Synapse

German: Synapse

	Table of contents
	1. Definition 2. Structure 3. Classification 4. According to neurotransmission 4.1. Electrical synapse 4.2. Chemical synapse 4.3. According to the kind of impulse 4.4. Stimulating synapse 4.5. Inhibiting synapse 4.6. According to site

Definition

A Synapse is a contact point between two neurons, or between a neuron and a muscle cell, which serves the transmission of stimuli. The term synapse was coined by Sir Charles Sherrington (British physiologist; 1857-1952).

Structure

A synapse consists of 3 elements:

• Presynapse or presynaptic part: The part that elicits the stimulus.
• Postsynapse or postsynaptic part: The part that receives the stimulus.
• Synaptic cleft: The space between the two structures.

Classification

Synapses are classified according to neurotransmission and kinds of stimulus:

According to neurotransmission

Electrical synapse

The electrical synapse or gap junction transmits the impulse through very close cell contact via ion channels directly from nerve cell to nerve cell. The synaptic cleft measures only about 3.5 nm. Electrical synapses work without delay. The neurotransmission is possible bidirectionally. They mainly occur where a rapid stimulus transmission is necessary (e.g. lid reflex).

Chemical synapse

In the chemical synapse, the presynapsis releases neurotransmitters from vesicles to transmit the impulse. This process, which makes necessary a complex interaction of various proteins, is also called exocytosis. The neurotransmitters diffuse through the synaptic cleft and bind to suitable receptors of the postsynapse. These receptors are either ionotropic or metabotropic. The synaptic cleft is wider than in the gap junctions and measures approx. 20-40 nm. Chemical synapses act with a slight time delay (about 1 millisecond). The neurotransmission is possible only unidirectionally.

According to the kind of impulse

Stimulating synapse

Stimulating synapses create an EPSP (excitatory postsynaptic potential) in the postsynaptic cell, which can trigger a depolarization.

Inhibiting synapse

Inhibiting synapses lead to a IPSP (inhibitory postsynaptic potential) in the postsynaptic cell, which suppresses a depolarisation.

According to site

Three types of synapses can be distinguished according to their exact contact point: ·

• Axo-somatic synapses: connection between axon and cytosome (soma)
• Axo-dendritic synapses: connection between axon and dendrite tree
• Axo-axonic synapses: connection between two axons