Substantia nigra
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The substantia nigra (Latin for "black substance") is a portion of the midbrain thought to be involved in certain aspects of movement and attention. It consists of two subdivisions, the pars compacta and the pars reticulata.
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Pars compacta
Anatomy
The pars compacta contains densely-packed neurons (brain cells) which, in humans, are coloured black by the pigment neuromelanin. This pigmentation is visible as a distinctive black stripe in brain sections and is the source of the name given to this area. The majority of these neurons send their axons along the nigrostriatal pathway to the striatum where they release the neurotransmitter dopamine.
Function
The function of the dopamine neurons in the substantia nigra pars compacta is essentially unknown. Current research suggests that dopamine neurons are involved in learning to predict which behaviours will lead to a reward (for example food or sex). In particular, it is suggested that dopamine neurons fire when a reward is greater than that previously expected; a key component of many reinforcement learning models. This signal can then be used to update the expected value of that action. Many drugs of abuse, such as cocaine, mimic this reward response—providing an explanation for their addictive nature.
Pathology
Degeneration of cells in this region is the principle pathology that underlies Parkinson's disease. In a few people the cause of Parkison's disease is genetic, but in most cases the reason for the death of these dopamine neurons is unknown. Parkinsonism can also be produced by viral infection (for example, certain forms of encephalitis including the "sleepy sickness" of the 1920s described in Oliver Sacks' book Awakenings), or toxins such as MPTP (a toxic substance which can be mistakenly produced during the synthesis of heroin). Pathological changes to the dopamine neurons of the pars compacta are also thought to be involved in Schizophrenia (see the dopamine hypothesis of schizophrenia) and psychomotor retardation sometimes seen in clinical depression.
Pars reticulata
Anatomy
Neurons in the pars reticulata are much less densely packed than those in the compacta. Most of these neurons produce the neurotransmitter gamma-aminobutyric acid (GABA), but there is also a small sub-population of dopamine neurons. The GABA neurons connect to portions of the thalamus and brainstem (the superior colliculus and pedunculopontine nucleus), and also make connections to the dopamine neurons in the pars compacta.
Function
The pars reticulata is considered to be one of the two primary output nuclei of the brain's basal ganglia (the other output is the internal segment of the globus pallidus). It is largely involved in orientation and the control of eye movements.
Pathology
The function of the neurons of the pars reticulata is profoundly changed in parkinsonism and epilepsy. These changes are thought to be mostly secondary to pathology elsewhere in the brain, but may be crucial to understanding the generation of the symptoms of these disorders.