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Chemical synapses are biological junctions through which ' signals can be sent to each other and to non-neuronal cells such as those in muscles or . 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 into a small space (the synaptic cleft) that is adjacent to another neuron. The neurotransmitters are contained within small sacs called , and are released into the synaptic cleft by exocytosis. These molecules then bind to neurotransmitter receptors on the postsynaptic cell. Finally, the neurotransmitters are cleared from the synapse through one of several Action potential including enzymatic degradation or Reuptake by specific transporters either on the presynaptic cell or on some other neuroglia to terminate the action of the neurotransmitter.
The adult human brain is estimated to contain from 1014 to 5 × 1014 (100–500 trillion) synapses. Every cubic millimeter of cerebral cortex contains roughly a billion (short scale, i.e. 109) of them. The number of synapses in the human cerebral cortex has separately been estimated at 0.15 quadrillion (150 trillion) Brain Facts and Figures Washington University.
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.
A typical neuron gives rise to several thousand synapses, although there are some types that make far fewer.
Most synapses connect to ,
but there are also other types of connections, including axon-to-cell-body, axon-to-axon,
Axons terminating on the postsynaptic cell body are axosomatic synapses. Axons that terminate on axons are axoaxonic synapses
and dendrite-to-dendrite. Axons connecting dendrite to dendrite are dendrodendritic synapses. Axons which connect axon to dendrite are called axodendritic synapses
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 enclosed in small membrane-bound spheres called (as well as a number of other supporting structures and organelles, such as mitochondria and endoplasmic reticulum). Synaptic vesicles are docked at the presynaptic plasma membrane at regions called .
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 (PSD).
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 .
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 (0.02 μ) wide. (2025). 9780128154809, Elsevier. ISBN 9780128154809 The small volume of the cleft allows neurotransmitter concentration to be raised and lowered rapidly.
An autapse is a chemical (or electrical) synapse formed when the axon of one neuron synapses with its own dendrites.
An exception to the general trend of neurotransmitter release by vesicular fusion is found in the type II receptor cells of mammalian . Here the neurotransmitter ATP is released directly from the cytoplasm into the synaptic cleft via voltage gated channels.
One example is neurons of the sympathetic nervous system (SNS), which release noradrenaline, which, besides affecting postsynaptic receptors, also affects presynaptic α2-adrenergic receptors, inhibiting further release of noradrenaline. (2025). 9780443071454, Churchill Livingstone. ISBN 9780443071454 This effect is utilized with clonidine to perform inhibitory effects on the SNS.
One example is again neurons of the sympathetic nervous system, which release noradrenaline, which, in addition, generates an inhibitory effect on presynaptic terminals of neurons of the parasympathetic nervous system.
On the other hand, a presynaptic neuron releasing an inhibitory neurotransmitter, such as GABA, can cause an inhibitory postsynaptic potential (IPSP) in the postsynaptic neuron, bringing the membrane potential farther away from the threshold, decreasing its excitability and making it more difficult for the neuron to initiate an action potential. If an IPSP overlaps with an EPSP, the IPSP can in many cases prevent the neuron from firing an action potential. In this way, the output of a neuron may depend on the input of many different neurons, each of which may have a different degree of influence, depending on the strength and type of synapse with that neuron. John Carew Eccles performed some of the important early experiments on synaptic integration, for which he received the Nobel Prize for Physiology or Medicine in 1963.
Recent work indicates that volume transmission may be the predominant mode of interaction for some special types of neurons. In the mammalian cerebral cortex, a class of neurons called neurogliaform cells can inhibit other nearby cortical neurons by releasing the neurotransmitter GABA into the extracellular space. Along the same vein, GABA released from neurogliaform cells into the extracellular space also acts on surrounding , assigning a role for volume transmission in the control of ionic and neurotransmitter homeostasis. Approximately 78% of neurogliaform cell boutons do not form classical synapses. This may be the first definitive example of neurons communicating chemically where classical synapses are not present.
