Cerebellum Physiology Notes

Parts Of Cerebellum

The cerebellum consists of a narrow, worm-like central body called vermis and two lateral lobes, the right and left cerebellar hemispheres. The part of vermis on the upper surface of the cerebellum is known as the superior vermis and the vermis on the under surface of the cerebellum is called the inferior vermis.

Vermis

• The vermis of the cerebellum is formed by nine parts. The parts of the superior vermis and inferior vermis are listed.
• Nodulus is continued on either side as an elongated and somewhat lobulated structure called flocculus. Modulus and flocculi are together called the flocculonodular lobe. On either side of the pyramid, there is another extension named para flocculus.

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• Superior vermis
  1. Lingula
  2. Central lobe
  3. Culmen
  4. Lobulus simplex
  5. Declive
• Inferior vermis
  1. Tuber
  2. Pyramid
  3. Uvula
  4. Modulus
• Fissures Present Over the Surface of Vermis
  • The primary fissure between culmen and lobulus simplex
  • Prepyramidal fissure between tuber and pyramid
  • The posterolateral fissure between the uvula and nodulus.

Cerebellar Hemispheres: The cerebellar hemispheres are the extended portions on either side of the vermis. Each hemisphere has two portions.

1. Lobulus uniforms or ensiform lobe: It is the larger portion of the cerebellar hemisphere
2. Lobulus paramedianus or paramedian lobe: It is the smaller portion of the cerebellar hemisphere.

Divisions Of Cerebellum

Division of cerebellum into different major parts is done by three methods

1. Anatomical divisions
2. Phylogenetic divisions
3. Physiological or functional divisions.
4. Anatomical Divisions: On a structural basis, the whole cerebellum is divided into three portions
   1. Anterior Lobe: This lobe includes the lingula, central lobe, and culmen. It is separated from the posterior lobe by the primary fissure.
   2. Posterior Lobe: It consists of lobulus simplex, declive, tuber, pyramid, uvula, para flocculi, and the two portions of hemispheres – ansiform lobe and paramedian lobe.
   3. Flocculonodular Lobe: It includes nodulus and the lateral extension on either side called flocculus. It is separated from the rest of the cerebellum by posterolateral fissure.
5. Phylogenetic Divisions: Depending upon phylogeny, the cerebellum is divided into two divisions
   1. Paleocerebellum: It is the phylogenetically oldest part of the cerebellum. It includes two divisions
      1. Archicerebellum which includes the flocculonodular lobe
      2. Paleocerebellum is proper which includes lingula, central lobe, culmen, lobulus simplex, pyramid, uvula, and para flocculi.
   2. Neocerebellum: Neocerebellum is the phylogenetically newer portion of the cerebellum. It includes declive, tuber, and the two portions of cerebellar hemispheres – lobulus ansiformis and lobulus paramedianus.
6. Physiological Or Functional Divisions: Based on the functions, the cerebellum is divided into three divisions
   1. Vestibulocerebellum: It includes the flocculonodular lobe that forms the archicerebellum.
   2. Spinocerebellum: It includes lingula, central lobe, culmen, lobulus simplex, declive, tuber, pyramid, uvula and paraflocculi, and medial portions of lobulus uniforms and lobulus paramedianus.
   3. Corticocerebellum: It includes the lateral portions of lobulus ansiformis and lobulus paramedianus.

Functional Anatomy Of Cerebellum

The cerebellum is made up of outer gray matter or cerebellar cortex and inner white matter. The white matter is formed by afferent and efferent nerve fibers of the cerebellum. The gray masses called cerebellar nuclei are located within white matter.

