Types of chemical synapse, classes of neurotransmitters, actions of neurotransmitters, signal cessation mechanisms. This video is available for instant download licensing here:
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Neurons communicate with each other mainly via chemical messages, or neurotransmitters. When a neuron is sufficiently stimulated, an electrical impulse called an action potential is generated and travels down the axon to the nerve terminal. Here, it triggers the release of a neurotransmitter into the synaptic cleft - a space between neurons. The neurotransmitter then binds to a receptor on a neighboring neuron, generating a signal in it, thereby transmitting the information to that neuron.
The neuron that releases the neurotransmitter is the pre-synaptic neuron, while the one that receives the signal is the post-synaptic neuron.
The axon of the pre-synaptic neuron may form a synapse with either a dendrite, the cell body, or the axon of the post-synaptic neuron, giving rise to an axodendritic, axosomatic, or axoaxonic synapse, respectively.
Chemical synapses exist not only between neurons, but also between a neuron and a target cell, such as muscle, or gland cell.
Over a hundred of neurotransmitters have been identified so far. Most of them can be grouped into classes according to their chemical structure. Major classes include:
- Amino acids, such as glycine, glutamate, aspartate, and GABA.
- Small peptides, called neuropeptides, such as beta-endorphin and substance P.
- Monoamines, such as epinephrine, norepinephrine, dopamine, serotonin, and histamine. Monoamines are basically amino acids with the acid group removed.
- And acetylcholine, an ester of choline, in its own class by itself.
Neurotransmitters are synthesized in the presynaptic neuron and stored in small sacs, called synaptic vesicles, at the axon terminal. Some of these vesicles are docked on plasma membrane, ready to release neurotransmitter on demand. When an action potential arrives at the nerve terminal, the resulting depolarization opens voltage-gated calcium channels, allowing calcium to flow in. Calcium causes the vesicles to fuse with plasma membrane, releasing the neurotransmitter in a process known as exocytosis.
Upon binding to their receptors on the postsynaptic cell, some neurotransmitters open ligand-gated ion channels, causing direct changes to membrane potential of the receiving neuron, while others act through second-messenger systems to exert their effect.
Some neurotransmitters are excitatory, others are inhibitory; and for some, the effect can be either excitatory or inhibitory depending on the receptor they bind to.