15.2 : Fascicle Arrangement in Skeletal Muscles

Fascicles are bundles of muscle fibers in a skeletal muscle. Muscle fascicle arrangement is directly associated with the power and range of motion of various muscles. The configuration of these fascicles can vary, leading to different functional outcomes.

The four primary types of muscle based on fascicle arrangement are:

1. Parallel Muscles: In this type, fascicles run parallel to the long axis of the muscle. Examples include the biceps brachii and the rectus abdominis. When parallel muscles contract, they shorten in length and increase in diameter, enabling movement. They can contract until they have shortened by about 30% in length. The tension developed during contraction depends on the muscle's total number of myofibrils.
2. Convergent Muscles: These muscles have fascicles that extend over a broad area but converge on a common attachment site. Examples include the pectoralis muscles. Convergent muscles can adapt to different activities as stimulating different muscle regions can change the direction they pull. However, when the entire muscle contracts, the muscle fibers do not pull as hard on the attachment site as a parallel muscle of the same size.
3. Pennate Muscles: In pennate muscles, the fascicles attach obliquely to a central tendon that runs the length of the muscle, resembling the pattern of feathers along a quill. This arrangement allows for a higher density of muscle fibers in a given area than parallel muscles. The increased fiber density in pennate muscles translates to greater force production, albeit at the cost of reduced range of motion and speed. Pennate muscles are further categorized based on the exact arrangement of their fascicles: unipennate muscles have fascicles attaching to one side of the tendon, bipennate have them attaching on both sides, and multipennate feature a complex, branching network of fascicles and tendons. This intricate architecture of pennate muscles is a perfect adaptation for tasks requiring strong, forceful contractions.
4. Circular Muscles: Also known as sphincters, circular muscles have fascicles arranged concentrically around an opening. This arrangement forms a ring-like structure that can constrict or relax to control the passage of substances through the body's various pathways. When the circular muscles contract, the orifice narrows or closes, and when they relax, the opening widens, allowing passage. This mechanism is essential in many physiological processes, such as controlling food movement through the gastrointestinal tract or regulating blood flow through blood vessels. The iris of the eye is another classic example of a circular muscle, where its contraction and relaxation change the size of the pupil, thereby controlling the amount of light that enters the eye.