Gray Matter: Gray matter or cerebellar cortex is made up of structures arranged in three layers. Each layer of gray matter is uniform in structure and thickness throughout the cerebellum. The three layers of gray matter

1. Outer molecular or plexiform layer
2. Intermediate Purkinje layer
3. Inner granular layer.

• Molecular or Plexiform Layer: It is the outermost layer of the cortex having the cells arranged in two strata. The superficial stratum contains a few star-shaped cells known as stellate cells. The deep stratum contains basket cells. In addition to stellate and basket cells, the molecular layer contains the following structures
  • Parallel fibers, which are the axons of granule cells, present in the granular layer
  • The terminal portions of climbing fibers (afferents from the medulla)
  • Dendrites of Purkinje cells and Golgi cells.
  • Cell junctions in the molecular layer: The molecular layer contains the following cellular junctions
    • The dendrites of stellate cells and basket cells synapse with parallel fibers, which are the axons of granule cells
    • The axons of stellate cells end on the dendrites of Purkinje cells. However, the axon of the basket cell descends down into the Purkinje layer and forms the transverse fiber that ends on the soma of Purkinje cells
    • The dendrites of Purkinje cells synapse with climbing fibers and parallel fibers
    • The dendrites of Golgi cells situated in the inner granular enter the molecular layer and end on parallel fibers.
• Purkinje Layer
  • It is situated between the outer molecular layer and the inner granular layer. It is the thinnest layer having a single layer of flask-shaped Purkinje cells.
  • The Purkinje cells are the largest neurons in the body. The dendrites of these cells ascend through the entire thickness of the molecular layer and arborize there.
  • These dendrites terminate either on climbing fibers or parallel fibers. The axons of the basket cells form the transverse fibers which descend down and end on the soma of Purkinje cells.
  • The axons of Purkinje cells descend into the white matter and terminate on the cerebellar nuclei and vestibular nuclei via the cerebella vestibular tract.
  • The Purkinje cells are termed the “Final common path” of the cerebellar cortex because the impulses from different parts of the cerebellar cortex are transmitted to other parts of the brain only through Purkinje cells.
• Granular Layer
  • It is the innermost layer of cerebellar gray matter and it is in between the Purkinje layer and the cerebellar white matter. It is formed by interneurons called granule cells and Golgi cells. The total number of interneurons in this layer is about half the number of all neurons in the whole nervous system.
  • The axon of the granule cell ascends into the molecular layer and forms the parallel fiber, which synapses with dendrites of Purkinje cells, stellate cells, basket cells, and Golgi cells.
  • The dendrites of granule cells and the axon and a few dendrites of a Golgi cell synapse with Mossy fiber. The synaptic area of these cells is called the glomerulus and it is encapsulated by the processes of glial cells.
    • Afferent Fibers to Cerebellar Cortex: The cerebellar cortex receives afferent signals from other parts of the brain through two types of nerve fibers
      1. Climbing fibers
         • The climbing fibers arise from the neurons of the inferior olivary nucleus situated in the medulla and reach the cerebellum via the olivocerebellar tract.
         • The inferior olivary nucleus relays the output signals from motor areas of the cerebral cortex and the proprioceptive signals from different parts of the body to the cerebellar cortex via climbing fibers.
         • The proprioceptive impulses from different parts of the body reach the inferior olivary nucleus through the spinal cord and vestibular system.
         • After reaching the cerebellum, the climbing fibers ascend into the molecular layer and terminate on the dendrites of Purkinje cells.
