Muscles are the powerhouses of the body. They are biological motors that convert chemical energy into mechanical energy, to transform messages from the central nervous system into physical movement. Muscles are involved in almost every movement of the body, from the blinking of an eye to the large motions involved in running and jumping. They are also responsible for movements within our bodies of which we are not even aware. For example, they help propel food into the stomach and along the gut, they are involved in contractions of the bladder and the womb, and every second one large muscle – the heart – contracts to pusharound the body. Some muscles do very little in terms of causing actual movement, but they have an important role in posture and in giving our bodies shape and protection. Muscles are kept in shape and working order only by using them. If muscles are not used through lack of exercise, or if the muscle is paralysed due to injury or disease, they start to waste away and die.
Types of muscle
There are three types of muscle in the body. The most abundant is called voluntary, striated, or skeletal muscle and is involved in making observable movements of the head, neck, trunk and limbs. A second type is involuntary, smooth, or visceral muscle which is responsible for internal body movements over which we have little or no conscious control’. Heart, or cardiac, muscle is the third type; it consists of the very specialized musclethat make up most of the structure of heart.
Anatomy of muscle
In addition to these relatively large groups of musclethere are some much smaller groups of musclelike cells, called myoepithelial cells. These are found surrounding sweat glands in the skin and the ducts of the milk glands in the breast. Muscles are made of cells, called muscle fibres. They are elongated cells, which lie lengthways along the line of muscle contraction. Most fibres are arranged in clusters or sheets gathered together in bundles called fasciculi which are enclosed in fibrous sheaths. Each whole muscle is composed of thousands of fibres and fasciculi. In skeletal muscles, involved in body movement, groups of muscle fibres are anchored at each end, usually to bones by tendons. If you examine voluntary muscle under a microscope, a series of stripes or bands can be seen running across it (which is why this kind of muscle is termed ‘striated’). These bands are caused by the special arrangement of two different types of tiny filament within each muscle cell, the actin and myosin filaments. To understand the molecular mechanism underlying muscle contraction we must look closely at the structure of a muscle fibre. The sarcomere is the basic contractile unit of the muscle fibre. It consists of actin and myosin filaments, which are actually long molecules, arranged lengthways in alternating layers. A nerve impulse causes these filaments slide into each other. Thus the length of the sarcomere is shortened, which results in contraction of the muscle. The sliding of the actin and myosin filaments is ratchet-like, because of the rapid forming and breaking of chemical bonds between the filaments. This uses energy which is supplied by the burning of the glycogen stores in the muscle. A chemical called adenosine triphosphate (ATP) is involved in this process. The energy suppliers are cell organelles called mitochondria, which are abundant in muscle fibres.
Cardiac muscle is similar to voluntary muscle in that it also appears striped (though not as regularly) when looked at under a microscope. However, it differs in that cardiac muscle fibres form branches which connect with other fibres as a network. This arrangement provides the contraction impulses to be conducted from fibre to fibre; therefore the heart can pump in a co-ordinated fashion and works as an entity rather than as a bunch of separated fibres. Involuntary or smooth muscle, as the latter name suggests, appears more uniform under the microscope without the regular bands of striated or cardiac muscle. The individual cells are spindle-shaped, thicker in the centre and tapering at the ends. The cells are surrounded by a fine membrane and are fitted intimately together to form sheets of muscle. They also contain actin and myosin filaments but these are not as clearly seen or regularly arranged as those in voluntary muscle.
Voluntary muscles are the muscles of body movement and are mostly attached to the skeleton. This type of muscle makes up about 25 per cent of body weight (slightly more in men than in women) and is found in almost every part: arms, legs, fingers, toes, back, chest, abdomen, and so on. The movements it causes are very much dependent on how and where it is attached to bones. Each muscle is usually attached to two or more bones. Contraction of the muscle moves one or both of the bones.
Most muscles work in pairs, so that contraction of one causes bending of a joint whereas contraction of its partner produces straightening. For example, the biceps muscle of your upper arm flexes your elbow, whereas the triceps muscle below it straightens this joint. Each muscle has a fixed resting length to which it returns after contraction.
Voluntary muscle responds very quickly to messages from the nervous system. The actual speed and contraction of a whole muscle depends on how many individual fibres are stimulated: the more fibres stimulated, the greater the contraction. For these muscles to respond so quickly they must use a lot of energy from stores within the muscle. The stores can be rapidly depleted, and gradually the muscle tires (’fatigues’). This is combined with a build-up of lactic acid (a waste product of metabolism) in the muscle, which may result in cramp.