Introduction to the Muscular System

The muscular system is a complex network of tissues designed to produce movement, maintain posture, and stabilize the body. While the system includes various types of muscle, this study focuses on skeletal muscles, which are under voluntary control and are responsible for moving our bones. To understand how these muscles function, we must look at how they are organized, how they attach to the skeleton, and the specific ways they are named by anatomists.

Divisions of the Muscular System

Skeletal muscles are broadly categorized into two main divisions based on their location and the roles they play in body mechanics:

1. Axial Muscles (60%): These muscles are located along the central axis of the body. They are responsible for positioning the head, the spinal column, and the rib cage. This group includes muscles of facial expression, mastication (chewing), the tongue, the pharynx, and the muscles of the vertebral column.
2. Appendicular Muscles (40%): These muscles support the limbs and the girdles (pectoral and pelvic). They are primarily responsible for moving the arms, legs, hands, and feet.

Muscle Organization and Fascicle Arrangement

A muscle is not just a single solid mass; it is organized into bundles called fascicles, which are composed of individual muscle fibers. The specific way these fascicles are arranged within the muscle determines its functional capabilities, such as its power, range of motion, and speed of contraction.

Patterns of Fascicle Organization

There are four primary patterns of fascicle arrangement:

• Parallel: The fibers run parallel to the long axis of the muscle. These muscles are often "straplike" and can shorten significantly while increasing in diameter during contraction. An example is the sartorius muscle.
• Convergent: The fibers spread out from a broad origin and converge toward a narrow insertion (often on a tendon or aponeurosis). Because the fibers pull in different directions depending on which are stimulated, they are not as strong as parallel muscles of the same size. The pectoralis major is a classic example.
• Pennate: These fibers form an angle with the tendon, much like the structure of a feather. Because they can pack more myofibrils into a given area, pennate muscles can develop much more force than parallel muscles, though they cannot move their tendons as far.
  • Unipennate: Fibers are on only one side of the tendon (e.g., extensor digitorum longus).
  • Bipennate: Fibers are on both sides of the tendon (e.g., rectus femoris).
  • Multipennate: The tendon branches repeatedly within the muscle (e.g., deltoid).
• Circular: Also known as sphincters, these muscles form rings that open and close to guard body entrances, such as the mouth (orbicularis oris).