Which Muscles Can Contract without the Need

At the cellular level, smooth muscle acts as an involuntary, untrired muscle. Smooth muscles contain thick, thin filaments that do not compete with sarcomeres, resulting in an unspoken pattern. On microscopic examination, it appears homogeneous. Smooth muscle cytoplasm contains large amounts of actin and myosin. Actin and myosin act as the main proteins involved in muscle contraction. Actin filaments attach to dense bodies distributed throughout the cell. Dense bodies can be observed under an electron microscope and appear dark. Another important structure is the calcium-containing sarcoplasmic reticulum, which helps maintain contraction. The shape of smooth muscles is fusiform, which is round in the middle and narrows at each end.

Smooth muscles can tense and relax, but have greater elastic properties than striped muscles. This quality is important in organ systems such as the bladder, where the preservation of contractile tone is a necessity. Figure 4. Contraction of skeletal muscles. (a) The active center on actin is exposed because calcium binds to troponin. b) The myosin head is attracted to actin, and myosin binds actin to its actin binding site, forming the transverse bridge. c) During the coup de force, the phosphate produced during the previous contraction cycle is released. This causes the myosin head to rotate towards the center of the sarcomere, after which the attached ADP group and phosphate group are released. d) A new ATP molecule attaches to the myosin head, loosening the transverse bridge. e) The myosin head hydrolyzes ATP to ADP and phosphate, causing the myosin to return to the tense position. Creatine phosphate is a molecule that can store energy in its phosphate bonds.

In a resting muscle, excess ATP transfers its energy to creatine and produces ADP and creatine phosphate. This acts as an energy reserve that can be used to quickly generate more ATP. When the muscle contracts and needs energy, creatine phosphate transfers its phosphate to ADP to make ATP and creatine. This reaction is catalyzed by the enzyme creatine kinase and occurs very quickly; Thus, ATP derived from creatine phosphate causes the first few seconds of muscle contraction. However, creatine phosphate can only provide energy worth about 15 seconds, after which another energy source must be used (Figure 7.14). Healthcare costs associated with asthma are estimated to have reached $81.9 billion in the United States in 2013. [16] With such a large health care burden, it is amazing to see that asthma results from something as simple as the contraction of smooth muscles. Smooth muscles are an integral part of the human body; Its function is vital and present in almost all organ systems. In the cardiovascular system, the smooth muscles of the vessels are used to maintain blood pressure and flow; in the lungs, it opens and closes the airways; in the gastrointestinal system, it plays a role in motility and nutrient collection; And yet, it also serves a purpose in almost every other organ system. The wide distribution of smooth muscles throughout the body and its many unique properties make it essential for healthcare professionals to have a thorough understanding of their physiology, function and disease applications. Because the ATP produced by creatine phosphate is depleted, muscles turn to glycolysis as a source of ATP.

Glycolysis is an anaerobic (non-oxygen-dependent) process that breaks down glucose (sugar) to produce ATP; However, glycolysis cannot produce ATP as quickly as creatine phosphate. Thus, the switch to glycolysis leads to a slower availability of ATP from the muscle. The sugar used in glycolysis can be provided by blood sugar or by metabolizing glycogen stored in the muscle. The breakdown of one glucose molecule produces two ATP and two pyruvic acid molecules, which can be used in aerobic respiration or converted to lactic acid at low oxygen levels (Figure 7.14b). Smooth muscle cells are variable in their function and play many roles in the body. They are spindle-shaped and smaller than skeletal muscles and contain fewer actin and myosin filaments. The filaments of actin and myosin are not organized into sarcomeres, so smooth muscles do not have a striped appearance. Unlike other types of muscles, smooth muscles can exert constant tension. This is called smooth muscle tone.

Smooth muscle cells have a metabolism similar to skeletal muscle and produce most of their aerobic energy. As such, they are not well suited to produce anaerobic energy.1 The sequence of events that lead to the contraction of a single muscle fiber begins with a signal – the neurotransmitter ACh – from the motor neuron that innervates that fiber. The local membrane of the fiber depolarizes when positively charged sodium ions (Na+) enter, triggering an action potential that propagates to the rest of the membrane, including the T tubules. This triggers the release of calcium ions (Ca++) from storage in the sarcoplasmic reticulum (SR). Ca++ then initiates a contraction that is maintained by ATP (Figure 7.10). As long as Ca++ ions remain in the sarcoplasm to bind to troponin, which keeps actin binding sites “unshielded,” and as long as ATP is available to drive the cross-bridge cycle and myosin pulling of actin strands, the muscle fiber will continue to shorten to an anatomical limit. Figure 6. Glycolysis and aerobic respiration. Each glucose molecule produces two ATP and two leachic acid molecules, which can be used in aerobic respiration or converted into lactic acid. When oxygen is not available, pyruvic acid is converted into lactic acid, which can contribute to muscle fatigue. This happens during intense exercise, when large amounts of energy are needed, but oxygen cannot be sufficiently supplied to the muscle. In order for thin filaments to continue to slide beyond thick filaments during muscle contraction, myosin heads must pull actin to the binding sites, detach it, strain it again, attach it to other binding sites, pull it, loosen it, cover it, etc.

This repeated movement is called the transverse bridge cycle. This movement of myosin heads is similar to that of rowing when a person rows a boat: pulling paddles from the oars (myosin heads) are lifted out of the water (detached), repositioned (rested), and then submerged again to shoot (Figure 7.13). Each cycle requires energy, and the action of myosin heads in sarcomeres that repeatedly pull on thin filaments also requires energy provided by ATP. DMD is an inherited disease caused by an abnormal X chromosome. It mainly affects men and is usually diagnosed in early childhood. DMD usually occurs first as a difficulty with balance and movement, and then develops into an inability to walk. It progresses higher in the body from the lower limbs to the upper body, where it affects the muscles responsible for breathing and circulation. It eventually causes death due to respiratory failure, and sufferers usually do not live beyond the age of 20. Research has shown that smooth muscles can contract without action potential. In multi-unit smooth muscles, action potentials usually do not occur. An example would be the smooth muscles of the iris of the eye, where norepinephrine and ACh produce a depolarization called junctional potential.