Synapses
definition
A synapse is the contact point between two nerve cells. It enables the transmission of stimuli from one neuron to another. A synapse can also exist between neuron and muscle cell or sensory cell and gland. There are two fundamentally different types of synapses, the electrical (gap junction) and the chemical. These each use a different type of excitation transmission. The chemical synapses can also be subdivided according to the messenger substances (neurotransmitters). These are used for transmission.
The synapses can also be subdivided according to the type of excitation. There is an exciting and an inhibiting synapse. Internural synapses (between two neurons) can also be subdivided according to localization, i.e. at which point on the neuron the synapse is attached. There are 100 trillion synapses in the brain alone. You can constantly build up and break down, this principle is called neural plasticity.
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Illustration of a nerve cell
Nerve cell -
Neuron
- Dendrites
- Synapse
(axodendritic) - Nucleus -
Nucleolus - Cell bodies -
Nucleus - Axon mounds
- Myelin sheath
- Ranvier lace-up
- Swan cells
- Axon terminals
- Synapse
(axoaxonal)
A - multipolar neuron
B - pseudounipolar neuron
C - bipolar neuron
a - Soma
b - axon
c - synapses
You can find an overview of all Dr-Gumpert images at: medical illustrations
Structure, function and tasks
The electrical synapse (gap junction) works instantly across a very small gap called a synaptic gap. With the help of ion channels, this enables the transmission of stimuli directly from nerve cell to nerve cell. This type of synapse is found in smooth muscle cells, heart muscle cells and in the retina. They are suitable for quick forwarding, such as for the eyelid reflex. Forwarding is possible in both directions (bidirectional).
The chemical synapse consists of a presynapse, a synaptic cleft and a postsynapse. The presynapse is usually the end button of a neuron. The postsynapse is a point on the dendrite of the adjacent neuron or a dedicated section of the adjacent muscle cell or gland. The synaptic gap is used to transmit excitations with the help of neurotransmitters. The previous electrical signal is converted into a chemical signal and then back into an electrical signal. This type of forwarding is only possible in one direction (unidirectional).
The electrical action potential is conducted to the presynapse via the axon of the neuron. In the presynaptic membrane, voltage-controlled Ca channels are opened by the action potential. There are small vesicles in the presynapse (Vesicle)that are filled with the transmitters. The increased calcium concentration causes the vesicles to fuse with the presynaptic membrane and the neurotransmitters to be released into the synaptic cleft. This type of transport is called exocytosis. The higher the action potential frequency, the more vesicles release their stored neurotransmitters. The neurotransmitters then diffuse through the synaptic gap, which is approx. 30 nm wide, and dock on neurotransmitter receptors. These are located on the postsynaptic membrane. These are channels that either ionotropic or metabotropic are.If the postsynapse is a motor end plate, it is an ionotropic channel that connects two molecules of the messenger substance (Acetylcholine) dock and open it like this. This allows cations to flow in (mainly sodium). This polarizes the postsynapse and creates an excitatory postsynaptic potential (EPSP). It takes several EPSPs to turn it into an action potential again. The EPSPs are summed up in terms of time and space and a postsynaptic action potential then arises on the so-called axon hill. This action potential can then be passed on via the axon of this nerve cell and the whole process starts all over again at the next synapse. This is the action of an exciting synapse.
An inhibiting synapse, on the other hand, is hyperpolarized and inspiratory postsynaptic potentials (IPSPs) arise. Inhibitory neurotransmitters such as glycine or GABA are used.
The transmission of information via chemical synapses takes a little longer due to the release of the neurotransmitter and its diffusion.
Incidentally, the neurotransmitters are recycled. They return from the synaptic cleft to the presynapse and are packaged again in vesicles. In the case of the transmitter substance acetylcholine, the enzyme cholinesterase plays an important role. It splits the neurotransmitter into choline and acetic acid (acetate). Thus the acetylcholine is inactive.
There are other ways to turn off synaptic transmission. For example, the cation channels of the postsynapse can be inactivated.
