what happens during a muscle contraction?
372330
For most 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. However, some muscles (such as the heart) do not contract as a result of conscious effort. These are said to be autonomic. Also, it is not always necessary for the signals to originate from the brain. Some reflexes are fast, unconscious muscular reactions that occur due to unexpected physical stimuli. Other actions such as locomotion, breathing, chewing have a reflex aspect to them; the brain will start the contractions, but continuation of the movements can become reflexive. The action potentials for reflexes to unexpected stimuli originate in the spinal cord instead of the brain.\nThere are three general types of muscle contractions: skeletal muscle contractions, heart muscle contractions, and smooth muscle contractions.\nFor 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.\nYou can read more here\nhttp://en.wikipedia.org/wiki/Muscle_contraction
949832
It depends what movements are being performed. There are three main types of muscle contraction; Isotonic, isometric and isokinetic. Look each up on Wikipedia or something and they'll give examples im sure. \nOne example in male gymnastics is holding the crucifix on the rings which causes an isometric contracting in the biceps and triceps brachii
105640
The heart cells are called myocytes that can contract.  Proteins inside the myocyte, actin and myosin, make this contraction possible.  Calcium ions are released very quickly (from the sarcoplasmic reticulum) and trigger the contraction. Then the heart relaxes and gets ready for the next beat.  The contraction is controlled from the top of the heart (the atria) and spreads down to the botton (the ventricles), like a water ripple through a pond but in this case it is through all the cells of the heart.  The coordinated contraction then results in blood being pumped into the lungs and forward through the body.\n\nIf you need a more detalied description let me know.  This is the basic idea though.  The billions of cells of the heart are coordinated to result in proper heart beats.
846724
You could have muscle spasm where your muscle does not stop firing.  This could be because of overload of work out or an injury.  It generally stops by itself.  But if it continues for more than two weeks you should consider seeing a doctor.
689075
The higher the EMG signal, the more action potentials there are innervating the muscle.  The stronger the contraction of the muscle, the more motor units are being used, so there are more AP's.  Thus the strong the contraction, the higher the EMG signal.  I believe it would be a linear relationship.
970442
A few hours after a person or animal dies, the joints of the body stiffen and become locked in place. This stiffening is called rigor mortis. Depending on temperature and other conditions, rigor mortis lasts approximately 72 hours. The phenomenon is caused by the skeletal muscles partially contracting. The muscles are unable to relax, so the joints become fixed in place. \nMore specifically, what happens is that the membranes of muscle cells become more permeable to calcium ions. Living muscle cells expend energy to transport calcium ions to the outside of the cells. The calcium ions that flow into the muscle cells promote the cross-bridge attachment between actin and myosin, two types of fibers that work together in muscle contraction. The muscle fibers ratchet shorter and shorter until they are fully contracted or as long as the neurotransmitter acetylcholine and the energy molecule adenosine triphosphate (ATP) are present. \n\nHowever, muscles need ATP in order to release from a contracted state (it is used to pump the calcium out of the cells so the fibers can unlatch from each other). ATP reserves are quickly exhausted from the muscle contraction and other cellular processes. This means that the actin and myosin fibers will remain linked until the muscles themselves start to decompose. \nRigor mortis can be used to help estimate time of death. The onset of rigor mortis may range from 10 minutes to several hours, depending on factors including temperature (rapid cooling of a body can inhibit rigor mortis, but it occurs upon thawing). Maximum stiffness is reached around 12-24 hours post mortem. Facial muscles are affected first, with the rigor then spreading to other parts of the body. The joints are stiff for 1-3 days, but after this time general tissue decay and leaking of lysosomal intracellular digestive enzymes will cause the muscles to relax.
541503
Nerve impulse is connected with Na and K while muscle contraction has to do with Ca and Mg
493844
the movement of skeletal muscles produces heat...when you are cold, shivering is your muscles 'shaking' which produces heat to keep you warmer..if you are producing a lot of heat (for ex.-during exercise) your body sweats to try and keep you cooler.
949981
Skeletal muscle and cardiac muscle are both striated tissue. However, the specific adaptations of cardia tissue allow it to remain contracted for a slightly longer time--but not tetanus (sustained muscle contraction).\nThe calcium release from muscle depolarization is allowed to stay in the muscle slightly longer, letting the myosin bind to the actin molecules. Once calcium is reabsorbed, the myosin can no longer hold onto the actin and the muscle relaxes. This longer contraction allows blood to be efficiently pumped through the heart.\nAlso, skeletal muscle's calcium channels are only activated by depolarization from an action potential from a connecting nerve. Cardiac muscle contracts because of a signal recieved from the pacemaker located at the top side of the right atrium. The cardiac muscle tissue can beat on it's own and does not require signals from the brain to contract. (unconscious operation).\n\nHope that helps!
200058
Rigor mortis happens few hours after death. The skeletal muscles remain in a state of contraction because the body cannot provide anymore ATP molecules needed for muscle relaxation. ATP is needed to break the cross bridges formed between thick and thin filaments that make the contraction happens. Rigor mortis subsides after a certain time.\nAs for tetanus, I suppose you are talking about tetanic muscular contractions which occur when a muscle receives 20 to 30 stimuli per second for incomplete tetanus and 35 to 50 stimuli per second for complete tetanus. The muscles remain totally contracted without any relaxation whatsoever. However the muscle will relax when the stimuli cease.Complete tetanus occurs when you are alive. Tetanus also refers to a disease caused by a bacteria entering the body by a puncture wound. One of the symptoms of the disease is severe cramping with continuous tetanic contractions in the muscles of the body.
