In one breath
The cerebellum occupies the posterior cranial fossa, just behind the brainstem, and is slung from it by three peduncles: superior to the midbrain, middle to the pons, inferior to the medulla. Functionally it is three organs in one: the vestibulocerebellum (flocculonodular lobe) for equilibrium, the spinocerebellum (vermis and the paravermal strip) for posture and for smoothing movement, and the cerebrocerebellum (the two hemispheres) for planning. Its cortex has three layers, and the Purkinje cell is the only cell that carries signals out of the cortex, to the four deep nuclei. Afferents arrive by just two routes: mossy fibres, which end on granule cells, and climbing fibres from the inferior olive, which end on Purkinje cells.
Builds on: Movement and organization of the motor system · Ascending tracts: dorsal column and anterolateral system · Cortical integration and motor cortex · Leads to: Cerebellar functions, lesions and tests · Basal ganglia: circuits and functions · Rubrospinal, vestibulospinal, reticulospinal and tectospinal tracts
Where it sits
- Posterior cranial fossa, behind the brainstem.
- It is attached to the brainstem by three cerebellar peduncles:
| Peduncle | Connects cerebellum to |
|---|---|
| Superior | Midbrain |
| Middle | Pons |
| Inferior | Medulla |
- Its surface area is about 75% of that of the cerebral cortex, but its weight is only about 10% of it. A large sheet packed into a small box means the cerebellar cortex must be heavily folded.
- Two fissures cut it into its major parts:
- Posterolateral fissure: separates the flocculonodular lobe from the rest of the cerebellum.
- Primary fissure: separates the anterior lobe from the posterior lobe.
The three functional divisions
| Division | Older name | Parts it is made of |
|---|---|---|
| Vestibulocerebellum | Archicerebellum | Flocculonodular lobe |
| Spinocerebellum | Paleocerebellum | Vermis + paravermal regions |
| Cerebrocerebellum | Neocerebellum | The two hemispheres |
The old names are simply their age: archi = the oldest part phylogenetically, paleo = intermediate, neo = the newest.
Vestibulocerebellum
- Made up of the flocculonodular lobe.
- Named for its rich two-way connection with the vestibular nuclei: it sends fibres to them and receives fibres from them.
- Functions: equilibrium, and the learned adjustment of the vestibulo-ocular reflex.
Spinocerebellum
- Made up of the vermis and the paravermal (intermediate) regions on either side of it.
- Named for its input: proprioceptive and other sensory signals from the whole body, arriving through the spinal cord.
- It also receives a copy of the motor plan from the motor cortex. Holding the plan against the proprioceptive report of what the limb is actually doing lets it smooth and coordinate the movement. This is the cerebellum’s comparator job.
- Its two strips act on different muscles:
| Strip | Projects to brainstem areas for | So it controls |
|---|---|---|
| Vermis | Axial and proximal limb muscles | Posture |
| Paravermal | Distal limb muscles | Skilled voluntary movement |
Cerebrocerebellum
- Made up of the two cerebellar hemispheres.
- Named for its connections with the cerebral cortex. The cortex reaches it through a relay in the pontine nuclei, so the pathway, and hence this division, is also called corticopontocerebellar.
- Because it works in a loop with the cortex, its business is the planning and programming of movement, not its execution.
Draw it: schematic of the divisions and their functions
Draw the cerebellum as seen from behind and unrolled: a midline strip (vermis), a paler strip on each side of it (paravermal), and a large hemisphere beyond that on each side. Add a small lobe hanging at the bottom and label it flocculonodular.
- Shade and label the three divisions: flocculonodular = vestibulocerebellum (archi), vermis + paravermal = spinocerebellum (paleo), hemispheres = cerebrocerebellum (neo).
- Draw an arrow in to each division from its source: vestibular apparatus and vestibular nuclei → flocculonodular; spinal cord (spinocerebellar tracts) → vermis and paravermal; cerebral cortex via pontine nuclei → hemispheres.
- Write the function beside each: equilibrium and the vestibulo-ocular reflex; posture (vermis) and skilled movement (paravermal); planning and programming.
- Mark the two fissures, posterolateral below and primary between the anterior and posterior lobes, and the three peduncles on the brainstem stump.
This single figure answers the first half of the standard 10-mark essay, so label it fully.
Functional histology
The cerebellum is built as an outer cortex with the deep cerebellar nuclei buried inside it.
