Lesions of the Motor System

Question 1

Outline the very general process of executing a movement, starting with the idea.

Answer 1

You have an idea to move
You turn that into a plan of action
This has inputs from sensory system and motor memory
Then you execute it and send the right signals down the right nerves to the right muscles
And you move.
This movement then send back information to the execution part and planning part of this process so adjustments can be made based on this new information.

Question 2

Give an overview of the motor system.

Answer 2

In the cerebral cortex you form the idea
Then you execute that idea by sending signals down your corticospinal tracts and corticobulbar tracts
These travel to the brainstem and then on down the spinal cord to the muscles
There is also feedback from the muscles back to cerebral cortex

Planning an action takes a long time in neurological terms ~ 800ms-1sec
It travels to the cerebellum and basal ganglia that process different aspects of the movement, then they feedback to the thalamus
The thalamus then sends the information to the cerebral cortex

There is also a feedback loop from cerebellum to the brain stem, and back to the cerebellum

Question 3

How can neurological lesions be defined? (Hughlings Jackson)

Answer 3

Positive sings
- The emergence of a feature
Negative signs
- The loss of a function or capacity

Question 4

Give me some of the anatomy of the cerebellum.

Answer 4

Highly folded - grey matter cortex - white matter core

3 peduncles - carry input/output fibres from and to the brainstem

Core contains three pairs of ‘deep’ nuclei
- Generate output projections to brain stem

Cerebellar cortex - 3 functional zones:

• Vestibulocerebellum (Archicerebellum)
• Spinocerebellum (Paleocerebellum)
• Cerebrocerebellum (neocerebellum)

Question 5

What are the functions of the cerebellum?

Answer 5

Vestibulocerebellum
- Main input from vestibular system
- Involved in balance and ocular reflexes
Spinocerebellum
- Involved in error correction
Cerebrocerebellum
- Involved in movement planning and motor learning
- Particularly in relation to visually guided movements
and coordination of muscle activation

Question 6

Name some effects of cerebellar dysfunction.

Answer 6

Ataxia
Dysmetria (past pointing)
Dysarthria ('scanning' speech)
Disequilibrium
Hypotonia
Writer's cramp
Dysdiadochokinesia
- Agonist/antagonist muscle coordination abnormal
Cannot learn new movements
Coarse nystagmus
No muscle atrophy/weakness

Question 7

What are common causes of cerebellar dysfunction?

Answer 7

Tumours
Cerebrovascular disease
Genetic - e.g. Friedreich’s ataxia

Question 8

Name the parts of the basal ganglia.

Answer 8

Caudate Nucleus

Putamen

Globus Pallidus

• External (GPe)
• Internal (GPi)

Substantia Nigra

• Pars Compacta (SNc)
• Pars Reticulata (SNr)

Subthalamic Nucleus (STN)

Caudate Nucleus + Putamen = (Neo)striatum
Putamen + Globus Pallidus = Lenticular Nucleus

Question 9

Describe the direct pathway through the basal ganglia.

Answer 9

Thalamus is inhibited by Global Pallidus internal and Substantia Nigra Pars Compacta (GPi/SNr)
GPi/SNr at rest are tonically active
GPi/SNr are inhibited by the striatum (D1 receptors are located here)
Striatum is activated by Cerebral Cortex
Cerebral Cortex is activated by Thalamus

(When activated)
Cerebral Cortex -> excites -> Striatum
Striatum -> inhibits -> GPi/SNr
GPi/SNr -STOPS inhibiting- Thalamus
Thalamus can feed back to Cerebral cortex
(And some feedback to striatum)

The direct pathway is the first to be activated.

Question 10

Describe the indirect pathway through the basal ganglia.

Answer 10

Thalamus is inhibited by Global Pallidus internal and Substantia Nigra Pars Compacta (GPi/SNr)
GPi/SNr is activated by Subthalamic nucleus (STN)
STN is inhibited by Global Pallidus external (GPe)
GPe is inhibited by Striatum (D2 receptors here)
Striatum is activated by Cerebral cortex

(When activated)
Cerebral Cortex -> Excites -> Striatum
Striatum -> Inhibits -> GPe
GPe -STOPS inhibiting- STN
STN -> Excites -> GPi/SNr

Indirect pathway follows direct, so it turns GPi/SNr back on

So Basal ganglia is determining amplitude and speed of execution

Question 11

Give a brief over view of the purposes of the direct and indirect pathway through the basal ganglia

Answer 11

Direct pathway is to initiate movement
Indirect pathway is to stop that movement
Basal ganglia takes information from the cerebral cortex and determines the speed and amplitude of execution of the action.

Question 12

How is the Substantia Nigra Pars Compacta involved in the basal ganglia pathways? And what is the significance of it’s involvement?

Answer 12

Substantia Nigra Pars Compacta (SNc) is a dopaminergic pathway
SNc has an excitatory input to D1 receptors in the Striatum which are involved in the direct pathway (Starting a movement) of the basal ganglia
AND
SNc has an inhibitory input to D2 receptors in the Striatum which are involved in the indirect pathway (Stopping a movement) of the basal ganglia
SNc acts to facilitate movement

The significance of this pathway is that it is the one affected in Parkinson’s disease

Question 13

What is the purpose of the basal ganglia?

Answer 13

To regulate the amplitude and velocity of the planned movement, particularly in relation to the use of internal (e.g. proprioceptive) information.

Question 14

What happens in basal ganglia dysfunction?

Answer 14

Generate hypo (Parkinson's) or hyper (Huntington's chorea) kinetic disorders
(Movement coordination is normal.

Question 15

What is Parkinson’s disease?

Answer 15

Progressive degeneration of the dopaminergic neurons of the substantia nigra
- The nigro-striatal pathway

There is a classic triad of signs:

• Tremor at rest, reduced by movement
• Increased tone - ‘lead pipe’ or ‘cog wheel’ rigidity
• Bradykinesia