ANATOMY OF THE NERVOUS SYSTEM
The basic functional unit of the brain is the neuron (Fig. 60-1). It is composed of a cell body, a dendrite, and an axon. The den-drite is a branch-type structure with synapses for receiving elec-trochemical messages. The axon is a long projection that carries impulses away from the cell body. Nerve cell bodies occurring in clusters are called ganglia or nuclei. A cluster of cell bodies with the same function is called a center (eg, the respiratory center). Neuroglial cells, another type of nerve cell, support, protect, and nourish neurons.
Neurotransmitters communicate messages from one neuron to another or from a neuron to a specific target tissue. Neurotrans-mitters are manufactured and stored in synaptic vesicles. They enable conduction of impulses across the synaptic cleft. The neuro-transmitter has an affinity for specific receptors in the post-synaptic bulb. When released, the neurotransmitter crosses the synaptic cleft and binds to receptors in the postsynaptic cell mem-brane. The action of a neurotransmitter is to potentiate, termi-nate, or modulate a specific action and can either excite or inhibit the target cell’s activity. There are usually multiple neurotrans-mitters at work in the neural synapse. There are various types of neurotransmitters (Bradley et al., 2000; Hickey, 2003); major neurotransmitters are described in Table 60-1.
Many neurologic disorders are due, at least in part, to an imbalance in neurotransmitters—that is, a lack of gamma-aminobutyric acid (GABA) and acetylcholine in Huntington’s disease (Bradley et al., 2000), low serotonin levels in some forms of epilepsy (Blows, 2000), and a decrease in dopamine in Parkin-son’s disease. In fact, probably all brain functions are modulated through neurotransmitter receptor site activity, including mem-ory and other cognitive processes.
There are two types of receptors: direct and indirect. Direct receptors are also known as inotropic because they are linked to ion channels and allow passage of ions when opened. They can be excitatory or inhibitory and are rapid-acting (measured in milliseconds). Indirect receptors affect metabolic processes in the cell, which can take from seconds to hours to occur. Receptor sites are an expanding area of research because they are often the target for the action and development of new medications. These medications either block or stimulate neurotransmitters at re-ceptor sites and thus provide relief from symptoms (Blows, 2000). Receptor sites are also sites for the action of addictive drugs.
Another important area of ongoing research is diagnostic test-ing that can detect abnormal levels of neurotransmitters in the brain. Positron emission tomography (PET), for example, can de-tect dopamine, serotonin, and acetylcholine (Gjedde et al., 2001). Single photon emission computed tomography (SPECT) is similar to PET.