VOL: 102, ISSUE: 47, PAGE NO: 28
Marion Richardson, BD, RN, Certed, DipNurs, RNT, is principal lecturer, University of Hertfordshire
Without muscles, human beings would be unable to live - our hearts would not beat, blood would not circulate, air would not be drawn into the lungs and we would be unable to move or to communicate. The importance of muscle is reflected in the fact that it makes up a huge 40-50% of the body’s mass. Muscle takes its name from the Latin word ‘mus’ (little mouse) because of the appearance of some muscles of moving under the skin (Marieb, 2006).
Muscle cells are specialised to contract. The cells are similar in structure to the other cells in the body but are elongated and, for this reason, are called muscle fibres. They consist mostly of water (about 75%), proteins and inorganic salts. There are around 700 different muscles in the human body and more than 250 million muscle fibres.
The body contains three distinct types of muscle (skeletal, cardiac and smooth), which will be outlined in this article. The following three articles in the series will address the structure and function of skeletal muscle, the largest muscle mass in the body.
Characteristics of muscle
Marieb (2006) reminds us that the characteristics of muscle allow it to perform its functions:
- The feature that sets muscle apart from other body tissues is that it is able to transform chemical energy from nutrient molecules into mechanical energy, which makes muscles capable of exerting force;
- Muscles are excitable or irritable. This means that they are capable of receiving and responding to a stimulus. The stimulus is usually a chemical - a neurotransmitter released by a nerve cell, a hormone or a local change in acidity (pH). In response, muscles generate an electrical impulse that causes the muscle cells to contract;
- Muscles can contract or shorten when they are stimulated. No other type of body tissue can do this;
- Muscles are extensible - that is, they can stretch or extend beyond their resting length;
- Muscle fibres are elastic and can return to their resting length after being stretched.
Types of muscle
Skeletal muscle comes in different shapes and sizes (Fig 1) and allows movement of the body and body parts. These muscles have the longest fibres. Skeletal muscles have obvious stripes, or striations, and can be controlled voluntarily, although we do not have to think about all the muscle movements. Skeletal muscles are able to contract very rapidly but they tire easily; they must also rest after relatively short periods of activity otherwise physical damage and muscle fatigue will occur.
Skeletal muscles can exert tremendous power and are remarkably adaptable. For example, hand muscles can exert a force of a few grams to pick up a feather but the same muscles can then exert a force of over 30kg or so to pick up a heavy object.
Cardiac muscle makes up most of the heart walls. The muscles are striated and are involuntary - we have almost no conscious control over how our heart beats. Cardiac muscle cells usually contract at a steady pace set by the heart’s pacemaker cells, though nerves can influence them to speed up or slow down.
The cardiac muscle tissue forms many branches, which then recombine (Fig 2) to form a continuous sheet (Thibodeau and Patton, 2005). This enables the tissue to contract as a unit and improves its efficiency. Cardiac muscle never rests - to do so would be fatal. Contractions of cardiac muscle move blood through the heart and around the body.
Smooth muscle tissue is found in the walls of the hollow visceral organs such as the respiratory passages, intestinal tract and urinary bladder. This is not striated and is involuntary. Contractions of smooth muscles are slow and sustained. The contraction of most of these muscles forces fluid and other substances through the internal body channels.
Functions of muscle
Humans are designed for movement. Our primitive hunter-gatherer ancestors’ lifestyles required constant activity - they did not exercise for fun but in order to live. Movements of the human body and parts of the body result from muscle contraction. This enables us to walk and sit and jump, to escape from danger and to express our emotions in a variety of ways.
Skeletal muscles move bones by pulling on them and work together to produce smooth movements of our limbs (see part four of this series) but these muscles also help to move the eyes and the tongue and to control the voluntary sphincters of the body (McLaren, 2005).
Cardiac muscle pumps blood around the body in order to supply the nutrients that are needed by the cells of the body.
Smooth muscle in the walls of the blood vessels helps to maintain blood pressure. In the hollow organs of the digestive, urinary and reproductive tracts it helps to propel or squeeze substances (for example, foodstuffs, urine, a baby respectively) through the organs and along the tracts (Marieb, 2006).
Maintaining posture or muscle tone
Although we are not generally aware of it, our skeletal muscles are continually working and making minor adjustments to allow us to maintain an upright posture despite the constant force of gravity trying to pull us down. This muscle tone is a result of relatively few of the muscle’s fibres shortening at any one time (tonic contraction) and maintaining just enough force to hold us in the required position.
Muscles and their associated tendons commonly cross joints and work together to stabilise them. This is particularly true of the knee and of the shoulder joint, which would be highly unstable were it not for the many muscles that hold it in place.
The body needs to be maintained at a constant temperature in order to function optimally - most of the heat required for this is produced by muscles, which generate heat as they contract. The energy needed to produce muscle contraction is obtained from adenosine triphosphate (ATP), which is manufactured in the muscle cells. Most of this energy is used to shorten the muscle fibres (see part three of this series) but some is ‘lost’ as heat during the reaction. Skeletal muscle accounts for at least 40% of body mass and is the muscle type responsible for generating most body heat.
Guarding entrances and exits
The openings of digestive and urinary tracts are encircled by skeletal muscles. These muscles provide voluntary control over swallowing, urination and defecation (Martini, 2005).
Supporting soft tissues
The abdominal wall and the floor of the pelvic cavity consist of sheets of skeletal muscle that support the weight of the visceral organs and protect the internal tissues from injury.
We have seen that muscles have a number of essential functions and without them, human life cannot exist. The next three articles will examine in detail the structure and function of skeletal muscle, the largest of the three muscle types in the body.
This article has been double-blind peer-reviewed