Neurotransmitters include acetylcholine, GABA, glycine, and glutamate. Neuromodulators include norepinephrine, epinephrine, serotonin, dopamine, TRH, ACTH, vasopressin, LHRH, CCK, VIP, substance P, and enkephalins.
Acetylcholine is an excitatory neurotransmitter, but it can also have inhibitory effects. Acetylcholine is found at numerous synapses within the nervous system and is an important chemical messenger for motor neurons in the peripheral nervous system, including the autonomic nervous system. Within the central nervous system, it excites skeletal muscles and inhibits cardiac muscle. GABA is secreted by nerve terminals in the spinal cord, the cerebellum, basal ganglia, and many areas of the cortex. Glycine is secreted mainly at synapses in the spinal cord. GABA and glycine are the primary inhibitory neurotransmitters. Glutamate is secreted by the presynaptic terminals in many of the sensory pathways as well as in many areas of the cortex. Glutamate is a major excitatory neurotransmitter.
Norepinephrine is secreted by many neurons in the brain stem and hypothalamus as well as by most of the postganglionic neurons of the sympathetic nervous system. In many areas of the brain, it causes excitation. Norepinephrine secreted by postganglionic neurons of the sympathetic nervous system excites some organs but inhibits others. The majority of the secretion of epinephrine is by sympathetic stimulation of the adrenal medulla. It causes vasoconstriction of all blood vessels of the body, increases cardiac activity, dilates pupils, etc.
Dopamine is secreted by the neurons that originate in the substantia nigra. It is usually an inhibitor. Serotonin is secreted by the brain stem. It is an inhibitor of pain pathways in the spinal cord, controls mood, and causes sleep. TRH (thyroid-stimulating hormone) is secreted by the anterior pituitary gland. It increases the secretion of thyroxine and triiodothyronine by the thyroid gland. VIP (vasoactive intestinal polypeptide) is released by the mucosa of the intestinal tract. VIP modulates cholinergic transmission and causes constriction of blood vessels in the intestine. ACTH (adrenocorticotropic hormone) is secreted by the anterior pituitary gland.
It causes the release of steroid hormones from the adrenal glands. CCK (cholecystokinin) is secreted by the small intestine and initiates the release of bile from the gall bladder. LHRH (luteinizing hormone-releasing hormone) is produced by the hypothalamus and causes the release of luteinizing hormone and follicle-stimulating hormone. Substance P is probably released by pain fiber terminals in the spinal cord. It causes excitation. Enkephalins may be secreted by nerve terminals in the spinal cord, brain stem, thalamus, and hypothalamus. They are excitatory transmitters.