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Neuromodulation occurs when a neurone uses a chemical to regulate diverse populations of neurones. Neuromodulators are secreted by a small group of neurons and diffused through large areas of the NS, instead of into a synaptic gap, affecting multiple neurones at the same time. Just one of these neurones can influence over 100,000 others through the neuromodulators that it secretes into the brain’s extracellular space.

Neuromodulators spend significant amounts of time in the cerebrospinal fluid (CSF), ‘modulating’ the activity of several other neurones. Some of the same chemicals that act as neurotransmitters are also neuromodulators, specifically serotonin, acetylcholine, dopamine and norepiniphrene. The neurons of the ‘hormonal brain’ differ from those of the ‘wired brain’ in several ways. The hormonal neurones are concentrated mainly in the brainstem and the central region of the brain. They form small masses of thousands of cells, but these cells project their axons into large areas of the forebrain and the midbrain. Many drugs and medications, including those prescribed for affective disorders and schizophrenia, act on the neuromodulators of the diffuse projection neurons in the brainstem. For this reason, the function and distribution of the projections of these neurons have been the subject of much research using tracing techniques, because the axons of these neurons are not myelinated and do not form readily identifiable bundles. The results have confirmed how widely diffused these projections are. For example, a single axon from one of these neurons may subdivide and innervate both the cortex and the cerebellum.

The four main neuromodulators are norepinephrine (diffused by the locus coereleus), serotinin (diffused by the Raphe nuclei), acetylcholine (diffused by the basal nucleus of Meynert, pedunculopontine and pontine nuclei) and dopamine (diffused by the substantia nigra and ventral tegmental area). Each of these groups of neurones projects axons into large areas of the CNS and thus modulates numerous behaviours. The diffusion of the four main neuromodulators is illustrated in Figure 2.6.