Muscles of the body
The functions of muscles
- Contracting and shortening muscle fibres produce movement
- Maintain posture and body position
- Support soft tissue
- Protect abdominal organs
- Assist lymph and blood circulation
There are three types of muscles found in the human body. Each type has a different role and is structurally different:
- Cardiac muscle
- Skeletal voluntary muscle
- Smooth involuntary muscle
- Cardiac muscle
This is the strongest type of muscle and is specialised involuntary muscle. It is not under the control of our will but controlled by nerves, which make it contract automatically. It consists of irregular short dark striped cylindrical branched muscle fibres with a central nucleus.
- Skeletal, voluntary or striated muscle
This is muscle which is under our control and contracts at will. It is cylindrical in shape with several nuclei. It has muscle fibres lying parallel to each other in dark and light bands creating the appearance of strips, and is surrounded by a membrane.
- Smooth, involuntary muscle
This is also not under conscious control and is found throughout the body in organs, eg bladder, alimentary and respiratory tracts and the walls of the blood and lymph vessels. The muscle fibres are spindle shaped cells with one central nucleus and are bundled together with a connective tissue sheath.
Muscle tone
Muscle tone is the slight but continuous tension or contraction in the muscles at all times. This keeps the body upright. Without this the muscles would all relax at once and the body would collapse.
Muscle contraction
Skeletal muscles only contract when stimulated by the axon of a motor neuron/nerve which carries the impulse or stimulus to the muscle fibre. The point at which a motor neuron enters the muscle is called the motor point.
Muscle fatigue
This occurs when the muscle is continuously stimulated. This leads to the muscle contraction gradually weakening until it can no longer contract, due to the build up of lactic acid and carbon dioxide which are waste products, and lack of adenosine triphosphate which provides the muscle with energy.
Each muscle has an origin and an insertion. When muscles contract they create an action.
- Origin: This is usually the attachment or most fixed point (proximal part).
- Insertion: This is usually the most movable point of attachment (distil part). The attachment of muscles can be by muscle fibrous bands (aponeuroses), tendons and muscle fibres, to each other, skin, fascia, bones, ligaments or cartilage.
- Action: When any movement takes place, groups or several muscles pull on bones at the joints to create movement.
Muscle tissues have several characteristics that help to function as muscle:
- Contractibility: when a muscle shortens and thickens.
- Extensibility: when the muscle relaxes, then the muscle stretches.
- Elasticity: the ability to return to its original shape after contracting.
- Irritability: when the muscle responds to stimuli provided by nerve impulses.