Cerebellum

• Cerebellum is the largest part of hindbrain and it develops from the metencephalon.
• Cerebellum is located in the posterior cranial fossa and its covered by the tentorium cerebelli (dura matter)
• Cerebellum is related:

  • superior to: tentorium cerebelli,
  • Anterior: brainstem (pons,medulla, and 4ventricle)
  • Posterior: Occipital lobe
• Cerebellum is an avoid shaped mass made of 2 well developed hemispheres joined by the vermis. Each hemisphere is further divided into 3 lobes: anterior, posterior and fluccunodular lobes.
• Structure of cerebellum

  
• Functions of cerebellum:

  • Motor control: cerebellum coordinates voluntary movement; it plays critical role in motor learning and helping fine-tune motor movements based on practice and feedback.
  • It also ensure that the movements are timed and precise
  • Its also involved in attention, focus, contributes ti language processing and higher cognitive functions
• The external surface of cerebellum is characterised by presence of numerous parallel fissures which separates slender rigdes called folia cerebelli.

  There are 3 main cerebellar fissues that divided each hemisphere into 3 loes:

  • Primary fissure: located in the sup suf of cerebellum. Divided anterior from posterior lobe
  • Horizontal fissure: runs horizontally along post surface. It separates superior and inferior parts of posterior lobe
  • Posterolateral fissure: in the inf surface of cerebellum, it separates posterior lobe from flucculonodular.
• The fissures subdivided cerebellum into leaf-like band known as folia. This architecture increases the surgace area of cerebellum allowing more neurons and increasing processing capacity
• cerebellum is further subdivided into a well-defined lobular organization. There are 8 lobules within the vermis, and except for the first one ach lobule has a corresponding lobule in the hemisphere.
• Lobules of cerebellum

  
• Aside from the 3 main fissure there Sre several other fissures that subd ide vermis and hemispheres into lobules.
• Anatomical subdivision of cerebellum

  
• Internal structure of cerebellum is composed of

  • thin surface layer of gray matter - cerebellar cortex
  • And a central core of white matter - cerebellar medulla

  Embedded inside the cerebellar medulla are 4 masses of gray matter - intracerebellar nuclei/ deep cerebellar nuclei.

  In transverse sections the white matter assumes a branching appearance of a tree- Arbor Vitae
• The deep cerebellar nuclei are embedded within the white matter, these are from lat to med:

  • Dentate nucleus: largest; nuclei of neocerebellum.
  • Emboliform nucleus: medial to dentate. Nucleus of paleocerebellum, receives afferent from it
  • Globose nucleus: btw embolisform and fastigial. Similar function to emboliform so they are collectively called nucleus interpositus.
  • Fastigial nucleus: near midline of cerebellum. Nucleus of archicerebellum
• Nuclei of cerebellum and fibers coming through and from them

  
• White matter of cerebellum concists of myelinated fibers that forms connections btw cerebelum and other parts of brain. It consists of 3 types of fibers:

  • Axons if purkinje cells: that leave cerebellar cortex towards deep cerebellar nuclei
  • Mossy fibers: these are all fibers entering cerebellum other than olivocerebellar fibers. These fibers originate from vestibular nuclei, pontine nucleus, and spinal cord and end in granular layer where they generate enlargement called rosetes within the glomeruli
  • climbing fibers
• Climbing fibers represents the terminations of axons reaching cerebellum from inferior olivary complex.

  They give off colaterals to deep cerebellar nuclei before entering cortex where they split into terminal branches each giving input to a single purkinje cell.

  • Stimulus to climbing fibers induce complex spike in purkinje cell which modulates activity of principlar neurons.
  • They do not directly synapse onto neurons in the deep cerebellar cortex but indirectly influence their activity through excitatory input of purkinje cells.
• Cerebellar nuclei

  
• Cerebellar cortex has 3 distinct layers: from out to inside

  • Molecular layer:
  • Purkinje cell layer
  • Granular layer
• The molecular layer is situated bellow pia matter.

  • It has two types of inhibitory GABA neurons, - stellate cells and basket cells.
  • dendritic arbors of purkinje neurons
  • and the parallel fibers tracts from granular cells.
• The basket cells are inhibitory GABanergic interneurons. They form basket-like structure around some of purkinje cells providing them with strong inhibitory input.
• Stellate cells are inhibitory interneurons that receive excitatory input from paralel fibers of granular cells causing them to release inhibitory neurotransmitters onto purkinje cells modulating their activity.
• Purkinje cell layer: single layer of purkinje cells in which dendrites form extensive trees that extend into molecular layer forming purkinje cell plexus.

