cerebellum Flashcards
what is the function of the cerebellum?
the coordination, calibration, learning and automating of skilled movements
what is the sequence of sensory-motor organisation? where do BG + C fit?
motivation
planning (BG)
programming
integration (C)
execution
what is the coordination aspect of the cerebellar function?
coordination + initiation: of multi-joint movements
what is the calibration aspect of the cerebellar function?
calibration: compares motor command (instruction from cortex) with feedback from ascending proprioceptor/sensory inputs to correct errors and modify movements to be more successful
what is the learning and automation aspect of the cerebellar function?
learning + automation of fast movements
what is the function of the vestibulocerebellum?
orientation of head/body, balance and posture, and eye movements
what is the function of the spinocerebellum?
control of axial and limb musculature: motor execution
what is the function of the cerebro-cerebellum?
planning and timing precise movements (motor planning)
slightly more cortical
what are the 3 deep output nuclei of the C, where are they found and where do they project?
fastigial: to medial
interposed: to lateral descending
^ both spinocerebellum ^
dentate: to motor and premotor cortices
^ cerebrocerebellum ^
all found embedded in white matter at heart of cerebellum
how does the cerebellum actually ‘learn’?
tunes and changes Purkinje cell responsiveness to specific sensorimotor situations
adjusts the stored parameters after comparing with intended performance
what are symptoms of general cerebellar disorders?
disorders of coordination, posture, limb tremor and eye movements:
incoordination of fine movement, dysmetria (poor accuracy of movement)
postural ataxia, staggering, wide-based gait, tremor (overshooting target then overcompensating or when in fixed posture)
nystagmus (involuntary eye movements)
what experiment/action is comparable to cerebellar disease?
drawing dot-to-dot with looking in the mirror
mimics tremor as cerebellum is not adapted to calibrate your movements in inverted visuospatial conditions
what allows for ordered connectivity in the cerebellum?
highly ordered structure within the cerebellar outer cortex
how do Purkinje cells calibrate movement?
finely tuned spiking via deep nuclei outputs to brainstem + cortex
what is the cytoarchitecture of the cerebellum?
3 layers: (outer to inner)
molecular (parallel fibres), Purkinje, granule
what are the cellular inputs to the cerebellum?
climbing fibres
mossy fibres
what are the inputs to the vestibular cerebellum?
to flocculonodular lobe
from primary sensory afferents with no relay in the brain stem
from secondary afferents of vestibular nuclei
what are the outputs of the vestibular cerebellum?
flocculus to fastigial deep nucleus and vestibular nuclei
from there to neck and back muscles for posture/balance
also to ocular motor nuclei for vestibulo-ocular reflex
what are the consequences of vestibulocerebellum lesions?
poor balance, nystagmus
what are the inputs to the spinal cerebellum?
sensory and motor cortex instructions
spinocerebellar tracts: neck, trunk and limbs
inferior olivary nucleus
what are the outputs of the spinal cerebellum?
leave the vermis via fastigial nucleus, leave the paravermis via interposed nucleus
FN to v/m brainstem descending systems (via thalamocortical relay)
IN to d/l brainstem descending systems
what are the consequences of lesions of the spinocerebellum?
medial zone: problems standing or walking
intermediate zone: poor accuracy, action tremor
what are the inputs to the cerebrocerebellum?
from the cortex via the pons
what are the outputs from the cerebrocerebellum?
from dentate nucleus
to motor cortical areas and PFC via VL thalamus
what are the consequences of lesions of the cerebrocerebellum?
disrupts multi-joint movements e.g. reaching + grasping objects
where does the motor planning process start?
cerebral cortex, particularly motor areas e.g. 1ry motor cortex (M1) and the PMC
these regions receive input from other areas of the cortex and integrate sensory information from: vision, proprioception and other sensory modalities.
what is motor planning?
involves the formation of a motor program, which represents a sequence of neural instructions for a specific movement
what does a motor plan specify?
program specifies the desired movement, the muscles involved, and the timing of muscle contractions
what are the crucial brainstem nuclei for motor movement?
red nucleus, reticular formation
coordinate motor commands and adjust muscle tone
what is the order of actions in the control of a motor movement?