You might also be interested in: Nerve fiber
Synaptic cleft
The synaptic cleft is part of the synapse and names the area between two consecutive nerve cells. This is where the signal is passed on with the help of action potentials. Is the synapse a motor end plate, i.e. the transition between nerve. and muscle cell the same term is used.
As can already be seen from the word "gap", there is a space between the cells, so there is no direct contact. The presynapse is located on one side of the synaptic cleft. This is where the electrical signal from the upstream nerve cell arrives. It leads to the release of neurotransmitters from the vesicles, i.e. it is converted into a chemical signal. These then migrate through the synaptic gap and reach the postsynaptic membrane of the downstream cell. This is where the other side of the synaptic gap is located. The signal is again converted into an electrical one by receptors in the membrane and thus reaches the second nerve cell. The excitement was thus passed on.
The neurotransmitters are, for example, acetylcholine, serotonin or dopamine.
You might also be interested in: Acetylcholine, serotonin, dopamine
Synapse poisons - botox
Typical synapse toxins are curare, botulinum toxin, tetanus toxin, atropine, insecticide parathion E605, sarin and alpha-lactrotoxin.
A synapse is a perfectly coordinated complex system. It is precisely because of this that it is relatively susceptible to interference with certain substances. These so-called synapse toxins are also called neurotoxins. They occur, for example, in the animal and plant world or are produced by bacteria.
Here are some examples of neurotoxins and how they work:
Curare: Curare is a poison from plants that grow in South America. The natives used it as an arrow poison for hunting. Curare is a competitive antagonist to the neurotransmitter acetylcholine. This occurs on the motorized end plate. Curare displaces the acetylcholine from the receptors of the postsynapse, but does not open the receptor. Accordingly, there is no EPSP and there is no forwarding of the action potentials. This paralyzes the muscles and the affected person dies of respiratory paralysis. So it's a deadly poison.
Botulinum toxin: This toxin is produced by the bacterium Clostirdium botulinum. It inhibits the release of the neurotransmitter acetylcholine from the vesicles by destroying the necessary enzymes. So there is no transfer of the action potentials to the downstream muscle cell and this is consequently paralyzed. The poison is used locally in cosmetic surgery to paralyze the facial muscles and thus minimize wrinkles. In this case it is known as "Botox". It is also used in the therapy of neuromuscular diseases such as spasticity. It is the most powerful neurotoxin known. For this reason, it should only be used in a very low concentration.
Read more on this topic at: Botox
Tetanus toxin: This toxin is also produced by a bacterium called Clostirdium tetani. These are often found on rusty metal. There are optimal conditions in wounds for the bacteria to endure. This is where the entry port for the toxin is located to get into the body. It will then retrograde transported to the anterior horns of the spinal cord. There it destroys enzymes that are responsible for the release of inhibitory transmitters from the vesicles. As a result, the inhibiting interneurons can no longer work. The lack of inhibition leads to overexcitation of the muscles. This leads to stretching cramps and the so-called devil's grin in those affected. The patients die from suffocation as a result of permanently tense respiratory muscles. Fortunately, there is a vaccination against this toxin.
Atropine: Atropine occurs in the black deadly nightshade. It displaces acetylcholine from the receptors on the postsynapse, but does not cause the channels to open. There is no sodium influx and so no action potential can be formed.
Insecticide Parathion E 605: The insecticide Parathion E 605 inhibits the enzyme cholinesterase, which is normally supposed to split acetylcholine in the synaptic cleft. Only in this way can this be transported back into the presynpse and stored again in vesicles. If this is not possible, there is consequently an excess of neurotransmitters and thus permanent depolarization of the postsynapse. The muscles are then in a permanent cramp. The permanent contraction of the respiratory muscles ultimately leads to death. The substance is banned in Germany. In addition to the insecticide, the chemical warfare agent sarin has the same mode of action. It is structurally similar to parathion and is absorbed through the airways and skin. It is fatal even at a low dose.
Alpha-lactrotoxin: This substance is the poison of a spider, the black widow. It causes the Ca canals in the presynapse to open permanently. This leads to a permanent transmission of supposed action potentials and thus to muscle cramps.
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