Basal Ganglia

Question 1

What is the collective term given to the basal ganglia and cerebellum?

Answer 1

Extrapyramidal system

Question 2

Broadly speaking, what is the role of this system?

Answer 2

It checks that the movement selected by the motor cortex is correct Both parts of the extrapyramidal system has functions aside from modulating motor control

Question 3

Where is the basal ganglion located?

Answer 3

White matter in the middle of the brain

Question 4

What are the different parts of the basal ganglia?

Answer 4

Striatum = caudate + putamen
Globus Pallidus = external segment (GPext) + internal segment (GPint)
Substantia Nigra = pars compacta (SNc) + pars reticulata (SNr)
Subthalamic Nucleus

Question 5

What is the function of the basal ganglia?

Answer 5

It is involved in planning and coordinating movement It elaborates associated movements (e.g. swinging arms when walking) Contribute to the smoothness of complex action Involved in moderating and coordinating movements

Question 6

Describe the structure of the direct pathway in the basal ganglia.

Answer 6

Fibres initially come from the motor cortex to the striatum (caudate and putamen) Direct Pathway: putamen –> globus pallidus internal segment + substantia nigra pars reticulata

Question 7

Describe the structure of the indirect pathway in the basal ganglia.

Answer 7

Fibres initially come from the motor cortex to the striatum (caudate and putamen) Indirect Pathway: putamen –> globus pallidus external segment –> subthalamic nucleus –> globus pallidus internal segment

Question 8

What is the difference in the function of the direct and indirect pathways?

Answer 8

Direct Pathway = excitatory on the motor cortex Indirect Pathway = inhibitory on the motor cortex

Question 9

What extra component modulates the function of the direct and indirect pathways?

Answer 9

Nigro-striatal pathway

Question 10

Where do the projections go after leaving the basal ganglion structures?

Answer 10

They go to the thalamus From the thalamus they go to the cortex (supplementary motor area and primary motor area = two regions involved in movement preparation and planning)

Question 11

Describe how the basal ganglia are involved in choosing correct motor programmes to carry out particular functions.

Answer 11

The basal ganglia and cortex form a processing loop. The basal ganglia enable proper motor programmes (stored in the cortex) via the direct pathway (excitatory) The basal ganglia inhibit the competing motor programmes via the indirect pathway
I.E the basal ganglia and its direct and indirect pathways make sure that appropriate motor commands get transmitted down the hierarchy.

Question 12

Connections with which parts of the brain allow the basal ganglia to have a role in enabling various cognitive, executive and emotional programmes?

Answer 12

Prefrontal association cortex Limbic cortex

Question 13

What causes Parkinson’s disease?

Answer 13

Parkinson’s disease is the neuronal degeneration of dopaminergic neurones in the substantia nigra pars compacta It is caused by the progressive depletion of dopaminergic neurones

NOTE: Symptoms only appear when 80% of the dopamine cells in the substantia nigra have died

Question 14

What are the consequences of this with regards to the circuitry of the basal ganglia?

Answer 14

The loss of nigro-striatal dopaminergic axons in the caudate and putamen mean that the connection between the striatum (caudate and putamen) and the substantia nigra pars compacta is lost. This means that the direct pathway is reduced and so the excitation of the motor cortex is reduced. The lack of excitatory input interferes with the ability of the motor cortex to generate commands for voluntary movement, resulting in poverty of movement.

Question 15

State the main signs of Parkinson’s disease.

Answer 15

Bradykinesia – slow movement 
Akinesia – difficulty in the initiation of movements 
Hypomimic face – expressionless face 
‘Pill-Rolling’ Tremor – 4-7 Hz 
Rigidity – increase in muscle tone

Question 16

Describe the Parkinsonian gait.

Answer 16

Walking slowly, small steps, shuffling feet, reduced arm swing Stooped posture with head and body bent forwards and downwards

Question 17

What is Huntington’s disease caused by?

Answer 17

Abnormality on chromosome 4 (autosomal dominant) Caused by the degeneration of GABAergic neurones in the striatum (firstly the caudate and then the putamen)

Question 18

What are the consequences of Huntington’s disease with regards to the circuitry of the basal ganglia?

Answer 18

The inhibitory effect of the indirect pathway no longer keeps the direct pathway under control so the cortex will be hyperexcitable. Patients will continuously have abnormal movements because the cortex is continuously sending involuntary commands for movements.

Question 19

What are the main signs of Huntington’s disease and how does the disease progress?

Answer 19

Choreic movements – rapid, jerky, involuntary movements of the body the hands and face are affected first
Chorea gradually increases over time until the patients are totally incapacitated by it
Later, the patients develop cognitive decline and dementia.