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Studies of the brain circuits that use dopamine and the locations
of the dopamine receptors in these circuits have identified eight
major dopaminergic pathways in the brain. Three of these pathways
are especially important. All three originate in the midbrain.
The
first is the mesolimbic pathway–the bundle of dopaminergic
fibres associated with the reward circuit. This pathway originates
in the ventral tegmental area and innervates several structures
of the limbic
system, including the nucleus accumbens. The mesolimbic
pathway is important for memory and for motivating behaviours.
By blocking this pathway, antipsychotic drugs reduce the intense
emotions caused by conditions such as schizophrenia.
The mesocortical pathway also originates in
the ventral tegmental area, but projects to the frontal cortex
and surrounding structures. Some evidence indicates that a
malfunction in this pathway might be the cause of some of the
symptoms of schizophrenia, such as hallucinations and disordered
thinking. Medications that block this pathway reduce psychotic
delirium, but also reduce the overall activity of the frontal
lobes.
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The third, nigrostriatal pathway projects
axons from the substantia nigra to the striatum (caudate nucleus
and putamen), which is involved in motor control. Degeneration
of the neurons in this pathway is associated with the trembling
and muscular rigidity symptomatic of Parkinson’s disease.
A fourth dopaminergic pathway worth mentioning is the tuberoinfundibular
pathway, which connects the hypothalamus to the pituitary gland, where
it influences the secretion of hormones such as prolactin. |
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