Chemical synapse


Chemical synapses are biological junctions through which neurons' signals can be sent to each other and to non-neuronal cells such as those in muscles or glands. Chemical synapses allow neurons to form circuits within the central nervous system. They are crucial to the biological computations that underlie perception and thought. They allow the nervous system to connect to and control other systems of the body.
At a chemical synapse, one neuron releases neurotransmitter molecules into a small space that is adjacent to the postsynaptic cell. The neurotransmitter molecules are contained within small sacs called synaptic vesicles, and are released into the synaptic cleft by exocytosis. These molecules then bind to neurotransmitter receptors on the postsynaptic cell. Finally, to terminate its action, the neurotransmitter is cleared from the cleft through one of several mechanisms, including enzymatic degradation or re-uptake, by specific transporters, either into the presynaptic cell or to neuroglia.
The adult human brain is estimated to contain from 1014 to 5 × 1014 synapses. Every cubic millimeter of cerebral cortex contains roughly a billion of them. The number of synapses in the human cerebral cortex has separately been estimated at 0.15 quadrillion
The word "synapse" was introduced by Sir Charles Scott Sherrington in 1897. Chemical synapses are not the only type of biological synapse: electrical and immunological synapses also exist. Without a qualifier, however, "synapse" commonly refers to chemical synapses.

Structure

Synapses are functional connections between neurons, or between neurons and other types of cells. A typical neuron gives rise to several thousand synapses, although there are some types that make far fewer. Most synapses connect axons to dendrites, but there are also other types of connections, including axon-to-cell-body, axon-to-axon, and dendrite-to-dendrite. Synapses are generally too small to be recognizable using a light microscope except as points where the membranes of two cells appear to touch, but their cellular elements can be visualized clearly using an electron microscope.
Chemical synapses pass information directionally from a presynaptic cell to a postsynaptic cell and are therefore asymmetric in structure and function. The presynaptic axon terminal, or synaptic bouton, is a specialized area within the axon of the presynaptic cell that contains neurotransmitters enclosed in small membrane-bound spheres called synaptic vesicles. Some synaptic vesicles are docked at the presynaptic plasma membrane at regions called active zones.
Immediately opposite is a region of the postsynaptic cell containing neurotransmitter receptors; for synapses between two neurons the postsynaptic region may be found on the dendrites or cell body. Immediately behind the postsynaptic membrane is an elaborate complex of interlinked proteins called the postsynaptic density.
Proteins in the PSD are involved in anchoring and trafficking neurotransmitter receptors and modulating the activity of these receptors. The receptors and PSDs are often found in specialized protrusions from the main dendritic shaft called dendritic spines.
Synapses may be described as symmetric or asymmetric. When examined under an electron microscope, asymmetric synapses are characterized by rounded vesicles in the presynaptic cell, and a prominent postsynaptic density. Asymmetric synapses are typically excitatory. Symmetric synapses in contrast have flattened or elongated vesicles, and do not contain a prominent postsynaptic density. Symmetric synapses are typically inhibitory.
The synaptic cleft—also called synaptic gap—is a gap between the pre- and postsynaptic cells that is about 20 nm wide. The small volume of the cleft allows neurotransmitter concentration to be raised and lowered rapidly.
An autapse is a chemical synapse formed when the axon of one neuron synapses with its own dendrites.

Signaling in chemical synapses

Overview

Some authors consider signal transmission at a chemical synapse as a special case of paracrine signaling., while others treat it a separate signaling mechanism.
Here is a summary of the sequence of events that take place in synaptic transmission from a presynaptic neuron to a postsynaptic cell. Each step is explained in more detail below. Note that with the exception of the final step, the entire process may run only a few hundred microseconds, in the fastest synapses.
  1. The process begins with a wave of electrochemical excitation called an action potential traveling along the membrane of the presynaptic cell, until it reaches the synapse.
  2. The electrical depolarization of the membrane at the synapse causes channels to open that are permeable to calcium ions.
  3. Calcium ions flow through the presynaptic membrane, rapidly increasing the calcium concentration in the interior.
  4. The high calcium concentration activates a set of calcium-sensitive proteins attached to vesicles that contain a neurotransmitter chemical.
  5. These proteins change shape, causing the membranes of some "docked" vesicles to fuse with the membrane of the presynaptic cell, thereby opening the vesicles and dumping their neurotransmitter contents into the synaptic cleft, the narrow space between the membranes of the pre- and postsynaptic cells.
  6. The neurotransmitter diffuses within the cleft. Some of it escapes, but some of it binds to chemical receptor molecules located on the membrane of the postsynaptic cell.
  7. The binding of neurotransmitter causes the receptor molecule to be activated in some way. Several types of activation are possible, as described in more detail below. In any case, this is the key step by which the synaptic process affects the behavior of the postsynaptic cell.
  8. Due to thermal vibration, the motion of atoms, vibrating about their equilibrium positions in a crystalline solid, neurotransmitter molecules eventually break loose from the receptors and drift away.
  9. The neurotransmitter is either reabsorbed by the presynaptic cell, and then repackaged for future release, or else it is broken down metabolically.

