Neurons, when stimulated, release neurotransmitters where they meet muscles or other neurons.
Possible effects, either stimulation or inhibition,
occur by changing membrane characteristics.
Neurons, when stimulated, release chemicals called neurotransmitters where they meet other neurons (at
synapses) or where they meet muscles. Some neurotransmitters
stimulate postsynaptic cells by depolarizing membranes; others
inhibit cells by increasing membrane polarity.
Most neurotransmitters accumulate in membrane-bounded vesicles in
the presynaptic neuron. They are released by exocytosis.
Some neurotransmitters (dopamine, serotonin) are recycled: the cell that
secreted them resorbs them (re-uptake) and reuses them.
- Acetylcholine (AC or ACh): the most common neurotransmitter,
secreted along with an enzyme (cholinesterase) that degrades it.
It occurs at all synapses between neurons in the peripheral nervous system and is
stimulatory. It also stimulates muscle contraction and slows the heartbeat.
- Norepinephrine and epinephrine: released in the brain and by
certain nerves (sympathetic nerves). They are stimulatory,
quickening the heartbeat and increasing the blood supply to most muscles.
- Dopamine: generally stimulatory, released primarily in the brain.
- Serotonin (5-HydroxyTryptamine, 5HT): stimulatory, found only
in the brain. More of it is made, released, and recycled during dreams.
- Gamma-Amino Butyric Acid (GABA): inhibitory.
- Glutamate and glycine: amino acids which may also be neurotransmitters.
- Endorphins and enkephalins: natural painkillers in the brain that
may also function as neurotransmitters.
- Nitric oxide (NO): a dissolved gas that stimulates smooth
muscle contraction.
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