<P> Neuromuscular junctions are the focal point where a motor neuron attaches to a muscle . Acetylcholine, (a neurotransmitter used in skeletal muscle contraction) is released from the axon terminal of the nerve cell when an action potential reaches the microscopic junction, called a synapse . A group of chemical messengers cross the synapse and stimulate the formation of electrical changes, which are produced in the muscle cell when the acetylcholine binds to receptors on its surface . Calcium is released from its storage area in the cell's sarcoplasmic reticulum . An impulse from a nerve cell causes calcium release and brings about a single, short muscle contraction called a muscle twitch . If there is a problem at the neuromuscular junction, a very prolonged contraction may occur, such as the muscle contractions that result from tetanus . Also, a loss of function at the junction can produce paralysis . </P> <P> Skeletal muscles are organized into hundreds of motor units, each of which involves a motor neuron, attached by a series of thin finger - like structures called axon terminals . These attach to and control discrete bundles of muscle fibers . A coordinated and fine tuned response to a specific circumstance will involve controlling the precise number of motor units used . While individual muscle units contract as a unit, the entire muscle can contract on a predetermined basis due to the structure of the motor unit . Motor unit coordination, balance, and control frequently come under the direction of the cerebellum of the brain . This allows for complex muscular coordination with little conscious effort, such as when one drives a car without thinking about the process . </P> <P> At rest, the body produces the majority of its ATP aerobically in the mitochondria without producing lactic acid or other fatiguing byproducts . During exercise, the method of ATP production varies depending on the fitness of the individual as well as the duration and intensity of exercise . At lower activity levels, when exercise continues for a long duration (several minutes or longer), energy is produced aerobically by combining oxygen with carbohydrates and fats stored in the body . During activity that is higher in intensity, with possible duration decreasing as intensity increases, ATP production can switch to anaerobic pathways, such as the use of the creatine phosphate and the phosphagen system or anaerobic glycolysis . Aerobic ATP production is biochemically much slower and can only be used for long - duration, low intensity exercise, but produces no fatiguing waste products that cannot be removed immediately from sarcomere and body and results in a much greater number of ATP molecules per fat or carbohydrate molecule . Aerobic training allows the oxygen delivery system to be more efficient, allowing aerobic metabolism to begin quicker . Anaerobic ATP production produces ATP much faster and allows near - maximal intensity exercise, but also produces significant amounts of lactic acid which render high intensity exercise unsustainable for greater than several minutes . The phosphagen system is also anaerobic, allows for the highest levels of exercise intensity, but intramuscular stores of phosphocreatine are very limited and can only provide energy for exercises lasting up to ten seconds . Recovery is very quick, with full creatine stores regenerated within five minutes . </P> <Table> <Tr> <Td> </Td> <Td> This section needs expansion . You can help by adding to it . (November 2017) </Td> </Tr> </Table>

What organs are a part of the muscular system