vestibular nerve receives information from the inner ear about position in relation to gravity from the utricle and saccule, as well as motion from the semicircular canals. Through the inferior cerebellar peduncle on the same side, the vestibular nerve delivers many afferent fibers directly to the cerebellum. Prior to synapsing and being sent to the cerebellum, other vestibular afferent fibers travel through the brainstem's vestibular nuclei. On the same side, they pass through the inferior cerebellar peduncle and enter the cerebellum. In the flocculonodular lobe of the cerebellum, all of the afferent fibers originating from the inner ear come to an end as mossy fibers.

Additional Afferent Fibers:

Furthermore, the tectum and the red nucleus send tiny bundles of afferent fibers to the cerebellum.

Additional Afferent Fibers:

Furthermore, the tectum and the red nucleus send tiny bundles of afferent fibers to the cerebellum. 

Cerebellar Efferent Fibres:

The axons of the Purkinje cells carry the cerebellar cortex's full output. The majority of Purkinje cell axons terminate by forming synapses with deep cerebellar nucleus neurons. The efferent outflow from the cerebellum is made up of the axons of the neurons that make up the cerebellar nuclei. A small number of axons from Purkinje cells exit the cerebellum and go straight to the lateral vestibular nucleus. The vestibular complex, reticular formation, thalamus, and red nucleus are all connected to the cerebellum's efferent fibers.

Rubral-Emboliform-Globose Pathway:

When the superior cerebellar peduncles decussate, the axons of the neurons in the globose and emboliform nuclei pass through the structure and cross the midline to reach the other side. The contralateral red nucleus cells with which the fibers synapse at the end give origin to the rubrospinal tract's axons. As a result, this pathway crosses twice: once in the superior cerebellar peduncle's decussation and once more in the rubrospinal tract near its origin. In this way, motor activity on the same side of the body is influenced by the globose and emboliform nuclei.

Dentatothalamic Pathway:

When the superior cerebellar peduncle decusses, axons from neurons in the dentate nucleus pass through it and cross the midline to the other side. The fibers terminate by forming synapses with cells located in the thalamus's contralateral ventrolateral nucleus. The thalamic neurons' axons climb via the corona radiata and internal capsule before coming to an end in the cerebral cortex's major motor region. By affecting the motor neurons in the opposing cerebral cortex, the dentate nucleus can affect motor activity via this pathway. Motor cortex impulses are then sent to the spinal segmental levels via the corticospinal tract.

Recall that the majority of the corticospinal tract fibers cross to the opposite side at the spinal segmental levels or later, during the pyramid decussation. As a result, the dentate nucleus can synchronize muscle contractions on the same side.

Vestibular Fastigial Pathway:

After passing via the inferior cerebellar peduncle, the axons of the neurons in the fastigial nucleus terminate by projecting onto the neurons on each side of the lateral vestibular nucleus. Keep in mind that certain axons from Purkinje cells project straight to the lateral vestibular nucleus. The vestibulospinal tract is made up of neurons from the lateral vestibular nucleus. Primarily affecting the tone of the ipsilateral extensor muscles, the fastigial nucleus has a facilitative effect.

Pathway Fastigial Reticular:

After passing via the inferior cerebellar peduncle, the axons of neurons in the fastigial nucleus synapse with neurons in the reticular formation. Through the reticulospinal tract, the axons of these neurons affect the motor activity of the spinal segments.

Functions of Cerebellum:

The cerebral cortex, as well as the muscles, tendons, and joints, provide the cerebellum with afferent information about voluntary movement. Additionally, it receives information from the vestibular nerve regarding balance and possibly from the tecto cerebellar tract regarding vision. The Purkinje cells are the point of convergence for the mossy and ascending fibers that feed all this information into the cerebellar cortex circuitry (see Fig. 6-8). With very few exceptions, the axons of Purkinje cells project onto the deep cerebellar nuclei. The globose and emboliform nuclei receive the output of the vermis, the dentate nucleus receives the output of the lateral part of the cerebellar hemisphere, and the intermediate parts of the cortex receive the output of the vermis.

Cerebellar Influence on Motor Control:

A small number of Purkinje cell axons exit the cerebellum directly and terminate on the brainstem's lateral vestibular nucleus. It is commonly accepted that Purkinje axons have an inhibitory effect on the neurons found in the lateral vestibular nuclei and the cerebellar nuclei.

The descending routes' sites of origin, which affect motor activity at the segmental spinal level, receive the cerebellar output. In this way, the cerebral cortex and brainstem allow the cerebellum to indirectly affect lower motor neurons while it lacks direct neural connections with them.

Descending Pathways and Motor Neuron Influence:

Physiologists have hypothesized that the cerebellum serves as a coordinator of precise movements by continuously comparing the proprioceptive information received from the site of muscle action with the output of the motor area of the cerebral cortex. It can then influence the activity of the lower motor neurons to bring about the necessary adjustments. The a and y motor neurons' firing patterns and timing are manipulated to achieve this. The amount of voluntary movement may be limited if the cerebellum provides the motor cerebral cortex with information that inhibits the agonist muscles and stimulates the antagonist muscles.