Neurotransmission (Latin: transmissio = passage, crossing; from transmitto = send, let through), also called synaptic transmission, is the process by which signaling molecules called neurotransmitters are released by a neuron (the presynaptic neuron), and bind to and activate the receptors of another neuron (the postsynaptic neuron). Neurotransmission usually takes place at a synapse, and occurs when an action potential is initiated in the presynaptic neuron. The binding of neurotransmitters to receptors in the postsynaptic neuron can trigger either short term changes, like changes in the membrane potential called postsynaptic potentials, or longer term changes by the activation of signaling cascades.
Nerve impulses are essential for the propagation of signals. These signals are sent to and from the central nervous system via efferent and afferent neurons in order to coordinate smooth, skeletal and cardiac muscles, bodily secretions and organ functions critical for the long-term survival of multicellular vertebrate organisms such as mammals.
Neurons form networks through which nerve impulses travel. Each neuron receives as many as 15,000 connections from other neurons. Except in the case of an electrical synapse through a gap junction, neurons do not touch each other, they have contact points called synapses. A neuron transports its information by way of a nerve impulse. When a nerve impulse arrives at the synapse, it releases neurotransmitters, which influence another cell, either in an inhibitory way or in an excitatory way. The next neuron may be connected to many more neurons, and if the total of excitatory influences is more than the inhibitory influences, it will also "fire", that is, it will create a new action potential at its axon hillock, in this way passing on the information to yet another next neuron, or resulting in an experience or an action.
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.
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.
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 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.
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.
- Acetylcholine (ACh)–vasoactive intestinal peptide (VIP) co-release.
- Acetylcholine (ACh)–calcitonin gene-related peptide (CGRP) co-release.
- Glutamate–dynorphin co-release (in hippocampus).
- Neuromuscular transmission
- Serpentine receptor