Basal ganglia or basal nuclei are collection of masses of gray matter situated within each cerebral hemisphere. They are the corpus striatum, the amygdaloid nucleus, and the claustrum. Basal ganglia are connected with the cerebral cortex, thalamus, brainstem, and several other areas of brain areas. The functions of basal ganglia include voluntary control of motor movements, learning, routine behaviors such as teeth grinding, eye movements, emotion and cognition.
Terminology of Basal ganglia
There are different terminologies to describe the basal nuclei. The subthalamic nuclei, the substantia nigra, and the red nucleus are functionally closely related to the basal nuclei, but they should not be included with them.
The interconnections of the basal nuclei are complex, but here the more important pathways are considered. The basal ganglia play an important role in the control of posture and voluntary movement.
|Terminology Commonly Used to Describe the Basal Nuclei|
The caudate nucleus is C-shaped mass of gray matter that is close to the lateral ventricle and lie lateral side to the thalamus. The laterally the nucleus is separated by internal capsule from the lentiform nucleus. The caudate nucleus can be divided into a head, a body, and a tail.
Head of caudate nucleus:
The caudate head is large and rounded and forms the lateral wall of the anterior horn of the lateral ventricle. The head is continuous inferiorly with the putamen of the lentiform nucleue. The caudate nucleus and the putamen are sometimes referred to as the neostriatum or striatum.
The body of caudate nucleus:
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.
Tail of caudate nucleus:
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. It terminates anteriorly in the amygdaloid nucleus
Lentiform Nucleus (Globus pallidus + Putamen )
The lentiform nucleus is a wedge shaped mass of gray matter. It is buried deep in the white matter of the cerebral hemisphere. The lentiform nucleus is related laterally to a thin sheet of white matter, the external capsule which 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.
There is a vertical plate of white matter which divides the lentiform nucleus into an inner lighter portion, the globus pallidus. The paleness of the globus pallidus is due to the presence of a high concentration of myelinated nerve fibers.
The larger, lateral and darker portion of the lentiform nucleus is called putamen. Inferiorly at its anterior end, the putamen is continuous with the head of the caudate nucleus
The amygdaloid nucleus is present in the temporal lobe close to the uncus. The amygdaloid nucleus is considered to be part of the limbic system. Amagdaloid nucleus has brain connections through which it can influence the body response to environmental changes. For example, during fear situation it can change the heart rate, blood pressure, skin color, and rate of respiration.
Substantia Nigra and Subthalamic Nuclei
The substantia nigra present in midbrain and the subthalamic nuclei of the diencephalon are functionally closely related to the activities of the basal ganglia.
The neurons of the substantia nigra are dopaminergic and inhibitory and have many connections to the corpus striatum.
The neurons of the subthalamic nuclei are glutaminergic and excitatory and have many connections to the globus pallidus and substantia nigra.
The claustrum is a thin sheet of gray matter that is separated from the lateral surface of the lentiform nucleus by the external capsule. Lateral to the claustrum is the subcortical white matter of the insula. The function of the claustrum is unknown.
Connections of the Corpus Striatum and Globus Pallidus
The caudate nucleus and the putamen form the main sites for receiving input to the basal nuclei. The globus pallidus forms the major site from which the output leaves the basal nuclei.
They receive no direct input from or output to the spinal cord
Connections of the Corpus Striatum
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. The largest input is from the sensory-motor cortex. Glutamate is the neurotransmitter of the corticostriate fibers
These intralaminar nuclei of the thalamus send large numbers of axons to the caudate nucleus and the putamen
These neurons 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 as the neurotransmitter. These fibers are inhibitory in function.
These fibers pass from the caudate nucleus and putamen to the globus pallidus. They have gamma-aminobutyric acid (GABA) as their neurotransmitter.
Striatonigral fibers pass from the caudate nucleus and putamen to the substantia nigra. Some of the fibers use GABA or acetylcholine as the neurotransmitter, while others use substance P.
Connections of the Globus Pallidus
Striatopallidal fibers pass from the caudate nucleus and putamen to the globus pallidus. As noted previously, these fibers have GABA as their neurotransmitter.
