Neurotransmission (Latin: transmissio "passage, crossing" from transmittere "send, let through") is the process by which signaling molecules called neurotransmitters are released by the axon terminal of a neuron (the presynaptic neuron), and bind to and react with the receptors on the dendrites of another neuron (the postsynaptic neuron) a short distance away. Changes in the concentration of ions, such as Ca2+, Na+, K+, underlie both chemical and electrical activity in the process. The increase in calcium levels is essential and can be promoted by protons. A similar process occurs in retrograde neurotransmission, where the dendrites of the postsynaptic neuron release retrograde neurotransmitters (e.g., endocannabinoids; synthesized in response to a rise in intracellular calcium levels) that signal through receptors that are located on the axon terminal of the presynaptic neuron, mainly at GABAergic and glutamatergic synapses. Neurotransmission is regulated by several different factors: the availability and rate-of-synthesis of the neurotransmitter, the release of that neurotransmitter, the baseline activity of the postsynaptic cell, the number of available postsynaptic receptors for the neurotransmitter to bind to, and the subsequent removal or deactivation of the neurotransmitter by enzymes or presynaptic reuptake. 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 synaptic cleft to be bind to receptors in the membrane of 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 complex biological neural networks through which nerve impulses (action potentials) travel. Neurons do not touch each other (except in the case of an electrical synapse through a gap junction); 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 (postsynaptic) 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" (i.e., generate an action potential), 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 (traditionally at the axon hillock), 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. Eg. acetylcholine Storage of the neurotransmitter in vesicles in the axon terminal. Before synaptic transmission there is a high concentration of Ca ions outside the axon terminals. Nerve impulse (depolarization) reaches the axon terminal. Depolarization of the presynaptic membrane causes voltage gated Ca2++ to open in the axon terminals Calcium diffuses into the axon terminal causing vesicles adjacent to axon terminals to fuse with the plasma membrane. Fusing results in release of the neurotransmitter into the synaptic cleft. After its release, the transmitter diffuses across the synpatic cleft and binds to neurotransmitter gated ion channels on the membrane of the postsynaptic dendrite (next nerve). Binding of the neurotransmitter stimulates adjacent Na voltage gates channels on the postsynaptic membrane to open and activates a receptor in the postsynaptic membrane. Na diffuses though Na voltage gated channels into postsynaptic dendrite triggering depolarization of the downstream neuron. 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.
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