Week 8a - basal ganglia - finished

Question 1

What is the basal ganglia?

Answer 1

A group of deep cerebral nuclei, networked together by several axonal bundles into separate circuits for various purposes.

Question 2

How do we test the basal ganglia?

Answer 2

Rapid alternating movements ie fingers to thumb as fast as you can.

Question 3

How does fatigue present with a basal ganglia dysfunction?

Answer 3

It presents as rapid onset fatigue, i.e. they can do 1-2 squats perfectly fine and then all of a sudden they fatigue and can’t so any more.

Question 4

What areas are termed within the name “basal ganglia”?

Answer 4

Caudate nucleus 
Putamen
Globus pallitus
Substantia nigra
Subthalamic nucleus.

Question 5

Between what structures does the fornix communicate?

Answer 5

The hippocampus and the hypothalamus

Question 6

What runs along the lateral border of the lateral ventricle?

Answer 6

The caudate nucleus

Question 7

Amygdala: what does it look like and what does it do? where is it located?

Answer 7

Almond shapes solid mass
Its about learning and recognising fear and danger, solely responsible for regulating the encoding of information about danger.

Its located anterior to the hippocampus

Question 8

Lentiform nucleus: what is it made of?

Answer 8

Putamen and the Globus pallidus

Question 9

Describe the motor loop

Answer 9

Although electrical stimulation of a healthy putamen can produce contralateral movement, the basal ganglia don’t normally initiate movement independent of the motor cortices.

The putamen appears to act as a reservoir for learned motor programs & assembles an appropriate motor sequence for desired movements & transmits this information to the Supplementary Motor Area (SMA).

As such, the basal ganglia (with the SMA) are fundamental in organizing the requisite sequence of excitation of M1 motor neurons for a given motor task.

Question 10

Describe the cognitive loop

Answer 10

The head of the caudate nucleus receives projections from the prefrontal cortex & appears to participate in motor learning.

These connections suggest that it participates in planning ahead, particularly for complex motor intentions

PET scan studies support this.

When a novel motor task has been practiced to the level of automatic execution, the motor loop becomes active instead.

Question 11

Describe the limbic loop

Answer 11

The limbic loop is likely to be involved in giving motor expression to emotions, for example through smiling or gesturing.

This loop is rich in dopaminergic fibres, & their decline may account for the mask-like facies & absence of spontaneous gesturing characteristic of Parkinson’s disease, and for the dementia that may set in after several years

Question 12

What is the caudate nucleus?

Answer 12

Is thought to be involved with ignition moreso than movement. This is substantiated by PET scans and a handful of cases of localised caudate damage

Question 13

What is the putamen?

Answer 13

Is considered collectively with the caudate as the striatum
Is thought to be most involved in movement
It has dense connections to motor & sensory cortices, the substantia nigra & thalamic nuclei.
The putamen also projects to the globus pallidus

Question 14

What does the putamen go together with to form the lentiform nucleus?

Answer 14

The globus pallidus

Question 15

How many portions does the globus pallidus have?

Answer 15

2 - internal and external

Question 16

What does the internal portion of the globus pallidus do? What does this mean? What happens if this doesn’t happen, e.g. in a basal ganglia lesion?

Answer 16

It holds the thalamus in tonic inhibition (spontaneous depolarisation).
The direct pathway releases this tonic inhibition and allows for activation of the thalamus and subsequently the cortex.
Without this we will see resting tremors.

Question 17

What does the external part of the globus pallidus do?

Answer 17

It plays a role in the indirect pathway and increases the tonic inhibition of the thalamus.

Question 18

How many parts does the substantia nigra? What do each of these parts do?

Answer 18

The substantia nigra is also divided into two distinct parts, the pars compacta & reticulata.
The cells of the reticula portion are GABA-ergic
The cells of the compact portion are dopaminergic & pigmented by neuromelanin.
These two regions also play different roles in the function of the basal ganglia in movement.

Question 19

What is the role of the subthalamis nucleus?

Answer 19

Plays an important role in the indirect pathway for movement. Has been implicated in huntingtons and hemiballism.

Question 20

How does the direct excitatory pathway work?

Answer 20

If the premotor and supplementary cortex is stimulated it releases glutamate. This excites the striatum which releases GABA, which inhibits the inhibition of the ventrolateral thalamus, which then in turn produces glutamate again to keep sending the movement signal.

Question 21

What does the striatum do for the direct pathway?

Answer 21

It releases GABA which inhibits the Globus Pallidus internal portion, which will cause the inhibition of the ventrolateral portion of the thalamus to be lifted and then movement can be initiated.

Question 22

What does the subthalamic nucleus do for the indirect pathway?

Answer 22

It releases glutamate which stimulates the Globus pallidus internal portion, which creates an increase in inhibition of the thalamus.

Question 23

Basic pathway of motor loop through the basal ganglia:

Answer 23

1 - Premotor and supplementary motor cortices plan movements and prime the primary motor cortex.
2 - To initiate the planned movement the premotor and supplementary motor cortices release glutamate and aspartate to stimulate the striatum.
3 - When stimulated the striatum releases GABA which inhibits the GPi and SNr, thus reducing their tonic GABA output and in turn reducing the tonic inhibition of the thalamus. This is known as disinhibition and it allows for cortical stimulation by the thalamus to initiate the planned movements.
4 - This is the direct pathway.
5 - When the striatum releases GABA it not only inhibits the GPi and SNr but also the GPe.

6 - The GPe releases GABA also (which has an inhibitory affect on its target tissue the subthalamic nucleus) thus decreasing the ability of the subthalamic nucleus to release glutamate to stimulate the GPi and SNr.
7 - By inhibiting the GPe the striatum is in effect stimulating the indirect pathway while stimulating the direct pathway. The two tend to cancel each other out thus the initiation of movement is difficult without the compact portion of the substantia nigra.
8 - The SNc releases dopamine to the striatum. Dopamine has an excitatory affect on some striatal cells and an inhibitory affect on others.
9 - Those striatal fibres acting in the direct pathway tend to be stimulated (thus exciting the direct pathway) while those acting in the indirect pathway tend to be inhibited.
10 - By inhibiting the ability of the striatum to disinhibit the subthalamic nucleus (via the GPe) the indirect pathway is in effect inhibited.
11 - The effect of dopamine on movement initiation is therefore two-fold.

Question 24

What are the putamen and the caudate nucleus collectively known as?

Answer 24

The striatum

Question 25

What do the premotor and supplementary motor cortices do?

Answer 25

They plan movements and prime the primary motor cortex (M1)

Question 26

How do the premotor and supplementary motor cortices initiate planned movement?

Answer 26

They release glutamate and aspirate to stimulate the striatum.

Question 27

What is parkinsonism?

Answer 27

Parkinsonism:  # of dopaminergic fibres   ability to initiate movement, hence known as a hypokinetic disorder.

Question 28

What is huntingtons disease?

Answer 28

Huntington’s: GPi neurons become abnormally underactive, striatal fibres to GPe degenerate  unable to disinhibit the subthalamic nucleus  known as a hyperkinetic disorder