Cerebellum and cerebellar peduncles

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Cerebellum combines with the basal ganglia to control function of descending motor pathways

  1. cerebellar lesions classically produce ataxia, nystagmus and tremor
    1. ataxia and nystagmus occur from damage to cerebellar afferents
    2. tremor occurs from damage from cerebellar efferents
    3. signs and symptoms are ipsilateral to the lesion
      1. due to the fact that both afferent and efferent paths cross making the overall effect ipsilateral
      2. PICA occlusion may cause inferior cerebellar peduncle infarct causing ipsilateral dysequilibrium signs
  2. cerebellum unconsciously controls coordination of muscle groups as a regulator of order, strength, duration, and timing of muscle contractions; it comes into play after motor plans from the cerebral cortex have been forumulated and sent to the basal ganglia; it compares the intended movement with actual performance, conferring coordination

Cerebellum comprised of 10 lobules; anterior lobe = lobules 1-5; posterior lobe = lobules 6-9; flocculonodular lobe = lobule 10; anterior cerebellar lobe is separated from the posterior cerebellar lobe by the primary fissure

  1. four nuclei imbedded in cerebellar white matter – fastigial, globose, emboliform, dentate (medial to lateral) Remember: Fat Girls Eat Donuts
    1. efferent projections of the nuclei course through the inferior and superior cerebellar peduncles
    2. blood supply of cerebellar nuclei is the superior cerebellar artery
    3. major efferent projection is to the pons
      1. fastigial nucleus sends projections to pons and medulla through the inferior cerebellar peduncle; one branch goes to the reticular formation and one goes to the vestibular nuclei
      2. each nuclei has projections in the superior cerebellar peduncle; superior cerebellar peduncle crosses at the inferior colliculus (caudal midbrain); the globose and emboliform nuclei (a.k.a. interposed nuclei due to their position in between the fastigial and dentate nuclei) send projections to the magnocellular portion of the red nucleus (contralateral) while neurons of the dentate nucleus synapse in the parvocellular portion of the red nucleus (rubrospinal tracts from the red nucleus then run contralateral and control flexor tone primarily in distal motor groups; rubrospinal tract also has a weigh station in the paravermal spinocerebellar pathway through the inferior olivary nucleus); dentate nucleus also projects to the contralateral ventrolateral (VL) nucleus of the thalamus where the head is represented medially and the body is represented laterally; extremities are represented ventrally and the back dorsally in the thalamus
      3. major outflow of the cerebellum is from the deep nuclei and is excitatory, the only inhibitory outflow of the cerebellar cortex is from the few Purkinje cells which bypass the fastigial nucleus going directly to the vestibular nuclei; Purkinje cells project only to the vestibular and cerebellar nuclei
    4. each cerebellar nuclei also has ascending projections to the VL nucleus of the thalamus (dentate contributes most) through the red nucleus
      1. VL projects to Broadman areas 4 and 6 (motor and premotor respectively)
      2. Stimulation of the VL nucleus produces a cortical response that grows rapidly in amplitude (known as an augmenting phenomenon)
      3. VL nucleus is divided by some texts into the VL, ventral oral (VO) and ventral intermediate (VIM) nuclei
    5. cerebellar cortex has three functional divisions
      1. spinocerebellum (vermis and paravermal cortex) - receives somatic sensory information from the spinal cord and is important in guiding limb movement and posture
    • receives input from spinocerebellar tracts and spinal nucleus of CN 5; also receives input from vestibular labyrinth and visual and auditory relays; intermediate hemisphere receives somatic sensory information from the limbs
        1. intermediate hemisphere receives input from dorsal spinocerebellar tract (from Clarke’s nucleus) relaying sensory information from the leg and lower trunk
          1. Clarke’s nucleus contains second order neurons relaying proprioceptive information from the lower extremity; these neurons run ipsilateral and do not cross (as opposed to the ventral spinocerebellar fibers that do cross and travel in a position just anterior to the dorsal spinocerebellar tract)
          2. Dorsal spinocerebellar tract carries impulses from both muscle spindles and Golgi tendon organs to the cerebellar vermis via the inferior cerebellar peduncle
          3. Dorsal spinocerebellar tract lies far lateral and anterior to the dorsal horn of the spinal cord; ventral spinocerebellar tract lies just anterior (far lateral) to the dorsal spinocerebellar tract; lateral spinothalamic tract lies just inside of the ventral spinocerebellar tract
        2. intermediate hemisphere also receives input from cuneocerebellar tract via the accessory cuneate nucleus which receives impulses from group Ia muscle fibers
        3. both dorsal spinocerebellar and cuneocerebellar (upper extremity equivalent of the dorsal spinocerebellar tract) enter through ipsilateral inferior cerebellar peduncle
        4. termination is distributed in a homunculus with rostral parts in the anterior lobe (mostly) and caudal parts in the posterior lobe
          1. fibers from the dorsal and ventral spinocerebellar and cuneocerebellar tract converge in the anterior lobe of the cerebellum
        5. ventral spinocerebellar tract of the lower limb and its partner the rostral spinocerebellar tract of the upper limb relay feedback about the amount of neural activity in descending motor pathways rather than sensory information; other texts state that the ventral spinocerebellar tract carries information from Golgi tendon organs
          1. after crossing, ventral spinocerebellar neurons terminate in the intermediate lobe through the superior cerebellar peduncle; some of the fibers cross again after reaching the cerebellum to the ipsilateral side – a ‘double cross’ – while others stay on the contralateral side
          2. the rostral spinocerebellar tract is an ipsilateral pathway and enters the cerebellum via both the inferior and superior cerebellar peduncles
          3. because the ventral spinocerebellar runs contralateral a hemisection of the spinal cord may result in a contralateral ataxia (NOTE: the ventral spinocerebellar tracts has first order neurons from L1-S2)
        6. vermis projects to the fastigial nucleus and the intermediate hemisphere projects to the interposed nuclei (a.k.a. the globose and emboliform nuclei)
          1. fastigial nucleus projects to the medial spinal cord (reticulosponal and vestibulospinal tracts)
          2. globose and emboliform nuclei project to the lateral descending systems including rubrospinal and lateral corticospinal tracts
          3. Remember: fastigial nucleus is most medial, receives input from the most medial cerebellar structure (vermis), and projects to the most medial of the spinal cord tracts
    1. paired cerebrocerebellum – (lateral) – receives input from the cerebral cortex via a relay in the pontine nuclei and participates in the planning of movement; there is somatotopically organized projection from the cerebral cortex with distal musculature represented laterally in the cerebellar hemispheres and proximal musculature represented in the vermian region
      1. major input is from the contralateral cerebral cortex via the pons and middle cerebellar peduncle
    • ends on dentate nucleus which in turn sends output via the superior cerebellar peduncle; dentatothalamic fibers invade the midbrain and decussate caudal to the red nucleus (in the caudal midbrain), which most fibers bypass, on their way to synapse in the VL nucleus of the thalamus which projects to motor and premotor cerebral cortex; those areas of the cerebral cortex contribute to the corticospinal and bulbar tracts; Remember: there are three crossed tracts that are used to influence contralateral movement (contralateral from the cerebral cortex) – first cross is pontocerebellar fibers from the cortex to the pons and then to the cerebellum, second cross – crossover from superior cerebellar peduncle back to cerebral cortex, third cross – corticospinal tract decussation at the pyramids
    1. vestibulocerebellum (flocculonodular) – receives input from the vestibular labyrinth and maintains balance and control of head and eye movement
      1. sends projections to fastigial nucleus and vestibular nuclei which in turn give rise to the medial vestibulospinal tract and the medial longitudinal fasciculus (MLF)

