ABSTRACT
The vertebrate nervous system is composed of the central nervous system (CNS), the brain and spinal cord, and the peripheral nervous system (PNS), which is responsible for relaying signals back and forth between the CNS and the rest of body. Both the CNS and the PNS contain two main cell types, the neurons (nerve cells) themselves and a number of supporting cells called glia. Glia in mammalian brains are about ten times as numerous as neurons and are responsible for such diverse functions as the physical and biochemical support of the neurons, axonal guidance during development, and the recycling of neurotransmitters (Nicholls, 1994). The brain itself is comprised of three major divisions: the caudal brainstem includes the medulla, midbrain, and cerebellum; the diencephalon consists of the thalamus, hypothalamus, and pituitary; and the rostral cerebral hemispheres contain the striatum, olfactory bulb, neocortex, hippocampus, and dentate gyrus (Nicholls, 1994). Communication between the neurons and between neurons and other cells depends primarily on the release of chemical messengers called neurotransmitters from the axonal bulbs of one cell (the presyanptic cell) that bind to receptors on the post-synaptic cell. Whereas more than 100 neuroactive compounds have been discovered, they are generally divided into three categories, including: Type I neurotransmitters, such as glutamate and gamma-aminobutyric acid (GABA) that may account for up to 90% of synaptic transmission; the “classical” Type II neurotransmitters, such as
acetylcholine and the catecholamines like dopamine and serotonin; and the neuropeptides (Type III neurotransmitters) that serve primarily as neuromodulators rather than acting primarily at synapses (Nicholls, 1994). The transmission of electrical signals within the nervous system (which depends on ion fluctuations across cell membranes) and the re-uptake and recycling of neurotransmitters, are both energetically expensive, and this high-requisite ATP consumption makes the brains of most animals critically dependent on a constant supply of oxygen.
