The muscular system, which includes exclusively skeletal muscles, is a complex and copious organ system. We will begin our analysis of this organ system by talking about its anatomy. Every muscle is considered its own organ and surrounding every organ is connective tissue called the epimysium which separates muscles from surrounding objects. Below the epimysium is another connective tissue called the perimysium which separates the muscle into sections called fascicles(bundles of muscle fibers). Separating the muscle fibers from each other is yet another connective tissue called the endomysium. The outside of the muscle fiber(cell) is called the sarcolemma. Attached to the sarcolemma is the transverse tubule which also attaches too the sarcoplasmic reticulum. Muscle fibers have several vital organelles which include mitochondria, sarcoplasmic reticulum, and contractile like organelles (filaments) such as the actin (found in the light band) and myosin (found in the dark band). Actin are attached to the Z-lines or a-discs, Actin is found in the light bands ( I-bands).Myosin is found in the dark bands(A-bands), on the myosin are matchstick like heads called the troponin, and the are in the middle of the myosin with no heads is called the H-zone. In the middle of the h-zone runs a line vertically called the m-line to indicate the middle of the myosin.
Enough about Anatomy, the physiology of the muscle cell is pretty cool also. There are a lot of steps to a muscle contraction and it all starts from an impulse from a nerve fiber. The impulse releases acetylcholine (enough is released to overcome the muscle cells threshold for change) across the neuro-muscluar junction to the sarcolemma making it extremely permeable allowing potassium to rush out and sodium to rush in reversing the electrochemical gradient (charges) causing depolarization which leads to a release of a action potential. The action potential then travels down the T-tubule to the sarcoplasmic reticulum stimulating it and allowing for the release of calcium ions. The calcium ions the attach to the tropomyosin moving it off the active sites found on the actin. Soon after this the troponin( heads of the myosin) slide into the active sites found on the actin, this is known as the cross bridge formation. Once the cross bridge is formed actin can slide past the myosin closer towards the M-line. This causes as the Z-lines to come closer towards one and other. The distance from z-line to z-line is known as a sarcomere, and when a sarcomere shortens this is known as a contraction( according to the sliding filament theory).
Muscles attain there energy for much of its processes from ATP. ATP is the muscles go to energy source for almost all activities, but there is a very small ready to go supply of ATP in the muscle. In order to create more ATP for muscle contractions creatine phosphate and glycogen are broken down to make more. Breaking down glycogen can create many to very few ATP depending on oxygen levels within a mucle cell, this can also be very time consuming. Creatine phosphate can be broken down in order to give its phosphate to and ADP molecule creating ATP very rapidly but in small amounts. Glycogen is usually found in adipose tissues where most carbohydrates are usually stored. The body beaks down its own fat supplies in order to reach the glycogen reservoirs.
The muscle system truly is inspiring yet complicated but it has its many roles. It is helpful in movement, the digestion of food, circulating blood, generating heat, support(tendons mostly), and breathing. All these abilities provided by the muscle help keep humans in a stable homeostatic range.