The Basal Ganglia

Functional aspects of cerebellum | Main Anatomy Index | The limbic system

Vu, D. (1998) Basal ganglia [Lecture handouts]. University of NSW. 21 September, 1998.

• The basal ganglia refer to those structures when damaged cause extrapyramidal syndromes.
• It includes:

1. The caudate nucleus;
2. The putamen,
3. The globus pallidus,
4. The subthalamic nucleus,
5. And substantia nigra.

Putamen + Globus Pallidus = Lenticular (or lentiform) Nucleus

Caudate Nucleus + Lenticular Nuclei = Corpus Striatum

Caudate Nucleus + Putamen = Neostriatum or Striatum

• The lenticular nucleus is shaped somewhat like a wedge cut out of a sphere.

• The putamen (L. husk) is approximately coextensive with the insula and forms the outermost portion of this wedge.

• It is separated from the more medial globus pallidus by a thin lateral medullary lamina.

• The globus pallidus is itself divided into medial and lateral portions.
• This is by the medial medullary lamina.

• The globus pallidus has a distinctive pale appearance (i.e., dark appearance in myelin-stained sections) due to the large numbers of myelinated fibres that traverse it, terminate in it and originate in it.

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• The caudate nucleus is in the wall of the lateral ventricle and grows with it in a C-shaped course.
• It has an enlarged head that bulges into the anterior horn.
• Its body that forms the lateral wall of the body of the ventricle.
• The slender tail of the caudate borders on the inferior horn.

• Embryologically, the caudate nucleus and putamen arise from the same mass of cells.
• In the adult, the caudate nucleus and the putamen retain their embryological continuity.
• This is just above the orbital surface of the frontal lobe.
• Here, the head of the caudate appears to be continuous with the anterior part of the putamen.

• In the temporal lobe, the tail of the caudate nucleus is continuous with the amygdala, which is in turn continuous with the putamen.
• These physical continuities are of no apparent significance, however.

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• The principle circuit of the basal ganglia is a loop:

Multiple cortical areas --> corpus striatum --> thalamus --> one of the cortical areas

• There are multiple versions of this loop.
• They are all similar in principle but each using different cortical areas and a distinctive portion of the corpus striatum.
• Each loop includes at least a portion of the frontal lobe and returns to a frontal area.

• The remaining connections of the basal ganglia fall into 3 categories:

1. GPL --> Subthalamic nuclei --> GPM
2. Striatum --> Substantia nigra --> Striatum
3. Striatum --> Pallidum --> CM of thalamus --> Striatum

• The striatum (caudate + putamen) receive inputs from 3 sources:

1. The cerebral cortex;
2. The intralaminar nuclei of the thalamus (centromedian, which receives input from the motor cortex);
3. And the substantia nigra.

• The fibres originating in the cerebral cortex (corticostriate fibres) are by far the most massive bundle.
• They end in a roughly topographical pattern in the striatum:

Motor and somatosensory cortex --> Putamen

Association areas --> Caudate Nucleus (esp. prefrontal cx --> head of caudate)

• Now the putamen projects to premotor and supplementary motor areas of cortex via the globus pallidus and thalamus.
• It is thus involved in most of the motor functions of the basal ganglia.

• The caudate nucleus projects to mostly prefrontal areas via the globus pallidus and thalamus.
• It is involved more prominently in cognitive functions and less is movement.

• The substantia nigra projects to all areas of the striatum.
• It does so in a point-to-point fashion by way of very fine dopaminergic fibres.

• Destruction of the nigrostriatal pathway is the major factor causing Parkinson's disease.

• The intralaminar nuclei, especially the centromedian and parafascicular nuclei project to the striatum.
• Many of these fibres have collateral branches that end in the cerebral cortex.

• The thalamostriate pathway is well-developed in primates but their function is mainly unknown.

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• Afferents to both segments of the pallidum arise in the striatum and the subthalamic nuclei.

• The subthalamic nucleus lies across the internal capsule from the globus pallidus.
• Small bundles of fibres cross the internal capsule and connect these two nuclei.
• These fibres are collectively called the subthalamic fasciculus.

• Although the two segments have similar inputs, their efferents are distinct and separate.

• The external segment projects through the subthalamic fasciculus to the subthalamic nucleus.

• The internal segment projects mainly to the thalamus through 2 collections of fibres.

