The Neuromuscular Junction Essay Example

skeletal muscle fibers at which they terminate to contract.

The junction between the terminal of the motor neuron and a fiber muscle is called a neuromuscular junction. The impulse reaches the terminal end of the motor neuron and breaks open the synaptic vesicles, releasing the neurotransmitter acetylcholine into the synaptic cleft. ACh then binds to receptor molecules in the motor end plate of the muscle fiber. Depolorization of the motor end plate triggers an action potential that propagates along the sacolemna and down the T-tubes. The action potential is started in the sarcolemma and passes along the membrane until it reaches the sarcoplasmic reticulum releasing calcium.

Calcium diffuses in sarcoplasm and binds to the troponin in the thin filaments. The binding sites on the actin molecules are freed, allowing the heads of myosin (thick filaments) to attach on the binding sites forming the cross-bridges connections. The cross-bridges shift their angle causing the thin filaments to slide over the thick filaments; this produces the contraction. ATP is needed for the last step. ATP is bound to myosin; calcium activates ATP breakdown into ADP and phosphate. Part of the energy produced is used to move bridges; the other part is released as heat, increasing the body temperature.

More ATP breaks the bond between thick and thin filaments in the cross bridge; calcium ions which trigger a contraction of the muscles cells returns to thesarcoplasmic reticulum and the thin filaments return to their original position, closing off the binding site to cross bridges and causing the muscle to relax. Several conditions can cause malfunctions at the site of a neuromuscular junction, leading to loss of muscle control. Ultimately,

lack of muscle control can cause death if it is severe, because the body's largest muscle, the heart, could stop beating.

The most common such illness is myasthenia gravis, which is caused by an autoimmune reaction against acetylcholine receptors. As a result, motor neurons cannot transmit the chemical to the muscles, which will cause muscle weakness and an ultimate loss of control. Myasthenia gravis tends to be more concentrated along skeletal muscles, and is especially noticeable in the face. Additionally, some toxins such as botulinum will inhibit the release of acetylcholine, which will lead to muscle paralysis.