Flashcards in Week 8a - basal ganglia - finished Deck (28)
What is the basal ganglia?
A group of deep cerebral nuclei, networked together by several axonal bundles into separate circuits for various purposes.
How do we test the basal ganglia?
Rapid alternating movements ie fingers to thumb as fast as you can.
How does fatigue present with a basal ganglia dysfunction?
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.
What areas are termed within the name "basal ganglia"?
Between what structures does the fornix communicate?
The hippocampus and the hypothalamus
What runs along the lateral border of the lateral ventricle?
The caudate nucleus
Amygdala: what does it look like and what does it do? where is it located?
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
Lentiform nucleus: what is it made of?
Putamen and the Globus pallidus
Describe the motor loop
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.
Describe the cognitive loop
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.
Describe the limbic loop
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
What is the caudate nucleus?
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
What is the putamen?
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
What does the putamen go together with to form the lentiform nucleus?
The globus pallidus
How many portions does the globus pallidus have?
2 - internal and external
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?
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.
What does the external part of the globus pallidus do?
It plays a role in the indirect pathway and increases the tonic inhibition of the thalamus.
How many parts does the substantia nigra? What do each of these parts do?
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.
What is the role of the subthalamis nucleus?
Plays an important role in the indirect pathway for movement. Has been implicated in huntingtons and hemiballism.
How does the direct excitatory pathway work?
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.
What does the striatum do for the direct pathway?
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.
What does the subthalamic nucleus do for the indirect pathway?
It releases glutamate which stimulates the Globus pallidus internal portion, which creates an increase in inhibition of the thalamus.
Basic pathway of motor loop through the basal ganglia:
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.
What are the putamen and the caudate nucleus collectively known as?
What do the premotor and supplementary motor cortices do?
They plan movements and prime the primary motor cortex (M1)
How do the premotor and supplementary motor cortices initiate planned movement?
They release glutamate and aspirate to stimulate the striatum.
What is parkinsonism?
Parkinsonism: # of dopaminergic fibres ability to initiate movement, hence known as a hypokinetic disorder.