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.
An example of excitatory neurotransmitter is glutamate. Upon binding, it triggers glutamate
receptors, ligand-gated ion channels, to open and allow positively-charged ions into the cell,
making it more positive, less polarized, and thus more likely to generate action potentials.
On the other hand, GABA, a major inhibitory neurotransmitter,
opens ligand-gated chloride channels to allow negatively-charged chloride to enter,
making the cell more negative, more polarized, and thus less likely to generate action potentials.
Acetylcholine is a neurotransmitter that can be either excitatory or inhibitory depending
on the receptor present on the target cell. At neuromuscular junctions, acetylcholine
released by motor neurons binds to nicotinic receptors on skeletal muscle cells and stimulates
them to contract. On the other hand, it inhibits cardiac muscle cells via muscarinic receptor M2,
causing heart rate to slow down, as part of the parasympathetic “rest and digest” response.
A neurotransmitter binds to its receptor for only a millisecond or so.
It then passively diffuses from the synapse and is taken up by nearby astrocytes for recycling.
If the presynaptic neuron continues to fire and release more neurotransmitter,
new molecules will bind and again activate the receiving neuron.
If the presynaptic signal stops coming, the transmission will eventually stop.
There also exist mechanisms to actively remove neurotransmitters from the synaptic space to avoid
overstimulation. Common mechanisms include: - degradation of the neurotransmitter
by an enzyme present in the synapse; - and reuptake, where the neurotransmitter
is transported back, by a transporter protein, to the presynaptic neuron for reuse.