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Depiction of smooth muscle contraction. Muscle contraction is the activation of tension-generating sites within muscle cells. [1] [2] In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such as when holding something heavy in the same position. [1]
When necessary, the maximal number of motor units in a muscle can be recruited simultaneously, producing the maximum force of contraction for that muscle, but this cannot last for very long because of the energy requirements to sustain the contraction. To prevent complete muscle fatigue, motor units are generally not all simultaneously active ...
The muscle fibers belonging to one motor unit can be spread throughout part, or most of the entire muscle, depending on the number of fibers and size of the muscle. [ 2 ] [ 3 ] When a motor neuron is activated, all of the muscle fibers innervated by the motor neuron are stimulated and contract.
An impulse from a nerve cell causes calcium release and brings about a single, short muscle contraction called a muscle twitch. If there is a problem at the neuromuscular junction, a very prolonged contraction may occur, such as the muscle contractions that result from tetanus. Also, a loss of function at the junction can produce paralysis. [5]
In physiology, medicine, and anatomy, muscle tone (residual muscle tension or tonus) is the continuous and passive partial contraction of the muscles, or the muscle's resistance to passive stretch during resting state. [1] [2] It helps to maintain posture and declines during REM sleep. [3]
The origin of a muscle is the bone, typically proximal, which has greater mass and is more stable during a contraction than a muscle's insertion. [14] For example, with the latissimus dorsi muscle, the origin site is the torso, and the insertion is the arm. When this muscle contracts, normally the arm moves due to having less mass than the torso.
Thickness is defined as the area between the aponeuroses of the muscle. A low gear ratio occurs when the contraction velocity of the whole muscle and individual fibers is approximately the same, resulting in a gear ratio of 1. Conditions resulting in a low gear ratio include high force and low velocity contraction of the whole muscle.
Within the muscle tissue of animals and humans, contraction and relaxation of the muscle cells is a highly regulated and rhythmic process. In cardiomyocytes, or cardiac muscle cells, muscular contraction takes place due to movement at a structure referred to as the diad , sometimes spelled "dyad."