Neuronal systems are groups of neurons that use the same neurotransmitter and the axons of which often form dense fiber bundles. They are described according to their neurotransmitter substance as cholinergic, noradrenergic, dopaminergic, serotoninergic, GABAergic, or peptidergic systems. The impulse can be transmitted to neurons of the same type or to neurons with a different neurotransmitter. In the sympathetic nervous system, for example, the neurons in the spinal cord are cholinergic; however, transmission in the peripheral ganglia switches to noradrenergic neurons.
Noradrenergic, dopaminergic, and serotoninergic neurons are located in the brain stem. Noradrenergic neurons form the
locus caeruleus and cell groups in the lateral part of the reticular formation of the medulla oblongata and the pons; their fibers project to the hypothalamus, to the limbic system, diffusely into the neocortex, and to the anterior horn and lateral horn of the spinal cord. Serotoninergic neurons lie in the raphe nuclei, especially in the posterior raphe nucleus; their fibers project to the hypothalamus, to the olfactory epithelium, and to the limbic system. Dopaminergic neurons make up the compact part of the substantia nigra from where the nigrostriatal
fibers extend to the striatum.
Peptidergic neurons are found in phylogenetically older brain regions, namely, in the central gray of the midbrain, in the reticular formation, in the hypothalamus, in the olfactory bulb, in the habenular nucleus, interpeduncular nucleus, and solitary nucleus. Numerous peptidergic neurons have also been demonstrated in the cerebral cortex, in the thalamus, in the striatum, and in the cerebellum. The significance of the different peptides is still largely unclear. It is assumed that they act as cotransmitters and have a modulating function. Many of these peptides are found in other organs as well, such as digestive system.
Glutamate is often the transmitter of projection neurons with long axons. Glutamatergic neurons are the projection neurons of the cerebral cortex, the pyramidal cells. GABAergic inhibitory neurons are often classified according to the target structures on which they form inhibitory synapses. GABAergic basket cells, which form synapses with cell bodies, are distinguished from axo-axonal cells. The latter develop inhibitory synapses at the beginning of the axon (initial segment) of a projection neuron. GABAergic neurons often form local circuits (interneurons). They often contain peptides and calcium-binding proteins apart from GABA as the classic transmitter.
Cholinergic neurons are found in the brain stem and also in the basal forebrain. As in the case of catecholaminergic neurons, farreaching projections originate from circumscribed groups of cells, for example, in the basal nucleus and in certain septal nuclei that supply, via fibers in the cingulate gyrus (limbic gyrus) and in the fornix, respectively, large regions of the neocortex and the hippocampus. These ascending cholinergic projections from the basal forebrain are thought to be associated with processes of learning and memory. They are affected in Alzheimer’s disease which is accompanied by disturbed learning and memory.