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Introduction: the cerebellum

  • Cerebellum is a component of the hindbrain
    • Located in the posterior cranial fossa below tentorium cerebelli
      • Overlies the dorsal and lateral sides of the 4th ventricle
      • Attached to the brainstem by the superior, middle and inferior cerebellar peduncles
    • Function- like the basal ganglia, it directs the motor outputs of the cortex (doesn't give rise to direct descending pathways, but exerts influence on nuclei in the brainstem and on the motor cortex, from which descending pathways originate)
      • Important in postural control and motor coordination, and balance, and coordination of movement (correct muscles, correct timing and correct sequence)
    • Inputs from the cortex, brainstem, inner ear and spinal cord
      • Thus synchronising muscle activity: their timing, sequence, and force
    • Damage manifests as disturbances in balance and equilibrium + a lack of motor coordination + decrease in muscle tone
      • But not paralysis
    • Has an unknown role in cognition
  • Cerebellar peduncles
    • Superior cerebellar peduncle – connects the cerebellum to the midbrain and thalamus
    • Middle cerebellar peduncle – connects the pontine nuclei to the cerebellum
    • Inferior cerebellar peduncle – connects in both directions the medulla and cerebellum, spinal cord and cerebellum

Cerebellum: gross anatomy

  • Made up of two hemispheres joined by a vermis at the midline
    • Layers:
      • Outer covering of cerebellar cortex
      • Medullary white centre
      • 4 pairs of deep cerebellar nuclei
  • Cortex
    • Highly folded forming transverse folia separated by sulci or fissures (deep sulci) (called folia rather than gyri). Unlike the cerebral cortex, the folia run parallel (except for the flocculus, which is a separate detached bit of cortex).
    • 3 lobes
      • anterior lobe (anterior 2/3 of superior surface)
        • separated from the posterior lobe by the primary fissure
      • posterior lobe (anterior third of the superior surface and most of the inferior surface)
        • cerebellar tonsils – lateral to the inferior vermis; on either side of the medulla
          • can herniate through foramen magnum with the brainstem with raised ICP; compressing the respiratory centres and causing death
      • flocculonodular lobe
        • made up of a nodule and paired flocculi
          • nodule – rostral end of the inferior vermis (covered on the ventricle side by the inferior medullary velum); dangles down into 4th ventricle (shows up on a sagittal section sliced down through the vermis)
        • flocculi – adjacent to the middle cerebellar peduncle, rostral to the inferior part of posterior lobe; the flocculi occupy the posterolateral fissure, which also sits behind the vermis
          • separated from the posterior lobe and inferior vermis by the posterolateral fissure
    • Vertical regions can be defined based on function
      • Vermis – convex on superior surface, concave on inferior surface; a deep area of the cerebellum
      • Paravermal zone (lateral to vermis)
      • Lateral zone (rest of hemisphere)

Deep cerebellar nuclei

  • Three pairs of deep nuclei embedded in the white matter
    • Fastigial nucleus – most medial; sits beneath the vermis
      • Either side of the midline dorsal to the apex (fastigium) of the roof of the 4th ventricle
    • Interposed nucleus – lateral to the fastigial nucleus (group of small intermediate nuclei); beneath the paravermal zone
    • Dentate nucleus – largest and most lateral, resembling the inferior olive; beneath the neocerebellum
  • Inputs to cerebellum go to cortex, then outputs come out of the deep nuclei
    • Via either climbing fibres or via mossy fibres (these are the two sources of input). A single purkinje cell only receives input from one climbing fibre. The climbing fibres wrap themselves around the dendrites of the purkinje cell, to make very many contacts with one cell. Climbing fibres originate in the inferior olive, cross over, come up the ICP, and branch to supply purkinje cells.
      • These fibres are very important for motor learning (programming activities into cells of cerebellum). The pattern of connection between climbing fibres and purkinje cells helps program the cerebellum.