Three layers of cerebellar cortex (external to internal) – molecular, Purkinje, granular Remember: the cerebellum gets many Miles Per Gallon

  1. Molecular
    1. Composed of axons of the granule cells (parallel fibers), stellate and basket cells, and dendrites of underlying Purkinje cells
  2. Purkinje
    1. Contains only one type of cell, the Purkinje cell (susceptible to alcohol especially in the superior vermis)
    2. Purkinje projects to the cerebellar nuclei (neurotransmitter is GABA)
    3. Purkinje cells receive inputs from climbing fibers and mossy fibers
      1. climbing fibers (using the neurotransmitter aspartate) originate in the inferior olive and are called the olivocerebellar tract as they enter through the inferior cerebellar peduncle (where they comprise the majority of fibers in the inferior cerebellar peduncle) on their way to the contralateral cerebellum (Remember: climbing fibers climb from the inferior olive to the opposite side)
        1. inferior olivary nucleus receives input from the red nucleus (motor) and spinal cord (sensory)
      2. each Purkinje cell receives input from a single climbing fiber but each climbing fiber touches more than 10 Purkinje cells (climbing fibers are excitatory)
    4. Mossy fibers provide second major input to Purkinje cells
      1. cell bodies of Mossy fibers are primarily in the spinal cord, pontine nuclei and reticular formation
      2. mossy fibers synapse on granule cells (excitatory neurons in the granular layer); the granule cells ascend through the Purkinje layer into the molecular layer where they form the parallel fibers which synapse on Purkinje cells and cerebellar interneurons; a single parallel fiber synapses with thousands of Purkinje cells and each Purkinje cell receives synapses from thousands of parallel fibers with weak excitatory effects NOTE: Granule cells are the only excitatory cells in the cerebellum and use glutamate as their neurotransmitter
    5. Purkinje cell is inhibited by two groups of interneurons
      1. stellate cells
        1. located in the outer portion of the molecular layer – primary input from parallel fibers
        2. synapse on the Purkinje dendrite and thus inhibition signal is slight
      2. basket cells
        1. located at the border between the molecular and Purkinje layers – primary input from parallel fibers
        2. synapse on the Purkinje cell body and so inhibition signal is very strong
  3. Granular
    1. Contains mostly granular cells (which utilize glutamate) and Golgi cells (an interneuron that inhibits the granule cell)
    2. stellate and basket cells directly inhibit Purkinje and Golgi cells while Golgi cells inhibit granule cells

Cerebellar peduncles

  1. superior (brachium conjunctivum) – efferent
  2. middle (brachium pontis) – afferent
  3. inferior (restiform body)- primarily afferent
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