Location
The substantia nigra is a major component of the basal ganglia, and in Image 10 it is shown on each side of the midbrain posterior to the cerebral peduncle.
The substantia nigra is located in the midbrain at the top of the brainstem, between the tegmentum posteriorly and each cerebral peduncle anteriorly. It is in close proximity to other basal ganglia components such as the subthalamus in the diencephalon, and is interconnected with other components of the basal ganglia (see caudate nucleus, putamen and globus pallidus text under Image 5). The proximity of the substantia nigra and subthalamic nucleus can be seen in Image 6.
Structure
Each substantia nigra has two parts: substantia nigra pars compacta (compact part) and substantia nigra pars reticulata (reticular part).
The compact part of substantia nigra contains densely packed dopaminergic neurons whose axons terminate in the caudate nucleus and putamen (striatum).
The reticular part of the substantia nigra contains GABAergic neurons that are less densely packed, and receives fibres from the striatum and sends axons to the thalamus, superior colliculus and reticular formation. Therefore, the pars reticulata is a basal ganglia output nucleus.
Function
Pars compacta – sends input to all the basal ganglia nuclei, especially the striatum (caudate and putamen). The ultimate effect of this input is to modify the output of the internal part of the globus pallidus and the pars reticulata of the substantia nigra. As the source of most output from the basal ganglia, these two nuclei are centrally involved in modulating cortical output. Where the axons from the internal globus pallidus and pars reticulata substantia nigra terminate determines their effect: output from these nuclei affects movement, learning and memory, sensory processing and many aspects of behaviour.
Pars reticulata – there is a significant resemblance between this part of the substantia nigra and the internal part of globus pallidus, with both receiving inputs from the striatum, external globus pallidus and subthalamic nucleus. While both project to the thalamus, it is only the pars reticulata that projects to the reticular formation and superior colliculus. By sending axons to the superior colliculus, the pars reticulata assists with the control of eye movement.
Parkinson's disease – it is degeneration of dopaminergic neurons in the substantia nigra, and the subsequent effect on inputs to the striatum, that causes the signs and symptoms of Parkinson's disease. These include: slow movement (bradykinesia), difficulty initiating movements (akinesia), increased muscle tone (rigidity), and tremors of the hands and jaw, more pronounced when the patient is resting and not attempting to move.
Schizophrenia – despite research attributing a role for dopamine and the substantia nigra in schizophrenia, there is limited evidence that there is disruption of domaminergic neurons in the cortex in this condition.