Histology of the cerebellar cortex

  • Outer molecular layer - relatively cell-free
  • Purkinje cells - 2D cells, with a very large dendritic tree
    • Receive synaptic inputs from various sources (200 000 synapses)
      • Direct – from inferior olive
      • Indirect – from all other sources
      • All inputs arrive via intermediate cells – granule cells
    • Form the output of the cerebellar cortex
      • Axons project to deep cerebellar nuclei
    • Span across the folium
  • Granule cells
    • Receive input via mossy fibres
      • Come from the brainstem, pons, spinal cord and synapse on a cluster (glomerulus) of granule cells. These have very small cell bodies, and have an axon that passes up to the surface of the cortex. At the molecular layer, it splits into parallel fibres, that run along the long axis of the folium (each fibre is 5mm). Each parallel fibre makes contact with 500 purkinje cells. These are excitatory to the cortex, but the purkinje cells leaving are inhibitory. There is a tonic activity in cerebellar nuclei, and inputs from the cortex will inhibit this activity to allow for movement to occur.
  • Vestibular nuclei
  • Reticular formation
  • Pontine nuclei
  • Spinal cord
    • Axons form parallel fibres
      • Each parallel fibre forms numerous synapses with Purkinje cells
  • Process
    • Stimulus enter the cerebellum via mossy fibres and stimulates the granule cells
    • Granule cells send projections up to the cortex with the parallel fibres
    • Parallel fibres give information to the purkinje cells
    • Purkinje cells project to the deep nuclei of the cerebellum which tehen project outwards to the motor areas of the cortex, the spinal cord, etc
  • Inferior olivary nuclei
    • Send axons to the contralateral cerebellum via the inferior cerebellar peduncle
      • Cortex: form climbing fibres that synapse directly with Purkinje cell
  • Each climbing fibre has inputs to one Purkinje cell
    • Input is important for learning new motor tasks
      • During rehearsal (practice), of a new task, climbing fibres program (sensitise) a certain pattern into the Purkinje cells
      • After learning complete, olive remains active to allow modification/refining of action to conditions

Functional subdivisions of the cerebellar cortex

  • General functions – maintaining balance, posture, muscle tone and coordinating voluntary movement
    • Receives inputs from the vestibular, sensory systems and cerebral cortex
  • 3 functional subdivisions:
    • Vestibulocerebellum
    • Spinocerebellum
    • Pontocerebellum
  • Vestibulocerebellum (archicerebellum) – flocculonodular lobe [vestibular]
    • Maintains balance and coordination of eye movements with head movements
    • Input from vestibular receptors and vestibular nuclei
      • Progress to the cortex before projecting to the fastigial nucleus and then the vestibular nuclei
      • Vestibular nuclei can thus influence motor neurons in the spinal cord (balance) and the extraocular nuclei CN3,4,6 (coordination of head/eye movements)
    • Lesions results in a loss of balance (fall to one side, wide-based gait)
      • Especially with closed eyes (visual can compensate)
      • Nystagmus
  • Spinocerebellum – vermis and paravermal zone (area just parallel to the vermis) [sensory]
    • Input from the somatosensory system via relays in the spinal cord (trunk/limbs) and the trigeminal nuclei (head)
      • Also input via pontine nuclei from the cortex
      • Spinal cord inputs travel via spinocerebellar tracts
    • Head and trunk inputs terminate topographically in the vermis
      • Limbs terminate in the paravermal zone
    • Outputs from vermis project to the fastigial nucleus which then travels to the reticular formation
      • Reticular formation influences lower motor neurons in the spinal cord
    • Outputs from the paravermal zone project to the interposed nucleus
      • From here, project to the red nucleus or thalamus
    • Thalamus projects the motor areas of the cerebral cortex
    • Receives input from both the periphery and cerebral cortex
      • Thus can compare commands with incoming sensory information about muscle positions
    • Monitors movements as they take place and correct errors in timing/muscle strength as they occur
    • Also control muscle tone through the fastigial nucleus
    • Lesions are rarely single
      • Often occur in conjunction with lesions of the neocerebellum (=the part of the cerebellum other than the spinocerebellum. Receives inputs from the cortex via the pontine nuclei).
  • Pontocerebellum (neocerebllum) – cerebellar hemispheres, lateral zone [motor]
    • Inputs from cerebral cortex via pontine nuclei relay (pontocerebellar tract)
      • Axons project to the dentate nucleus
    • This then goes to the contralalteral thalamus via the SCP
      • Thalamus (VL nucleus) projects to motor cortex
    • Functions:
      • Plays a coordinating role in movement
        • Acts on areas of the cortex rather than communicating with individual motor neurons
      • Stores learned information about movements (esp. speed, direction and force of muscle contractions required)
    • Signalpattern
      • Intention to move signals relay from cortex to the neocerebellum by the cortico-ponto- cerebellar pathway before movement takes place
      • Neoerebellum provides the movement pattern required and relays via the dentate nucleus the relevant information back to motor cortex
      • This activates muscle via corticospinal
    • Lesions always affect the ipsilateral side of the body
      • Result in decreased muscle tone and loss of motor coordination (ataxia)
        • Causes intention tremor – tremor while movements are taking place)

Other notes

  • SCP = to midbrain; the two SCPs are connected by the superior medullary velum, which forms the roof of the 4th ventricle
  • MCP = to pons
  • ICP = to medulla
  • Diagram with horizontal bar at the bottom represents flocculonodular lobe. This part of the cerebellum receives vestibular inputs, and so is also known as the vestibulocerebellum

Table of cerebellar pathways

CerebellumTable.png