question: why when going to bed your leg or arm will suddenly jerk
comment: It's not unusual, and it's related to how the body handles a chemical called dopamine (this is an important chemical in the brain). The process of getting tired and going to sleep causes changes in the level of this chemical, and the body is also preparing to paralyse itself later (if you've ever had the can't-run-away nightmare - this is it! Your body literally freezes itself, and its common to become aware of this during a dream). The process of relaxing the muscles can sometimes cause them to misfire, and randomly shoot out. Another popular theory is that this sudden relaxation of muscles causes the brain to initiate an ancient reflex. The brain accidentally recognises this as falling, and reflexively orders the limbs to panic-contract in order to stop this. Regardless of the reason, it's quite common, and caffeine can make it a lot more common (as can not exercising enough or similar things).
passages: Jerks of muscle groups, much of the body, or a series in rapid succession, which results in the person jerking bolt upright from a more relaxed sitting position is sometimes seen in ambulatory patients being treated with high doses of morphine, hydromorphone, and similar drugs, and is possibly a sign of high and/or rapidly increasing serum levels of these drugs. Myoclonic jerks caused by other opioids, such as tramadol and pethidine, may be less benign. Medications unrelated to opioids, such as anticholinergics, are known to cause myoclonic jerks.
  
  Section::::Pathophysiology.
   BULLET::::- Sleep myoclonus occurs during the initial phases of sleep, especially at the moment of dropping off to sleep, and include familiar examples of myoclonus such as the hypnic jerk. Some forms appear to be stimulus-sensitive. Some people with sleep myoclonus are rarely troubled by it, or need treatment. If it is a symptom of a more complex and disturbing sleep disorders, such as restless legs syndrome, it may require medical treatment. Myoclonus can be associated with patients with Tourette syndrome.
  
  Section::::Cause.
   Sometimes, hypnic jerks are mistaken for another form of movement during sleep. For example, hypnic jerks can be confused with restless leg syndrome, periodic limb movement disorder, hypnagogic foot tremor, rhythmic movement disorder, and hereditary or essential startle syndrome, including the hyperplexia syndrome. But some phenomena can help to distinguish hypnic jerk from these other conditions. For example, the occurrence of hypnic jerk arises only at sleep onset and it happen without any rhythmicity or periodicity of the movements and EMG bursts. Also, other pertinent history allows to differentiate it.
   BULLET::::- Rigidity is characterized by an increased muscle tone (an excessive and continuous contraction of the muscles) which produces stiffness and resistance to movement in joints. Rigidity may be associated with joint pain, with such pain being a frequent initial manifestation of the disease. When limbs of the person with PD are passively moved by others, a "cogwheel rigidity" is commonly seen. Cogwheel-like or ratchety jerks are characterized by the articulation moving as opposed to the normal fluid movement; when a muscle is externally tried to move, it resists at first, but with enough force, it is partially moved until it resists again, and only with further force, will it be moved. The combination of tremor and increased tone is considered to be at the origin of cogwheel rigidity.
   Myoclonus can occur by itself, but most often as one of several symptoms associated with a variety of nervous system disorders, including multiple sclerosis, Parkinson's disease, Alzheimer's disease, Opsoclonus Myoclonus, Creutzfeldt–Jakob disease, Lyme Disease and lupus. Myoclonic jerks commonly occur in persons with epilepsy, a disorder in which the electrical activity in the brain becomes disordered leading to seizures. It is also found in MERRF (Myoclonic Epilepsy with Ragged Red Fibers), a rare mitochondrial encephalomyopathy.
   Periodic limb movement disorder is characterized by recurrent episodes of frequent limb movements while sleeping. It mostly happens in the lower parts of the body like the toes, ankles, knees and hips. It can also, in some cases, appear in the upper extremities of the body. These movements can lead the patient to wake up, and if so, sleep interruption can be the origin of excessive daytime sleepiness.
   Myoclonus is a brief, involuntary, irregular (lacking rhythm) (different from clonus, which is rhythmic/ regular) of a muscle or a group of muscles. It describes a medical sign and, generally, is not a diagnosis of a disease. These myoclonic twitches, jerks, or seizures are usually caused by sudden muscle contractions ("positive myoclonus") or brief lapses of contraction ("negative myoclonus"). The most common circumstance under which they occur is while falling asleep (hypnic jerk). Myoclonic jerks occur in healthy persons and are experienced occasionally by everyone. However, when they appear with more persistence and become more widespread they can be a sign of various neurological disorders. Hiccups are a kind of myoclonic jerk specifically affecting the diaphragm. When a spasm is caused by another person it is known as a "provoked spasm". Shuddering attacks in babies fall in this category.
   BULLET::::1. Do you or your bed partner believe that you move your arms, legs, or body too much, or have unusual behaviors during sleep?
  
  BULLET::::2. Do you move while dreaming, as if you are simultaneously attempting to carry out the dream? l Have you ever hurt yourself or your bed partner during sleep?
  
  BULLET::::3. Do you sleepwalk or have sleep terrors with loud screaming?
  
