Cerebellum

Describe the neural circuit, connections, functions and effect of lesion of cerebellum. List and describe the cerebellar function tests done in brief.

(NB: Must Draw Relevant Diagrams from your standard textbooks)

(NB: On smartphones best read in landscape mode)

Cerebellar Inputs/Efferent Connection

• Functionally, Cerebellum is divided into 3 divisions-
  • Vestibulocerebellum/Archicerebellum-
  • Spinocerebellum/Paleocerebellum-
  • Cerebrocerebellum/Corticocerebellum/Neocerebellum-
• Cerebellum receives somatosensory inputs from almost all parts of body via 3 routes:-
  • Mossy Fibres-
    • Major source of inputs to cerebellum
    • Carries direct proprioceptive inputs from all body parts
    • Conveys input mainly to cerebral cortex
    • Project mainly to granule cells
  • Climbing fibres-
    • Conveys input from inferior olivary nucleus which receives proprioceptive input from all parts of body
    • Project to Purkinje cells of cerebellum
  • Other inputs-
    • Monoaminergic inputs
    • Inputs from thalamus
    • Project to deep cerebellar nuclei
• Cerebellar afferents through Climbing fibres are:-
  • Olivocerebellar tract-
    • Through Climbing Fiber system
    • Proprioceptive inputs from whole body via relay in inferior olivary nucleus(ION) in the rostral medulla.
    • ION receives input from vestibular system, spinal cord and cerebral cortex.
• Cerebellar Afferents through Mossy Fibres are-
  • Vestibulocerebellar Tract-
    • Through mossy fibres
    • Enters through inferior cerebellar peduncle
    • Inputs from ipsilateral vestibular nuclei about position and movement of head
    • Projects to Vestibulocerebellum via deep cerebellar nuclei
  • Dorsal spinocerebellar tract-
    • Through mossy fibres
    • Enters through ipsilateral inferior cerebellar peduncle
    • Inputs from ipsilateral Clarke’s Column of cells
    • Uncrossed tracts carrying proprioceptive and exteroceptive inputs from same side.
    • Projects to Spinocerebellum
  • Ventral spinocerebellar tract-
    • Through Mossy Fibres
    • Enters through Superior Cerebellar peduncle
    • Arises from marginal cells in dorsal grey horn of spinal cord, crosses the midline, and ascends in the opposite side.
    • Carries proprioceptive and exteroceptive inputs from the limbs of opposite side
    • Projects to the lower limb area of the cortex of spinal cord.
  • Cuneocerebellar tract-
    • Through Mossy fibres
    • Enters through ipsilateral inferior cerebellar peduncle
    • Inputs from lateral/accessory cuneate nucleus in the caudal medulla
    • Carries proprioceptive inputs from upper limb, upper trunk, head, and neck of same side.
    • Distributed to spinocerebellum
  • Tectocerebellar tract-
    • Through Mossy fibres
    • Enters via superior cerebellar peduncle
    • Conveys visual information from superior colliculus and auditory information from inferior colliculus
  • Pontocerebellar tract-
    • Cortico-Pontine fibres arise from cerebral motor cortex and ends in pontine nuclei.
    • Ponto cerebellar fibres then cross to enter the opposite side  through the middle cerebellar peduncle
    • In association with cerebrum affects the proper coordination of muscular activity in voluntary movements.
  • Trigeminocerebellar tract-
    • Fibers from mesencephalic nucleus of trigeminal nerve.
    • Enters via Ipsilateral superior cerebellar peduncle
    • Conveys proprioceptive information from Jaw muscles and TMJ along with sensory impulses from periodontal tissues.
    • Projects to spinocerebellum
  • Reticulocerebellar tract-
    • Arises from lateral reticular nucleus
    • Enters through ipsilateral inferior cerebellar peduncle
    • Projects to whole of cerebellar cortex
  • Rubrocerebellar tract-
    • Arises from Red Nucleus
    • Transmits motor cortex impulses relayed to red nucleus
    • Enters through superior cerebellar peduncle
    • Can be crossed or uncrossed
    • Projects mainly to dentate nucleus
• Cerebellar Afferents through other Fiber Systems-
  • Includes Monoaminergic inputs.
  • Serotonergic inputs from nucleus raphe magnus
  • Noradrenergic nucleus from nucleus locus ceruleus
  • Inputs form thalamus and other parts of brain
  • Projects to the deep cerebellar nuclei

Cerebellar Outputs/Afferent Connections

Outputs from Vestibulocerebellum-

•  Directly controls Vestibulospinal tract activity by directly projecting to Vestibular nuclei, without relaying to the deep cerebellar nuclei.

