The muscular system is an organ system consisting of skeletal, smooth and cardiac muscles. It provides support and mobility of the body and circulates blood throughout the body. The muscles maintain your posture when you sit or stand and they also stabilize joints to help prevent dislocation. You are able to move because of skeletal muscles. Your guts move because of smooth muscles. Your blood flow because of pumping action of the heart (cardiac muscle). Contraction of skeletal muscles around the deep veins help to squeeze the blood through peripheral veins. Diaphragm contraction or breathing sucks blood into the chest cavity, and also squeezes on abdominal veins. It aids in thermoregulation through a shivering reflex. The shivering reflexes is done when the muscles generate heat by you shivering in response to the cold. The muscular system in vertebrates is controlled through the nervous system, although some muscles (such as the cardiac muscle) can be completely autonomous. Skeletal muscles are striated (striped) and are responsible for voluntary movement in the body. They are shaped like long fibers and are multinucleated. Skeletal muscles, like other striated muscles, are composed of myocytes, or muscle fibers, which are in turn composed of myofibrils, which are composed of sarcomeres, the basic building block of striated muscle tissue. Smooth muscles are nonstriated and are responsible for involuntary movement. They are shaped like almonds and have one nucleus per cell. Cardiac muscles are striated like skeletal muscles but they are responsible for involuntary movement like smooth muscles, and are only found in the heart. They are branched and shaped like fibers ,cross-linked to one another. Normally, only one nucleus per cell.
The striated muscle feature is due to the presence of the sarcomere structure (A bands of the sarcomere are dark in colour while I bands are much lighter). Each sarcomere is defined by two parallel and very dark colored elongated bands called Z-lines. These Z-lines are made of dense protein discs that do not easily allow the passage of light, resulting in their dark appearance under a microscope. The adjacent area between the Z-lines is further divided into two lighter colored bands at either end called the I-bands, which is where actin myofilaments are not superimposed by myosin myofilaments. A darker, grayish band in the middle called the A band is the length of a myosin myofilament within a sarcomere. The I-bands appear lighter due to these areas containing thin actin filaments, allowing the passage of light between them because of their smaller diameter. The A band, on the other hand, contains mostly myosin filaments whose larger diameter restricts the passage of light. The M-line is the the line at the center of a sarcomere to which myosin myofilaments bind. H-band is the area adjacent to the M-line, where myosin myofilaments are not superimposed by actin myofilaments. Skeletal and cardiac muscles have sarcomeres. Smooth muscles don’t have sarcomeres so they’re not striated. They still have myosin, actin, and use the sliding filament mechanism. They just are not organized into sarcomeres. The sliding filament mechanism is a theory by which muscles are thought to contract based on muscle proteins that slide past each other to generate movement. At a basic level, each myocyte is made up of smaller fibres called myofibrils which themselves contain even smaller structures called actin and myosin filaments.
These filaments slide in and out between each other to form a muscle contractions, hence the name sliding filament mechanism. Muscle contraction begins when a nervous impulse arrives at the neuromuscular junction. This triggers the release of a chemical called acetylcholine. Acetylcholine causes the motor end plate to depolarise which continues to travels throughout the muscle by the transverse tubules. This event causes calcium ions to be released from the sarcoplasmic reticulum. Due to the high concentrations of calcium ions, these ions binds to troponin, causing it to changing its shape and it moves tropomyosin from the active site of actin. The myosin filaments are now able to attach themselves to actin to form a cross-bridge. ATP is broken down to release energy which enables the myosin filamets to pull the actin filaments inwards and shorten the muscle. This occurs along the entire length of every myofibril in the muscle cell. The myosin then detaches from the actin and an ATP molecule binds to the myosin head, breaking the cross-bridge. The ATP can then be broken down, allowing the myosin filament head to again attach to an actin binding site further along the actin filament and repeat the power stroke. This repeated pulling of the actin over the myosin is often known as the ratchet mechanism. As long as there is adequate ATP and calcium ions, this process of muscular contraction can last for as long as is necessary. Calcium ions are pumped back to the sarcoplasmic reticulum once the impulse stops and the actin filament returns to its resting position causing the muscle to lengthen and relax. A single power stroke results in only a shortening of about 1% of the entire muscle. Based on this fact, to achieve an overall shortening of up to 35%, the whole process must be repeated several times. One theory is that whilst half of the cross-bridges are active in pulling the actin over the myosin, the other half are looking for their next binding site.
Motor neurons are nerve fibers that carries nerve impulses away from the central nervous system toward the peripheral effector organs, mainly muscles and glands. It signals muscles or relevant organs to carry out an action. It is the opposite of sensory neurons, which are neurons that convert a specific type of stimulus, via their receptors, into action potentials. Autonomic motor neurons control involuntary muscles, such as the smooth muscles and cadiac muscles. At the neuromuscular junction the axon terminal of the nerve the motor end plate. The motor end plate is the part of the muscle cell membrane, or sarcolemma, that synapse with the motor neuron and it has receptors for the neurotransmitters. At the neuromuscular junction, an action potential of the nerve reaches the axon terminal. This iniatiates the release of neurotransmitters into the synapse unto its receptors on the motor end plate. The sarcolemma picks this signal and establishes a graded potential. If it reaches threshold, then action potential created and it travels down the sarcolemma and causes the muscle to contract.
The sympathetic nervous system is the fight or flight system. It is a physiological reaction that occurs in response to a perceived harmful event, attack, or threat to survival. When activated, it causes the heart to beat faster, the pupils to dilate and it raises the blood pressure. It also increases the flow of blood to muscles and constricts the blood supply in certain areas that are not required for the stimuli, such as the digestive system. The parasympathetic nervous system is known as the rest and digest system. It is seen as the opposite of sympathetic system as its activation results in opposite effects of the sympathetic system such as slower heart beat and lower blood pressure.
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