Neurotransmission

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. 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

 * 1) Synthesis of the neurotransmitter. This can take place in the cell body, in the axon, or in the axon terminal.
 * 2) Storage of the neurotransmitter in storage granules or vesicles in the axon terminal.
 * 3) Calcium enters the axon terminal during an action potential, causing release of the neurotransmitter into the synaptic cleft.
 * 4) After its release, the transmitter binds to and activates a receptor in the postsynaptic membrane.
 * 5) 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 is connected 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; but if the neuron gets as many inhibitory as excitatory impulses, the inhibition cancels out the excitation and the nerve impulse will stop there. Summation takes place at the axon hillock.

Spatial summation means several firings on different places of the neuron, that in themselves are not strong enough to cause a neuron to fire. However, if they fire simultaneously, their combined effects will cause an action potential.

Temporal summation means several firings at the same place, that won't cause an action potential if they have a pause in between, but when there are several firings in rapid succession, they will cause the neuron to reach the threshold for excitation.

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. 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.
 * Acetylcholine (ACh)–vasoactive intestinal peptide (VIP) co-release.
 * Acetylcholine (ACh)–calcitonin gene-related peptide (CGRP) co-release.
 * Glutamate–dynorphin co-release (in hippocampus).