Output from Spinocerebellum-

• Fastiglobular tract: Vermis -> Fastigial nucleus -> Superior Cerebellar Peduncle -> Pontine reticular formation and Vestibular Nuclei thus controlling Pontine Reticulospinal Tract and Vestibulospinal tract.
• Cerebelloreticular Tract: Emboliform & Globosus Nuclei -> Ipsilateral Superior Cerebellar Peduncle -> Reticular Formation -> Reticulospinal Tract -> Gamma Motor Neurons of Spinal Cord.
• Cerebello-olivary tract: Emboliform & Globosus Nuclei -> Superior Cerebellar Peduncle -> Ipsilateral Inferior Olivary Nucleus -? Olivospinal Tract -> Alpha Motor Neurons of Spinal Cord
• Paravermal -> Nucleus Interpositus -> Red Nucleus and thus controls Rubrospinal Tract

Output from Cerebrocerebellum-

• Cerebellar Hemisphere -> Dentate Nucleus -> Dentatothalamic tract -> Superior Cerebellar Peduncle -> Cross the Midline -> Decussation with fibres of opposite side-> Thalamus ->Thalamocortical Fibres -> Motor Area of Cerebral Cortex and thus controls Corticospinal Tract.

(Include here diagram showing major output pathways from cerebellum)

Neuronal Circuits/Internal Connections

• Internal Circuitry is designed mainly to modulate the excitatory output of the deep cerebellar nuclei.
• Cerebellum receives inputs from climbing fibres and mossy fibres
• Climbing Fibres stimulate
  • Purkinje Cells-
    • Involved in motor learning
    • Produces long term adjustment to motor response.
    • Complex multi peak spike action potential is generated
    •  Include here diagram of multi-spike action potential
  • Deep cerebellar nuclei

• Mossy Fiber stimulate
  • Deep Cerebellar nuclei
  • Granule Cell Parallel Fiber which stimulates
    • Basket Cells which inhibit
      • Purkinje Cells (feed forward inhibition)
    • Stellate Cells which inhibit
      • Purkinje Cells (feed forward inhibition)
    • Golgi Cells(interneurons) which inhibit
      • Granule Cells (local feedback inhibition)
    • Purkinje Cells which inhibit–
      • Deep cerebellar nuclei through release of GABA
      • Simple spike action potential is generated
      •  Include here diagram of single spike action potential

• Deep cerebellar nuclei output is thus always excitatory despite inhibition by Purkinje cells.
• So, lesions of cerebellum leads to Hypotonia
• Include here diagram showing internal connections of Cerebellum

Functions

• Vestibulocerebellum-
  • Regulates tone, posture, and equilibrium
  • Receives information regarding gravity, linear movement, and angular acceleration through vestibulocerebellar tract
  • Sends these information to spinal cord via vestibulospinal and reticulospinal tracts.
• Spinocerebellum-
  • Regulates tone, posture and equilibrium.
  • Receiving area for sensory impulses from tactile, proprio, visual and auditory receptors
  • Localisation of tactile and proprioceptive impulses
  • Regulation of postural reflexes.
  • Adjustment of posture and equilibrium in response to visual and auditory impulses
• Corticocerebellum-
  • Integration and regulation of well-coordinated muscular activities.
  • Damping action
  • Control of ballistic movements
  • Timing and programming of movements
  • Servomechanism
  • Comparator function

Lesions

• In unilateral lesions, symptoms appear on the same side.
• Lesion of Vestibulocerebellum-
  • Hypotonia- reduction of muscle tone
  • Failure to maintain posture and equilibrium
• Lesion of Spinocerebellum-
  • Stoppage of discharge from gamma motor neurons from spinal cord leading to hypotonia and disturbance in posture
• Attitude changes in unilateral lesions like face rotated towards healthy side, shoulder lowered on affected side, outward rotation and abduction of leg on involved side.
• Arms deviated laterally when held out straight with eyes closed
• Deep tendon reflexes become weak and pendular
• Wide base walking, drunken gait.
• Positive Romberg’s sign- Unable to walk in straight line with closed eyes.
• Ataxia- lack of coordination of movements characterised by asynergia and dysmetria.
• Intention tremors which become more marked as the hand approaches the object.
• Nystagmus- rhythmic, to and fro, involuntary oscillating movement of eye.
• Dysarthria- Slurred, prolonged, scanning speech/ lalling speech, like a baby.
• Astasia- Unsteady voluntary movements.
• Charcot’s Triad Syndrome- Nystagmus + Intention tremors + scanning speech. Seen in disseminated sclerosis

Function Tests

• Finger Nose Test- Patient has great difficulty in bringing the finger of outstretched arm to touch the tape of his nose because of intention tremors.
• Making Circle in air
• Kneel Heel test
• Adiadochokinesia – Patient is unable to rapidly perform alternating supination and pronation of forearm.
• Rebound phenomena- – On attempting to do movement against resistance, the limb moves forcibly in that direction of resistance when the resistance is suddenly removed. This is due to absence of breaking action of antagonistic muscle.
• Gait Test- Unable to walk in straight line even with eyes open
• Past pointing/Overshooting- Movement goes beyond intended point due to dysmetria.