The basal ganglia is the name given to a group of nerve nuclei located below the cerebral cortex in pairs in each of the two hemispheres of the brain. The basal ganglia perform important functions in the control and regulation of processes within the peripheral nervous system. Their functions are important in all voluntary and reflex motor activity, as well as in cognitive processes. The basal ganglia also play an important role in the formation of emotional processes generated by the limbic system, such as motivation, spontaneity, willpower, and in affect.
What are the basal ganglia?
Basal ganglia, which are usually referred to as basal nuclei (nuclei basales) according to more recent nomenclature, consist of a collection of neuronal bodies located in the brain below the cerebral cortex (subcortical). Some basal ganglia are identical to specific nuclei such as the curved nucleus (nucleus caudatus), while others are composed of several nuclei and form a functional unit such as the lentiform nucleus (nucleus lentiformis), which is composed of the shell body (putamen) and the pallidum (globus pallidus). The globus pallidus (pale sphere), in turn, is divided into an internal and an external segment, each with different functions. Although the basal ganglia perform important tasks in complex movement processes, they are not connected to the pyramidal cells, the pyramidal system, which is essentially used to coordinate movement in humans for voluntary and involuntary movement processes. The basal ganglia are instead classified as part of the extrapyramidal motor system (EPMS), but in addition, beyond their influence on movement coordination, they perform much broader tasks in the area of emotional performance.
Anatomy and structure
The caudate nucleus, assigned to the basal ganglia, represents a paired large cluster of cerebral nerve nuclei. From the laterally adjacent putamen, the caudate nucleus is demarcated by nerve fibers visible as white stripes. Both nuclei together are therefore also called the striate body (corpus striatum or simply striatum). The globus pallidus externa and interna, which lie laterally adjacent to the putamen, are also sometimes included in the striatum. In the literature, the putamen and the pallidum are often grouped together as the nucleus lentiformis. The nucleus accumbens, which also performs important tasks in the brain‘s reward system, acts as the connecting link between the putamen and the caudate nucleus. In addition to the pallidum, the so-called substantia nigra, also known as the Soemmering ganglion, assumes an important function in the activation-deactivation regulatory circuits. It is a core complex of the midbrain consisting of the pars compacta and the pars reticularis. The pars compacta contains a high concentration of iron and melanin, making it appear almost black. Most often, the nucleus subthalamicus is also included in the basal ganglia because it occupies the function of the amplifier in the control circuit of the basal ganglia. The basal ganglia work with different neurotransmitters for activation or for antagonistic inhibition. The major neurotransmitters in the basal ganglia regulatory circuits are gamma-aminobutyric acid, glutamate, and dopamine.
Function and tasks
The basal ganglia are part of complex regulatory circuits that are involved in many executive “commands” of the cortex. For example, in the motor area, complex movement sequences that are only possible with the participation of many different muscles are assembled. At the same time, the basal ganglia take over reinforcing (excitatory) and inhibiting (inhibitory) functions within the control circuits with respective feedback. They compose quasi highly integrative processes – also in the non-motor area – and simultaneously exert a filtering effect. Although not all functions and tasks of the basal ganglia are fully understood, there is at least agreement on the most important reporting pathways within the control circuit. The regulatory circuit is activated by messages from the cortex to the striatum via nerve connections that have glutamate receptors (glutamaterg). From the pars reticularis of the substantia nigra and from the pars interna of the globus pallidus, the already processed information reaches the thalamus directly in case of inhibition via connections that react to gamma-butyric acid as a messenger.The thalamus reports directly back to the cortex via glutamatergic nerve connections. In the case of intended reinforcements, the pars compacta of the substantia nigra stimulates the striatum via dopaminergic connections. The nucleus subthalamicus may act in the reinforcement process via glutamatergic connections to the substantia nigra and globus pallidus.
Diseases
The complex and fluid “normal” movement patterns generated by basal ganglia control circuits can be significantly impaired when basal ganglia dysfunction occurs. Functionally impaired basal ganglia typically result in dystonia, which manifests as prolonged uncontrollable muscular tension of the skeletal muscles with corresponding abnormal postures. Another symptom complex consists in the expression of hyperkineses. These are involuntary, uncontrollable movements of the limbs, head and neck. One of the best known dystonias is Parkinson’s disease, which is caused by a progressive degenerative process of the substantia nigra. There is a breakdown of melanin-containing neurons, resulting in a dopamine deficiency that makes fluid movement difficult and, as the disease progresses, impossible. The main symptoms of Parkinson’s disease are muscle rigidity, muscle tremor, slowed movements and increasing instability in posture. Attention deficit hyperactivity disorder (ADHD), which is relatively frequently diagnosed in children, is also triggered by a disorder in the regulatory circuit of the basal ganglia. Athetoses, which are associated with involuntary, slow-extending movements and often lead to joint hyperextension, are also associated with dysfunction of the striatum. In this case, the damage to the striatum is usually due to birth processes. So-called tics such as Tourette’s syndrome, which are also due to dysfunction of the basal ganglia, clearly show that the basal ganglia not only cover the extrapyramidal motor area. Tics, in addition to compulsive and repetitive movements, may be coupled with a compulsion to, for example, incoherently pronounce or exclaim certain words.
Typical and common nervous disorders
- Nerve pain
- Nerve inflammation
- Polyneuropathy
- Epilepsy