         • While passing through the cerebellum, the climbing fibers of the olivocerebellar tract send collaterals to the cerebellar nuclei.
         • So, the impulses from the cerebral cortex and proprioceptors of the body are conveyed not only to the cerebellar cortex but also to the cerebellar nuclei through the climbing fibers. Each climbing fiber innervates one single Purkinje cell.
      2. Mossy fibers
         • Unlike climbing fibers, the mossy fibers have many sources of origin, namely motor areas of the cerebral cortex, pons, medulla, and spinal cord. Fibers arising from all these areas send collaterals to the cerebellar nuclei before reaching the cerebellar cortex.
         • So, like climbing fibers, the mossy fibers also convey afferent impulses to both cerebellar nuclei and cerebellar cortex. Some of the mossy fibers arise from cerebellar nuclei.
         • Mossy fibers reach the granular layer of the cerebellar cortex and divide into many terminals. Each terminal enters a specialized structure called a glomerulus and ends in a large expanded structure that forms the central portion of the glomerulus.
         • The dendrites of granule cells and the axon and dendrites of Golgi cells synapse on the mossy fiber giving a thick bushy appearance. The word ‘mossy’ refers to the appearance of a plant called moss, which grows into dense clumps, hence, these fibers are called mossy fibers.
    • Neuronal Activity in Cerebellar Cortex and Nuclei
      • The functions of the cerebellum are executed mainly by the impulses discharged from the cerebellar nuclei. However, the cerebellar cortex controls the discharge from the nucleus constantly via the fibers of Purkinje cells.
      • It is done in accordance with the signals received by the cerebellar cortex from different parts of the brain and body via climbing and mossy fibers. The entire process involves a series of neuronal activities as mentioned in Table.
        1. The climbing fibers excite the Purkinje cells directly and the cerebellar nuclei via the collaterals by releasing aspartate. The excitatory effect of climbing fiber on Purkinje cells is very strong because each climbing fiber ends on a single Purkinje cell.
        2. Mossy fibers excite the Purkinje cells indirectly. In the glomeruli, the mossy fibers release glutamate and excite the granule cells and Golgi cells. The collaterals of mossy fibers activate the cerebellar nuclei also by glutamate.
        3. The granule cells, which are activated by mossy fibers in turn, excite the Purkinje cells, stellate cells, and basket cells through the parallel fibers. The neurotransmitter utilized by granule cells is glutamate or aspartate. The granule cells are the only excitatory cells in the cerebellar cortex while all the other cells are inhibitory. Each mossy fiber innervates many Purkinje cells indirectly via granule cells. So, the excitatory effect of mossy fibers on Purkinje cells is weak.
        4. The stellate cells and basket cells, which are activated by the granule cells, inhibit the Purkinje cells by releasing gamma-aminobutyric acid (GABA).
        5. The Golgi cell that is activated by mossy fibers, in turn, provides feedback inhibition to granule cells by releasing GABA, i.e. it inhibits the transmission of impulse from mossy fiber to granule cell.
        6. The cerebellar nuclei are excited by collaterals from climbing and mossy fibers. In turn, the nuclei send excitatory impulses to the thalamus and different nuclei in brainstem.
        7. However, the signals discharged from Purkinje cells inhibit the cerebellar nuclei via GABA. The Purkinje cells inhibit the activities of vestibular nuclei also. Thus, it is clear that the cerebellar cortex plays an important role in modulating the excitatory signals of the following pathways:
           • From cerebellar nuclei to the cerebral cortex via the thalamus
           • From the final common motor pathway via the brainstem and spinal cord.
           • Because of this activity of the cerebellar cortex, the movements of the body are well organized and coordinated.