• The lenticular fasciculus runs directly through the internal capsule and then passes medially as a sheet of fibres between the subthalamic nuclei and zona incerta.
• At the medial edge of the zona incerta, it makes a hairpin turn in a dorsolateral direction and enters the thalamus.

• The ansa lenticularis (L. ansa, loop) loops around the medial edge of the internal capsule.

• It joins the lenticular fasciculus in the thalamic fasciculus, which enters the thalamus.

• The thalamic fasciculus ends in a variety of thalamic nuclei:

1. Fibres related to movement control --> VL/VA
2. Related to the caudate nucleus and prefrontal cortex --> DM and part of VA
3. The others --> CM and parafascicular nuclei.

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• The principle contacts of the subthalamic nucleus consist of:

1. Interconnections with the globus pallidus;
2. And some efferent projections to the substantia nigra.

• These connections form the substrate of an indirect route through the basal ganglia.
• This plays a major role in determining the output of the globus pallidus.

• The substantia nigra is actually composed of 2 parts:

1. A dorsal compact part, containing closely packed, pigmented neurons;
2. And a reticular part, nearer the cerebral peduncle containing more loosely packed neurons, which are non-pigmented.

• This resembles in many respects a displaced portion of the globus pallidus.

• It receives inputs from the striatum and the subthalamic nucleus.
• It projects to the VL/VA and DM nuclei of the thalamus.
• In fact, it is a more important route for information from the caudate nucleus to reach the thalamus than is the globus pallidus.

• Also, there are projections from the substantia nigra to the superior colliculus and reticular formation.
• This is probably one way in which the basal ganglia influences eye movements.

• The pigmented neurons in this part use dopamine as their neurotransmitter.
• They project in a precisely organised topographic fashion to the caudate nucleus and putamen.
• The dopaminergic endings in the striatum ultimately modulate the output from the globus pallidus.

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Inhibitory to GPM & Substantia nigra --> less inhibition to Thalamus --> movements are facilitated by exciting Premotor and Supplementary motor areas which then project to the Motor cortex

Inhibitory to GPL --> less inhibition of Subthalamic nucleus --> more excitation of GPM --> more inhibition of Thalamus --> less excitation of Premotor and Supplementary motor areas --> movements are inhibited.

• Lesion of subthalamic nucleus --> less inhibition of movements --> hemiballismus.

• This is from the substantia nigra via the nigrostriatal pathway.

• It excites the direct pathway.
• It inhibits the indirect pathway.
• Thus, it facilitates movement.

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• Patients with chorea (G. dance) exhibit a series of nearly continuous rapid movements of the face, tongue and limbs (distal portions).
• These movements often resemble fragments of normal voluntary movements.

• There are 2 types of chorea, Sydenham's and Huntington's.

• This type of chorea (also known as Vitu's dance) may be caused by acute rheumatic fever.

• This is an autosomal dominant hereditary disorder characterised by neuronal degeneration that is particularly severe in the striatum, especially the caudate nucleus.
• Typically, symptoms first appear as between the ages of 30-50.

• Symptoms follow:

1. Firstly, personality changes;
2. Then choreiform movements which gradually become more pronounced;
3. Which are followed by developing dementia.

• The defective gene has been localised to the short arm of chromosome 4.

• Athetosis (G. without position) is characterised by slow, writhing movements that are most pronounced in the hands and fingers.
• The patient may be unable to keep the affected limb in a fixed position.

• The responsible lesion seems to be in the striatum.
• All intermediate forms between chorea and athetosis are seen.

• The most prominent characteristic of hemiballismus (G. jumping about) is wild, flailing movements of one arm or leg.
• The responsible lesion is in the contralateral subthalamic nucleus (indirect pathway).

• The symptoms to this are variable in relative severity and onset.
• They include:

1. A resting tremor, characteristically involving the hands in a "pill-rolling" movement that diminishes during voluntary movement and increases during emotional stress;
2. The rigidity is caused by increased tone in all muscles though strength is nearly normal and reflexes are not particularly affected;
3. And difficulty in moving (bradykinesia, or slow movements and hypokinesia, or few movements) is shown by decreased blinking, expressionless face and absence of arm movements associated with walking.

• The rigidity in Parkinsonism may:

1. Be uniform throughout a range of movements imposed by the examiner (plastic or lead-pipe rigidity);
2. Or be interrupted by a series of brief relaxations (cog-wheel rigidity).

• Also, bradykinesia and hypokinesia are fundamental deficits and not the result of rigidity.
• Patients whose rigidity is not pronounced can nevertheless have great difficulty moving.

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