<P> As explained above, the only direct action of a neurotransmitter is to activate a receptor . Therefore, the effects of a neurotransmitter system depend on the connections of the neurons that use the transmitter, and the chemical properties of the receptors that the transmitter binds to . </P> <P> Here are a few examples of important neurotransmitter actions: </P> <Ul> <Li> Glutamate is used at the great majority of fast excitatory synapses in the brain and spinal cord . It is also used at most synapses that are "modifiable", i.e. capable of increasing or decreasing in strength . Modifiable synapses are thought to be the main memory - storage elements in the brain . Excessive glutamate release can overstimulate the brain and lead to excitotoxicity causing cell death resulting in seizures or strokes . Excitotoxicity has been implicated in certain chronic diseases including ischemic stroke, epilepsy, amyotrophic lateral sclerosis, Alzheimer's disease, Huntington disease, and Parkinson's disease . </Li> <Li> GABA is used at the great majority of fast inhibitory synapses in virtually every part of the brain . Many sedative / tranquilizing drugs act by enhancing the effects of GABA . Correspondingly, glycine is the inhibitory transmitter in the spinal cord . </Li> <Li> Acetylcholine was the first neurotransmitter discovered in the peripheral and central nervous systems . It activates skeletal muscles in the somatic nervous system and may either excite or inhibit internal organs in the autonomic system . It is distinguished as the transmitter at the neuromuscular junction connecting motor nerves to muscles . The paralytic arrow - poison curare acts by blocking transmission at these synapses . Acetylcholine also operates in many regions of the brain, but using different types of receptors, including nicotinic and muscarinic receptors . </Li> <Li> Dopamine has a number of important functions in the brain; this includes regulation of motor behavior, pleasures related to motivation and also emotional arousal . It plays a critical role in the reward system; Parkinson's disease has been linked to low levels of dopamine and schizophrenia has been linked to high levels of dopamine . </Li> <Li> Serotonin is a monoamine neurotransmitter . Most is produced by and found in the intestine (approximately 90%), and the remainder in central nervous system neurons . It functions to regulate appetite, sleep, memory and learning, temperature, mood, behaviour, muscle contraction, and function of the cardiovascular system and endocrine system . It is speculated to have a role in depression, as some depressed patients are seen to have lower concentrations of metabolites of serotonin in their cerebrospinal fluid and brain tissue . </Li> <Li> Norepinephrine which focuses on the central nervous system, based on sleep patterns, focus and alertness . It is synthesized from tyrosine . </Li> <Li> Epinephrine which is also synthesized from tyrosine takes part in controlling the adrenal glands . It plays a role in sleep, with ones ability to become and stay alert, and the fight - or - flight response . </Li> <Li> Histamine works with the central nervous system (CNS), specifically the hypothalamus (tuberomammillary nucleus) and CNS mast cells . </Li> </Ul> <Li> Glutamate is used at the great majority of fast excitatory synapses in the brain and spinal cord . It is also used at most synapses that are "modifiable", i.e. capable of increasing or decreasing in strength . Modifiable synapses are thought to be the main memory - storage elements in the brain . Excessive glutamate release can overstimulate the brain and lead to excitotoxicity causing cell death resulting in seizures or strokes . Excitotoxicity has been implicated in certain chronic diseases including ischemic stroke, epilepsy, amyotrophic lateral sclerosis, Alzheimer's disease, Huntington disease, and Parkinson's disease . </Li>

What is the most abundant neurotransmitter in the nervous system