Cerebellar Nuclei: Cerebellar nuclei are the masses of gray matter scattered in the white matter of the cerebellum. There are four nuclei on either side.

1. Fastigial Nucleus: It is also known as nucleus fasting. Phylogenetically, it is the oldest cerebellar nucleus. It is placed near the midline on the roof of 4 ventricles.
2. Globosus Nucleus: This nucleus is situated lateral to the nucleus fasting. This is also known as nucleus globosus.
3. Emboliform Nucleus: Emboliform nucleus is also called nucleus emboliforrrvs This nucleus is below the nucleus fasting and nucleus globosus.
4. Dentate Nucleus: Dentate nucleus is also called the nucleus dentatus. It is the largest cerebellar nucleus. As it is created, it is called dentate nucleus. It is situated lateral to all the other nuclei.

White Matter Of Cerebellum: White matter of the cerebellum is formed by afferent and efferent nerve fibers. These nerve fibers are classified into three groups.

1. Projection fibers: Projection fibers are the afferent and efferent nerve fibers that connect the cerebellum with other parts of the central nervous system.
2. Association fibers: Association fibers connect different regions of the same cerebellar hemisphere.
3. Commissural fibers: The commissural fibers connect the areas of both halves of the cerebellar cortex.
   • The projection fibers of the cerebellum that form the afferent and efferent fibers are arranged in three bundles:
     • Inferior cerebellar peduncles between cerebellum and medulla oblongata
     • Middle cerebellar peduncles between cerebellum and pons.
     • Superior cerebellar peduncles between cerebellum and midbrain.
     • Inferior Peduncles: Tine inferior cerebellar peduncles are otherwise called restiform bodies and contain predominantly afferent fibers. These nerve fibers transmit the impulses from tactile receptors, proprioceptors, and receptors in the vestibular apparatus to the cerebellum.
     • Middle Peduncles: The middle cerebellar peduncles are otherwise called brachia pontis. These peduncles contain predominantly afferent fibers. Most of the fibers of the middle cerebellar peduncles are commissural fibers, which connect the areas of both the halves of the cerebellar cortex.
     • Superior Peduncles: The superior cerebellar peduncles are otherwise called brachia conjunctivae and contain predominantly efferent fibers.

Vestibulocerebellum Archicerebellum

This part of the cerebellum is connected with the vestibular apparatus and so it is known as the vestibulocerebellum. Since the vestibulocerebellum is the phylogenetically oldest part of the cerebellum, it is also called the archicerebellum. It is concerned with the maintenance of posture and equilibrium.

Components Of Vestibulocerebellum

• The vestibulocerebellum includes the flocculonodular lobe that is formed by the modulus of the vermis and its lateral extensions called flocculi.
• The uvula of the vermis is also considered as the part of vestibulocerebellum by some physiologists.

Connections Of Vestibulocerebellum

• Afferent Connections
  • Vestibulocerebellar tract
    • This tract is formed by the fibers arising from the vestibular nuclei situated in the pons and medulla. It passes through the inferior cerebellar peduncle of the same side and reaches the cerebellar nuclei – nucleus globosus, nucleus emboliformis, and nucleus fasting. Fibers from these nuclei, reach the vestibulocerebellum (flocculonodular node).
    • Vestibular nuclei in turn receive fibers from the vestibular apparatus situated in the inner ear through the vestibular division of the cochlear (8 cranial) nerve.
• Efferent Connections
  1. Cerebellovestibular tract: Fibers of this tract arise from the flocculonodular lobe, pass through the inferior cerebellar peduncle of the same side, and terminate on the vestibular nuclei in the brainstem.
     • The fibers from vestibular nuclei form medial and lateral vestibulospinal tracts, which terminate on the medial group of alpha motor neurons in the spinal cord. This way forms part of a medial system of the motor pathway (extrapyramidal system).
  2. Fastigibulbar tract
     • The efferent fibers from the modulus reach the fastigial nucleus. The fibers arising from here pass through the inferior cerebellar peduncle of the same side and terminate on the vestibular nuclei in the medulla oblongata and medullary reticular formation.
     • From the vestibular nuclei, vestibulospinal tracts (mentioned above) arise and terminate on alpha motor neurons. From the reticular formation, the reticulospinal tract arises and terminates on the gamma motor neurons in the spinal cord forming the part of the medial motor system (extrapyramidal system).

Functions Of Vestibulocerebellum: The Vestibulocerebellum regulates tone, posture, and equilibrium by receiving impulses from the vestibular apparatus. From the vestibular apparatus, information regarding gravity, linear movement, and angular acceleration reaches the vestibulocerebellum through the vestibulocerebellar tract. The vestibulocerebellum, in turn, sends signals to the spinal cord via vestibulospinal and reticulospinal tracts.

• Mechanism of Action of Vestibulocerebellum
  • Normally, the vestibular nuclei facilitate the movements of the trunk, neck, and limbs through vestibulospinal tracts and alpha motor neurons. The medullary reticular formation inhibits the muscle tone through the reticulospinal tract and gamma motor neurons.
  • However, the vestibulocerebellum inhibits both vestibular nuclei and medullary reticular formation. As a result, the movements of the neck, trunk, and limbs are checked and the muscle tone increases. Because of these effects, any disturbance in posture and equilibrium is corrected.
  • In the lesion of the vestibulocerebellum, there is a reduction of muscle tone (hypotonia) and failure to maintain posture and equilibrium.

Spinocerebellum Paleocerebellum

Spinocerebellum is connected with spinal cord and hence the name. It forms the major receiving area of the cerebellum for sensory inputs. It is concerned with the maintenance of muscle tone and anticipatory adjustment of muscle contraction during movement. Spinocerebellum is also a phylogenetically older part of the cerebellum: It is otherwise called paleocerebellum.

