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Muscle fiber generates tension through the action of actin and myosin cross-bridge cycling. While under tension, the muscle may lengthen, shorten or remain the same. Although the term 'contraction' implies shortening, when referring to the muscular system, it means muscle fibers generating tension with the help of motor neurons (the terms twitch tension, twitch force, and fiber contraction are also used).
Voluntary muscle contraction is controlled by the central nervous system. Voluntary muscle 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 several muscle fibers. 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 proceeding in the body to the muscle itself.
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 several muscle fibers. 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, and chewing have a reflex aspect to them: the contractions can be initiated consciously or unconsciously.
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: by increasing the number and size of contractile units simultaneously, called multiple fiber summation, and by increasing the frequency at which action potentials are sent to muscle fibers, called frequency summation.
- Multiple fiber summationâ€“ 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 summationâ€“ 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. In a typical circumstance, when a human is exerting a muscle as hard as he/she is consciously able, roughly one-third of the fibers in that muscle will be firing at once, yet can be affected by various physiological and psychological factors (including Golgi tendon organs and Renshaw cells). 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.
Skeletal muscle contractions
Skeletal muscles contract according to the sliding filament model:
- An action potential originating in the CNS reaches an alpha motor neuron, which then transmits an action potential down its own axon.
- The action potential propagates by activating voltage-gated sodium channels along the axon toward the synaptic cleft. Eventually, the action potential reaches the motor neuron terminal and causes a calcium ion influx through the voltage-gated calcium channels.
- The Ca2+ influx causes vesicles containing the neurotransmitter acetylcholine to fuse with the plasma membrane, releasing acetylcholine out into the extracellular space between the motor neuron terminal and the motor
From Yahoo Answers
Answers:There are many...many...many muscle "cells" in a muscle. Many long strands. When stimulated, of course, they contract. If 100% of the muscle fibres in the muscle are stimulated, the limb moves as far as it can. If 50% are stimulated, the limb moves a certain distance (dependant on restraint, distance, etc...) and stops. Graded contractions are basically how much stimuli the muscle fibres are getting, and how much movement is accomplished. The less the stimuli, the weaker the grade.
Answers:Maybe Ach binding could relate to adding the cheese... or melting cheese could represent the depolarization wave down the T-tubule. The tropomyosin movement to expose actin to the myosin heads seems kind of like when you bite into a (poorly made) pizza, and all the cheese slides off (but then cheese couldn't represent Ach).
Answers:what kind of popcorn?
Answers:1. Each myosin has: A. a binding site for an ATP molecule B. a binding site for an actin molecule C. the ability to swivel when powered by ATP > D. A and B E. A, B, and C 2. As ATP binds to the myosin head at the beginning of a muscle contraction cycle, > A. The myosin head detaches from actin B. The myosin head initiates binding with actin C. the myosin head tightens its bond to actin D. ATP does not bind to the myosin head E. None of these complete the statement correctly 3. At the conclusion of the power stroke, A. inorganic phosphate has been released from the myosin > B. actin has been moved toward the M line C. ADP is released from the myosin head D. the myosin head is tightly bound to actin E. All of these are correct 4. Which statement is true? A. The sarcomere shortens, the two Z disks at each end move closer together, and the I band and H zone narrow B. The thin filaments slide along the thick filaments as they move toward the M line in the center of the sarcomere C. When a muscle contracts, myosin molecules coil up like springs to shorten the length. D. A and B are true. > E. A, B, and C are true.