  • Receives input from: Parallel fibers: excitatory input from granular cells; and climbing fibers: excitatory input from inferior olivary nucleus
  • Purkinje cell axons travel to the deep cerebellar nuclei providing them with inhibitory input regulating their activity.
• Granular layer is the innermost layer and has 3 types of cells:

  • Granular cells: excitatory cells whose activation cause excitatory input onto purkinje cells. Their axons( parallel fibers) extends into molecular layer where they synapse with purkinje cells and interneurons.
  • Golgi cells: inhibitory neurons. Their dendrites enter molecular layer and synapse with parallel fibers. Axons take part in formation of glomeruli feedback inhibition of granular cells.
• Microstructure of cerebellar cortex

  
• Incoming signal to cerebellum are from:

  • Mossy fibers (sensory + motor) info from spinal cord, cortex, brainstem. Enter cerebellum and synapse with granullar cells
  • Climbing fibers from inf olivary nucleus and excites purkinje cells
  • Granule cells receives excitatory input from mossy fibers and they send out their axons into molecular layer biforcating and forming parallel fibers which them provide excitatory input onto purkinje cells
• Purkinje cell receive excitatory input from parallel fiber and also excitatory input from a single climbing fibers. Purkinje cells then send inhibitory output to deep cerebellar nuclei which play crutial role in modulating and fine-tuning motor commands
• • Basket and stellate cells in molecular layer are interneurons that receives excitatory input from parallel fibers and provide inhibitory output to purkinje cell modulating their activity.
  • Golgi cells in granular layer receive input from parallel fibers and mossy fibers and provide inhibitory output to granular cell modulating their activity.
• Afferent and efferent fibers of cerebellum are grouped together into 3 large bundles named cerebellar peduncles
• The superior cerebellar peduncle connects cerebellum with midbrain. It primarily includes efferent fibres going from the dentate nucleus to the red nucleus, thalamus and cerebral cortex of the opposite side. It’s the main efferent pathway from the cerebellum and its fibres originate primarily in the dentate nucleus.
• The middle cerebellar peduncle connects cerebellum to pons.

  The middle cerebellar peduncle comprises only afferent fibres which originate from the pontine nuclei of the opposite side.
• The inferior cerebellar peduncles connects cerebellum to to medulla.

  • consists primarily of afferent fibres to the cerebellum from the spinal cor: dorsal spinocerebellar, cuneocerebellar, vestibulocerebellar, olivicerebellar, reticulocerebellar tracts.
  • It also carries a couple of efferent fibres from the cerebellum to the medulla, mainly to the vestibular nuclei and reticular formation.(cerebelloreticular, cerebellovestibular)
• The cerebellum is supplied by 3 pairs of cerebellar arteries:

  • Superior cerebellar artery: a branch of basilar artery
  • Anterior inferior cerebellar artery: a branch of basilar artery.
  • Posterior inferior cerebellar artery: a branch of vertebral artery.
• Based upon phylogenetic and functional standards, the cerebellum is split into the following 3 parts:

  • Archicerebellum (Vestibulocerebellum)
  • Paleocerebellum (Spinocerebellum)
  • Neocerebellum (Cerebrocerebellum)
• • Archicerebellum: earliest part of the cerebellum. Found in floculonodular lobe, contains fastigial nucleus is concerned with the maintenance of equilibrium.
  • Paleocerebellum: it’s the next part of the cerebellum to appear. The paleocerebellum is primarily spinocerebellar in links and is concerned with the tone, stance and primitive movements of the limbs.
  • Neocerebellum: it’s the latest part of the cerebellum to grow. Found in the lateral hemispheres. Contains dentate nucleus and is concerned with coordination of voluntary movements and plans future movements.
• Phylogetenic and functional subdivision of cerebellum

  
• Superior cerebellar peduncle Afferents

  • tecto- cerebral fibers
  • trigemino-cerebellar fibers
  • hypothalamo-cerebellar fibers
  • rubro-cerebellar fiber
  • Ventro-spino-cerebellar fibers
• Superior cerebellar peduncle efferents

  • cerebellar-thalamic fiber
  • cerebello-rubro fibers
  • cerebello-reticular fibers
  • cerebello-nuclear fibers
• Inferior cerebellar peduncle efferent

  • cerebello vestibular fiber
  • cerebello reticular
  • cerebello olivary fiber
• Inferior cerebellar peduncle afferent

  • post spinothalamic tract
  • cuneo cerebellar fibers
  • vestibulo cerebellar fibers
  • reticulo cerebellar fibers
  • olivo cerebellar fibers