Motorized end plate

definition

The motorized end plate (neuromuscular endplate) is a chemical synapse that can transmit electrical excitation from the end of a nerve cell to a muscle fiber.

Task of the motorized end plate

The task of the motor end plate is to generate an excitation, i.e. a Action potentialthat through the Nerve fiber to be transferred from this to the muscle cell, causing it to muscle becomes possible to contract (contract).

construction

The motor end plate usually becomes three parts counted:

• The End button the nerve fiber, which is an expansion at the end of the axon of this fiber, or the membrane present here, which is also called presynaptic membrane (= membrane located in front of the synapse),
• the opposite portion of the membrane of the muscle fiber cell, which one also postsynaptic membrane (= Membrane after the synapse) calls and
• of the synaptic cleftlocated between the two membranes.

A process of excitation

When a Action potential reaches the end button of the nerve cell, this end button opens in the membrane voltage gated calcium channels. The then flowing into the cell Calcium ions to bind on small vesicles (Vesicle) that are in the cytoplasm and those with the Transmitter fabric (Transmitter) Acetylcholine are filled. Because the calcium ions are now bound to the vesicles, these are caused to towards the presynaptic membrane to move and to merge with it. This process is under the name Exocytosis known and has the consequence that the contents of the vesicles, i.e. in this case the acetylcholine, are emptied to the outside. It is now in the synaptic cleft.

The postsynaptic membrane is associated with a variety of Receptors For this Neurotransmitters fitted.
These receptors are known as ionotropicsince they are with a Ion channel are linked, which opens after the receptors are occupied.
The acetylcholine receptors found here are nicotinic acetylcholine receptors, a term that comes from the substance nicotine can also dock to these receptors (whereby the concentration of nicotine, which is achieved, for example, by smoking, is not sufficient to open the channels).
There is also another receptor for acetylcholine called the muscarinic acetylcholine receptor which, however, does not occur on muscle cells but in the parasympathetic nervous system.

If acetylcholine now binds to the nicotinic receptor, a channel opens that is responsible for Cations (i.e. positively charged ions) is continuous. Due to the concentration of these ions inside and outside the muscle cell and the resulting driving forces, this leads to the fact that above all Sodium ions and Calcium ions in the Muscle fiber pour in.
As a result, this becomes Endplate potential the postsynaptic membrane always more positive, one speaks of one Depolarization the cell. This turns the so-called Resting potential the cell first Generator potential, which passively spreads electrotonically along the muscle fiber. However, when a certain threshold is exceeded, they also open voltage-dependent sodium channels.
This process does that Creation of an action potentialwhich can spread much faster. The action potential also reaches the tubule system of the muscle cell via the membrane.
Here voltage-controlled calcium channels are opened because of the incoming action potential, whereby the Ryanodine receptors of the sarcoplasmic reticulum (which corresponds to the endoplasmic reticulum of body cells) are activated.
The result is that a massive release of calcium ions he follows. The calcium in turn ensures that the binding site of actin and myosin is released, which causes the Sliding filament mechanism is set in motion: the muscle fiber shortens and the muscle contracts.
This process is also known as electromechanical coupling, since an originally electrical signal (namely the action potential) leads to a mechanical reaction (namely the contraction of the muscle).

The acetylcholine, which was previously released into the synaptic cleft, cannot as such get back into the terminal button of the nerve cell. That's why it gets through a enzyme, the Acetylcholinesterase, first split into its components acetate and choline, which can migrate separately through the presynaptic membrane, unite and are now packed into vesicles as acetylcholine.
The concentration of acetylcholinesterase in the synaptic cleft can, among other things, control the length and intensity of the muscle contraction, as it has a direct effect on how long the acetylcholine remains there and can cause a contraction. That's why it's this Point of attack of some drugs as well as some poisons.