In this article we will discuss about:- 1. Meaning of Cerebellum 2. Special Features of Cerebellum 3. Division 4. Connection 5. Function.
Meaning of Cerebellum:
Cerebellum is also termed as little brain. Present in the posterior part of cranial fossa below occipital lobe.
Connected to other parts of brainstem by three pairs of peduncles, namely:
a. Superior peduncle
b. Middle peduncle
c. Inferior peduncle
In cerebellum, there is a lot of white matter just like in cerebral cortex. Deep below the white matter many nuclei of cerebellum are present.
They are:
a. Dentate
b. Emboliform
c. Globose (the nucleus emboliform and globose together sometimes refer to as nucleus interpositus)
d. Fastigial
Special Features of Cerebellum:
a. Though cerebellum receives a lot of afferent inputs fibers of proprioceptors from various parts of body, conscious perception of sensation will not be possible.
b. It cannot exert its control directly over LMN. However, influence from cerebellum on LMN is essential for proper co-ordination of any movement.
c. Unlike cerebral cortex that controls activity of contralateral half of body, cerebellum controls motor activity of ipsilateral half of body.
d. Apart from nuclei, we find Purkinje, granular, basket, and Golgi cells are also present. There is lot of connection between these various cells.
Divisions of Cerebellum:
Formed by important parts (anatomically) Fig. 9.37:
a. Anterior lobe
b. Flacculonodular lobe
c. Posterior lobe
d. Vermis
Phylogenetically (i.e. depending on order of development during evolution), cerebellum is subdivided into:
a. Archicerebellum (primitive)
b. Paleocerebellum (old)
c. Neocerebellum (newest)
Functionally cerebellum is subdivided into (Fig. 9.38):
a. Flocculonodular lobe (corresponds to archicerebellum) also known as vestibulocerebellum.
b. Vermis and part of cerebellum immediately lateral to vermis on either side in anterior and posterior lobes is known as spinocerebellum.
c. Most lateral part of cerebellar hemisphere is known as cerebrocerebellum/corticocerebellum. This part is very highly developed in human beings.
Connections of Cerebellum (Fig. 9.39):
Afferent Connections:
Cerebellum has a lot of afferent inputs coming from proprioceptors (receptors involved in sense of position and movement). However, when impulse reaches cerebellum, it will not come to conscious perception.
a. The proprioceptive inputs from head and neck region reach cerebellum through cuneocerebellar tract.
b. Vestibulocerebellar pathway carries proprioceptive inputs from vestibular apparatus present in inner ear. Proprioceptive impulses from vestibular apparatus reach cerebellum through vestibular nuclei present in brainstem.
c. Dorsal and ventral spinocerebellum tracts carry proprioceptive impulses from both contralateral and ipsilateral sides of various parts of body.
d. Tectocerebellar pathway carries afferent impulses from superior and inferior colliculi of tectum of midbrain. This colliculus receives afferent inputs from visual and auditory areas.
e. Olivocerebellar tract takes origin from inferior olivary nucleus. Inferior olivary nucleus receives afferent proprioceptive inputs from almost all parts of body.
f. Also there are corticopontocerebellar fibers which come from motor and premotor (area number 4 and 6) areas of cerebral cortex.
Efferent Connections:
The cerebellum can exert its influence either on nuclei in brainstem or neurons present in motor and premotor areas of cerebral cortex through the following efferent connections namely:
a. Cerebellovestibular
b. Dentatorubrothalamocortical
c. Cerebelloreticular, etc.
Cerebellum has no direct influence on LMN present in spinal cord. But, influence on LMN of spinal cord is through various descending tracts that take origin from brainstem regions (Fig. 9.40).
Functions of Cerebellum:
a. Regulation of posture and equilibrium:
Flocculonodular lobe is involved in this function. It receives afferent inputs from vestibulocerebellar tract and proprioceptors from all over the body. Afferent inputs coming to flocculonodular lobe are processed, and efferent impulses from cerebellum are sent to vestibular and reticular nuclei present in brainstem.
From these nuclei, impulses are sent to lower motor neurons through vestibulospinal and reticulospinal tracts. Impulses coming from flocculonodular lobe control activities of axial (midline trunk) muscles and proximal limb muscles (muscles which attach limb to trunk part of the body).
Hence cerebellum has an important role to play in maintenance of posture and equilibrium.
b. Also controls activities of extraocular muscles which control eye movements through medial longitudinal bundle. This bundle controls activity of 3, 4, and 6 cranial nerve motor nuclei. Thus flocculonodular lobe of cerebellum helps in fixing gaze/vision. Even though position of head is fixed, we can move eyeballs to focus on any particular object/subject that we want to see.
c. Cerebellum also brings about coordination of movements. For smooth movements to be brought about, there will be involvement of three groups of muscles.
