There are almost 700 muscles in the body, which can make up almost half of your body weight.
Muscles have three main functions: movement, maintaining posture and the production of heat.
Each movement requires the co-ordinated action of several muscles. The muscular system is also involved in the movement of fluids such as blood, lymph and urine.
In order to stand upright, the fibres of some muscles create tension and rigidity. Heat is created by the movement generated by the muscles.
Each muscle is approximately 75% water, 20% protein and 5% mineral salts, glycogen and fat. Skeletal muscles are composed of bundles of muscles fibres called fasciculi. These bundles and muscles are surrounded by connective tissue sheaths. The epimysium is the outer layer which encircles the entire muscle.
The perimysium surrounds collective bundles of muscles, whilst the endomysium surrounds the individual muscle fibre. This layer contains blood capillaries that supply blood containing oxygen and nutrients, and remove waste products. These layers extend beyond the muscle fibres and become tendons or aponeuroses and attach the muscle to bone.
The three main types of muscle are voluntary, which is mainly attached to the bone, involuntary, which is found inside the digestive and urinary tracts as well as the walls of blood vessels, and finally cardiac, which is only found in the walls of the heart.
Voluntary (skeletal) muscles consist mainly of muscle fibres, each of which is enclosed in connective tissue called the endomysium. Most voluntary muscles are made up of fast-twitch fibres which react quickly but also tire easily, and slow-twitch fibres which have greater endurance. Voluntary muscles contract if a stimulus is received from a motor nerve and they are all attached to the skeleton.
Involuntary (smooth) muscles are not under the control of a conscious part of the brain and are found in the walls of hollow organs such as the stomach. These muscles are controlled by the autonomic nervous system and therefore work below the level of consciousness.
The muscle cells are spindle shaped and tapered at one end, with each cell containing a central nucleus. These muscles are controlled and contract in response to nerve impulses and neurotransmitters, and some by autorhythmic cells (self excitable).
The cardiac muscle contracts to create the pumping action required for blood flow. These contractions are automatic and never tire, and are regulated by nerves and hormones such as adrenaline.
It is branched in structure and has discs between each muscle cell. It is an involuntary muscle, regulated by the autonomic nervous system and its own pacemaker, the sinoatrial node. It cannot work without oxygen, and a shortage will create pain, muscle spasm and cardiac arrest.
Muscle attachments are known as origins and insertions. The end of the muscle closest to the centre of the body is the origin and is fixed, whilst the insertion is further away and moveable. Origins are shorter and broader, whilst insertions are longer and more dense. On contraction, the insertion generally moves toward the origin.
Muscle contraction is the ability to shorten and thicken. The speed at which they do this can vary depending on the type of muscle it is. Voluntary muscles move by a stimulus from the brain. Each muscle fibre is connected to a fibre of a nerve cell which ends in a motor point where the stimulus is given, like a tiny electrical current. When an impulse is received, the motor nerve fibre secretes a substance called acetylcholine which stimulates the muscle to contract.
The energy required to perform a muscle contraction comes from carbohydrates. Glucose, the smallest form of carbohydrate, is converted into glycogen which is broken down in the presence of oxygen during a contraction and energy is released enabling the muscle to move.
An adequate blood supply is essential if the muscles are to carry out their movements. A lack of oxygen and nutrients will result in muscle cramp, which is a painful spasm.