Components Of Spinocerebellum: Spinocerebellum consists of medial portions of the cerebellar hemisphere, para flocculi, and the parts of the vermis, viz. lingula, central lobe, culmen, lobulus simplex, declive, tuber, pyramid, and uvula. However, some physiologists do not consider the uvula as part of the spinocerebellum.

Connections Of Spinocerebellum

• Afferent Connections
  1. Dorsal spinocerebellar tract: This tract arises from Clarke’s column of cells in the dorsal gray horn of the spinal cord. It is an uncrossed tract and reaches the spinocerebellum through the inferior peduncle of the same side. This tract conveys the proprioceptive information from the limbs of the same side regarding the position and movements.
  2. Ventral spinocerebellar tract: The fibers of this tract arise from the marginal cells in the dorsal gray horn of the spinal cord. After taking the origin, the fibers cross the midline, ascend on the opposite side, and reach the spinocerebellum through the superior cerebellar peduncle. This tract conveys information about the position and movements of opposite limbs to the spinocerebellum.
  3. Cuneocerebellar tract: This tract arises from the accessory cuneate nucleus situated lateral to the cuneate nucleus in the medulla. It reaches the spinocerebellum through the inferior cerebellar peduncle of the same side. This tract conveys the proprioceptive impulses from the upper limb, upper trunk, and neck to the spinocerebellum.
  4. Olivocerebellar tract: It is formed by the climbing fibers arising from the inferior olivary nucleus in the medulla. After taking origin, these fibers cross the midline and reach the spinocerebellum through the inferior cerebellar peduncle of the opposite side. This tract also gives collaterals to the cerebellar nuclei particularly, the globose nucleus and emboliform nucleus. The inferior olivary nucleus receives afferent fibers from three sources:
     1. The brainstem nuclei on the same side
     2. The spinal cord through the spinoolivary tract on the same side
     3. Cerebral cortex of opposite side.
        • The olivocerebellar tract conveys proprioceptive impulses from the body and output signals from the cerebral cortex to the spinocerebellum.
  5. Pontocerebellar tract: This tract arises from pontine nuclei, crosses the midline, and reaches the spinocerebellum through the middle cerebellar peduncle of the opposite side. The pontine nuclei receive afferents from the cerebral cortex. The pontocerebellar tract conveys information to the spinocerebellum about the motor signals discharged from the cerebral cortex.
  6. Tectocerebellar tract: The tectocerebellar tract arises from the superior and inferior colliculi of the tectum in the midbrain. It reaches the spinocerebellum through the superior cerebellar peduncle of the same side. This tract carries visual impulses from the superior colliculus and auditory impulses from the inferior colliculus to the spinocerebellum.
  7. Trigeminocerebellar tract: It is formed by the fibers arising from the mesencephalic nucleus of the trigeminal nerve. It reaches the spinocerebellum via the superior cerebellar peduncle of the same side. This tract conveys proprioceptive information from jaw muscles and temporomandibular joint to the spinocerebellum. It also carries the sensory impulses from the periodontal tissues (tissues around the teeth) to the spinocerebellum.
• Efferent Connections: The cortex of spinocerebellum is projected into the cerebellar nuclei — fastigi, emboliformis and globosus. Fibers from these nuclei pass through the following tracts:
  1. Fastigiobulbar tract: It arises from the fastigial nucleus, passes through the superior cerebellar peduncle of the same side, and ends in the reticular formation.
  2. Cerebelloreticular tract: The fibers of this tract arise from the emboliform and globose nuclei, pass through the superior cerebellar peduncle of the same side, and terminate in the reticular formation.
     • From the reticular formation, the reticulospinal tract arises and terminates on the gamma motor neurons of the spinal cord.
  3. Cerebello-olivary tract: This tract arises from the emboliform and globose nuclei and reaches the inferior olivary nucleus of the same side by passing through the superior cerebellar peduncle. From the olivary nucleus, the olivospinal tract arises, and the fibers of this tract end on the alpha motor neurons of the spinal cord.

