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Basal Nuclei (Basal Ganglia)

 

INTRODUCTION  

The term basal nuclei is applied to a collection of masses of gray matter situated within each cerebral hemisphere. They are the corpus striatum, the amygdaloid nucleus, and the claustrum. The basal nuclei play an important role in the control of posture and voluntary movement.

The purpose of this is to describe briefly the basal nuclei, their connections, and their functions.

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CORPUS STRIATUM

The corpus striatum is situated lateral to the thalamus . It is almost completely divided by a band of nerve fibers, the internal capsule, into the caudate nucleus and the lentiform nucleus.

Caudate Nucleus

The caudate nucleus is a large C‑shaped mass of gray matter that is closely related to the lateral ventricle and lies lateral to the thalamus. The lateral surface of the nucleus is related to the internal capsule, which separates it from the lentiform nucleus. For purposes of description, it can be divided into a head, a body and a tail.

The head of the caudate nucleus is large and rounded and forms the lateral wall of the anterior of the lateral ventricle. The head is continuous inferiorly with the putamen of the lentiform nucleus.* Just superiorly to this point of union, strands of gray matter pass through the internal capsule, giving the region a striated appearance, hence the term corpus striatum.

*The caudate nucleus and the putamen are sometimes referred to as the neostriatum or striatum.

 

The body of the caudate nucleus is long and narrow and is continuous with the head in the region of the interventricular foramen. The body of the caudate nucleus forms part of the floor of the body of the lateral ventricle.

The tail of the caudate nucleus is long and slender and is continuous with the body in the region of the posterior end of the thalamus. It follows the contour of the lateral ventricle and continues forward in the roof of the inferior horn of the lateral ventricle. It terminates anteriorly in the amygdalold nucleus.  

Lentiform Nucleus

The lentiform nucleus is a wedge‑shaped mass of gray matter, whose broad convex base is directed laterally and its blade medially. It is buried deep in the white matter of the cerebral hemisphere and is related medially to the internal capsule, which separates it from the caudate nucleus and the thalamus. The lentiform nucleus is related laterally to a thin sheet of white matter, the external capsule, that separates it from a thin sheet of gray matter, called the claustrum. The claustrum, in turn, separates the external capsule from the subcortical white matter of the insula. A vertical plate of white matter divides the nucleus into a larger, darker lateral portion, the putamen, and an inner lighter portion, the globus pallidus. Inferiorly at its anterior end, the putamen is continuous with the head of the caudate nucleus.  

AMYGDALOID NUCLEUS

The amygdaloid nucleus is situated in the temporal lobe close to the uncus.The amygdaloid nucleus is considered to be part of the limbic system.  

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CLAUSTRUM

The claustrum is a thin sheet of gray matter that is separated from the lateral surface of the lentiform nucleus by the external capsule (Fig. Lateral to the claustrum is the subcortical white matter of the insula. The function of the claustrurn is unknown.

CONNECTIONS

 

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CONNECTIONS OF THE CORPUS STRIATUM

Afferent Fibers

CORTICOSTRIATE FIBERS

All parts of the cerebral cortex send axons to the caudate nucleus and the putamen .  Each part of the cerebral cortex projects to a specific part of the caudate.putamen complex. Most of the projections are from the cortex of the same side. The largest input is from the sensory‑motor cortex. Glutamate is the neurotransmitter of the corticostriate fibers.

 THALAMOSTRIATE FIBERS

Intralaminar nuclei of the thalamus send large numbers of axons to the caudate nucleus and the putamen.  

NIGROSTRIATE FIBERS

Cells in the substantia nigra send axons to the caudate nucleus and the putamen and liberate dopamine at their terminals as the neurotransmitter. It is believed that these fibers are inhibitory in function.

 

BRAINSTEM STRIATAL FIBERS

Ascending fibers from the brainstem end in the caudate nucleus and putamen and liberate serotonin at their terminals at the neurotransmitter. It is thought that these fibers are inhibitory in function.

 

Efferent Fibers

STRIATOPALLIDAL FIBERS

These fibers pass from the caudate nucleus and putamen to the globus pallidus. They have gammaaminobutyric acid (GABA) as their neurotransmitter.

 

STRIATONIGRAL FIBERS

Fibers pass from the caudate nucleus and putamen to the substantia nigra. Some of the fibers use GABA as the neurotransmitter, while others use substance P

 

 

CONNECTIONS OF THE GLOBUS PALLIDUS

 

Afferent Fibers

 STRIATOPALLIDAL FIBERS

 

These fibers pass from the caudate nucleus and putamen to the globus pallidus. As noted previously, these fibers have GABA as their neurotransmitter.  

Efferent Fibers

 PALLIDOFUGAL FIBERS  

These fibers can be divided into four groups: (1) the lenticularis, (2) the fasciculus lenticularis, (3) the pa tegmental fibers, and (4) the pallidosubtlialamic fiber.

 

FUNCTIONS OF THE BASAL NUCLEI

 

Basal nuclei are joined together and connected with different regions of the nervous system by a very complex number of neurons.

 

Basically the corpus striatum receives afferent information from most of the cerebral cortex, the thalamus, the subthalamus, and the brainstem, including the substantia nigra. information is integrated within the corpus striatum and the outflow passes back to the areas listed above. This circular pathway is believed to function as follows.

 

The activity of the basal nuclei is initiated by information received from the sensory cortex, the thalamus, and the brainstem. The outflow from the basal nuclei is channeled through the globus pallidus, which then influences the activities of the motor areas of the cerebral cortex or other motor centers in the brainstem. Thus the basal nuclei can control muscular movements by influencing the cerebral cortex rather than through direct descending pathways to brainstem and spinal cord. In this way the basal nuclei assist in the regulation of voluntary movement and the learning of motor skills.

 

Destruction of the motor cerebral cortex prevents the individual from performing fine discrete movements of the hands and feet on the opposite side of the body However, .the individual is still capable of performing gross crude movements. If destruction of the corpus striatum then occurs paralysis of the remaining movements of the opposite of the body takes place.

 

It has been shown that the basal nuclei not only influence the execution of a particular movement of say the limbs, but play a role in the preparation of the movements. This be achieved by controlling the axial and girdle movements of the body and the positioning of the proximal parts of the limbs. The activity in certain neurons of the globus pallidus increases before movement takes place in the distal limb muscles. This preparatory function enables the trunk and limbs to be placed in appropriate positions before the primary motor part of the cerebral cortex activates discrete movements of the hands and feet.