  BULLET::::4. Do your legs feel restless or begin to twitch a lot or jump around when you are drowsy or sleepy, either at bedtime or during the day?
   BULLET::::- Cortical reflex myoclonus is thought to be a type of epilepsy that originates in the cerebral cortex – the outer layer, or "gray matter," of the brain, responsible for much of the information processing that takes place in the brain. In this type of myoclonus, jerks usually involve only a few muscles in one part of the body, but jerks involving many muscles may occur. Cortical reflex myoclonus can be intensified when patients attempt to move in a certain way or perceive a particular sensation.
   Section::::In the geometric design of roads and tracks.
   Scientists do not know exactly why this phenomenon occurs, so they are still trying to understand it. Therefore, several theories have attempted to explain the actual causes behind it. Yet for the moment, none of them are certified as the right one.
   BULLET::::- Reticular reflex myoclonus is thought to be a type of generalized epilepsy that originates in the brainstem, the part of the brain that connects to the spinal cord and controls vital functions such as breathing and heartbeat. Myoclonic jerks usually affect the whole body, with muscles on both sides of the body affected simultaneously. In some people, myoclonic jerks occur in only a part of the body, such as the legs, with all the muscles in that part being involved in each jerk. Reticular reflex myoclonus can be triggered by either a voluntary movement or an external stimulus.
   The name "jerk" was borrowed from dynamics, where it means the rate of change of the acceleration of a body, that is, the third derivative of its position with respect to time (the acceleration being the second derivative); or, more specifically, a sudden and momentary spike (or dip) in that rate.
  
  Section::::Description.
   Muscle atrophy is defined as a decrease in the mass of the muscle; it can be a partial or complete wasting away of muscle, and is most commonly experienced when persons suffer temporary disabling circumstances such as being restricted in movement and/or confined to bed as when hospitalized. When a muscle atrophies, this leads to muscle weakness, since the ability to exert force is related to mass. Modern medicine's understanding of the quick onset of muscle atrophy is a major factor behind the practice of getting hospitalized patients out of bed and moving about as active as possible as soon as is feasible, despite sutures, wounds, broken bones, and pain.
   Section::::History.
  
  In 1780, Luigi Galvani discovered that the muscles of dead frogs' legs twitched when struck by an electrical spark. This was one of the first forays into the study of bioelectricity, a field that still studies the electrical patterns and signals in tissues such as nerves and muscles.
   Myoclonic jerks may occur alone or in sequence, in a pattern or without pattern. They may occur infrequently or many times each minute. Most often, myoclonus is one of several signs in a wide variety of nervous system disorders such as multiple sclerosis, Parkinson's disease, dystonia, Alzheimer's disease, Gaucher's disease, subacute sclerosing panencephalitis, Creutzfeldt–Jakob disease (CJD), serotonin toxicity, some cases of Huntington's disease, some forms of epilepsy, and occasionally in intracranial hypotension.
   abdominal muscles. It later affects the proximal leg and abdominal wall muscles. The stiffness leads to a change in posture, and patients develop a rigid gait. Persistent lumbar hyperlordosis often occurs as it progresses. The muscle stiffness initially fluctuates, sometimes for days or weeks, but eventually begins to consistently impair mobility. As the disease progresses, patients sometimes become unable to walk or bend. Chronic pain is common and worsens over time but sometimes acute pain occurs as well. Stress, cold weather, and infections lead to an increase in symptoms, and sleep decreases them.
   BULLET::::- Pain on movement is commonly caused by osteoarthritis, often in quite minor degrees, and other forms of arthritis. It may also be caused by injury or overuse and rarely by more complex causes of pain such as infection or neoplasm. The range of motion may be normal or limited by pain. "Morning stiffness" pain which eases up after the joint has been used, is characteristic of rheumatoid arthritis.
   Patellar reflex
  
  The patellar reflex or knee-jerk (myotatic) (monosynaptic) (American spelling knee reflex) is a stretch reflex which tests the L2, L3, and L4 segments of the spinal cord.
  
  Section::::Mechanism.
   In SPS patients, motor unit neurons fire involuntarily in a way that resembles a normal contraction. Motor unit potentials fire while the patient is at rest, particularly in the stiff muscles. The excessive firing of motor neurons may be caused by malfunctions in spinal and supra-segmental inhibitory networks that utilize GABA. Involuntary actions show up as voluntary on EMG scans; even when the patient tries to relax, there are agonist and antagonist contractions.
   Since forces, changing at a suitable rate in time (that is, "suitable" jerk) are the cause of vibrations, and vibrations significantly impair the quality of transportation, there is good reason to simply "minimize" jerk in transportation vehicles.
  
  As an everyday example, driving in a car can show effects of acceleration and jerk. More skilled and experienced drivers can accelerate smoothly, but beginners often provide a "jerky" ride.
   Jerks seem to occur in irregular intervals, on average about once every 10 years. In the period between jerks, each component of the field at a specific location changes with time "t" approximately as a fixed polynomial of the second degree, "A" "t" + "B" "t" + "C". Each jerk is a relatively sudden change (spread over a period of a few months to a couple of years) in the "A" coefficient of this formula, which determines the second derivative; and usually in "B" and "C" coefficients as well.
   Section::::Pathophysiology.
  
  Muscle cells work by detecting a flow of electrical impulses from the brain which signals them to contract through the release of calcium by the sarcoplasmic reticulum. Fatigue (reduced ability to generate force) may occur due to the nerve, or within the muscle cells themselves. New research from scientists at Columbia University suggests that muscle fatigue is caused by calcium leaking out of the muscle cell. This causes there to be less calcium available for the muscle cell. In addition an enzyme is proposed to be activated by this released calcium which eats away at muscle fibers.
   Muscle cells work by detecting a flow of electrical impulses from the brain, which signals them to contract through the release of calcium by the sarcoplasmic reticulum. Fatigue (reduced ability to generate force) may occur due to the nerve, or within the muscle cells themselves. New research from scientists at Columbia University suggests that muscle fatigue is caused by calcium leaking out of the muscle cell. This makes less calcium available for the muscle cell. In addition, the Columbia researchers propose that an enzyme activated by this released calcium eats away at muscle fibers.
   Section::::Types.:Peripheral muscle fatigue.
  