The input area of the basal ganglia is the striatum, also known as the striate body. This part of the brain is interconnected with the motor neural pathways and is the first switching point for the circuitry of specific movements. Degeneration of the striatum can occur in the context of Parkinson’s disease or Huntington’s disease and usually affects the brain as either hypo- or hyperkinesis.
What is the striatum?
The striatum, or actually corpus striatum, belongs to the basal ganglia and thus to the forebrain. It is also called the striate body in German and forms the part on the side to the thalamus in each cerebral hemisphere. The ray body plays a role specifically for the motor nervous system. The five motor nerves of the spinal cord can be divided into the two pyramidal tracts and the three extrapyramidal tracts. These nerves have their switching points in the brain. Especially for the extrapyramidal pathways, the striatum is one of the most important switching points. It forms the entrance to the basal ganglia, where motivation, cognition, emotion and movement behavior are neuronally organized. As such, the basal ganglia are particularly responsible for goal-directed actions, control, decisions, and movement plans. Output from this system excites specific muscles, enabling the realization of voluntary movement plans.
Anatomy and structure
The caudate nucleus and the putamen each form a striatum. The putamen is part of the so-called gray matter. The caudate nucleus is its associated part of the white matter and in this sense belongs to the end brain. The capsula interna demarcates the nucleus caudatus from the putamen. This is a collection of nerve fibers that grows around the unit of putamen and caudate nucleus during late embryonic development. This nerve fiber pathway is the longest stimulus-receiving pathway of the central nervous system. There is a connection between the putamen and the caudate nucleus, despite the capsula interna, in the form of fine strips of gray matter. On the ventral side, the nucleus accumbens connects the putamen with the nucleus caudatus. The nucleus accumbens is part of the mesolimbic system and, together with the nucleus basalis and part of the limbic amygdala, forms the substantia innominata. The striatum is equipped with many excitatory or glutamatergic nerve fibers from the cortex. In addition, there are dopaminergic fibers from the substantia nigra. In the case of the nerve cells of the stratium, physicians also speak of spiny neurons with a particularly fine dendrite structure.
Function and tasks
The striatum is the first switching point of the basal ganglia and thus provides input to the basal ganglia system for processing specific projections. Thus, the circuitry of specific movements begins in the striate. In particular, the incoming projections to the straitum originate from the cerebral cortex, substantia nigra, and the core areas of the central nervous system. The input of these projections to the striatum is biochemical. The neurotransmitter glutamate plays an important role with respect to the excitatory fibers from the cortex. In contrast, the dopaminergic fibers from the substantia nigra are controlled by the neurotransmitter dopamine. In this system, the stratium performs an inhibitory function on movements of the extrapyramidal system. This inhibition runs via the release of the neurotransmitter GABA. Thus, the striatum inhibits the globus pallidus through efferent fibers and the substantia nigra through negative feedback. Thus, from the cortex, the striatum receives an action plan that relates to a movement execution. This intention to move is communicated to the ray body via glutamate and shifts the spiking neurons of the stratum. These inhibitory spiny neurons then release the inhibitory transmitter GABA on the pale and black nuclei of the brain. The black nucleus now releases dopamine, thereby feedback inhibiting the movement inhibitory spiny neurons. Output from the basal ganglia passes through the pale nucleus and movement occurs through the intermediary of the inhibited spiny neurons. The nucleus accumbens from the putamen and caudate nucleus, on the other hand, plays a role primarily in the brain’s reward system and thus in the development of addiction. This area rewards certain behaviors with feelings of happiness and is the link between motor activity and emotion.
Diseases
When structures of the feedback loop between substantia nigra and striatum are defective, so-called hypokinesia sets in.In this clinical picture, there is reduced movement. Spontaneous motor activity decreases and the general movement becomes mask-like and small. Such hypokinesia may be due to degenerative diseases such as Parkinson’s disease. In hypokinesia due to Parkinson’s disease, the dopaminergic cells of the black nucleus are destroyed. Movements are slowed down and the initiation of purposeful movements is accompanied by tremor. Once the movement goal is reached, the tremor often subsides. For example, when reaching for a glass, tremor may occur during this process. Despite this, however, drinking can often be done normally after the goal has been reached. Similar to Parkinson’s disease, Huntington’s disease is often associated with degeneration of the striatum. Instead of hypokinesia, hyperkinesia develops in this clinical picture. This form of movement disorder is also called motor restlessness. Such phenomena are usually related to degeneration of GABA neurons in the striatum. Tremor in general may also be due to a disorder of the striatum. A third example of disorders of this area of the brain is the so-called striatum syndrome.
