Hierarchy of Motor Control - Cerebellum

Question 1

What is the cerebellum responsible for?

Answer 1

Co-ordination of voluntary movements

Question 2

What do we mean by co-ordination of voluntary movements?

Answer 2

Control of the timing of sequences of contractions in different muscles to make a movement smooth and reliable.

Question 3

What is the structure of the cerebellum like?

Answer 3

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.

Question 4

Functionally there are 3 parts of the cerebellum (based on embryological and physiological criteria.
What are these?

Answer 4

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

Question 5

Where does the input to the cerebellum go?

Answer 5

ALL input to the cerebellum is to the cortex of the cerebellar hemispheres

Question 6

Where is output from the cerebellum from?

Answer 6

ALL output is from the nucleus (dentate in the case of neocerebellum), relayed via thalamus to the motor cortex

Question 7

What are the two routes information reaches the cerebellar cortex through?

Answer 7

1. Direct route via the pons of the brainstem
2. Indirect route, which transverses several brain areas and finally make connections in the inferior olive nucleus in the brainstem

Question 8

What are the names of the fibres from the two information routes into the cerebellar cortex?

Answer 8

Connections from the pons via the direct route are the messy fibres.

Connections from the inferior olive via the indirect route are climbing fibres

Question 9

How do collaterals of the corticospinal tract interact with the cerebellum?

Answer 9

Collaterals of the CS tract neurones pass to the cerebellum (copy of signals sent to the motoneurones)

Question 10

What is the major input for the cerebellum?

Answer 10

Readiness potential through the direct route

copy of type of movement desired

Question 11

What happens if there is a discrepancy with all the inputs into the cerebellum?

Answer 11

Error signal is sent to the motor cortex

Question 12

The cerebellum has been likened to the computer of the motor system because it has circuits.
What is cool about these circuits?

Answer 12

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

Question 13

Describe very basically how the cerebellum is set up for its function

Answer 13

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

Question 14

How does motor learning work?

Answer 14

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

Question 15

How does synaptic strengthening make a movement easier?

Answer 15

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

Question 16

How does the Purkinje fibres control co-ordination?

Answer 16

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

Question 17

Importance of cerebellum is seen in patients with cerebellar damage.
Give some examples of cerebellar conditions

Answer 17

Ataxia
Decomposition of Movement
Asynergia
Dysmetria
Intentional Tremor
Dysdiadochokinesia
Posture, gait and balance deficits

Question 18

What is ataxia?

Answer 18

Caused by global deficit
Incoordination
Specific effects depend on area of cerebellum damaged

Question 19

What is decomposition of movement?

Answer 19

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

Question 20

What is asynergia?

Answer 20

Lack of cooperation between agonists and antagonists

May contract together

Question 21

What is dysmetria?

Answer 21

“Past pointing”
Goal directed movements lack accuracy
Point past, short or to the side of a target

Question 22

What is intentional tremor?

Answer 22

Occurs during movement and particularly towards the end

NOT at rest like Parkinson tremor

Question 23

What is Dysdiadochokinesia?

Answer 23

Inability to make rapidly alternating opposite movements

Question 24

Explain posture, gait and balance deficits

Answer 24

Related to use of information from the vestibular system.

Adopt a wide swaying stance with a tendency to fall over (drunken sailor’s gait)

Question 25

How does cerebellum contribute to higher cognitive function?

Answer 25

Autism, altered sense of self may be due to cerebellar impairment.

Question 26

Summarise the hierarchy of motor control

Answer 26

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