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How Does the Muscles Work

By Mr Ghaz, January 22, 2011

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How Does the Muscles Work

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Skeletal, or voluntary, muscles are activated by motor nerves in the spinal cord-the bundle of nerve fibers that runs down from the brain through a channel in the spinal column. These motor nerves split into several strands, where they enter, or innervate, a voluntary muscle. Each strand then makes contact with a different muscle cell. An electrical impulse runs down the nerve from the brain and on reaching the tip of the nerve allows a tiny amount of the chemical acetyl choline, to be shed from granules where it is stored. The short distance between the nerve endings and the muscle is crossed by the acetyl choline where it alights on special areas of the muscle surfaces known as receptors. Once the receptor is occupied by the acetyl choline the muscle then contracts and remains in this state as long as the chemical is in touch with the receptor. In order to ensure that the muscles can relax, an enzyme that neutralizes acetyl choline comes into action.

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The simplest reflex movements happen through direct activation of motor nerves by signals arriving at the spinal cord from sensory receptors, the nerves that receive sensations. For example, in the ‘knee-jerk’ reflex, a tap just below the knee cap is sensed by receptors inside one of the tendons that runs across the knee joint. These receptors send signals to the spinal cord, and in turn activate motor nerves running from the spinal cord to the thigh muscles. As a result, the thigh muscle rapidly contracts, and the lower part of the leg jerk forward.

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Conscious movements of the voluntary muscles, in contrast, are set off by signals sent down the spinal cord from the brain. Some of these signals act to stimulate particular motor nerves, and others act to dowse them so a pattern is worked out which will cause some muscles to contract, and others to relax.

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The activity of the myosin and actin filaments during muscular contraction is a complicated process, in which a series of chemical bonds between them is continually formed and broken. This requires energy, provided by the burning of oxygen and food-fuel in the mitochondria, and stored and transferred as a compound called ATP (adenosine triphosphate), which is very rich in high energy phosphate. The process of muscular contraction is started by a flow of calcium (one of the common minerals of the body) into the muscle cells through a whole series of little tubes running between the myofibrils, called the microtubules.

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At any given time, several cells in a muscle will be contracting, giving the muscle a degree of tension, or tone. When enough muscle fibers contract, the whole muscle shortens, reducing the distance between its attachment points, so that two or more bones move in relation to others.

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Individual muscles can act only to shorten, and not to lengthen, the distance between two attachment points-they can pull but not push. For movement in the opposite direction, another muscle must be activated. For example, the biceps in the upper arm can flex the elbow, but extension of the arm is brought about by another muscle, the triceps, on the underside of the upper arm. Muscles such as biceps and triceps are called antagonistic-they ‘work against each other’.

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Smooth muscle is also supplied with motor nerves. However, instead of one motor nerve stimulating one muscle cell, stimulation spreads in a wave over several of the cells. This wavelike action helps, for example, in moving food through the intestines.

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Contraction of cardiac muscle is not brought about by motor nerves but by pulses from a special pulsemaker tissue within the heart. These pulses pass over the heart about 72 times every minute, causing the heart to contract and to squeeze out blood.

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Above: Muscles of the lower leg and foot. When walking, the body is thrown forward by the powerful muscles in the lower leg-the gastrocenemius and soleus. These muscles pull on the ankle joint, which is used as a lever. The extensor muscles in the foot bend the toes for the final thrust forward.

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