Neurotransmission, also called synaptic transmission, is the process by which
signaling molecules called neurotransmitters are released by a
neuron, and bind to and activate the receptors of another neuron.
Neurotransmission is essential for the process of communication between two
neurons. Synaptic transmission relies on: the availability of the
neurotransmitter; the release of the neurotransmitter by exocytosis; the
binding of the postsynaptic receptor by the neurotransmitter; the functional
response of the postsynaptic cell; and the subsequent removal or deactivation
of the neurotransmitter. In response to a threshold action
potential or graded electrical potential, a neurotransmitter is
released at the presynaptic terminal. The released neurotransmitter may then
move across the synapse to be detected by and bind with receptors in the
postsynaptic neuron. Binding of neurotransmitters may influence the
postsynaptic neuron in either an inhibitory or excitatory way. The
binding of neurotransmitters to receptors in the postsynaptic neuron can
trigger either short term changes, such as changes in the membrane potential
called postsynaptic potentials, or longer term changes by the activation of
signaling cascades. Neurons form elaborate networks through
which nerve impulses travel. Each neuron has as many as 15,000 connections with
other neurons. Neurons do not touch each other; instead, neurons interact at
close contact points called synapses. A neuron transports its information by way
of an action potential. When the nerve impulse arrives at the synapse, it may
cause the release of neurotransmitters, which influence another neuron. The
postsynaptic neuron may receive inputs from many additional neurons, both
excitatory and inhibitory. The excitatory and inhibitory influences are
summed, and if the net effect is inhibitory, the neuron will be less
likely to "fire", and if the net effect is excitatory, the neuron will be more
likely to fire. How likely a neuron is to fire depends on how far its membrane
potential is from the threshold potential, the voltage at which an
action potential is triggered because enough voltage-dependent sodium channels
are activated so that the net inward sodium current exceeds all outward
currents. Excitatory inputs bring a neuron closer to threshold, while
inhibitory inputs bring the neuron farther from threshold. An action
potential is an "all-or-none" event; neurons whose membranes have not reached
threshold will not fire, while those that do must fire. Once the action
potential is initiated, it will propagate along the axon, leading to
release of neurotransmitters at the synaptic bouton to pass along
information to yet another adjacent neuron.
Stages in neurotransmission at the synapse
Synthesis of the neurotransmitter. This can take place in the cell body, in the
axon, or in the axon terminal. Storage of the neurotransmitter in
storage granules or vesicles in the axon terminal.
Calcium enters the axon terminal during an action potential, causing release of
the neurotransmitter into the synaptic cleft.
After its release, the transmitter binds to and activates a receptor in the
postsynaptic membrane. Deactivation of the neurotransmitter.
The neurotransmitter is either destroyed enzymatically, or taken back into the
terminal from which it came, where it can be reused, or degraded and removed.
Summation Each neuron connects with numerous other
neurons, receiving numerous impulses from them. Summation is the adding
together of these impulses at the axon hillock. If the neuron only gets
excitatory impulses, it will also generate an action potential. If instead
the neuron gets as many inhibitory as excitatory impulses, the inhibition
cancels out the excitation and the nerve impulse will stop there. Action
potential generation is proportionate to the probability and pattern of
neurotransmitter release, and to postsynaptic receptor sensitization.
Spatial summation means that the effects of impulses received at different places
on the neuron add up, so that the neuron may fire when such impulses are received
simultaneously, even if each impulse on its own would not be sufficient to cause
firing. Temporal summation means that the
effects of impulses received at the same place can add up if the impulses are
received in close temporal succession. Thus the neuron may fire when multiple
impulses are received, even if each impulse on its own would not be
sufficient to cause firing. Convergence and divergence
Neurotransmission implies both a convergence and a divergence of
information. First one neuron is influenced by many others, resulting in
a convergence of input. When the neuron fires, the signal is sent to many other
neurons, resulting in a divergence of output. Many other neurons are
influenced by this neuron. Cotransmission
Cotransmission is the release of several types of neurotransmitters from a single
nerve terminal. At the nerve terminal, neurotransmitters
are present within 35–50 nm membrane-encased vesicles called
synaptic vesicles. To release neurotransmitters, the synaptic vesicles
transiently dock and fuse at the base of specialized 10–15 nm cup-shaped
lipoprotein structures at the presynaptic membrane called porosomes.
The neuronal porosome proteome has been solved, providing the molecular
architecture and the complete composition of the machinery.
Recent studies in a myriad of systems have shown that most, if not all,
neurons release several different chemical messengers. Cotransmission
allows for more complex effects at postsynaptic receptors, and thus allows
for more complex communication to occur between neurons.
In modern neuroscience, neurons are often classified by their cotransmitter.
For example, striatal "GABAergic neurons" utilize opioid peptides or
substance P as their primary cotransmitter.
Some neurons can release at least two neurotransmitters at the same time, the
other being a cotransmitter, in order to provide the stabilizing negative
feedback required for meaningful encoding, in the absence of inhibitory
interneurons. Examples include: GABA–glycine co-release.
Dopamine–glutamate co-release. Acetylcholine–glutamate co-release.
ACh–vasoactive intestinal peptide co-release.
ACh–calcitonin gene-related peptide co-release.
Glutamate–dynorphin co-release. See also
Autoreceptor G protein-coupled receptor
Neuromuscular transmission Molecular neuropharmacology
Neuropsychopharmacology Neurotransmitter
Biological neuron model#Synaptic transmission
References External links
Historical evolution of the neurotransmission concept