Functions Of Spinocerebellum

• The spinocerebellum regulates tone, posture, and equilibrium by receiving sensory impulses from tactile receptors, proprioceptors, visual receptors, and auditory receptors.
• Spinocerebellum is the receiving area for the tactile, proprioceptive, auditory, and visual impulses. It also receives cortical impulses via pontine nuclei. The tactile and proprioceptive impulses are localized in the spinocerebellum.
• Localization of tactile and proprioceptive impulses in the spinocerebellum is determined by stimulating the tactile receptors and the proprioceptors and by recording the electrical responses in different parts of the spinocerebellum. The different parts of the body are represented in the spinocerebellum in the manner
  • Lingula — Coccygeal region
  • Central lobe — Hind limb
  • Cuimen — Forelimb
  • Lobulus simplex — Face and head
• In the cerebral cortex, different parts of the body are represented in an inverted manner. But, in the cerebellum, the different parts are represented in an upright manner.
• The spinocerebellum regulates the postural reflexes by modifying muscle tone. It facilitates the discharge from gamma motor neurons in the spinal cord via cerebellar-vestibulospinal and cerebello-reticulospinal fibers.
• Increased discharge from gamma motor neurons increases muscle tone. The lesion, destruction, or abolishing of the function of the spinocerebellum by cooling, causes stoppage of discharge from the gamma motor neurons resulting in hypotonia and disturbances in posture.
• The spinocerebellum also receives impulses from the optic and auditory pathways and helps in the adjustment of posture and equilibrium in response to visual and auditory impulses.

Corticocerebellum Neocerebellum

Corticocerebellum is largest part of cerebellum. Because of its connection with the cerebral cortex, it is called corticocerebellum orcerebrocerebellum. It is a phylogenetically newer part of the cerebellum. So, it is also called neocerebellum. It is concerned with planning, programming, and coordination of skilled movements.

Components Of Corticocerebellum: Corticocerebellum includes the lateral portions of cerebellar hemispheres.

Connections Of Corticocerebellum

• Afferent Connections
  1. Pontocerebellar tract
     • It arises from the pontine nuclei. It crosses the midline and enters the corticocerebellum via the middle cerebellar peduncle. It is the largest tract in the body having about 20 million nerve fibers.
     • The pontocerebellar tract is also called the cortico-pontocerebellar circuit. Because it receives signals from the motor area of the cerebral cortex and conveys those signals to corticocerebellum. It helps the cerebellum in planning the movements initiated by the cerebral cortex.
  2. Olivocerebellar tract
     • This tract arises from the inferior olivary nucleus situated in the medulla. It crosses the midline and enters the corticocerebellum via the inferior cerebellar peduncle of the opposite side. There it terminates on the dentate nucleus and cerebellar cortex. This tract is formed by climbing fibers.
     • The inferior olivary nucleus receives impulses from the brainstem, spinal cord, and cerebral cortex and conveys these impulses to the corticocerebellum through the olivocerebellar tract.
• Efferent Connections
  • The output signals are relayed mainly through the dentate nucleus. The fibers from the dentate nucleus pass through the superior cerebellar peduncle, cross the midline, and form decussation with the fibers of the opposite side. After forming the decussation, these fibers divide into two tracts
    1. Dentatothalamic tract: After crossing, some of the fibers pass through the red nucleus without having any synapse and terminate in the lateral ventral nucleus of the thalamus. The thalamus in turn projects into the motor area of the cerebral cortex via thalamocortical fibers.
    2. Dentatorubral tract: The remaining fibers terminate in the red nucleus of the opposite side. Three tracts arise from the red nucleus
       1. Rubrothalamic tract: From the red nucleus, this tract ascends and terminates in the lateral ventral nucleus of the thalamus. From here, the thalamocortical fibers arise and reach the cerebral cortex.
       2. Rubroreticular tract: It descends down and ends in the reticular formation. Reticular formation projects into the spinal cord via the reticulospinal tract.
       3. Rubrospinal tract: The red nucleus also projects directly into the spinal cord through the rubrospinal tract.

