Muscle contraction



A muscle contraction (also known as a muscle twitch or simply twitch) is a biological process that occurs when a muscle cell (called a muscle fiber) shortens. Locomotion in most higher animals is possible only through the repeated contraction of many muscles at the correct times.

Functions
Movement is a important function in our body, without it, we can not do anything, even breathe. The integrated action of joints, bones, and skeletal muscles produces obvious movements such as walking and running. Skeletal muscles also produce more subtle movements that result in various facial expressions, eye movements, and respiration. In addition to movement, muscle contraction also fulfills some other important functions in the body, such as posture, joint stability, and heat production. Posture, such as sitting and standing, is maintained as a result of muscle contraction.

The skeletal muscles are continually making fine adjustments that hold the body in stationary positions. The tendons of many muscles extend over joints and in this way contribute to joint stability. This is particularly evident in the knee and shoulder joints, where muscle tendons are a major factor in stabilizing the joint. Heat production, to maintain body temperature, is an important by-product of muscle metabolism. Nearly 85 percent of the heat produced in the body is the result of muscle contraction.

Skeletal muscle contraction


Skeletal muscle is also called striated muscle because of its striped appearance. It carries out all voluntary movements, such as playing a piano, and generates the movements of breathing. Skeletal muscle cells named muscle fibers are large. A muscle such as your biceps is composed of many muscle fibers bundled together by connective tissue.

Each muscle fiber is packed with myofibrils, which is bundles of contractile filaments made up of actin and myosin. A longitudinal view of a myofibril reveals the reason for the striated appearance of skeletal muscle and cardiac muscle. The myofibril consists of repeating units, called sarcomeres, which are the units of contraction. Each sarcomere is made of overlapping filaments of actin and myosin, which create a distinct band pattern. As the muscle contracts, the sarcomeres shorten, and the appearance of the band pattern changes.

When the muscle contracts, the sarcomere shortens. The H zone and the I band become much narrower, and the Z lines move toward the A band as if the actin filaments. This observation led Huxley and Huxley to propose the sliding filament theory of muscle contraction: Actin and myosin filaments slide past each other as the muscle contracts.

Internal organs' slow contraction
Most of our internal organs provided the contractile force by the smooth muscle. Internal organs are under the control of the autonomic nervous system. Smooth muscle cells are the simplest muscle cells, they are usually long and spindle-shaped.

Smooth muscle tissue, such as that from the wall of the digestive tract, has interesting properties. The cells are arranges in sheets, and individual cells in a sheet are in electrical contract with one another through gap junctions. So as a result, an action potential generated in the membrane of one smooth muscle cell can spread to all the cells in the sheet of tissue. Then the cells in the sheet can contract in a balanced fashion.

Another property of smooth muscle cells is that their plasma membranes are sensitive to being stretched. That means is the wall of the digestive tract is stretched in one position, the membranes of the stretched cells depolarize, reach threshold, and fire action potentials, which cause the cells to contract. Therefore, the smooth muscle contracts after being stretched, and the harder it stretched, the stronger the contraction.

Other causes that change the membrane potential of smooth muscle cells are the neurotransmitters of the autonomic nervous system. In the case of the digestive tract, acetylcholine causes smooth muscle cells to depolarize and hence makes them more likely to fire action potentials and contract. Norepinephrine causes these muscle cells to hyperpolarize and then makes them less likely to fire action potentials and contract. (Purves, p 905)

Heart beat

 * Main Article: Heart



Cardiac muscle are striated because of the regular arrangement of their actin and myosin filaments, that's why they looks different from smooth muscle or skeletal muscle when viewed under the microscope. The heart walls can withstand high pressures while pumping blood without the danger of developing leaks because cardiac muscle cells also branch. Those branches of bordering cells interdigitate into a meshwork that allows cardiac muscle to resist tearing.

The individual cardiac cells are in electrical contact with the others, gap junctions in the intercalated discs offer low resistance to ions or electric currents. Regular beating of the heart is achieved as a result of the inherent rhythmicity of cardiac muscle. (Purves, p 905)

Some specialized cardiac muscle cells which called pacemaker cells start the rhythmic contractions of the heart. Because of these pacemaker cells. the heartbeat is myogenis-generated by the heart muscle itself. The autonomic nervous system changes the rate of the pacemaker cells, but it is not necessary for their continued rhythmic function. A heart removed from an animal continues to beat with no input from the nervous system. (Purves, p 905)

The myogenic nature of the heartbeat is a major factor in making heart transplants possible. (Purves, p 905) The pacemaker refers to the cell mass that stimulates the heart’s actions. The pacemaker is only responsible for the actual beating of the heart, not the heart rate. The heart rate is stimulated and regulated via autonomic innervation. Spontaneous depolarization can not be handled by the pacemaker.