They are:
i. Agonists/protogonists—directly involved in performing a movement.
ii. Antagonists—muscle group involved in opposing movements are made to relax.
iii. Synergistic—not directly involved but needed for coordination of smooth movement.
In cerebral cortex, only movements are represented and not individual muscles.
During coordination of movement, the following characteristic features have to be taken care off:
i. Force generated during movement
ii. Rate of movement
iii. Range of movement
iv. Direction of movement
If there is an error in any one or all of characteristics, there is said to be in-coordinated movement (ataxia). Ataxia is characteristic feature in cerebellar lesion and this is called as motor ataxia. Ataxia can also occur due to some sensory deficits in which case it is called as sensory ataxia (posterior or afferent nerve root lesion, thalamic syndrome).
Coordination of movement is possible because cerebellum acts as servo comparator.
Whenever impulses are sent from motor and pre- motor areas of cerebral cortex to LMN, a copy of the command is sent to cerebellum through the corticopontocerebellar pathway (Fig. 9.41).
Cerebellum behaves as a servo comparator. Movements are initiated in the body due to impulses coming to part of body through corticospinal tracts. A copy of the command sent to LMN by cerebral cortex is also sent to cerebellum through cortioponto- cerebellar pathway.
During every step of movements, proprioceptive impulses arising from muscles and joints are sent back to cerebellum from the part of body that is involved in movement. These inputs keep informing cerebellum about various aspects of movement that is taking place (like degree of movement, direction, force, etc.).
If movement is not according to motor command, cerebellum compares command for intended movement with the movement that is going on (afferent inputs will be informing about this) and any rectification of error is faithfully relayed back to motor cortex through dentato-rubro-thalamo-cortical pathway. This leads to modification/rectification of movements so that target is reached accurately.
d. Cerebellum also helps in maintenance of muscle tone:
Vestibulocerebellum influences activity of vestibular nucleus and pontine reticular formation. Spinocerebellum influences activities of nuclei present in brainstem. Rubrospinal, vestibulospinal and pontine reticular formation extend their influence over LMNs present in spinal cord.
The efferent impulses coming from cerebellum through these nuclei are generally excitatory to LMNs. Because of this, if there is cerebellar lesion it leads to hypotonia (decreased muscle tone).
e. Cerebellum also has a role to play in learning and memory especially some of movements which are to be excelled for a short time by repeated trials.
Features of Cerebellar Lesion:
1. Imbalance in posture and equilibrium
2. Incoordination of movements
3. Hypotonia
4. Knee jerk gets affected (pendular knee jerk)
5. Dysarthria
6. Nystagmus
Applied Aspects:
Vestibulocerebllar damage:
Unsteady gait/posture. The person will have drunken gait (feet far apart from each other).
Control of subconscious associated movements will be absent, for example, swinging of arms while walking will be absent.
Ataxia or incoordination of movement is also seen:
Incoordination leads to features, like.
a. Dysmetria:
Rate of movement (it may over shot or under shoot the target). Unable to gauge length of movement efficiently.
b. Adiadochokinesia:
Inability to perform rapid alternating movements (alternatively supinating and pronating the palm, or flexion and extension fingers, etc.) cannot be brought about.
c. Decomposition of movement:
When one has to perform any complex movement involving many joints, normally the movement can be quite brisk and smooth. But when there is in-coordination or ataxia, complex movement will occur, but in steps and slowly, that is movement at each joint will be performed as a separate entity one after the other.
d. Dysarthria:
Speech is also affected. Speech is usually a complex movement involving articulation of lips, laryngeal muscles, tongue, etc. In cerebellar lesion, slow staccato speech is observed that is a single word is broken into many syllables.
Tremors:
As long as the person is at rest, tremors will be absent. Tremors are seen when any voluntary movement starts and the intensity of tremors increase during the course of movements. So these are known as intention/kinetic tremors.
Knee jerk is an example of deep reflex. When knee jerk is elicited in a person suffering from cerebellar lesion, the knee jerk becomes pendular. Muscle tone will be less than normal that is hypotonia in the case of human beings.
Tests for Cerebellar Lesion:
i. Test for muscle tone and tremors
ii. Stand with eyes closed
iii. Walking along a straight line
iv. Finger nose test
v. Heel knee test
vi. Knee jerk test—look for pendular jerk.