The three cortical layers
From the surface inwards:
| Layer | Cells in it |
|---|---|
| Molecular (outer) | Basket cells, stellate cells |
| Purkinje cell (middle) | Purkinje cells |
| Granular (inner) | Granule cells, Golgi cells |
Basket, stellate and Golgi cells are all interneurons: they act only within the cortex.
Molecular layer
- Holds the interneurons: basket cells and stellate cells.
- It is also the layer the dendrites of the Purkinje cells spread into, which is why the interneurons here can act on them.
Purkinje cell layer
- The Purkinje cell is the largest neuron of the cerebellum, with a huge, richly branched dendritic tree.
- Those dendrites run up into the molecular layer, where the axons of the molecular-layer interneurons end on them.
- Purkinje cells also take input straight from the climbing fibres.
- They are the only cells whose axons leave the cerebellar cortex, and they carry that output to the deep cerebellar nuclei. So every calculation the cortex makes must be read out through the Purkinje cell: it is the bridge between cortex and deep nuclei.
Worth knowing, though not in your pages: the Purkinje cell is inhibitory (GABAergic), so the cortex acts on the deep nuclei by turning them down. The deep nuclei themselves are excitatory and fire steadily, and the cortex sculpts that steady output. Granule cells are the only excitatory neurons of the cerebellar cortex.
Granular cell layer
- Holds granule cells and Golgi cells (interneurons).
- Golgi cells project back onto the granule cells and so trim the granule cell output.
- Granule cells receive the mossy fibres, and send their axons up into the molecular layer, where each divides into parallel fibres that run along the folium and end on Purkinje cells, basket cells, stellate cells and Golgi cells.
Two inputs, two different targets
Mossy fibres end on granule cells; climbing fibres end directly on Purkinje cells. This is the most frequently asked one-mark question from the chapter, and the two are easily swapped. Remember that a climbing fibre “climbs” the Purkinje dendritic tree, while a mossy fibre must be relayed through granule cells and their parallel fibres before it reaches the same Purkinje cell.
Deep cerebellar nuclei
There are four, lying deep in the white matter, and each is fed by the strip of cortex lying above it:
| Nucleus | Lies deep to | Receives cortical input from |
|---|---|---|
| Fastigius | Vermis | Vermal spinocerebellum |
| Globosus + emboliformis | Paravermal region | Paravermal spinocerebellum |
| Dentatus | Hemisphere | Cerebrocerebellum (neocerebellum) |
- Nucleus fastigius sits in the deep vermal part of the cerebellum.
- Nucleus globosus and nucleus emboliformis are taken together as the nucleus interpositus.
- Nucleus dentatus lies in the hemispheric part. It is called dentate because its outline is serrated, like a row of teeth.
- All four then project onward to various parts of the brainstem and to the thalamus, which is how the cerebellum influences the descending pathways (see Cerebellar functions, lesions and tests).
Cortex to nuclei, nuclei to the world
The cerebellar cortex has no direct output to the brainstem or cord. Every signal must pass cortex → Purkinje cell → deep nucleus → brainstem or thalamus. An answer that has the cerebellar cortex projecting straight to the spinal cord has lost a mark.
Cerebellar inputs
The cerebellum is fed somatosensory signals from almost the whole body, and signals of every sensory modality, the special senses included. The afferent tracts are:
| Afferent tract | What it brings in |
|---|---|
| Vestibulocerebellar | Direct from vestibular apparatus and vestibular nuclei |
| Dorsal spinocerebellar | Proprioception + exteroception from the body |
| Ventral spinocerebellar | Proprioception + exteroception from the body |
| Cuneocerebellar | Proprioception from head and neck |
| Tectocerebellar | Vision and hearing from the colliculi |
| Pontocerebellar | Motor cortex signals, relayed in pontine nuclei |
| Olivocerebellar | Proprioception from whole body via inferior olive |
Two of them need their relay named:
- Cuneocerebellar tract: begins in the lateral cuneate nucleus of the caudal medulla.
- Tectocerebellar tract: brings visual information from the superior colliculus and auditory information from the inferior colliculus.
- Olivocerebellar tract: the inferior olivary nucleus of the rostral medulla is itself fed by the vestibular system, the spinal cord and the cerebral cortex, and it projects to the cerebellum as climbing fibres.
