Neuromuscular junction
The motor end plate (also neuromuscular junction or anat. Terminatio neuromuscularis ) transfers the excitation of a nerve fiber to the muscle fiber. It provides a chemical synapse with the transmitter acetylcholine dar.
It consists of a microscopically visible Synapsenendknöpfchen in the form of a swelling at the end of an axon ( presynaptic ), and - separated by the synaptic cleft and a basal lamina - a tightly fitting, specifically structured membrane part of a muscle ( postsynaptic ). The surface of the postsynaptic membrane is increased by a plurality of folds. This is known as folding subneuraler apparatus. At the presynaptic part of the synapse incoming action potentials, the opening of voltage-gated calcium channels cause. The inflowing calcium (Ca2 ) is vesicles that are located in the Synapsenendknöpfchen and contain acetylcholine, in motion. This causes which - filled with acetylcholine - vesicles move toward the synaptic gap to the membrane of the Endknöpfchens. Already at the membrane docked vesicles fuse with it ( exocytosis ) and pour the neurotransmitter acetylcholine into the synaptic cleft from. Acetylcholine binds to acetylcholine receptors in the postsynaptic membrane of the muscle cell, thereby opening the ion channels these receptors are ionotropic.
The ionotropic acetylcholine receptors are also known as nicotinic acetylcholine receptors, since nicotine agonist acting on it. Nicotine reached the smoker but locally sufficient concentration to achieve a relevant effect. The acetylcholine receptor is a non-specific cation channel, which is conductive for sodium, calcium and potassium ions. It consists of five subunits that are arranged around a central pore. By the different driving forces for these ions, especially by a supported sodium and calcium ion current flowing through this channel. The result is a depolarization of the muscle cell, which can rise to a potential of generator, the so-called end plate potential of a resting membrane potential. This generator is an electrostatic potential tonic potential, that is, it spreads passively through the muscle fiber membrane ( in contrast to the action potential ). Exceeds the generator potential, the threshold potential, action potential, voltage -gated sodium channels and action potential arises Open. The action potential triggers by the then following calcium influx muscle contraction from:
About the opening of voltage- activated calcium channels in the transverse tubules of the muscle cell and activation of intracellular ryanodine receptors leads to a release of calcium ions from the sarcoplasmic reticulum, the endoplasmic reticulum of the muscle cell. The resulting sharp increase in the intracellular calcium concentration causes contraction of the muscle cell. Thus, calcium ions exert a key role in the electro-mechanical coupling, the connection of electrical excitation to the diaphragm up to the contraction.
Acetylcholine is hydrolyzed from the synaptic cleft by the enzyme acetylcholinesterase to choline and acetate. Acetate then diffuses out of the gap, choline, however, is taken up by the presynaptic cell and reused there. Acetylcholinesterase is anchored to the basal lamina of the synaptic cleft.
When the function of acetylcholine receptors is disrupted by autoimmune antibodies, for example, there may be muscular fatigability and weakness ( myasthenia gravis ) can occur.