Hierarchy of Motor Control - Cerebellum Flashcards
What is the cerebellum responsible for?
Co-ordination of voluntary movements
What do we mean by co-ordination of voluntary movements?
Control of the timing of sequences of contractions in different muscles to make a movement smooth and reliable.
What is the structure of the cerebellum like?
Very complex structure
There is a highly convoluted grey matter cerebellar cortex, beneath which is white matter containing axons going to and from the cortex.
Embedded in the white matter are the deep cerebellar nuclei.
Functionally there are 3 parts of the cerebellum (based on embryological and physiological criteria.
What are these?
Archicerebellum (Vestibulocerebellum):
-Oldest part concerned with posture and balance, major input from the vestibular system
Paleocerebellum:
-Important in 4-legged animals, control of gait
Neocerebellum: (90% of the cerebellum in man)
-Unconsious co-ordination of voluntary movements
Where does the input to the cerebellum go?
ALL input to the cerebellum is to the cortex of the cerebellar hemispheres
Where is output from the cerebellum from?
ALL output is from the nucleus (dentate in the case of neocerebellum), relayed via thalamus to the motor cortex
What are the two routes information reaches the cerebellar cortex through?
- Direct route via the pons of the brainstem
- Indirect route, which transverses several brain areas and finally make connections in the inferior olive nucleus in the brainstem
What are the names of the fibres from the two information routes into the cerebellar cortex?
Connections from the pons via the direct route are the messy fibres.
Connections from the inferior olive via the indirect route are climbing fibres
How do collaterals of the corticospinal tract interact with the cerebellum?
Collaterals of the CS tract neurones pass to the cerebellum (copy of signals sent to the motoneurones)
What is the major input for the cerebellum?
Readiness potential through the direct route
copy of type of movement desired
What happens if there is a discrepancy with all the inputs into the cerebellum?
Error signal is sent to the motor cortex
The cerebellum has been likened to the computer of the motor system because it has circuits.
What is cool about these circuits?
They are modified by experience so that motor learning can take place, involves setting up of PREFERENTIAL PATHWAYS.
They enable the precise TIMING of movements to be calculated
Describe very basically how the cerebellum is set up for its function
The cerebellum carries a motor humunculus, in which the same simple circuitry is repeated millions and millions of times.
Purkinje cells are the key (only output of the cortex)
Direct pontine input is via mossy fibres. Each one makes contact with >1000 Purkinje cells.
This raised their level of excitation, but does NOT cause APs to be generated in the Purkinje cells
Indirect climbing fibre input makes powerful excitatory 1:1 contact with Purkinje cells -> APs in Purkinje cells which inhibit the dentate nucleus
How does motor learning work?
Motor learning is all about selecting the right climbing fibre to give you the effect you want, using all of the proprioceptive, visual, auditory, affirmative feedback.
Once movement begins to go right, repeat it hundreds and hundreds of times “practice makes perfect”
As a result of joint activation of mossy and climbing fibre inputs to a Purkinje cell there is a synaptic strengthening, so that eventually mossy fibre input on its own is enough to cause APs to be discharges in Purkinje cells
How does synaptic strengthening make a movement easier?
After practising hundreds and hundreds of times mossy fibre input on its own (no climbing fibre needed) is enough to cause AP. Movement becomes automatic
Climbing fibre not needed so signal doesn’t need to take indirect route for action (doesn’t need to go through all this brain areas) -> happens without you really having to think about it.
Synaptic strengthening is permanent, involves increased permeability to Ca and protein synthesis
How does the Purkinje fibres control co-ordination?
Essence of co-ordination is timing.
As mossy fibre excitation sweeps along the array of Purkinje cells there is a separation of time between their excitation as first one then another is activated.
The parking cells picked out and strengthened by learning will therefore discharge in a temporal pattern
These cells inhibit the discharge of the dentate nucleus caused by mossy fibre excitation. Thus a TEMPORAL CODE of excitation is passed to the motor cortex to activate the correct motoneurons in the right sequence to produce a movement which is smooth, reliable and automatic
Importance of cerebellum is seen in patients with cerebellar damage.
Give some examples of cerebellar conditions
Ataxia Decomposition of Movement Asynergia Dysmetria Intentional Tremor Dysdiadochokinesia Posture, gait and balance deficits
What is ataxia?
Caused by global deficit
Incoordination
Specific effects depend on area of cerebellum damaged
What is decomposition of movement?
Instead of a movement being carried out smoothly, it is decomposed into simpler units, e.g. joint by joint.
For speech = dysarthria or staccato speech, there is no aphasia but the mechanical aspects of speech are impaired, slow, syllable by syllable.
Te precise timing of movements is lost preventing smooth movement
What is asynergia?
Lack of cooperation between agonists and antagonists
May contract together
What is dysmetria?
“Past pointing”
Goal directed movements lack accuracy
Point past, short or to the side of a target
What is intentional tremor?
Occurs during movement and particularly towards the end
NOT at rest like Parkinson tremor
What is Dysdiadochokinesia?
Inability to make rapidly alternating opposite movements
Explain posture, gait and balance deficits
Related to use of information from the vestibular system.
Adopt a wide swaying stance with a tendency to fall over (drunken sailor’s gait)
How does cerebellum contribute to higher cognitive function?
Autism, altered sense of self may be due to cerebellar impairment.
Summarise the hierarchy of motor control
For a voluntary movement to occur the readiness potential has to be processed by the basal ganglia in order for the movement to be initiated and also has to be processed by the cerebellum in order for the desired movement to be coordinated.
In Parkinson’s disease the problem is with initiation, once the movement has started, the cerebellum takes over
