Subthalamic nucleus

The subthalamic nucleus is a small lens-shaped nucleus in the brain where it is a part of the basal ganglia system. As suggested by its name, the subthalamic nucleus is located ventral to the thalamus. It is also dorsal to the substantia nigra and medial to the internal capsule. It was first described by Jules Bernard Luys in 1865, and the term corpus Luysi or Luys' body is still sometimes used.

Anatomy

Structure

The principal type of neuron found in the subthalamic nucleus has rather long dendrites devoid of spines . The dendritic arborizations are ellipsoid, replicating in smaller dimension the shape of the nucleus. The dimensions of these arborizations (1200,600 and 300 μm) are similar across many species—including rat, cat, monkey and man—which is unusual. However, the number of neurons increases across evolution as well as the external dimensions of the nucleus. Due to the bending of dendrites at the border, the subthalamic nucleus is a close nucleus,able to receive information only in its space. The principal neurons are glutamatergic, which give them a particular functional position in the basal ganglia system. In humans there are also a small number (about 7.5%) of GABAergic interneurons that participate in the local circuitry.

Afferent axons

The subthalamic nucleus receives its main input from the external segment of the globus pallidus (84.2% of its axons), not so much through the ansa lenticularis as often said but by radiating fibers crossing the medial pallidum first and the internal capsule (see figure). This afference is GABAergic, inhibiting the neurons of the subthalamic nucleus. Excitatory, glutamatergic inputs come from the cerebral cortex (particularly the motor cortex), and from the pars parafascicularis of the central complex.The subthalamic nucleus also receives neuromodulatory inputs, notably dopaminergic axons from the substantia nigra pars compacta.

Efferent targets

The axons of subthalamic nucleus neurons leave the nucleus dorsally. The efferent axons are glutamatergic (excitatory). Except for the connection to the striatum (17.3% in macaques), most of the subthalamic principal neurons are multitargets and directed to the other elements of the core of the basal ganglia. Some send axons to the substantia nigra medially and to the medial and lateral nuclei of the pallidum lateraly (3-target, 21.3%). Some are 2-target with the lateral pallidum and the substantia nigra (2.7%) or the lateral pallidum and the medial (48%). Less are single target for the lateral pallidum. In the pallidum, subthalamic terminals end in bands parallel to the pallidal border. When all axons reaching this target are added, the main afference of the subthalamic nucleus is, in 82.7% of the cases, clearly the lateral pallidum (external segment of the globus pallidus).

Some researchers have reported internal axon collaterals. However, there is little functional evidence for this.

Physiology

The subthalamic neurons are "fast-spiking pacemakers", spontaneously generating action potentials at rates of 80 to 90Hz in primates.

Lateropallido-subthalamic system

Strong reciprocal connections link the subthalamic nucleus and the external segment of the globus pallidus. Both are fast-spiking pacemakers. Together, they are thought to constitute the "central pacemaker of the basal ganglia" with synchronous bursts.

The connection of the lateral pallidum with the subthalamic nucleus is also the one in the basal ganglia system where the reduction between emmiter/receiving elements is likely the strongest. In terms of volume, in humans, the lateral pallidum mesures 808 mm³, the subthalamic nucleus only 158 mm³. This translated in numbers of neurons represents a strong compression with loss of map precision.

The systemic position of this circuit is particular in the basal ganglia system. There are two output paths starting from the stiatum. The first has a first relay in the medial pallidum (GABAegic inhibitory) and the second in the nigra reticulata (GABA). These two output subsystems do not send regulatory messages either to the striatum, lateral pallidum or subthalamic nucleus. The lateropallido-subthalamic subsystem is particular in that it does the reverse. It does not send axons to the thalamus and from there to the cortex. All efferent axons of the subsystem are returning inside the basal ganglia system.This topologically makes it a regulator. Some axons from the lateral pallidum go to the striatum. The activity of the medial pallidum is influenced by afferences from the lateral pallidum and from the subthalamic nucleus. The same for the nigra reticulata. The subthalamic nucleus sends axons to another regulator: the pedunculo-pontine complex (id).

The lateropallido-subthalamic system is thought to play a key role in the generation of the patterns of activity seen in Parkinson's disease.

Physiopathology and interventions

The chronic stimulation of the nucleus leads to a clear improvement of Parkinsonian symptoms. The first to be stimulated are the terminal arborisations of afferent axons which modifies the activity of subthalamic neurons. However trigger zones may also send fastly the signals to the output axons.The effects of stimulation are the objects of many works.

Pathology

Unilateral destruction or disruption of the subthalamic nucleus – which can commonly occur via a small vessel stroke in patients with diabetes, hypertension, or a history of smoking – produces hemiballismus.

References

External links

Primate basal ganglia system

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