    Neurotransmitter release

The release of a neurotransmitter is triggered by the arrival of a nerve impulse and occurs through an unusually rapid process of cellular secretion. Within the presynaptic nerve terminal, vesicles containing neurotransmitter are localized near the synaptic membrane. The arriving action potential produces an influx of calcium ions through voltage-dependent, calcium-selective ion channels at the down stroke of the action potential. Calcium ions then bind to synaptotagmin proteins found within the membranes of the synaptic vesicles, allowing the vesicles to fuse with the presynaptic membrane. The fusion of a vesicle is a stochastic process, leading to frequent failure of synaptic transmission at the very small synapses that are typical for the central nervous system. Large chemical synapses, on the other hand, have a synaptic release probability, in effect, of 1. Vesicle fusion is driven by the action of a set of proteins in the presynaptic terminal known as SNAREs. As a whole, the protein complex or structure that mediates the docking and fusion of presynaptic vesicles is called the active zone. The membrane added by the fusion process is later retrieved by endocytosis and recycled for the formation of fresh neurotransmitter-filled vesicles.
An exception to the general trend of neurotransmitter release by vesicular fusion is found in the type II receptor cells of mammalian taste buds. Here the neurotransmitter ATP is released directly from the cytoplasm into the synaptic cleft via voltage gated channels.

Receptor binding

Receptors on the opposite side of the synaptic gap bind neurotransmitter molecules. Receptors can respond in either of two general ways. First, the receptors may directly open ligand-gated ion channels in the postsynaptic cell membrane, causing ions to enter or exit the cell and changing the local transmembrane potential. The resulting change in voltage is called a postsynaptic potential. In general, the result is excitatory in the case of depolarizing currents, and inhibitory in the case of hyperpolarizing currents. Whether a synapse is excitatory or inhibitory depends on what type of ion channel conduct the postsynaptic current, which in turn is a function of the type of receptors and neurotransmitter employed at the synapse. The second way a receptor can affect membrane potential is by modulating the production of chemical messengers inside the postsynaptic neuron. These second messengers can then amplify the inhibitory or excitatory response to neurotransmitters.

Termination

After a neurotransmitter molecule binds to a receptor molecule, it must be removed to allow for the postsynaptic membrane to continue to relay subsequent EPSPs and/or IPSPs. This removal can happen through one or more processes:
  • The neurotransmitter may diffuse away due to thermally-induced oscillations of both it and the receptor, making it available to be broken down metabolically outside the neuron or to be reabsorbed.
  • Enzymes within the subsynaptic membrane may inactivate/metabolize the neurotransmitter.
  • Reuptake pumps may actively pump the neurotransmitter back into the presynaptic axon terminal for reprocessing and re-release following a later action potential.

    Synaptic strength

The strength of a synapse has been defined by Bernard Katz as the product of release probability pr, quantal size q, and n, the number of release sites. "Unitary connection" usually refers to an unknown number of individual synapses connecting a presynaptic neuron to a postsynaptic neuron.
The amplitude of postsynaptic potentials can be as low as 0.4 mV to as high as 20 mV. The amplitude of a PSP can be modulated by neuromodulators or can change as a result of previous activity. Changes in the synaptic strength can be short-term, lasting seconds to minutes, or long-term, lasting hours. Learning and memory are believed to result from long-term changes in synaptic strength, via a mechanism known as synaptic plasticity.