21.5 Cerebellum Flashcards
What are the different functional subdivisions of the cerebellum? What is the function of each?
- Vestibulocerebellum -> Balance/posture + Eye movement
- Spinocerebellum -> Motor execution (i.e. control of axial and limb muscles)
- Cerebrocerebellum (a.k.a. pontocerebellum) -> Motor planning (i.e. planning and timing precise movements)
What is the anatomical location of the vestibulocerebellum?
Flocculonodular lobe
What is the anatomical location of the spinocerebellum?
Vermis (and intermediate zone)
What is the anatomical location of the cerebrocerebellum?
Lateral cerebellar hemispheres
What are the three layers in the cytoarchitecture of the cerebellum, from innermost to outermost?
Granule cell layer
Purkinje cell layer
Molecular layer
What are the different layers of the cerebellum and what is found in each?
[IMPORTANT]
- Inner layer (Granule cell layer):
- Granule cells
- Mossy fibre inputs
- Golgi interneurons
- Middle layer (Purkinje cell layer):
- Purkinje cells
- Outer layer (molecular layer):
- Purkinje cell dendrites
- Granule cell parallel fibres (note the perpendicular fibres)
- Climbing fibres
- Stellate and basket interneurons
Describe the principle of the molecular layer of the cerebellar cortex.
- It is the outer layer where the Purkinje fibres send their dendrites to receive information from climbing fibres and granule cell parallel fibres. None of these cells actually have their cell bodies here.
- There are also stellate and basket interneurons here.
What are the different cell types found in the cerebellum?
Purkinje, Golgi, granule, stellate, basket.
What are the outputs from the cerebellar cortex to the deep nuclei?
Purkinje cells are the sole output.
*GABA (inhibitory cells)
In which layer does most motor processing take place?
Molecular layer
Mossy fibres provide input to the…
Granule cells
and
Deep cerebellar nuclei
What are the functions of the basket and stellate cells?
- GABAergic inhibitory interneurons
- Send input to Purkinje cells and parallel fibres to induce lateral inhibition and sharpen borders of area activated by the parallel fibres
Between the basket cells and stellate cells, which is long-range and which is short-range?
Basket cells = long-range (large basket)
Stellate cells = short-range (small star)
What are the functions of the Golgi interneurons?
Receive excitatory input from mossy fibres - synapse onto granule cells and their parallel fibres
Feedforward and feedback inhibition
What is the function of the granule cells?
Stimulated by mossy fibres to generate simple spikes in Purkinje cells
What is the most direct route from input to output?
Mossy fibres –> granule cell –> Purkinje neuron –> deep nucleus neuron –> output
Which side of the body does the cerebellum work with?
- The ispilateral side (think about the ascending spinocerebellar tract).
- This is unlike the cortex and basal ganglia, which work with the contralateral side.
Label the different functional parts of the cerebellum and give their functions.
[IMPORTANT]
- Spinocerebellum
- Occupies vermis
- Responsible for motor execution
- Cerebrocerebellum (a.k.a. pontocerebellum)
- Occupies lateral hemispheres
- Responsible for motor planning
- Vestibulcerebellum
- Occupies flocculonodular lobe
- Responsible for balance and eye movement
Note that this is an unfolded diagram. The cerebellum is folded so that the top of this diagram is the superior side, while the bottom of this diagram is the inferior side.
Describe the output nuclei that the cerebellum outputs to and what the function of each of these is.
[IMPORTANT]
Spinocerebellum outputs to:
- Fastigial nucleus -> To medial descending systems
- Interposed nuclei (globose and emboliform nuclei) -> To lateral descending systems + Red nucleus
Cerebrocerebellum (pontocerebellum) outputs to:
- Dentate nucleus -> To motor and premotor cortices (via ventral anterior and ventral lateral thalamus)
Vestibulocerebellum outputs to:
- Fastigial nucleus -> To vestibular nuclei
- Vestibular nuclei (directly)
Compare the type of firing that mossy fibres/granule cells and climbing fibres stimulate in the Purkinje neurons.
[IMPORTANT]
Mossy fibres and granule cells:
- Generate simple spikes in Purkinje cells
- The mossy fibres and therefore granule cells fire at around 50-100Hz
- Each Purkinje cell receives input from 200,000 granule cell parallel fibres
- When around 200 (0.1% of the total) of these fire simultaneously on a Purkinje cell, they summate to trigger an action potential in the Purkinje cell (simple spike)
Climbing fibres:
- Generate complex spikes in Purkinje neurons
- The climbing fibres fire at around 1-10Hz
- Each Purkinje cell receives input from only 1 climbing fibre, but at 300 synapses
- An action potential in the climbing fibres triggers a calcium-dependent EPSP (not action potential) in the Purkinje cell
What is the function of simple spikes in Purkinje neurons?
