Muscular system
The muscular system is made up by the muscles. These are organs that are able to move the bones connected by the joints, the skin and the internal organs. Muscles are similar to machines that can transform static inactive chemical energy into active and dynamic mechanical movement (see muscular energy). There are three main types of muscle.
- Striated or skeletal muscles: they are composed of striated muscle tissue (under the microscope they appear as a series of light and dark bands). They adhere to the skin and the bones forming what is commonly called the flesh. They contract rapidly and are able to develop a considerable power for short periods of time and only «on demand»: thus, they are called voluntary muscles. Striated muscles are in charge of voluntary movement and automatic reflexes, and they are controlled by the central nervous system. They make up most of the muscles in our body and they are arranged along the skeleton. The tendons (the collagenous tissue that is highly resistant to extension), or a sheetlike connective tissue called aponeurosis merges these muscles to the bones: when a muscle contracts, the connected bones move along. Each muscle has tendons attached at each end, whereas their central part is called venter. Muscles consist of many bundles of elongated cells, arranged in parallel, called muscle fibres. Each fibre bundle is held together by a connective tissue sheath, which is innervated and perfused by capillaries carrying oxygen and glucose for its functioning. by bundles of connective tissue, called ligaments;
- Smooth muscles: they are composed of smooth fusiform muscle fibres, which are smaller than the striated ones. They are slow twitch muscles and function independently of the will: thus, they are called involuntary muscles. They cover the internal organs responsible for respiration, digestion, circulation, excretion, reproduction and allow their activities. Their movements are ruled by the autonomic nervous system.
- Cardiac muscle: the cardiac or heart muscle is found only in the heart, it is regulated and innervated by the autonomic nervous system, but it is similar to the striated musculature, although acting involuntary. It is much more resistant than skeletal muscle and it is able to contract continuously, rhythmically, without showing signs of fatigue.
