Basal Ganglion-Physiological anatomy

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                                                                    Basal Ganglion

Keywords: Caudate nucleus, putamen, globus pallidus,substantia nigra, subthalamic nucleus, corpus striatum.

Definition: The term basal ganglia is a subcortical nuclei group in the forebrain and upper part of the midbrain. The basal ganglions are present bilaterally around the thalamus and are the virtual image of each other.  

The term basal is due to its presence at the base of the forebrain. But the term ganglion is not appropriate according to the definition of the ganglion. A collection or group of neurons outside the central nervous system is a ganglion.

Nuclei are collections or groups of neurons in the central nervous system surrounded by white matter. Basal ganglion is present in the central nervous system and surrounded by white matter of the brain; therefore, basal nuclei seem better term. 

It is the primary motor area in lower animals and is called the head ganglia of neural control.

                                                 Figure 1 Relative position of Basal Ganglion

                                    Figure 2  Position of Different parts of Basal Ganglion.

The basal ganglia include: 

  1.Caudate Nucleus 

  2.Putamen 

  3.Globus pallidus or palladium or paleostriatum (pale globe).with a minor ventral extension called the ventral pallidum. 

  4.Subthalamic Nucleus 

  5.Substantia nigra,

Caudate, Putamen and Globus pallidus develops from the telencephalon of the prosencephalon. Caudate, Putamen and Globus pallidus are large. The subthalamic nucleus and substantia nigra are small compared to caudate, putamen, and globus pallidus. They lie posteriorly in the brain, then caudate, putamen, and globus pallidus.

The subthalamic nucleus develops from the diencephalon.

The substantia nigra develops from the mesencephalon.

Caudate Nucleus 

1. It possesses a head and a tail.

2. The corpus striatum or neostriatum has two components, the caudate nucleus, and the putamen. It is a phylogenetically more recent part.

3. Between the head of the caudate nucleus and the putamen anterior limb of the internal capsule.

4. Numerous grey strands extended between the caudate nucleus and putamen, passing across the internal capsule, hence the name corpus striatum.

Putamen and Globus pallidus:

1. These two nuclei are phylogenetically older parts of the basal ganglion. They appear like a lens hence the name.

2. Globus pallidus is also called paleostriatum. 

3. Globus pallidus is subdivided into external and internal segments by a medial medullary lamina.

4. Pallida neurons operate using a disinhibition principle. Pallidal neurons fire at steady high rates automatically without any input. And has inhibitory effects on their targets. The net result of striatal information to the pallidum is a reduction of the tonic inhibition exerted by pallidal cells on their targets –disinhibition-with an increased rate of firing in the targets. 

  Substantia nigra                                              

 It has two parts: the dorsomedial pars compacta and the ventrolateral pars reticulate. It is particularly rich in copper content.

Subthalamic Nucleus: this is present ventral to the thalamus, dorsal to the substantial nigra, and medial to the internal capsule. Jules Bernard Luys, in 1865, described this nucleus; it is named after him corpus Luysi or Luys’ body. The subthalamic nucleus is a small nucleus in the subthalamus region of the brain.

Histology 

Upon microscopic examination, the basal ganglia contain three main types of cells:

1. Small cells. They are sensory cells, i.e., they receive impulses from other structures of the brain.

2. Large cells. They are motor cells, i.e.they send impulses to other parts of the brain.

3. Interneurons are present in the striatum.

The substantia nigra secrete neurotransmitters dopamine, enkephalin, dynorphin, and substance P.

Main neurotransmitters in Basal Ganglion:

1.Glutamic acid                                 stimulatory

2.Gama aminobutyric acid GABA     inhibitory

3.Dopamine                                         inhibitory

4.Acetylcholine                                   stimulatory

5.serotonin                                            neuromodulator

Interneurons secrete different types of neurotransmitters like nitric oxide, acetylcholine, parvalbumin, and calretinin. 

 External link:

1.https://www.britannica.com

2. https://www.medicinenet.com>pleura

3. https://en.m.wikipedia.org>wiki