Neurochemistry

Several classes of neurotransmitters have been identified in the cerebral cortex, including excitatory amino acids, g-aminobutyric acid (GABA), neuroactive pep-tides, and monoamines. The excitatory amino acids, glutamate and aspartate, are the primary neurotrans-mitters of cortical projections. They are quick acting and potent to enable fast transfer of information with high fidelity. These neurotransmitters act primarily on N-methyl-d-aspartate (NMDA), a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), kainate, l-ammonophosphonobutyric acid, and metabotropic receptors, all of which are more prevalent in sensory than motor regions.

GABA is localized in a wide variety of nonpyramidal cells and is often colocalized with neuroactive peptides. It acts on GABAA and GABAB receptors to regulate information within small regions throughout the cortex. GABA is the primary inhibitory neuro-transmitter of the central nervous system.

Cholecystokinin, vasoactive intestinal polypeptide, neuropeptide Y, somatostatin, substance P, and corticotrophin-releasing factor are all considered to be neuroactive peptides that influence neurotransmission by modulating the effects of excitatory amino acids and GABA. Moreover, some of these peptides may mediate their effects by altering blood flow and metabolism in local regions of the brain.

The fourth class of chemical mediators found in the cerebral cortex are monoamines and include noradre-naline, serotonin [or 5-hydroxytryptamine (5-HT)], and dopamine. These monoamines are the primary source of input to the cortex from extrathalamic sources and have different patterns of distribution throughout the cortex. Noradrenergic axons (mostly originating from neurons in the locus coeruleus in the pons) innervate primary motor and somatosensory areas and the nearby regions in the frontal and parietal lobes. Noradrenaline acts on ai, a2, bi, and b2 receptors, which are more prominent in prefrontal, motor, and somatosensory regions. Serotonergic fibers originate primarily from the raphe nuclei and the midbrain tegmentum of the brain stem. They innervate diffuse areas of the cerebral cortex to act on the 5-HT (5-HT1-5-HT7) receptor. Dopamine fibers preferentially innervate motor and association cortical areas to modulate the connections between association areas and descending motor systems. These fibers, which act on the dopamine receptor (Di and D2 receptor subtypes), originate in the substantia nigra and ventral tegmental area of the midbrain.

Another class of neurotransmitter is acetylcholine. Cholinergic fibers, which originate in the basal nucleus of Meynert and diagonal band of Broca, innervate diffuse cortical areas, including limbic, primary sen-sorimotor, and association areas. Acetylcholine acts on muscarinic and nicotinic receptors mainly on pyramidal but also on nonpyramidal cells. The mono-amines and acetycholine are not necessarily excitatory or inhibitory; they act to modulate and enhance the inputs from the thalamus and other cortical areas.

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