Sir Charles Scott Sherringtonin coined the word 'synapse' and the history of the word was given by Sherrington in a letter he wrote to John Fulton:
Integration of synaptic inputs
In general, if an excitatory synapse is strong enough, an action potential in the presynaptic neuron will trigger an action potential in the postsynaptic cell. In many cases the excitatory postsynaptic potential (EPSP) will not reach the action potential for eliciting an action potential. When action potentials from multiple presynaptic neurons fire simultaneously, or if a single presynaptic neuron fires at a high enough frequency, the EPSPs can overlap and summate. If enough EPSPs overlap, the summated EPSP can reach the threshold for initiating an action potential. This process is known as summation, and can serve as a high pass filter for neurons.
Volume transmission
When a neurotransmitter is released at a synapse, it reaches its highest concentration inside the narrow space of the synaptic cleft, but some of it is certain to diffuse away before being reabsorbed or broken down. If it diffuses away, it has the potential to activate receptors that are located either at other synapses or on the membrane away from any synapse. The extrasynaptic activity of a neurotransmitter is known as volume transmission. It is well established that such effects occur to some degree, but their functional importance has long been a matter of controversy.
Relationship to electrical synapses
An electrical synapse is an electrically conductive link between two abutting that is formed at a narrow gap between the pre- and postsynaptic cells, known as a gap junction. At gap junctions, cells approach within about 3.5 Nanometre of each other, rather than the 20 to 40 nm distance that separates cells at chemical synapses. As opposed to chemical synapses, the postsynaptic potential in electrical synapses is not caused by the opening of ion channels by chemical transmitters, but rather by direct electrical coupling between both neurons. Electrical synapses are faster than chemical synapses. Electrical synapses are found throughout the nervous system, including in the retina, the reticular nucleus of the thalamus, the neocortex, and in the hippocampus. While chemical synapses are found between both excitatory and inhibitory neurons, electrical synapses are most commonly found between smaller local inhibitory neurons. Electrical synapses can exist between two axons, two dendrites, or between an axon and a dendrite.
In some fish and , electrical synapses can be found within the same terminal of a chemical synapse, as in Mauthner cells.
Effects of drugs
One of the most important features of chemical synapses is that they are the site of action for the majority of psychoactive drugs. Synapses are affected by drugs, such as curare, strychnine, cocaine, morphine, alcohol, LSD, risperidone, and countless others. These drugs have different effects on synaptic function, and often are restricted to synapses that use a specific neurotransmitter. For example, curare is a poison that stops acetylcholine from depolarizing the postsynaptic membrane, causing paralysis. Strychnine blocks the inhibitory effects of the neurotransmitter glycine, which causes the body to pick up and react to weaker and previously ignored stimuli, resulting in uncontrollable muscle spasms. Morphine acts on synapses that use endorphin neurotransmitters, and alcohol increases the inhibitory effects of the neurotransmitter GABA. LSD interferes with synapses that use the neurotransmitter serotonin. Risperidone is a blocker of various receptors including several dopamine and serotonin receptors, and it can bind with high affinity to some types of serotonin receptors. Cocaine blocks reuptake of dopamine and therefore increases its effects.
History and etymology
During the 1950s, Bernard Katz and Paul Fatt observed spontaneous miniature synaptic currents at the frog neuromuscular junction. Based on these observations, they developed the 'quantal hypothesis' that is the basis for our current understanding of neurotransmitter release as exocytosis and for which Katz received the Nobel Prize in Physiology or Medicine in 1970. In the late 1960s, Ricardo Miledi and Katz advanced the hypothesis that depolarization-induced influx of calcium ions triggers exocytosis.
See also
Notes
(2025). 9780838577011, McGraw-Hill. ISBN 9780838577011
(2025). 9780781739443, Lippincott Williams & Wilkins. ISBN 9780781739443
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