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Rigor mortis is the reason why the word "stiff" is a slang term for a dead body. Two or three hours after a person or animal dies, the muscles start to stiffen. This phenomenon progresses in a downward, head-to-toe direction. In 12 to 18 hours the body is, as the saying goes, stiff as a board. At this stage, you can move the joints only by force, breaking them in the process.

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The heart automaticity is a fundamental physiological function in higher organisms. The spontaneous activity is initiated by specialized populations of cardiac cells generating periodical electrical oscillations. The exact cascade of steps initiating the pacemaker cycle in automatic cells has not yet been entirely elucidated.

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For a muscle to contract and do work, its cells must be stimulated, most often by the nerves supplying them. Nervous impulses cause the release of the neurotransmitter acetylcholine at the nerve-muscle junction, and the acetylcholine activates receptors on the surface of the muscle cell. This results in an influx of positively charged sodium ions into the muscle cell and a depolarization of the muscle cell membrane, which in the resting state is quite negatively charged. If the membrane becomes sufficiently depolarized, an action potential results; the muscle cell is then "excited" from an electrochemical standpoint.

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When something is described as elastic, this is simply a statement that it can be stretched or contracted by some amount above or below its resting or default length without damaging it, and that it will return to this resting length once the stimulus for stretching or contraction is removed. Your muscles require the property of elastic recoil for them to be able to do their jobs. If, say, your biceps muscles failed to recoil to their resting length after being stretched during a series of curling exercises, they would become slack, and slack muscles with no tension are unable to generate any force and are therefore useless as levers.

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Most of your body's cells lack the capacity to stretch; attempting to do so only damages or destroys them. Your long, cylindrical muscle cells, however, are a different story. Muscle cells contract, and in order for them to retain this ability, they must accordingly possess extensibility, or the capacity to lengthen. Your muscle cells can be stretched to about three times their contracted length without rupturing. This is important because in a lot of coordinated movements, so-called antagonistic muscles operate such that one is lengthening while the other is contracting. For example, when you run, the hamstring in the back of your thigh contracts while your quadriceps are extended and conversely.

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In the case of skeletal muscles, muscle cells contract when stimulated by neural input; smooth and cardiac muscles do not require this input. When a muscle cell is excited, the impulse travels along various membranes of the cell to its interior, where it leads to the opening of calcium channels. Calcium ions flow toward and bind to a protein molecule called troponin, leading to sequential changes in shape and position of the associated proteins tropomyosin, myosin and actin. The upshot is that myosin binds to small strands within the cell called myofilaments and pulls them along, causing the cell to shorten, or contract. Since this is going on simultaneously and in a coordinated fashion in many thousands of myocytes at the same time, the muscle as a whole contracts.

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Vertebrate striated muscle contraction is controlled (regulated) by the action of the proteins troponin and tropomyosin on the actin filaments. Nervous stimulation causes a depolarisation of the muscle membrane (sarcolemma) which triggers the release of calcium ions from the sarcoplasmic reticulum.

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Myosin is composed of several protein chains: two large "heavy" chains and four small "light" chains. The structures available in the PDB, such as the one shown above, contain only part of the myosin molecule. In the illustration above, from PDB entry 1b7t , atoms in the heavy chain are colored red on the left-hand side, and atoms in the light chains are colored orange and yellow. The whole molecule is much larger, as shown on the next page, with a long tail that has been clipped off to allow the molecule to be studied. Fortunately, the crystal structures include most of the "motor" domain, the part of the molecule that performs the power stroke, so we can look at this process in detail.

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Actin, protein that is an important contributor to the contractile property of muscle and other cells. It exists in two forms: G-actin (monomeric globular actin) and F-actin (polymeric fibrous actin), the form involved in muscle contraction.In muscle, two long strands of beadlike actin molecules are twisted together to form a thin filament, bundles of which alternate and interdigitate with bundles of thick filaments formed of myosin, the most abundant protein found in muscle. When a signal for muscle contraction is sent along a nerve to a muscle cell, actin and myosin are activated. Myosin works as a motor, hydrolyzing adenosine triphosphate (ATP) to release energy in such a way that a myosin filament moves along an actin filament, causing the two filaments to slide past each other. Two other muscle proteins, tropomyosin and troponin, regulate the temporary fusion of actin and myosin that results in the contraction of muscle.

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Sliding filament theory A proposed mechanism of muscle contraction in which the actin and myosinfilaments of striated muscle slide over each other to shorten the length of the muscle fibres (see sarcomere). Myosin-binding sites on the actin filaments are exposed when calcium ions bind to troponinmolecules in these filaments. This allows bridges to form between actin and myosin, which requires ATP as an energy source. Hydrolysis of ATP in the heads of the myosin molecules causes the heads to change shape and bind to the actin filaments. The release of ADP from the myosin heads causes a further change in shape and generates mechanical energy that causes the actin and myosin filaments to slide over one another (see illustration).

