BASAL NUCLEI

 

The basal nuclei or telecephalic nuclei are collections of gray matter included in the white matter of the cerebral hemispheres. They are also known as basal ganglia. However, this terminology is improper because a ganglion is a collection of neuronal bodies outside the central nervous system. When the neuronal bodies group inside they are referred as nuclei. The basal nuclei are: The basal nuclei are: the caudate nucleus[1], the accumbens[2], the lenticular nucleus[3], the endopeduncular (is a caudal extension of the pallidum towards the subthalamus), the claustrum and the amygdaloid[4] body.  The lenticular nucleus is composed of two parts: the putamen[5] and the globus pallidus[6]. Together these structures are wrongly named corpus striatum. The caudate nucleus and the putamen develop from a single nucleus, the lateral striated body in the diencephalic ventricular lumen.  As they develop, the fibers of the internal capsule separate the two nuclei[7]. However, there are fibers remain attaching both nuclei. In the adult brain, the combination of the caudate nucleus and the putamen is properly termed striated body or corpus striatum[8] due to the striated appearance of the myelinated fibers that unite these two nuclei.

 

According to the phylogenetic origin, the basal nuclei are classified in: paleostriatum (refers to the globus pallidus), neostriatum (refers to the striated body), and archistriatum (refers the amygdaloid body). The paleostriatum receives fibers from the striated body and projects to the thalamus and subthalamus, the striated body is related with the majority of the neocortex, and the archystriatum to the piriform lobe (paleocortex).

 

The caudate nucleus is a long and curved nucleus that receives this name because it has a head, a body and a tail. It remains attached, by strands of myelinated fibers, to the putamen to form the striated body. It receives cortical axons, and projects to the pallidum and substantia nigra. The caudate, putamen and pallidum are related to motor functions. The pallidum has two portions: a medial and a lateral one. Both receive axons from the striated body and project to the thalamus. The medial portion (endopeduncular nucleus) facilitate wanted movements and the lateral portion inhibits unwanted ones. The accumbens is located ventromedially to the head of the caudate nucleus, close to the telecephalic septum. It is involved in motivation in order to codify motor functions. The amygdaloid body is located in the piriform lobe and close to the insula. This is a hidden neocortex inside the pseudosylvian fissure in the carnivores (for the rest of mammals is termed sylvian fissure). The insula and the amygdaloid boy are involved in emotional reactions through its connections with limbic related structures.

 

The basal nuclei principally exert their function over the cerebral cortex, allowing mechanical movements in the animal, such as running, eating, changing posture, etc.[9] For this reason, when the cerebral cortex is experimentally removed from a cat and leaving the basal nuclei intact, the cat is capable of walking, arching its back and realizing the majority of its normal movements.  On the other hand, in humans, a similar lesion in the cerebral cortex without affecting the basal nuclei, would result in a loss of gross movements of the trunk and limbs[10]. They also play an important role in behavior.  In this sense, the bilateral extirpation of the caudate nucleus in cats does not cause a motor deficit but does cause changes in behavior that disappear after six or nine months[11].

 

Electrophysiological studies have demonstrated two ways that the striatum is capable of controlling the activity of the rest of the basal nuclei. These are: a direct method and an indirect method[12].

 

Direct Method:

                                                                                                                                   

When a movement is initiated, the excitatory glutaminergic fibers of the cerebral cortex discharge in the pyramidal tract and the corpus striatum. The fibers from the striate body induce direct inhibition over the neurons of the medial portion of the globus pallidus. Since these neurons are inhibitory over the thalamus, the striate body fibers cause disinhibition (inhibition of an inhibitor causes excitation) of the neurons of the thalamus over the cerebral cortex. This mechanism allows the initiated movement to be maintained and facilitates the initiation of the movement.

 

Indirect Method:

 

The striate body fibers that terminate in the lateral portion of the pallidum induce inhibition of its neurons. The principal projection of this portion of the pallidum is the subthalamic nucleus. Because the activity of this projection has itself been inhibited, the subthalamic nucleus is disinhibited. This situation causes this nucleus to activate the medial portion of the pallidum which, consequently, causes the inhibition of the thalamus (the pallidothalamic fibers are inhibitory) and its activity over the cerebral cortex. This method impedes undesired movements.

 

[1] From the Latin caudatus, “with tail”.

[2] From the Latin accumbo, is, “to recline, to lie back”.

[3] From the Latin lens, “lentil”.

[4] From the Latin amygdale, “almond”.

[5] From the Latin putamen, “shell, peel, cortex”.

[6] From the Latin pallidus, “pale”.

[7] Pages 166 and 167 of “Anatomia humana” by Orta Llorca, F.

[8] From the Latin corpus, “body”; striatus, “grooved, striped”.

[9] Page 152 of “Physiological and clinical anatomy of the domestic animals. Vol. 1, Central nervous system” by King, A. S.

[10] Page 18 of “Basic neuroscience. Anatomy and physiology” by Guyton, A. C

[11] Page 145 of “Veterinary neuroanatomy and clinical neurology” by De Lahunta, 2nd ed.

[12] Page 158 of “Neuroanatomy” by Crossman, A. R. and Neary, D.