Pallidofugal fibers are complicated and they are divided into groups:
(1) Ansa lenticularis, which pass to the thalamic nuclei;
(2) Fasciculus lenticularis, which pass to the subthalamus;
(3) Pallidotegmental fibers, which terminate in the caudal tegmentum of the midbrain.
(4) Pallidosubthalamic fibers, which pass to the subthalamic nuclei.
Functions of the Basal Ganglia
The basal ganglia are joined together and connected with many different regions of the nervous system by a very complex number of neurons. The corpus striatum receives afferent information from most of the cerebral cortex, the thalamus, the subthalamus, and the brainstem, including the substantia nigra. The information is integrated within the corpus striatum, and the outflow passes back to that areas. The activity of the basal nuclei is initiated by information received from the premotor and supplemental areas of the motor cortex, the primary 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 control muscular movements by influencing the cerebral cortex and have no direct control through descending pathways to the brainstem and spinal cord. In this way, the basal ganglia assist in the regulation of voluntary movement and the learning of motor skills. If destruction of the corpus striatum then takes place, paralysis of the remaining movements of the opposite side of the body occurs. The activity in certain neurons of the globus pallidus increases before active movements take place in the distal limb muscles. This important 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 in the hands and feet. The basal ganglia integrate the function of eye movements, role in decision making and working memory.
Clinical significance of Basal ganglia
Disorders of the basal ganglia are of two general types. Hyperkinetic disorders are those in which there are excessive and abnormal movements, such as seen with chorea, athetosis, and ballism. Hypokinetic disorders include those in which there is a lack or slowness of movement. Parkinson disease includes both types of motor disturbances.
In these patient exhibits involuntary, quick, jerky, irregular movements that are nonrepetitive. Swift grimaces and sudden movements of the head or limbs.
Huntington disease is an autosomal dominant inherited disease. The disease has been traced to a single gene defect on chromosome 4. This gene encodes a protein, huntingtin, the function of which is not known. The codon (CAG) that encodes glutamine is repeated many more times than normal. Choreiform movements first appear as involuntary movements of the extremities and twitching of the face. Later, more muscle groups are involved.There is a degeneration of the GABA-secreting, substance P–secreting, and acetylcholine-secreting neurons of the striatonigral-inhibiting pathway. This results in the dopa-secreting neurons of the substantia nigra becoming overactive. This inhibition produces the abnormal movements seen in this disease. Computed tomography scans show enlarged lateral ventricles.
Sydenham chorea (St. Vitus’ dance) is a disease of childhood in which there are rapid, irregular, involuntary movements of the limbs, face, and trunk. The condition is associated with rheumatic fever. The antigens of the streptococcal bacteria are similar in structure to the proteins present in the membranes of striatal neurons. The host’s antibodies attack the membranes of the neurons of the basal ganglia. This results in the production of choreiform movements.
Hemiballismus is a form of involuntary movement confined to one side of the body. It usually involves the proximal extremity musculature, and the limb suddenly flies about out of control in all directions. The lesion occurs in the opposite subthalamic nucleus or its connections.
Parkinson disease is a progressive disease of unknown cause. It is associated with neuronal degeneration in the substantia nigra and, to a lesser extent, in the globus pallidus, putamen, and caudate nucleus. The degeneration of the neurons of the substantia nigra that send their axons to the corpus striatum results in a reduction in the release of the neurotransmitter dopamine within the corpus striatum. This leads to hypersensitivity of the dopamine receptors in the postsynaptic neurons in the striatum. Patients have tremor, rigidity, bradykinasia and postural disturbances.
Athetosis consists of slow, sinuous, writhing movements that most commonly involve the distal segments of the limbs. Degeneration of the globus pallidus occurs in athetosis.
Development of Basal ganglia
Prosencephalon give rise to Telencephalon and diencephalon which form On each side of the brain: the cerebral cortices, caudate, putamen Globus pallidus, ventral pallidum, thalamus, hypothalamus, subthalamus, epithalamus, subthalamic nucleus
Mesencephalon give rise to midbrain substantia nigra pars compacta (SNc), substantia nigra pars reticulata (SNr).