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Subthalamic nucleus

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487pxCoronal slices of human brain showing the basal ganglia, globus pallidus: external segment (GPe), subthalamic nucleus (STN), globus pallidus: internal segment (GPi), and substantia nigra (SN).

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[1], and the term corpus Luysi or Luys' body is still sometimes used.

File:Horizontal section showing the subthalamic nucleus, the pallidum and the fibers linking them in a case of capsular degeneration

from Foix and Hillemand (1925) Masson.Paris

Anatomy

Structure

The principal type of neuron found in the subthalamic nucleus have rather long dendrites devoid of spines . The dendritic arborizations are ellipsoid, replicating in smaller dimension the shape of the nucleus[2]. 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[3].

Afferent axons

The subthalamic nucleus receives its main input from the external segment of the globus pallidus (84.2% of its axons[4])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[5].

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[4]. 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[6] and Smith et al.1990). 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[7]. However, there is little functional evidence for this.

Physiology

The subthalamic neurons are "fast-spiking pacemakers"[8], spontaneously generating action potentials at rates of 80 to 90Hz in primates (they are only 4–40 Hz in rodents).

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"[9] with synchronous bursts. The connection of the lateral pallidum with the subthalamic nucleus is also that 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 808mm3, the subthalamic only 158mm3 (Yelnik, 2002). 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), sends axons to a particular place of the thalamus, the nucleus ventralis oralis VO (again GaBaergic and inhibitory). VO sends its axons to the accessory motor and the motor cortex (this time with glutamate as the mediator). The second output subsystem follows exactly the same pattern, this time starting from the nigra reticulata (GABA) to the nucleus ventralis anterior VA (GABA again) and from there to the frontal cortex and oculomotor areas (again glutamate). 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 are returning inside the basal ganglia system. Some from the lateral pallidum go to the striatum (Sato et al.2000). The activity of the medial pallidum is influenced by afferences from the lateral pallidum and from the subthalamic nucleus (Smith, Y., Wichmann,T.,DeLong, M.R. 1994).So is that of the nigra reticulata (Smith, Y., Hazrati, L_N, Parent, A. 1990). 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[10].

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

  1. ^ Luys, Jules Bernard (1865). Recherches sur le système cérébro-spinal, sa structure, ses fonctions et ses maladies (in French). Paris: Baillière.
  2. ^ Yelnik, J. & Percheron, G. (1979). "Subthalamic neurons in primates : a quantitative and comparative anatomy". Neuroscience. 4 (11): 1717–1743. PMID 117397.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  3. ^ Levesque J.C. & Parent A. (2005). "GABAergic interneurons in human subthalamic nucleus". Movement Disorders. 20 (5): 574–584. PMID 15645534.
  4. ^ a b Sato F.; Parent M.; Levesque M.; & Parent A. (2000). "Axonal branching pattern of neurons of the subthalamic nucleus in primates". Journal of Comparative Neurology. 424 (1): 142–152. PMID 10888744.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  5. ^ Cragg S.J.; Baufreton J.; Xue Y.; Bolam J.P.; & Bevan M.D. (2004). "Synaptic release of dopamine in the subthalamic nucleus". European Journal of Neuroscience. 20 (7): 1788–1802. PMID 15380000.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  6. ^ Nauta, H.J.W. & Cole, M. (1978). "Efferent projections of the subthalamic nucleus : an autoradiographic study in monkey and cat". Journal of Comparative Neurology. 180 (1): 1–16. PMID 418083.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  7. ^ Kita, H.; Chang, H.T.; & Kitai, S.T. (1983). "The morphology of intracellularly labeled rat subthalamic neurons: A light microscopic analysis". Neuroscience. 215 (3): 245–257. PMID 6304154.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  8. ^ Surmeier D.J.; Mercer J.N.; & Chan C.S. (2005). "Autonomous pacemakers in the basal ganglia: who needs excitatory synapses anyway?". Current Opinion in Neurobiology. 15 (3): 312–318. PMID 15916893.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  9. ^ Plenz, D. & Kitai, S.T. (1999). "A basal ganglia pacemaker formed by the subthalamic nucleus and external globus pallidus". Nature. 400 (6745): 677–682. PMID 10458164.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  10. ^ Bevan M.D.; Magill P.J.; Terman D.; Bolam J.P.; & Wilson CJ. (2002). "Move to the rhythm: oscillations in the subthalamic nucleus-external globus pallidus network". Trends in Neurosciences. 25 (10): 525–531. PMID 12220881.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  • Sato, F., Lavallée, P., Levesque, M. and Parent, A. (2000) Single-axon tracing study of neurons of the external segment of the globus pallidus in primate. J. Comp. Neurol. 417: 17-31
  • Smith, Y., Hazrati, L-N. and Parent, A. (1990) Efferent projections of the subthalamic nucleus in the squirrel monkey as studied by the PHA-L anterograde tracing method. J. Comp. Neurol. 294: 306-323
  • Smith, Y., Wichmann, T. and DeLong, M.R. (1994) Synaptic innervation of neurones in the internal pallidal segment by the subthalamic nucleus and the external pallidum in monkeys. J. Comp. Neurol. 343: 297-318
  • Yelnik, J. (2002) Functional anatomy of the basal ganglia. Mov. Disord.17:S15-S21


see also

[[Primate basal ganglia ]]

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