Mode of inputs
Whatever the tract, an afferent reaches the cerebellum by one of three routes:
| Route | Tracts using it | Ends on |
|---|---|---|
| Climbing fibres | Olivocerebellar | Purkinje cells |
| Mossy fibres | The other six tracts | Granule cells |
| Other fibres | Monoaminergic, thalamic | Deep nuclei |
Mossy fibre inputs
- The major source of input to the cerebellum.
- They carry direct proprioceptive signals from every part of the body, together with the input coming down from the cerebral cortex.
- They end mainly on granule cells.
Climbing fibre inputs
- They carry signals from the inferior olivary nucleus only.
- The olive collects proprioceptive input from the whole body before passing it on.
- They end on Purkinje cells.
Worth knowing, though not in your pages: one Purkinje cell receives just one climbing fibre, but that one fibre makes hundreds of contacts on it, so a single climbing fibre impulse always fires the Purkinje cell, as a complex spike. Climbing fibre activity is the error signal used in motor learning; mossy fibre activity, arriving through thousands of parallel fibres, is what drives the cell moment to moment (simple spikes).
Other inputs
These bypass the cortex and go straight to the deep cerebellar nuclei:
- Monoaminergic fibres: serotonergic from the nucleus raphe magnus, noradrenergic from the nucleus locus ceruleus.
- Thalamic fibres, and fibres from other parts of the brain.
Draw it: the cerebellar cortical circuit
Draw three horizontal bands and label them, top to bottom: molecular, Purkinje, granular. Put the deep cerebellar nucleus in a box below them.
- Draw one big Purkinje cell in the middle band, with its dendritic tree fanning up into the molecular layer and its axon going down into the deep nucleus box. This axon is the only line leaving the cortex, so draw no other.
- Bring a climbing fibre up from below, labelled from inferior olive, and let it wind up the Purkinje dendritic tree.
- Bring a mossy fibre up from below, labelled all other afferents, and end it on a granule cell in the granular layer.
- Take the granule cell axon up into the molecular layer, split it into a T, and label the two arms parallel fibres. End them on the Purkinje dendrites, and on basket and stellate cells drawn in the molecular layer, whose own axons you then bring back onto the Purkinje cell.
- Add a Golgi cell in the granular layer with an arrow back to the granule cell, labelled “trims granule output”.
- Add the arrow for the other inputs, monoaminergic and thalamic, entering the deep nucleus box directly. Worth knowing, though not in your pages: the mossy and climbing fibres also send collaterals straight into that box, so dot those in as well.
- Mark the two afferent routes at the bottom with a bracket: mossy → granule, climbing → Purkinje.
Exam-answer skeleton: "Describe the divisions, connections and functions of the cerebellum" (long essay)
- Where it is and what holds it: posterior cranial fossa, behind the brainstem; the three peduncles and what each connects to; the folded cortex (75% of the cortical area, 10% of its weight).
- Anatomical landmarks: posterolateral fissure and flocculonodular lobe, primary fissure and the anterior/posterior lobes.
- The three functional divisions with their diagram: vestibulo-, spino- and cerebrocerebellum, each with its older name, its parts and its function; separate the vermal (posture) from the paravermal (skilled movement) strip.
- Functional histology: the three cortical layers and their cells; the Purkinje cell as the sole output; the four deep nuclei and the cortex that feeds each.
- Inputs: the seven afferent tracts in a table, then the three modes (mossy, climbing, other) with the cell each ends on.
- Internal circuit: draw it, and say that the cortex acts on the deep nuclei through the Purkinje cell while the afferents excite those nuclei directly.
- Outputs: fastigius, interpositus and dentatus to the brainstem and thalamus, and the descending pathways each steers (see Cerebellar functions, lesions and tests).
- Functions: equilibrium, muscle tone, posture, coordination of voluntary movement, planning and motor learning, with the comparator explanation.
- Close with applied value: the features of a cerebellar lesion, in two or three lines.
Asked in exams
- Cerebellum: divisions, connections, functions: Jul 2012, 10 marks · Aug 2015, 10 marks · Aug 2016, 10 marks · Jan 2021, 5 marks · May 2022, 10 marks · Oct 2024, 15 marks
- Cerebellar circuits: mossy, climbing and Purkinje fibres: May 2022, MCQ · Dec 2023, MCQ · Dec 2023, MCQ