The substantia nigra represents a nuclear area in the midbrain that is dark in color and belongs to the extrapyramidal motor system. It thus contributes to the control of movements. Atrophy of the substantia nigra occurs in Parkinson’s syndromes and leads to the development of the cardinal symptoms of rigor, tremor, bradykinesia, and postural instability.
What is substantia nigra?
The substantia nigra is located symmetrically in both halves of the brain (hemispheres) and belongs to the midbrain. There, it borders the cerebral peduncles (crura cerebri) and the midbrain cap (tegmentum mesencephali). The substantia nigra gets its name from its black coloration, which is due to the high amount of melanin and iron in this area. Dopamine functions in the substantia nigra as the most important neurotransmitter, occurs as a messenger substance exclusively in the central nervous system and belongs to the group of biogenic amines. These are neurotransmitters that are formed from the amino acid tyrosine and lose a carbon dioxide molecule through decarboxylation. In addition to dopamine, biogenic amines include serotonin, epinephrine and norepinephrine.
Anatomy and structure
Anatomically, the substantia nigra can be divided into two areas: the pars compacta, also known as the zona compacta, and the pars reticulata. The pars compacta consists of closely arranged nerve cells that contain a large amount of the pigment melanin. Nerve fibers connect the pars compacta to the striatum. In addition, the pars compacta is part of the black system (nigrostriatal loop). It also includes the nucleus ruber, which is also located in the midbrain, and nuclei of the striatum. The neurons of the pars reticulata are less closely spaced compared to the neurons of the pars compacta and contain a lot of iron, which gives the tissue a reddish color. This area also includes the pars lateralis, which some experts consider an independent part. The pars reticulata of the substantia nigra has connections to the striatum and the ventrolateral thalamus. Other nerve fibers lead from the substantia nigra to the cerebral cortex and the nucleus subthalamicus, among others.
Function and tasks
The substantia nigra belongs to the extrapyramidal motor system and is thus involved in the control of movements. In this context, its main function is that of a starter, as it is particularly involved in movement initiation and planning. The extrapyramidal motor system also includes the basal ganglia, the motor cortex and various nuclear areas in the brain, including the nucleus ruber in the midbrain and the formatio reticularis, which runs through the rhombus, midbrain and diencephalon. All of these structures, like the substantia nigra, rely on dopamine as a neurotransmitter: Nerve cells produce the neurotransmitter in their terminal nodules and store it in vesicles. When an electrical impulse – a so-called action potential – reaches the end of the nerve fiber and thus the terminal nodules, the cell releases the dopamine into the synaptic cleft. The messenger substance crosses the gap between the presynaptic and postsynaptic nerve cells and attaches to receptors in the postsynaptic membrane, whereupon ion channels open in it. Charged sodium particles can flow into the cell through the channels and change the electrical charge of the neuron. If the change exceeds the threshold potential, a new action potential is generated in the postsynaptic neuron. Dopamine deficiency leads to a disruption in this process, thereby affecting motor function in humans. As a whole, the extrapyramidal motor system is primarily responsible for gross motor movements.
Diseases
Parkinson’s disease is associated with atrophy of the substantia nigra, which leads to the development of the characteristic symptoms of the disease. Parkinson’s disease is a neurodegenerative disorder and is also known as shaking palsy. In 1917, James Parkinson was the first to describe the syndrome; today, approximately 250,000 people in Germany suffer from the disease, three quarters of whom have idiopathic Parkinson’s syndrome. The cardinal symptoms are rigor, tremor, bradykinesia/kinesthesia, and postural instability. Rigor is muscle rigidity or stiffness that occurs due to increased resting tone: The affected muscles are excessively tense. In contrast, the second leading symptom, tremor, manifests as muscle tremor and primarily affects fine motor movements.Affected persons also typically suffer from slowed movements; this phenomenon is referred to in medicine as bradykinesis. Whereas patients with bradykinesis can basically perform movements – albeit at a slower pace – in akinesia they are only able to do so partially (lack of movement) or not at all (immobility). Postural instability leads to unsteady posture and, as a result, often a slightly bent-over gait. The combination of bradykinesia with rigor, tremor, and/or postural instability often results in gait disturbances and other functional impairments. In addition to idiopathic Parkinson’s syndrome, medicine distinguishes three other forms. Familial Parkinson’s syndrome is due to errors in the genetic material – various genes can be considered as the cause. In contrast, symptomatic or secondary Parkinson’s syndrome develops as a result of another underlying disease, such as Binswanger’s disease or Wilson’s disease, or as a result of medication, drugs, poisoning or injury. The fourth form of Parkinson’s syndrome is also a result of other diseases; however, these are specifically neurodegenerative conditions that manifest in the loss of nerve cells. These include Lewy body dementia, multisystem atrophy, progressive supranuclear gaze palsy, and corticobasal degeneration. L-dopa is often used to treat Parkinson’s disease. The precursor of dopamine can cross the blood–brain barrier and at least partially compensate for the dopamine deficiency in the brain, leading to symptom relief. Causative treatment is not possible.