Afferent-Efferent Circuit (Cerebro-Cerebello-Cerebral Connections)

• It is an important neuronal pathway, involved in cerebellar control of voluntary movements initiated by the motor area of cerebral cortex.
• Fibers from motor areas 4 and 6 in the frontal lobe of the cerebral cortex enter the pontine nuclei. These fibers are called corticopontine fibers. From pontine nuclei, the pontocerebellar fibers arise and pass through the middle cerebellar peduncle of the opposite side and terminate in the cerebellar cortex. This pathway is called the cerebropontocerebellar tract.
• The cerebellar cortex is, in turn, connected to the dentate nucleus. Fibers from the dentate nucleus pass via the superior cerebellar peduncle and end in the red nucleus on the opposite side.
• These fibers are called fibers. From the red nucleus, the subthalamic fibers go to the thalamus. The thalamus is connected to areas 4 and 6 in the motor cortex of the cerebrum by thalamocortical fibers. This pathway is called the dentate rubro thalamocortical tract.

Functions Of Corticocerebellum: Corticocerebellum is concerned with the integration and regulation of well-coordinated muscular activities. It is because of its afferent-efferent connection with the cerebral cortex through the cerebrum-cerebellum-cerebral circuit. Apart from its connections with the cerebral cortex, the cerebellum also receives feedback signals from the muscles through the nerve fibers of proprioceptors.

Mechanism of Action of Corticocerebellum

1. Damping action
   • Damping action refers to the prevention of exaggerated muscular activity. This helps in making the voluntary movements smooth and accurate. All voluntary muscular activities are initiated by motor areas of the cerebral cortex. Simultaneously, the cortical cerebellum receives impulses from the motor cortex as well as feedback signals from the muscles as soon as the muscular activity starts.
   • The corticocerebellum, in turn, sends information (impulses) to the cerebral cortex to discharge only appropriate signals to the muscles and to cut off any extra impulses. Because of this damping action of corticocerebellum, the exaggeration of muscular activity is prevented and the movements become smooth and accurate. Literally, the word damping means any effect that decreases the amplitude of mechanical oscillation.
2. Control of ballistic movements: Ballistic movements are the rapid alternate movements, which take place in different parts of the body while doing any skilled or trained work like typing, cycling, dancing, etc. Corticocerebellum plays an important role in preplanning the ballistic movements during the learning process.
3. Timing and programming the movements
   • The corticocerebellum plays an important role in timing and programming the movements, particularly during the learning process. While using a typewriter or while doing any other fast skilled work, a chain of movements occurs rapidly in a sequential manner.
   • During the learning process of these skilled works, corticocerebellum plans the various sequential movements. It also plans a schedule of the duration of each movement and the time interval between movements.
   • All the information from the corticocerebellum is communicated to the sensory-motor area of the cerebral cortex and stored in the form of memory. So, after the learning process is over, these activities are executed easily and smoothly in a sequential manner.
4. Servomechanism
   • A servomechanism is the correction of any disturbance or interference while performing skilled work. Once the skilled works are learned, the sequential movements are executed without any interruption.
   • The cerebellum lets the cerebral cortex discharge the signals, which are already programmed and stored in the sensory-motor cortex, and, does not interfere much. However, if there is any disturbance or interference, the corticocerebellum immediately influences the cortex and corrects the movements.
5. Comparator function
   • The comparator function of the corticocerebellum is responsible for the integration and coordination of the various muscular activities.
   • On one side, the cerebellum receives the information from cerebral cortex regarding the cortical impulses that are sent to the muscles. On the other side, it receives the feedback information (proprioceptive impulses) from the muscles regarding their actions under the instruction of the cerebral cortex.
   • By receiving messages from both ends, corticocerebellum compares the cortical commands for muscular activity and the actual movements carried out by the muscles. If any correction is to be done, then, corticocerebellum sends instructions (impulses) to the motor cortex.
   • Accordingly, the cerebral cortex corrects or modifies the signals to muscles, so that the movements become accurate, precise, and smooth. This function of corticocerebellum is known as the comparator function.
   • Simultaneously, it also receives impulses from tactile receptors, the eye, and the ear. Such additional information facilitates the comparator function of corticocerebellum.