- motor plan + initiation
- motor execution
- control loop (simultaneous w execution)
- learning + adaptation
what is the spinal cord’s role in the initiation of a movement?
relay station, transmits signals to appropriate motor neurons, located in anterior horn of spinal cord, send impulses for contraction
what is the feedback that the brain relies on during motor execution for monitoring + adjusting?
proprioceptive information, tactile sensations, and visual feedback
what is the function of feedback loops in the cortex, BG and C
allow comparison of desired movement with the actual movement and enable adjustments to be made in real-time
what does motor learning involve?
acquisition of new motor skills and the refinement of existing ones
simply, how is the cerebellum involved in motor learning?
stores and retrieves motor memories and facilitates skill acquisition
what is found in the molecular layer of the cerebellum?
purkinje cell dendrites, granule cell parallel fibres, climbing fibres, stellate and basket interneurons
what are the connections between mossy fibres and purkinje cells?
1 mossy fibre contacts 10 granule cells
granule cell axons form parallel glutamatergic fibres
each P cell receives input from 200,000 parallel fibres: convergence
what cells do mossy fibres interact with?
granule cells and deep cerebellar neurons
what spikes do Mossy fibre inputs generate in Purkinje cells? how?
simple spikes
M fibres fire at 50-100 Hz
need ~200 parallel fibre inputs from GCs to P cells to summate to cause Na/K spikes in P cells (20-50 Hz)
what is each P cell tuned to? what does it do?
a particular source/type of input
its activity inhibits output nuclei to offset motor over/undershoot
how do climbing fibres contact purkinje cells?
axons from neurons in contralateral inferior olive
each P cell receives inputs from 1 climbing fibre, CF winds through dendritic tree making ~300 synapses
what cells do climbing fibres contact?
purkinje cells and deep cerebellar neurons
what cells do climbing fibres contact?
purkinje cells and deep cerebellar neurons
what type of spikes do CFs generate in PCs?
a single AP in CF (1-10Hz) generates a large EPSP causing a prolonged Ca2+ dependent ‘complex spike’ in PCs
what is the outcome of a complex spike in PCs?
opens Ca2+ channels and mediates a change in efficacy of Mossy Fibre inputs: causes long term depression (LTD) of parallel fibre-PC synapses
what is the function of the LTD?
CFs report ‘error’ to P cells: error = detection of difference in motor command vs sensory detection
CFs use this error to ‘teach’ P cells which set of PFs they should become LESS responsive
source of instructive signals controlling induction of cerebellum-dependent motor learning
what are the 3 types of interneurons
basket, golgi, stellate
what are the 3 types of interneurons
basket, golgi, stellate
describe basket cells?
input: excitatory granule cell input
axons project to: neighbouring P cells (forming a basket)
excitation causes lateral inhibition
what is the role of stellate cells?
short-range inhibition of P cells
describe golgi cells
input: from parallel fibres to dendrites in molecular layer
output: project back to granule cells
provide feedback inhibition to shorten duration of excitement of granule cells but M fibres
how can the cerebellum engage in motor learning (vestibulocerebellum)
if prism glasses are worn, cerebellum has to adapt vestibular-ocular reflex to calibrate foveation of an object
in the prism case, have to look straight to look directly at object off to the side, in the normal case would just look towards object
learning of VOR reversal prevented by lesions of V-cerebellum in animals
what is the evidence for the role of the cerebellum in motor learning?
in the Purkinje cells of monkeys learning a new motor task there is:
- stereotyped activity in simple spikes (M fibres), then gradual decrease
- increased frequency of complex spikes (C fibres)
as the task is learned:
- frequency of complex spikes returns to control (CFs)
- frequency of simple spikes remains decreased (MFs)
- can see learned adaptation in P cell output due to error detection by C fibres
[Gilbert and Thach 1977]
what happens to motor performance when you learn a new task?
initially bad, then increases back to normal/good over time + practice
what are some cerebellar disorders?
alcoholic cerebellar degeneration
essential tremor
multiple sclerosis
acquired ataxia
problems with what part of the cerebellum lead to postural ataxia + ataxic gait?
vestibulo + spinocerebellum
what percentage of MS patients have a tremor?
dysmetria
25-60%
