Motor System

Question 1

Briefly describe the general location of the upper and lower motor neurones

Answer 1

Upper- entirely in the CNS
Lower- cell body and proximal part of axon in CNS
- distal part of axon in the PNS

Question 2

Briefly describe why the lower motor neurone is called the final path

Answer 2

Activation of the neurone results in activation of skeletal muscle

Question 3

Briefly describe the more specific location of the UMN and LMN cell bodies

Answer 3

UMN- primary motor cortex (in the precentral gyrus of the frontal lobe)
LMN- ventral horn of the spinal cord
- also in motor nuclei of brainstem

Question 4

Briefly describe how the lentiform nucleus is defined

Answer 4

Anatomically defined:
Putamen
Globus pallidus- internal and external

Question 5

Briefly describe the nigrostriatal pathway

Answer 5

The pathway from the substantia nigra to the striatum nucleus

Question 6

Briefly describe how the striatum is defined

Answer 6

Functionally and evolutionarily defined
Made up of:
Putamen and caudate nucleus (they are separated by the CST)

Question 7

Briefly describe the structure of the cerebellum

Answer 7

2 cerebellar hemispheres

Midline vermis

Question 8

Briefly describe how the cerebellum attaches to the brainstem

Answer 8

The hemispheres attach via 3 peduncles:
Superior cerebellar peduncle- attaches to midbrain
Middle cerebellar peduncle- attaches to the pons
Inferior cerebellar peduncle- attaches to the medulla

Question 9

Briefly explain which cerebellar peduncle is largest and why

Answer 9

The middle cerebellar peduncle

It contains the corticoponto cerebellar pathway which connects the motor cortex to the cerebellum.

Question 10

Briefly describe the basal ganglia direct pathway

Answer 10

Putamen inhibits the internal globus pallidus, which would normally inhibit the thalamus.
As the thalamus isn’t inhibited, it can continue to stimulate the cortex.
Producing a net excitatory effect on the motor cortex.

Question 11

Briefly describe the basal ganglia indirect pathway

Answer 11

The putamen inhibits the external globus pallidus, which would otherwise have inhibited the subthalamic nucleus.
Without this inhibition, the subthalamic nucleus stimulates the internal globus pallidus, which in turn inhibits the thalamus, preventing stimulation to the cortex.
Producing a net inhibitory effect on the motor cortex.

Question 12

Describe the dopaminergic effect on basal ganglia pathways

Answer 12

Dopamine is released from the substantia nigra.
It acts on D1 receptors at the putamen to stimulate the direct pathway, causing an excitatory effect at the cortex.
It also acts on D2 receptors at the putamen to inhibit the indirect pathway. This also has an excitatory effect on the motor cortex (by inhibiting the inhibition of the cortex).

Question 13

Briefly describe the cause of Parkinson’s disease and how the mechanism links to symptoms

Answer 13

Degeneration of the substantia nigra reduces dopamine release.
This means the net excitatory effect of dopamine on the primary motor cortex is lost. Decreases movement.
Causes bradykinesia.
Rigidity may be due to a lack of agonist and antagonist muscle coordination.
Psychiatric features are due to similar affected loops in the frontal lobe, involved in cognition and emotion.

Question 14

Briefly describe the cause of Huntington’s chorea and how the mechanism links to symptoms

Answer 14

Autosomal dominant inheritance.
Degeneration of inhibitory influences from the putamen to the external globus pallidus.
So, the external globus pallidus is able to inhibit the subthalamic nucleus, which is then unable to stimulate the internal globus pallidus. Thus, the thalamus is not inhibited and is able to stimulate the cortex.
Resulting in hyperkinesis and dystonia.

Question 15

Briefly describe the cause of Hemiballismus and how the mechanism links to symptoms

Answer 15

Lesion of subthalamic nucleus.
Globus pallidus isn’t stimulated, so is unable to inhibit the thalamus. The cortex is then stimulated by the thalamus.
This leads to involuntary, explosive unilateral movements.

Question 16

Are basal ganglia lesions uni or bilateral?

Answer 16

Sometimes unilateral but often bilateral due to neurodegeneration.

Question 17

Are cerebellar lesions uni or bilateral?

Answer 17

Unilateral

Question 18

Briefly explain if basal ganglia lesions are ipsilateral or contralateral and why

Answer 18

Contralateral.
The corticostriatal and pallidothalamocortical pathway do not decussate.
The UMN then decussates at the level of the pons to supply the contralateral LMN and muscle.

Question 19

Briefly explain if cerebellar lesions are ipsilateral or contralateral and why

Answer 19

Ipsilateral
The corticoponto cerebellar and cerebello-thalamo-cortico pathway both decussate to the contralateral cerebellum.
This means the UMN on that side then decussates at the pons, supply the same side that the previous pathways originated from.
Double decussation.
Also, sensory pathways (e.g. SCT) run to the ipsilateral cerebellar hemisphere.

Question 20

What are the symptoms of a lesion in the cerebellar hemisphere?

Answer 20

Vomiting
Vertigo
Difficulty walking

Question 21

What are the symptoms of a lesion in the vermis of the cerebellum?

Answer 21

DANISH - affecting the trunk
Dysdiadochokinesis- can’t perform rapid alternating movement e.g. pronation to supination
Ataxia
Nystagmus- fast towards lesion
Intention tremor- opposite to Parkinson’s
Dysarthria- larynx and pharynx muscle coordination lost
Hypotonia

Question 22

What produces similar symptoms to a lesion in the vermis of the cerebellum?

Answer 22

Occlusions of the three cerebellar arteries- may also damage brainstem so test CNs.

Question 23

Briefly describe the basic function of the basal ganglia

Answer 23

Inhibits inappropriate movement, via the indirect pathway to the primary motor cortex
Stimulates appropriate movement, via the direct pathway to the primary motor cortex.

Question 24

Briefly describe the basic function of the cerebellum

Answer 24

Determines sequence of movement by integrating information form the primary motor cortex, sensory motor cortex and the spinocerebellar tracts (proprioceptors) and feeding back to the primary motor cortex.