Calibration/optimisation:
- Each Purkinje cell is sensitive to a particular source/type of input, based on the sensory and motor inputs it receives
- The frequency of simple spikes is determined by granule cell parallel fibre firing, which in turn is determined by sensory/motor inputs (from visual, vestibular, etc.)
- Thus, when the frequency of Purkinje cell simple spikes is increased, negative feedback on the deep cerebellar nuclei is increased, correcting any motor overshoot
What is the function of complex spikes in Purkinje neurons?
Long-term depression of mossy fibre/granule cell inputs:
- The number of complex spikes is relatively small compared to the frequency of firing of simple spikes, so climbing fibres have only a small effect on Purkinje cell firing
- However, the complex spikes involve calcium influx, which has downstream effects
- The calcium causes long term depression of the synapses between granule cell parallel fibres and Purkinje cells, so that their firing is decreased
- This is shown in the diagram, where complex spikes lead to long-term decreases in EPSPs caused by granule cells
Summarise the function of climbing fibres from the inferior olive.
Motor learning -> Recalibration, optimisation and automation:
- They respond to mismatches between sensory feedback and motor activity (i.e. when there is motor error)
- In response, they stimulate the Purkinje fibres, producing complex spikes, leading to calcium influx
- This leads to long-term depression of the signals from the granule cell parallel fibres that have fired in the last 200ms
- So the Purkinje cell learns to be less responsive to the sets of granule cell parallel fibres that have caused the recent motor imbalance
Describe the function, location, inputs and outputs of the vestibulocerebellum.
Functions:
- Controls balance via axial and proximal limb muscles
- Controls eye movement
Location:
- Flocculonodular lobe (little tail on the inferior side that has been curled up)
Inputs:
- Direct input from primary sensory afferents from vestibular system (only sensory system that doesn’t have to relay in the brainstem)
- Input from the vestibular nuclei (secondary afferents)
Outputs (from flocculus):
- To the fastigial nucleus and vestibular nuclei, which then output to:
- Medial and lateral vestibulospinal tracts to neck and back muscles (for posture/balance)
- Oculomotor nuclei for the ‘vestibulo-ocular reflex (VOR)’ which moves eyes accordingly (not illustrated)
Note that the vestibular nuclei output to descending tracts, not to the vestibular apparatus or anything like that.
Describe the function, location, inputs and outputs of the spinocerebellum.
Functions:
- Controls posture and locomotion via axial and proximal limb muscles
Location:
- Vermis (including paravermis)
Inputs:
- Sensory and motor cortex (instructions for movement)
- Spinocerebellar tracts:
- From neck and trunk
- From limbs
- Inferior olivary nucl.*
Outputs:
- Vermis outputs to fastigial nucleus, which outputs to:
- Ventromedial brainstem descending systems (vestibulo-spinal, reticulo-spinal and cortico-spinal tracts (via thalamo-cortical relay))
- Paravermis outputs to interposed nuclei, which outputs to:
- Dorsolateral brainstem descending systems (rubro-spinal and cortico-spinal tracts)
Describe the function, location, inputs and outputs of the cerebrocerebellum (a.k.a. pontocerebellum).
Functions:
- Initiation, planning and timing of movements
Location:
- Lateral hemispheres
Inputs:
- From cortex, via the pons
Outputs:
- Dentate nucleus -> To VL thalamus and then motor cortical areas + prefrontal cortex
What are the two main input types into the cerebellum?
[IMPORTANT]
- Mossy cells
- From vestibular system, spinal cord and pons
- To granule cells and deep cerebellar nuclei
- Climbing fibre inputs
- From inferior olive
- To Purkinje neurons and deep cerebellar nuclei
What is incoordination?
- Incoordination of fine movement -> Movements break down to subcomponents
What is postural ataxia?
- Incoordination of axial muscles, postural instability, staggering wide-based ataxic gait (similar to being drunk)
- This is a symptoms of cerebellar disorders
What is intention tremor?
- Low frequency high amplitude oscillations of a limb as it approaches target (this is due to overshooting and overcompensating as the individual ‘hunts’ to find the target)
- Seen in patients with cerebellar disorders
What is nystagmus?
- Involuntary, rapid oscillation of the eyeballs
- Seen in patients with cerebellar disorders
What are the possible effects of lesions to the cerebellum?
*Incoordination
*Postural ataxia
*Intention tremor
*Nystagmus
