Now I shall discuss the whys and wherefores of NTs as is currently understood by myself. The main catagories of CNS NTs are as Follows:
Gamma-Amino Butyric Acid (GABA)
Aspartic Acid/Aspartate (Asp)
Glutamic Acid/Glutamate (Glu)
Serotonin or 5-Hydroxytryptamine(5-HT)
NTs are produced and packaged as vesicles or small globules in the neuron cell body and then transported down the axon to the terminal region called the bouton. Neurons have a distinctive shape. Upstream to the cell body are thin processes called dendrites. The neuron is rather angular on a globular shape due to the presence of these numerous dendrites and the one large and long downstream transmission cable, the axon. When sufficient stimulation is received by the cell body region from the sum total of all membrane activities taking place due to dendritic stimulation (+ or -), an action potential is generated by the cell body where the axon starts (hillock) and travels down the axon to the bouton. At the bouton, vesicles fuse with the membrane of the bouton region and empty their contents into the synaptic cleft between the upstream neuron and the downstream neuron. The upstream neuron spitting its NT onto the membrane surface of the downstream neuron. NTs make their way across the minute space called the synapse and interact with the membrane of the downstream dendrite or cell body. Thus while the conduction of the nerve impulse is electro-chemical in nature involving fluctuations of sodium, potassium and chloride ions in one individual neuron, neurons "talk" to each other by dumping various chemicals onto the membranes of each other. The book by M.D. Chavitz contains an excellent and readily understandable account of neurophysiology. It is highly recommended for obtaining an overall understanding of the neurotransmitters and behavior as well as the neuroanatomy of the CNS.
The monoamine (DA, NE, 5-HT) neurons have their origins in the brainstem and project to higher centers in the midbrain, cerebellum and cortex as well as descending to the Peripheral NS (PNS) via the spinal cord. ACh neurons appear to be universally present throughout the brain but have a general upwards ascending tendency. ACh neurons terminate at a lower level of the neocortex than do the monoamines. The excitatory amino acid (Asp, Glu) neurons in contrast have descending pathways from the neocortex, mid-brain and cerebellum. GABA and Glycine neurons are interneurons dispersed throughout the Asp/Glu network and modulate the stimulatory effects of the Asp/Glu neurons. This is an extremely important negative feedback system as overstimulation of neurons causes cell death. GABA neurons are found in the brain while Glycine neurons are found in the spinal cord. In the brain, it seems that excitatory glutamate neurons and aspartate neurons from the frontal cortex (the primary CNS outflow area) are modulated by GABA and in the ventral spinal cord, aspartate neurons are modulated by glycine. It also seems that in regards to voluntary frontal cortex primary brain outflow, stimulation is a relative mixture of aspartate and glutamate neurons modulated by GABA that yields a graded overall type of stimulation.