547015
Muscle cells undergo hypertrophy when they are exercised (resistance training).  Hypertrophy means each fibre that makes up the muscle increases in width but not in number.  The current theory is that small tears are produced in the muscle when it is worked out.  As these tears are healed during recovery (~48hrs after exercise) the muscle lays down new contractile filaments (protein) and begins to increase in diameter.  Some research does suggest that hyperplasia (increased muscle cells) can occur when the muscle reaches a very large size, but this has not been completely proven yet.  Satellite cells, which are stem-like cells in the muscle can also be stimulated by hormones produced by exercise (like IGF-2) to migrate into the muslce and join with existing muscle cells.  This adds nuclei to the existing cells along with contractile filaments allowing the muscle to increase in diameter (hypertrophy).
72657
All animals have a charged neuromuscular system. Thatis what allows for normal contractions of your bodies muscles. Every time a muscle contracts for what ever reason there is a polority shift and a measurable amount of current flows through your nerve and the muscle that is to contract.
980721
Heat is generated through inefficient energy reactions in our body.\n\nAs you may remember from physics class, no reaction is 100% efficient - for example: In a light bulb, not all of the electrical energy in converted into light; some is converted into heat.\n\nThe same is true of our cells, and a some of our energy is released as heat (instead of what is intended!) Muscle movement is especially inefficient, with a great deal of the energy used for muscle cell contraction given off as heat. This is why you get hot when you do exercise!!
1157941
The contraction of a skeletal muscle is caused by the movement of actin and myosin fibers resulting from influx of exracelluar calcium caused by surface depolarization of the cell initiated by presynaptic discharge of a motor neuron. If this is a homework question, I suppose a one word answer would depend on the nature of the course discussion to which this question is addressed.
440066
A neuromuscular junction is the synapse or junction of the axon terminal of a motoneuron with the motor end plate, the highly-excitable region of muscle fiber plasma membrane responsible for initiation of action potentials across the muscle's surface, ultimately causing the muscle to contract. The signal passes through the neuromusclar junction via the neurotransmitter acetylcholine.
713095
The heart IS a muscle, but a heart attack isn't a cramp like you would get in say a leg muscle. \n A heart attack, also called a myocardial infarction, refers to the death of a certain segment of the heart muscle (myocardium).  It's usually the result of a complete blockage in one of the main coronary arteries or a branch which supplies oxygentated blood to the heart muscle.  The main cause of myocardial infarction is atherosclerosis (the build up of plaque on the inside of the blood vessels due to fat substances, cholesterol, calcium and other substances)  in the coronary arteries which results in impaired contractility of the heart muscle within seconds, and is initially restricted to the affected segment.\n\nThe myocardial ischemia or infarction begins in the endocardium (the inner lining of the heart) and spreads to the epicardium (the outer lining of the heart). Irreversible heart damage will occur if the blockage is complete for at least 15-20 minutes. Irreversible damage occurs maximally in the area at risk, and when the occlusion is maintained for 4-6 hours. Most of the damage occurs in the first 2-3 hours.  Restoration of flow within the first 4-5 hours is associated with salvage of the heart muscle, but the salvage is greater if flow is restored in the first 1-2 hours.\n\nA major determinant of death and illness is the size of the infarct. Increasing the oxygen supply to the involved site of blockage by coronary reperfusion can be done through use of certain medications and angioplasty, stents or artherectomy.\n\nAddendum: \n(Impaired contractility means a muscle is unable to contract the way it should or can't contract at all.)\n\nCramps:\nA muscle cramp is an involuntary, painful CONTRACTION of the muscles which produce a hard, bulging muscle. Muscle twitching, also called fasciculation is the result of spontaneous local muscle contractions that are involuntary. The muscle cramp usually only affects the individual muscle groups connected to a particular motor neuron.  Some common causes of cramps are:\n\nMuscle fatigue\nHeavy exercise \nDehydration \nPregnancy \nHeat Cramps\n\nThere may be some temporary loss of circulation to the muscle group that is cramped because of muscle contraction, but there is NO death of tissue due to a clot or embolus.  There are no clots or emboli involved in a cramp therefore they aren't the same as an infarction (as explained in the answer below).     :-)
1125727
When the old uterine lining begins to break down, molecular compounds called prostaglandins are released. These compounds, specifically prostaglandin F2alpha (PGF2alpha), cause the muscles of the uterus to contract. When the uterine muscles contract, they constrict the blood supply (vasoconstriction) to the endometrium. This contraction blocks the delivery of oxygen to the tissue of the endometrium which, in turn, breaks down and dies. After the death of this tissue, the uterine contractions literally squeeze the old endometrial tissue through the cervix and out of the body by way of the vagina. \n\nThe difference between menstrual cramps that are more painful and those that are less painful is apparently related to a woman's prostaglandin levels. Women with menstrual cramps have prostaglandin blood levels that are 5-13 times higher than women who do not experience cramps. Menstrual cramps are very similar to those a pregnant woman experiences when she is given prostaglandin as a medication to induce labor.