Cerebellum and Basal Ganglia

Cerebellum

Gross Anatomy

  • Located just below the occipital lobe in the posterior fossa of the skull
  • Joined to the brainstem via 3 stalks
    • Superior, middle and inferior peduncles
  • Surrounded by the falx cerebri and tentorium cerebelli
  • Has an anterior and posterior lobe and a Flocculus
    • Primary fissure separates the ant/post lobes
    • Horizontal fissure splits the posterior lobe
  • Grey matter (folia) and white matter (Arbor vitae)

Functional regions of the cerebellum

  • Vestibulocerebellum (archicerebellum- oldest part- comprises flocculonodular lobe- receives input from vestibular system)- primary function: balance
  • Spinocerebellum (paleocerebellum-  comprises most of the vermis and paravermian zone- receives input from the spinal cord, spinocerebellar tract)- primary function: integration of proprioception for stance/gait; muscle tone (antigravity muscles)
  • Cerebrocerebellum (neocerebellum- cerebellar hemispheres- connects with the cerebrum)- primary function: smooth and precise control of fine movements

Microanatomy

3 cell layers:

  • Outer molecular layer
    • Mostly nerve fibres (parallel fibres- granule cell axons; purkinje cell dendrites)
    • A few inhibitory cells (stellate/golgi/basket cells)
  • Middle Purkinje cell layer
    • Cell bodies of Purkinje cells only
      • These are the only efferent cells of the cerebellum- projecting mainly to the deep nuclei (inhibitory) but also to elsewhere in the body
  • Inner Granule cell layer
    • Cell bodies of granule cells (make up 95% of cerebellum).  They are the only excitatory cells of the cerebellum.

Afferent Inputs

  • Mossy fibres (Spinocerebellar; Pontocerebellar; Vestibulocerebellar)- synapse diffusely on the granule cells in the granular cell layer (inhibitory)
  • Climbing fibres (Olivocerebellar- originating from the oliviary nucleus of the medulla)

Cerebellar Nuclei

  • Within the Arbor vitae are four important grey matter nuclei
    • Fastigial nucleus (medially)
      • Receives afferents from the Purkinje cells in the vestibulocerebellum and sends efferents to vestibular nuclei (ipsilaterally) and reticular formation (contralaterally- to the vestibular nuclei on the opposite side) (will eventually reach the reticulo/vestibulospinal tracts to control balance)
    • Globose nucleus (lateral to that) and emboliform nucleus (together the interposed nucleus)
      • Receive afferents from the spinocerebellum and send efferents to the contralateral red nucleus and to the thalamus/reticular formation/vestibular nuclei to influence output to antigravity muscles
    • Dentate nucleus (most laterally)
      • Receives afferents from the cerebrocerebellum and pontine nuclei and sends efferents to the thalamus (then to the cortex)

NB Most of the output from the cerebellum is from the hemispheres via the dentate nucleus, through the superior peducle to the ventrolateral thalamus.

Arrangement of the cells in the cerebellum

This is (A) well known about and (B) functionally important.

Basal Ganglia

Function

  • Part of the motor system (the extrapyramidal motor system)
  • Important in the initiation & modulation of movement and in control of muscle tone (lesions of the basal ganglia cause dyfunction of these)

Components

  • Corpus striatum
    • Striatum
      • Caudate Nucleus
      • Putamen
    • Pallidum
      • Globus pallidus

NB Because of their anatomical proximity the putamen and the globus pallidus are often grouped together and referred to as the lentiform nucleus.

  • Subthalamic nucleus
  • Substantia nigra

Connections

  • Striatum
    • Receives afferents from the cerebral cortex (corticostriatal); thalamus (thalamostriatal- which has received afferents from the cerebellum) and substantia nigra (nigrostriatal)
    • Sends efferents to the globus pallidus (striatopallidal) and substantia nigra (striatonigral)
  • Globus pallidus
    • Recieves afferents from the striatum and subthalamic nucleus (subthalamopallidal)
    • Sends efferents to the thalamus (pallidothalamic) and subthalamic nucleus (pallidosubthalamic)

Basal Ganglia Dysfunction

Unilateral lesions of the BG affect the contralateral side of the body.  Lesions of the basal ganglia generally do not cause paralysis, sensory loss, loss of power or ataxia.  Lesions DO cause abnormal muscle control, changes in muscle tone and dyskinesias (abnormal involuntary movements) including tremor, chorea or myoclonus.

Disorders of the BG include

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