Rule of cerebellar connections
- Afferent fibres terminate in the cortex.
- Exception
- Some vestibular fibres project directly to the cerebellar nuclei.
- Efferent fibres arising in the cortex end in cerebellar nuclei.
- Exception
- Some cortical fibres project directly to vestibular nuclei (vestibular nucleus is considered as displaced cerebellar nucleus)
- Fibres arising in the nuclei project to centres outside the cerebellum.
- Inhibitory GABA neurons
- Excitatory neurons
- Granule cells
- Interneurons: Unipolar brush cells (UBC)
- Afferent fibre system (Climbing and mossy fibers)
Afferent Fibres (Enter cerebellar cortex)
Mainly from
- Afferent input (Periphery)
- Proprioception
- Exteroception
- Vision
- Vestibular apparatus.
- Integrative input
- Cerebral cortex
- Reticular formation
- Inferior olivary complex
The afferent fibres to the cerebellar cortex are of three different types:
Mossy Fibres
- All fibres entering the cerebellum end as mossy fibres
- Except olivocerebellar and par-olivocerebellar tracts
- Originate from the
- Vestibular nuclei (vestibulocerebellar)
- Pontine nuclei (pontocerebellar)
- Spinal cord (spinocerebellar)
- Terminate
- Granular layer of the cortex within the glomeruli.
- Branch profusely within the granular layer
- Terminates in an expanded terminal called a rosette
- Have multiple indirect connection with Purkinje cells
- Afferent inputs through mossy fibres pass through granule cells to reach the Purkinje cells.
- Indirectly influence a large number of Purkinje cells.
- To accomplish this, the mossy fibers branch extensively in the granule cell layer, where they establish synaptic contacts with the granule cells.
- Rosettes are the sites of synapses between mossy fibers and the claw-like terminals of granule cell dendrites.
- Mossy fiber rosettes also establish synapses with Golgi type II cell bodies.
- Mossy fibers form excitatory synapses onto
- Deep cerebellar nuclei AND
- Granule cells, which send their axons (parallel fibers) to form excitatory synapses with multiple Purkinje cells (equally, each Purkinje cell also receives input from multiple different granule cell parallel fibers), which then reduces output from cerebellar nuclei
- The granule cells project superficially to the molecular layer, where they bifurcate in a T-shaped manner to form the so-called parallel fibers.
- Each parallel fiber establishes contacts with a large number of Purkinje cell dendrites.
- Glomeruli
- Complex synaptic structure
- 10 µm in diameter
- Surrounded by a neuroglial capsule
- Core of each glomerulus formed by
- Mossy fiber
- Rosette (the expanded termination of a mossy fibre-dendrites)
- Numerous (up to 20) dendrites of granule cells synapse with the rosette
- Axon
- Dendritic
- Excitatory
- Golgi cells
- Axons
- Dendrites
- Inhibitory
- Granule cells
- Dendrite
- Pathology: Wnt Medulloblastoma
Climbing Fibres
- Are terminations of axons reaching the cerebellum
- Arising from the inferior olivary complex
- Olivocerebellar tract
- Par-olivocerebellar tract
- Climb up the granular layer and the Purkinje cell layer to reach the molecular layer.
- Have one to one contact with Purkinje cells:
- One climbing fibre will send excitatory signal to deep cerebellar nuclei and also send excitatory signal to one Purkinje cell. The Purkinje cell will send inhibitory signal to the deep cerebellar nuclei. Since it is only one Purkinje cell with inhibitory signalling and one climbing fibre sending excitatory signal to the deep cerebellar nuclei the deep cerebellar nuclei will have increased deep cerebellar nuclei output.
- See mossy fibre for contrast
Aminergic fibers
- Distinct both from climbing and mossy fibers
- Contain biogenic amines.
- Serotonin-containing axons that originate in the raphe nuclei of the brainstem
- Norepinephrine-containing axons that originate in the locus ceruleus.
Inter neurons
- The afferent fibers (climbing fibers, mossy fibers, and aminergic fibers) are modified by intracortical circuits formed by three types of interneurons that modulate Purkinje cell activity
- Golgi
- Basket
- Stellate cells
- Like the Purkinje cells, these interneurons are inhibitory in nature, and they contain the neurotransmitter gamma-aminobutyric acid (GABA).
Efferent Fibres
- The fibres from cerebellar cortex
- Mainly originate from of axons of Purkinje cells
- Terminate in the cerebellar nuclei.
- Inhibitor signals to cerebellar nuclei
- Some purkinje cell efferent of Flocculonodular lobe bypass the cerebellar nuclei and terminate in the vestibular nuclei of brainstem. (see the top images have two black lines)
- The fibres from dentate, emboliform and globose nuclei leave the cerebellum through the superior cerebellar peduncle.
- The fibres from the fastigial nucleus leave the cerebellum through inferior cerebellar peduncle.
- Ending in:
- Thalamus → cerebral cortex.
- Red nucleus
- Reticular formation
- Vestibular nuclei
- Modulation of Purkinje cell inhibition/disinhibition
- Via the parallel fibers in two main ways:
- Feed-back inhibition
- Due to granule cell axon (parallel fiber) excitation of Golgi cells forming inhibitory synapses on other granule cells
- Feed-forward inhibition
- Due to mossy fiber direct excitation of Golgi cells forming inhibitory synapses on granule cells
- Other mechanisms for inhibition of Purkinje cell output include
- Parallel fiber activation of inhibitory interneurons synapsing on the dendritic arbor (Stellate cells) or body (basket cells) of Purkinje cells.
- The net effect of these complex interactions is continuous modification of the level of inhibition/disinhibition of Purkinje cell output to the cerebellar nuclei, which is combined with the excitatory input from cerebellar afferent collaterals, to dynamically vary the excitatory outflow from each cerebellar nucleus.
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