Skeletal+Muscle+Contractions

=Skeletal Muscle Contractions= Skeletal muscles take only a matter of seconds to contract. The [|nervous system]requires hardly any time at all to send an impulse to the muscle to tell it to contract in the way that it is needed. However, the time that it takes for a muscle to contract is deceivingly simple. While it only takes a short amount of time for the contraction to take place, there are many things that are happening in the body to send the signal down to the muscle and then for the muscle to actually contract. The nerve impulse is send down the nervous system to the [|axon] of the muscle and then to the [|neuromuscular junction]. After the nerve impulse reaches the neuromuscular junction the voltage-regulated calcium channels open which then allows the calcium to enter the axon. Once the calcium ions are inside the axon the axonal, or semantic, vesicles fuse with the membrane of the axon. [|Acetylcholine] is released when the axonal vesicles fuse to the axonal membrane. The acetylcholine is released into the synaptic cleft which is called [|exocytosis]. The acetylcholine diffuses and moves across the synaptic cleft to the sarcolemma and binds to it. The binding of the acetylcholine to the sarcolemma causes an action potential in the muscle. After the action potential is created acetylcholinesterase is released so that it will destroy the acetylcholine. This is a very important part of the process because without the acetylcholinesterase the muscle would be in a continual state of contraction. The acetylcholine will not form another action potential without additional stimuli because of the acetylcholinesterase. This part of the process is called the neuromuscular junction. The next part in the process to get a muscle to contract is called [|excitation-contraction coupling]. This part of the process consists of the action potential that was generated by the acetylcholine to spread along the sarcolemma and then to travel down the [|T-tubules]. Calcium is released from the terminal cisternae when the action potential reaches that far down the T-tubules. The released calcium then binds to the [|troponin] and covers causes the [|tropomyosin]to move and reveal the active actin potential sites. The cross bridge cycling part of the process of getting a muscle contract starts when the myosin cross bridges attach and detach alternatively which then makes the thin filaments move closer towards the center of the sarcomere. When the muscle is finished contracting the calcium ions are pumped back into the [|sarcoplasmic reticulum]which then allows the tropomyosin to move back into its place covering the active actin binding sites. Once the actin binding sites are covered again the muscle fiber is able to relax. The process of sending an impulse to the axon to send an action potential to the muscle and the actual contraction of the muscle all happen in nanoseconds, though the actual process seems like it would take much longer for anything to happen.