Muscular System

Muscular System

After studying this chapter, readers should be able to:

1. Distinguish between the origin and insertion of a skeletal muscle.

2. Differentiate between flexion and extension.

3. Differentiate between parallel, convergent, pennate, and circular muscles.

4. Which type of lever system is most common in the skeletal muscles?

5. Identify the actions of the rhomboid major, subclavius, and trapezius muscles.

6. Explain the movements of the deltoid, pectoralis major, and teres major.

7. Name the muscles of the abdominal wall and explain the action of the rectus abdominis.

8. Explain the muscles that flex and extend the thigh.

9. Describe the quadriceps femoris group and the function of the muscles it contains.

10. Describe the function of the gastrocnemius.

Overview

Skeletal muscles usually function in groups, with the nervous system stimulating the desired muscles to perform the intended function. A muscle that con-tracts to provide most of a desired movement is called a prime mover or agonist. A good example is the pectoralis major muscle, which is a prime mover of arm flexion. Other muscles, known as synergists, work with a prime mover to make its action more effective by adding a small amount of additional force. For example, when you bend your forearm, the ago-nist muscles are the biceps and the synergists are the triceps. In muscles that cross several joints, contrac-tion causes movement at all the spanned joints unless other muscles act as stabilizers of the joints. Other flex-ors may cause some undesirable movements in a joint, but synergists prevent this and allow the total force of the prime mover to occur in the desired directions. Some synergists, known as fixators, may also assist an agonist by preventing another joint from moving to stabilize the origin of the agonist. Fixator muscles run-ning from the axial skeleton to the scapula cause the scapula to be immobilized. Only desired movements can then occur at the shoulder joint. The muscles that help maintain upright posture are fixators.

Other muscles act as antagonists to prime mov-ers. They cause movement in the opposite direction. In the earlier example, the triceps are the antagonists to the biceps. Smooth body movement depends on antagonists relaxing while prime movers contract. Muscles may work opposite to each other or together to control various movements. Antagonists and their prime movers are situated on the opposite sides of joints across which they function. An example is the ­pectoralis major, which acts as an antagonist to the latissimus dorsi, the prime mover that extends the arm. It is important to understand that antagonists can actually also be prime movers such as latissimus dorsi when it acts as the prime mover of arm extension.

Origins and Insertions

One end of a skeletal muscle usually is fastened to a relatively immovable part (origin) at a movable joint. The other end connects to a movable part (insertion) on the other side of the joint. As contraction occurs, the insertion is pulled toward the origin. There may be more than one origin or insertion such as in the biceps brachii muscle of the arm. When this muscle contracts, the insertion being pulled toward its origin causes the forearm to flex at the elbow (FIGURE 10-1). Muscle con-traction produces specific actions or movements.

The head of a muscle is the part closest to its origin­. The term flexion describes a decrease in the angle of a joint, for example, a movement of the fore-arm that causes it to bend at the elbow. The term extension describes an increase in the angle of a joint, for ­example, a movement of the forearm that straightens the elbow.

Arrangement of Skeletal Muscles

Skeletal muscles, according to the arrangement of their fascicles, are divided into four distinct types: parallel muscles, convergent muscles, pennate mus-cles, and circular muscles. The pennate muscles are subdivided into unipennate, bipennate, and multipennate­ muscles.

Parallel Muscles

Most skeletal muscles are classified as parallel muscles, in which the fascicles are parallel to the long axes. Some are flat muscular bands with broad attachments called aponeuroses at each end, whereas others are thick and cylindrical having tendons at one or both ends. An example of a flat parallel muscle is the sartorius muscle, located in the thigh. When they are thick and cylindrical, they have a spindle shape with a central body. An example of a parallel muscle is the biceps brachii, which has a spindle shape and an expanded body. Sometimes, such spindle-shaped muscles are referred to as fusiform­ muscles.

Convergent Muscles

In a convergent muscle, the muscle fascicles extend over a broad area, converging on a single attachment site. The muscle may pull on a tendon, aponeurosis, or a slender band of collagen fibers. This band is known as a raphe. An example of a convergent muscle is the pectoralis major, which is triangular or fan-shaped.

Pennate Muscles

In a pennate muscle, the fascicles create a com-mon angle with the tendon and muscle fibers pull at an angle. This means pennate muscles do not move tendons as far as parallel muscles do, but they have more tension because they have more muscle fibers and myofibrils. The fascicles and muscle fibers are short and obliquely attached to the central tendon funning the entire length of the muscle. An example of a unipennate muscle is the extensor digitorum of the leg, which has its fascicles inserted into just one side of the tendon. One example of a bipennate muscle is the rectus femoris in the thigh, which has fascicles inserted into the tendon from opposite sides. It resembles the shape of a feather. One multipennate muscle is the del-toid of the shoulder, which appears as many feather located side by side, with each of them inserted into one large tendon.

Circular Muscles

In a circular muscle or sphincter, the fascicles are arranged around an opening in a concentric pattern. Muscle contractions cause a decrease in the diameter of the opening, such as the orbicularis oris muscle of the mouth or the orbicularis oculi muscle of the eye.

Arrangement of Fascicles

The range of motion and strength of a muscle are based on the arrangement of its fascicles. Skeletal muscle fibers can shorten to approximately 70% of their resting length when they contract. Therefore, longer and more parallel muscle fibers, in compar-ison to a muscle’s long axis, means the more the muscle is able to shorten. Parallel fascicle arrange-ments offer the most ability to shorten, but are not as strong as other types of fascicle arrangements. The power of a muscle is based on the number of muscle fibers in it. More muscle fibers offer more strength. For example, bipennate and multipennate muscles have more fibers and are very strong while only shortening slightly.

1. Define the terms origin and insertion.

2. Differentiate between flexion and extension.