Corpus: Synapse

A synapse is a contact structure for the transmission of a chemical or electrical signal from one nerve cell to another nerve cell (interneuronal synapses) or another target cell (muscle cell, gland cell, sensory cell). The term synapse goes back to Sir Charles Sherrington (English physiologist, 1857-1952).

In a broader sense, the term is also applied to cells of the immune system (immunological synapse).

A synapse consists of 3 elements:

• Presynapse or presynaptic part: the part from which the signal originates
• Postsynapse or postsynaptic part: The part that receives the signal
• Synaptic cleft: The space between the two structures

Synapses are categorised according to the form of excitation transmission and the type of excitation:

...by type of excitation transmission[Bearbeiten]

Electrical synapse[Bearbeiten]

The electrical synapse (gap junction) transmits the excitation directly from nerve cell to nerve cell through very close cell contact via ion channels. The synaptic gap measures only around 3.5 nm. Electrical synapses work without delay. Excitation can be transmitted in both directions ("bidirectional"). They are mainly found where rapid stimulus transmission is required (e.g. eyelid reflex).

Chemical synapse[Bearbeiten]

In the chemical synapse, the presynapse releases neurotransmitters from vesicles in order to transmit the signal, e.g. an action potential. This process, which requires a complex interplay of different proteins, is also known as exocytosis. The neurotransmitters diffuse through the synaptic cleft and dock onto suitable receptors in the postsynapse. These receptors can be ionotropic or metabotropic. The synaptic cleft is wider than the gap junctions and measures around 20 to 30 nm. Chemical synapses work with a short time delay (about 1 millisecond). Excitation can only be transmitted in one direction ("unidirectional").

Examples of chemical synapses are

...by type of excitation[Bearbeiten]

Excitatory synapse[Bearbeiten]

Excitatory synapses or excitatory synapses form an EPSP (excitatory postsynaptic potential) in the postsynaptic cell, which can trigger depolarisation.

Inhibitory synapse[Bearbeiten]

Inhibitory synapses or inhibitory synapses lead to an IPSP (inhibitory postsynaptic potential) in the postsynaptic cell, which suppresses depolarisation.

...by organ of success[Bearbeiten]

• interneural synapse (interneuronal synapse): Synapse between two nerve cells
• neuromuscular synapse (= motor end plate): Synapse with a muscle cell
• neuroglandular synapse: synapse with a gland cell
• neurosensory synapse: synapse with a secondary sensory cell

...according to localisation[Bearbeiten]

In the case of interneural synapses, a distinction is made between different types of synapses according to their exact point of contact:

• axo-somatic synapses: Connection between axon and cell body (soma)
• axo-dendritic synapses: Connection between axon and dendrite tree
• axo-axonic or axo-axonal synapses: Connection between two axons
• dendro-dendritic synapses: Connection between dendrites of different neurones
• dendro-somatic synapses: Connection between dendrites of one neuron and the body of another
• somato-somatic synapses: Connection between the cell bodies of one neuron and an immediately neighbouring neuron.
• somato-dendritic synapses: Connection between the neuronal cell body and the dendrites of another neuron.
• somato-axonal synapses: Connection between a nerve cell body and the axon of another nerve cell.

...according to other characteristics[Bearbeiten]

• Synapse en passant
• Synapse par distance
• Shaft synapses
• Spine synapses

As part of neuronal plasticity, new synapses are continuously formed in the nervous system. This process is known as synaptogenesis. The reverse process, namely the breakdown of unused synapses, is called synapse elimination.

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