Applied Physiology Cerebellar Lesions

• Cerebellar lesions may be due to a tumor, abscess, or injury. Excess alcohol ingestion also leads to cerebellar lesions. The loss of functions of the cerebellum also occurs due to degenerative changes in the cerebellar cortex, cerebellar nuclei, cerebellar peduncles, and spinocerebellar tracts.
• In general, during cerebellar lesions, there are disturbances in posture, equilibrium, and movements. In unilateral lesions, symptoms appear on the affected side because the cerebellum controls the same (ipsilateral) side of the body.
  Most of the disturbances are due to the damage to corticocerebellum (neocerebellum) because in human beings, it is larger than other divisions.

Disturbances In Tone And Posture

1. Atonia or Hypotonia
   • Atonia is the loss of tone and hypotonia is the reduction in tone of the muscle. The cerebellar lesion causes atonia or hypotonia depending upon the severity. Atonia or hypotonia due to cerebellar lesions causes disturbances in the postural reflexes.
   • The cause for atonia or hypotonia during cerebellar lesions is the loss of facilitatory impulses to gamma motor neurons in the spinal cord via cerebellovestibulospinal and cerebelloreticulospinal fibers.
2. Attitude: The attitude of the body changes in the unilateral lesion of the cerebellum. The changes in the attitude are
   1. Rotation of head towards the opposite side (unaffected side)
   2. Lowering of the shoulder on the same side
   3. Abduction of the leg on the affected side. The leg is rotated outward
   4. The weight of the body is thrown on the leg of the unaffected side. So, the trunk is bent with concavity towards the affected side.
3. Deviation Movement: It is the lateral deviation of arms when both arms are stretched and held in front of the body with closed eyes. In bilateral lesions, both arms deviate and in unilateral lesions arm of the affected side deviates.
4. Effect on Deep Reflexes
   • movements occur while eliciting a tender; jerk. The pendular movements are very common while eliciting the knee jerk or patellar tendon reflex in patients affected by cerebellar lesions.
   • A tap on the patellar tendon, when the leg is hanging freely, causes a brisk extension of the leg due to the contraction of the quadriceps muscle. In normal conditions, after the extension, the leg returns back to the resting position immediately. In the cerebellar lesion, the leg shows pendular movements.

Disturbances In Equilibrium

• While Standing: While standing, the legs are spread to provide a broad base. And, the body sways side-to-side with the oscillations of the head.
• While Moving – Gait: Gait means the manner of walking. In the cerebellar lesions, a staggering, reeling, and drunken-like gait is observed.

Disturbances In Movements

1. Ataxia: It is the lack of coordination of movements.
2. Asynergia: Asynergia is the lack of coordination between different groups of muscles such as protagonists, antagonists, and synergists.
3. Asthenia: It refers to weakness of muscles, easy fatigability, and slowness of muscles.
4. Dysmetria: Dysmetria is the inability to check the exact strength and duration of muscular contractions required for any voluntary act. While reaching for an object, the arm may overshoot (past pointing) or it may fall short of the object. Overshooting is called hypermetria and falling short is known as hypometria.
5. Intention tremor: Intention tremor is the tremor that occurs while attempting to do any voluntary act.
6. Astasia: It is a condition characterized by unsteady voluntary movements.
7. Nystagmus: To and fro movement of the eyeball is called nystagmus.
8. Rebound phenomenon: When the patient attempts to do a movement against a resistance, and if the resistance is suddenly removed, the limb moves forcibly in the direction in which the attempt was made. It is called the rebound phenomenon. It is due to the absence of breaking action of antagonistic muscle.
9. Dysarthria: The disturbance in speech is called dysarthria. It is due to the incoordination of various muscles and structures involved in speech.
10. Adiadochokinesis: Diadochokinesis is the ability to do rapid alternate successive movements such as supination and pronation of the arm. The inability to do these movements is called adiadochokinesis, which is a common feature of cerebellar lesions. It is also called disdiadochokinesia.