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Skeletal muscle, also called voluntary muscle, in vertebrates, most common of the three types of muscle in the body. Skeletal muscles are attached to bones by tendons, and they produce all the movements of body parts in relation to each other. Unlike smooth muscle and cardiac muscle, skeletal muscle is under voluntary control. Similar to cardiac muscle, however, skeletal muscle is striated; its long, thin, multinucleated fibres are crossed with a regular pattern of fine red and white lines, giving the muscle a distinctive appearance. Skeletal muscle fibres are bound together by connective tissue and communicate with nerves and blood vessels. For more information on the structure and function of skeletal muscle, see muscle and muscle system, human.

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Cardiac muscle tissue is one of the three types of muscle tissue in your body. The other two types are skeletal muscle tissue and smooth muscle tissue. Cardiac muscle tissue is only found in your heart, where it performs coordinated contractions that allow your heart to pump blood through your circulatory system.

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Smooth muscle, also called involuntary muscle, muscle that shows no cross stripes under microscopic magnification. It consists of narrow spindle-shaped cells with a single, centrally located nucleus. Smooth muscle tissue, unlike striated muscle, contracts slowly and automatically. It constitutes much of the musculature of internal organs and the digestive system.

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Tetany is a symptom. Like many symptoms, it can be brought on by a variety of conditions. This means that it’s sometimes difficult to find what’s causing this symptom. While there are effective treatments for the condition, preventing it often depends on pinpointing what caused it in the first place.

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Tetanus is a serious bacterial disease that affects your nervous system, leading to painful muscle contractions, particularly of your jaw and neck muscles. Tetanus can interfere with your ability to breathe and can threaten your life. Tetanus is commonly known as "lockjaw."

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Tonic seizures are brief seizures (usually < 60 seconds) consisting of the sudden onset of increased tone in the extensor muscles. If standing, the patient typically falls to the ground. These seizures are invariably longer than myoclonic seizures. Occasionally tonic seizures terminate with a clonic phase.

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Generalized onset clonic seizures are a relatively rare type of motor (convulsive) seizures mainly affecting infants and very young children. They also occur in metabolic and electrolyte derangements. Clinically, they manifest with bilateral, rhythmic clonic convulsions, with each clonic event lasting fewer than 100 ms at a repetitive rate of 1 to 3 Hz. Consciousness is usually lost or impaired. There is significant misperception in terminology and definition and particularly in their differentiation from myoclonus. In this article, the author details the clinical manifestations, pathophysiology, EEG, differential diagnosis, and management of patients with generalized onset clonic seizures.

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Otherwise known as muscle cramps, spasms occur when a muscle involuntary and forcibly contracts and cannot relax. These are very common and can affect any muscle. Typically, they involve part or all of a muscle, or several muscles in a group. The most common sites for muscle spasms are the thighs, calves, foot arches, hands, arms, abdomen and sometimes along the ribcage. When occurring in the calves, especially, such cramps are known as “charley horses.”

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Muscle fatigue is a symptom that decreases your muscles’ ability to perform over time. It can be associated with a state of exhaustion, often following strenuous activity or exercise. When you experience fatigue, the force behind your muscles’ movements decrease, causing you to feel weaker.

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5 Basic Functions of the Muscular System

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5 Basic Functions of the Muscular System

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The primary function of muscular system is to produce voluntary gross and fine movements.The abdominal muscles and the muscles of the lower back help protect the vital organs of the body.The contraction of the heart muscle is involuntary and primarily controlled by the heart’s own electrical system.The smooth muscles of the stomach and intestines work to process the food we ingest.There are smooth muscles in the walls of the blood vessels. When the heart contracts, the arteries expand to accept blood.

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Fascia is a specialized system of the body that has an appearance similar to a spider's web or a sweater. Fascia is very densely woven, covering and interpenetrating every muscle, bone, nerve, artery and vein, as well as, all of our internal organs including the heart, lungs, brain and spinal cord. The most interesting aspect of the fascial system is that it is not just a system of separate coverings. It is actually one continuous structure that exists from head to toe without interruption. In this way you can begin to see that each part of the entire body is connected to every other part by the fascia, like the yarn in a sweater.

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A tendon is a tough yet flexible band of fibrous tissue. The tendon is the structure in your body that connects a muscle to a bone. Tendons are very organized. The body creates very tightly packed fibers of collagen in parallel arrays that are flexible but very strong. The strength of tendons is important as these tight structures are required to resist the forces of very heavy loads.