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من طرف al_zoubi الإثنين يوليو 14, 2008 1:16 pmA muscle contraction (also known as a muscle twitch or simply twitch) occurs when a muscle fibre generates tension through the action of actin and myosin cross-bridge cycling. While under tension, the muscle may lengthen, shorten or remain the same. Though the term 'contraction' implies a shortening or reduction, when used as a scientific term referring to the muscular system contraction refers to the generation of tension by muscle fibers with the help of motor neurons. Locomotion in most higher animals is possible only through the repeated contraction of many muscles at the correct times. Contraction is controlled by the central nervous system (CNS), which comprises the brain and spinal cord. Voluntary muscle contractions are initiated in the brain, while the spinal cord initiates involuntary reflexes.
1 Contractions, by muscle type
1.1 Skeletal muscle contractions
1.1.1 Classification of voluntary muscular contractions
1.2 Smooth muscle contraction
1.2.1 Invertebrate smooth muscles
2 Contractions
2.1 Concentric contraction
2.2 Eccentric contraction
2.2.1 Eccentric contractions in movement
3 Force-length and Force-velocity relationships
4 See also
5 Additional images
6 References
7 External links
[edit] Contractions, by muscle type
For voluntary muscles, contraction occurs as a result of conscious effort originating in the brain. The brain sends signals, in the form of action potentials, through the nervous system to the motor neuron that innervates the muscle fiber. In the case of some reflexes, the signal to contract can originate in the spinal cord through a feedback loop with the grey matter. Involuntary muscles such as the heart or smooth muscles in the gut and vascular system contract as a result of non-conscious brain activity or stimuli endogenous to the muscle itself. Other actions such as locomotion, breathing, chewing have a reflex aspect to them; the contractions can be initiated consciously or unconsciously, but are continued through unconscious reflex.
There are three general types of muscle tissues:
Skeletal muscle responsible for movement
Cardiac muscle responsible for pumping blood
Smooth muscle responsible for sustained contractions in the blood vessels, gastrointestinal tract and other areas in the body
Skeletal and cardiac muscles are called striated muscle because of their striped appearance under a microscope which is due to the highly organized alternating pattern of A band and I band.
While nerve impulse profiles are, for the most part, always the same, skeletal muscles are able to produce varying levels of contractile force. This phenomenon can be best explained by Force Summation. Force Summation describes the addition of individual twitch contractions to increase the intensity of overall muscle contraction. This can be achieved in two ways: (1) by increasing the number and size of contractile units simultaneously, called multiple fiber summation, and (2) by increasing the frequency at which action potentials are sent to muscle fibers, called frequency summation.
Multiple fiber summation – When a weak signal is sent by the CNS to contract a muscle, the smaller motor units, being more excitable than the larger ones, are stimulated first. As the strength of the signal increases, more motor units are excited in addition to larger ones, with the largest motor units having as much as 50 times the contractile strength as the smaller ones. As more and larger motor units are activated, the force of muscle contraction becomes progressively stronger. A concept known as the size principle allows for a gradation of muscle force during weak contraction to occur in small steps, which then become progressively larger when greater amounts of force are required.
Frequency summation - For skeletal muscles, the force exerted by the muscle is controlled by varying the frequency at which action potentials are sent to muscle fibers. Action potentials do not arrive at muscles synchronously, and during a contraction some fraction of the fibers in the muscle will be firing at any given time. Typically when a human is exerting a muscle as hard as they are consciously able, roughly one-third of the fibers in that muscle will be firing at once, but various physiological and psychological factors (including Golgi tendon organs and Renshaw cells) can affect that. This 'low' level of contraction is a protective mechanism to prevent avulsion of the tendon - the force generated by a 95% contraction of all fibers is sufficient to damage the body.
[/color][/left]1 Contractions, by muscle type
1.1 Skeletal muscle contractions
1.1.1 Classification of voluntary muscular contractions
1.2 Smooth muscle contraction
1.2.1 Invertebrate smooth muscles
2 Contractions
2.1 Concentric contraction
2.2 Eccentric contraction
2.2.1 Eccentric contractions in movement
3 Force-length and Force-velocity relationships
4 See also
5 Additional images
6 References
7 External links
[edit] Contractions, by muscle type
For voluntary muscles, contraction occurs as a result of conscious effort originating in the brain. The brain sends signals, in the form of action potentials, through the nervous system to the motor neuron that innervates the muscle fiber. In the case of some reflexes, the signal to contract can originate in the spinal cord through a feedback loop with the grey matter. Involuntary muscles such as the heart or smooth muscles in the gut and vascular system contract as a result of non-conscious brain activity or stimuli endogenous to the muscle itself. Other actions such as locomotion, breathing, chewing have a reflex aspect to them; the contractions can be initiated consciously or unconsciously, but are continued through unconscious reflex.
There are three general types of muscle tissues:
Skeletal muscle responsible for movement
Cardiac muscle responsible for pumping blood
Smooth muscle responsible for sustained contractions in the blood vessels, gastrointestinal tract and other areas in the body
Skeletal and cardiac muscles are called striated muscle because of their striped appearance under a microscope which is due to the highly organized alternating pattern of A band and I band.
While nerve impulse profiles are, for the most part, always the same, skeletal muscles are able to produce varying levels of contractile force. This phenomenon can be best explained by Force Summation. Force Summation describes the addition of individual twitch contractions to increase the intensity of overall muscle contraction. This can be achieved in two ways: (1) by increasing the number and size of contractile units simultaneously, called multiple fiber summation, and (2) by increasing the frequency at which action potentials are sent to muscle fibers, called frequency summation.
Multiple fiber summation – When a weak signal is sent by the CNS to contract a muscle, the smaller motor units, being more excitable than the larger ones, are stimulated first. As the strength of the signal increases, more motor units are excited in addition to larger ones, with the largest motor units having as much as 50 times the contractile strength as the smaller ones. As more and larger motor units are activated, the force of muscle contraction becomes progressively stronger. A concept known as the size principle allows for a gradation of muscle force during weak contraction to occur in small steps, which then become progressively larger when greater amounts of force are required.
Frequency summation - For skeletal muscles, the force exerted by the muscle is controlled by varying the frequency at which action potentials are sent to muscle fibers. Action potentials do not arrive at muscles synchronously, and during a contraction some fraction of the fibers in the muscle will be firing at any given time. Typically when a human is exerting a muscle as hard as they are consciously able, roughly one-third of the fibers in that muscle will be firing at once, but various physiological and psychological factors (including Golgi tendon organs and Renshaw cells) can affect that. This 'low' level of contraction is a protective mechanism to prevent avulsion of the tendon - the force generated by a 95% contraction of all fibers is sufficient to damage the body.
