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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
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Answers:YES! Action potentials are how electrical signals propagate through our nervous systems. When you touch or smell something, you "feel" or "smell" it because action potentials are sent from your skin/nostrils to the brain. When you walk, you send signals from your brain to your muscles - via action potentials - that lead to the release of neurotransmitters (such as acetylcholine) at the neuromuscular junction, that leads to contraction of muscles. Action potentials in the heart are quite different than those in the nervous system (APs in the heart are mainly due to calcium channels and last for ~100ms; those in the nerves are mainly die to sodium channels, and last ~1ms), but they are considered APs, nonetheless. I do not know squat about the digestive tract. Just as how electrical currents flow through copper wires in your apartment via the movement of electrons, electrical currents flow through nerves in your body via the movement of (mainly) sodium ions - via the propagation on action potentials.
Answers:Striated muscle cells are found in skeletal muscles and are mainly associated with voluntary movement, such as walking or bending your elbow. They have many parallel fibers and appear striped, or striated, under a microscope. Smooth muscle cells have no striations and are used in involuntary bodily functions, mainly found in the muscles of your digestive tract. Your stomach is a smooth muscle that contracts to break down food and push it along through your intestines.
Answers:Yes, there is. Skeletal (voluntary), smooth(involuntary), and cardiac(heart).
Answers:Noooooo. Dont listen to the above. There are 3 kinds of muscle. Voluntary, Involuntary, and Cardiac. Voluntary muscles are the ones you have control over- such as the ones in your arms or legs that you can move. These tend to be striated (have layers of tissue that are visible- if you ever see raw salmon cut into fillets, it looks a bit like that) Involuntary muscles are the opposite- you cant control them willingly, they move themselves. This is also considered smooth muscle. ex- intestines. Cardiac muscle is in the heart. It is considered a mixture of the two, because it is striated like cardiac muscle, yet involuntary because we cannot willingly control our hearts/