7.2: Muscles
Muscles make up the bulk of an animal’s body and account for about half its weight. The meat on the chop or roast is muscle and is composed mainly of protein. The cells that make up muscle tissue are elongated and able to contract to a half or even a third of their length when at rest. There are three different kinds of muscle based on appearance and function: smooth, cardiac and skeletal muscle.
Types of Muscle
- Smooth muscle : Smooth or Involuntary muscle carries out the unconscious routine tasks of the body such as moving food down the digestive system, keeping the eyes in focus and adjusting the diameter of blood vessels. The individual cells are spindle-shaped, being fatter in the middle and tapering off towards the ends with a nucleus in the center of the cell. They are usually found in sheets and are stimulated by the non-conscious or autonomic nervous system as well as by hormones (see Chapter 3).
- Cardiac muscle : Cardiac muscle is only found in the wall of the heart. It is composed of branching fibers that form a three-dimensional network. When examined under the microscope, a central nucleus and faint stripes or striations can be seen in the cells. Cardiac muscle cells contract spontaneously and rhythmically without outside stimulation, but the sinoatrial node (natural pacemaker) coordinates the heart beat. Nerves and hormones modify this rhythm (see Chapter 3).
- Skeletal muscle : Skeletal muscle is the muscle that is attached to and moves the skeleton, and is under voluntary control. It is composed of elongated cells or fibers lying parallel to each other. Each cell is unusual in that it has several nuclei and when examined under the microscope appears striped or striated. This appearance gives the muscle its names of striped or striated muscle. Each cell of striated muscle contains hundreds, or even thousands, of microscopic fibers each one with its own striped appearance. The stripes are formed by two different sorts of protein that slide over each other making the cell contract (see diagram 7.1).
Diagram 7.1 - A striped muscle cell
Muscle Contraction
Muscle contraction requires energy and muscle cells have numerous mitochondria. However, only about 15% of the energy released by the mitochondria is used to fuel muscle contraction. The rest is released as heat. This is why exercise increases body temperature and makes animals sweat or pant to rid themselves of this heat.
What we refer to as a muscle is made up of groups of muscle fibers surrounded by connective tissue. The connective tissue sheaths join together at the ends of the muscle to form tough white bands of fiber called tendons . These attach the muscles to the bones. Tendons are similar in structure to the ligaments that attach bones together across a joint (see diagrams 7.2a and b).
Diagram 7.2 a and b - The structure of a muscle
Tendons Tie muscles to bones and Ligaments Link bones at joints
Structure of a muscle
A single muscle is fat in the middle and tapers towards the ends. The middle part, which gets fatter when the muscle contracts, is called the belly of the muscle. If you contract your biceps muscle in your upper arm you may feel it getting fatter in the middle. You may also notice that the biceps is attached at its top end to bones in your shoulder while at the bottom it is attached to bones in your lower arm. Notice that the bones at only one end move when you contract the biceps. This end of the muscle is called the insertion . The other end of the muscle, the origin , is attached to the bone that moves the least (see diagram 7.3).
Diagram 7.3 - Antagonistic muscles, flexion and extension
Antagonistic muscles
Skeletal muscles usually work in pairs. When one contracts the other relaxes and vice versa. Pairs of muscles that work like this are called antagonistic muscles . For example the muscles in the upper forearm are the biceps and triceps (see diagram 7.3). Together they bend the elbow. When the biceps contracts (and the triceps relaxes) the lower forearm is raised and the angle of the joint is reduced. This kind of movement is called flexion . When the triceps is contracted (and the biceps relaxes), the angle of the elbow increases. The term for this movement is extension .
When you or animals contract skeletal muscle it is a voluntary action. For example, you make a conscious decision to walk across the room, raise the spoon to your mouth, or smile. There is however, another way in which contraction of muscles attached to the skeleton happens that is not under voluntary control. This is during a reflex action , such as jerking your hand away from the hot stove you have touched by accident. This is called a reflex arc and will be described in detail in chapter 14-15.