Anatomy & Physiology

Anatomy & Physiology Study Guide

Chapter 11: The Muscular System Nearly all skeletal muscle fibers contract at similar rates and shorten to the same degree. Variations in macroscopic and microscopic organization dramatically affect the speed, range, and power of movement produced when muscles contract. Skeletal muscle fibers form bundles called fascicles . Fascicle arrangement is associated with muscle power and range of motion. The patterns of fascicle organization determine the skeletal muscle classification, which can be convergent muscles, parallel muscles, pennate muscles, and circular muscles. 11.1 Muscle Classification Parallel Muscles Fascicles that are parallel to the long axis of a muscle, is classified as a parallel muscle. A majority of skeletal muscles are parallel muscles. A skeletal muscle fiber can contract until it has shortened by roughly 30 percent. In a parallel muscle, when the fibers contract together, the entire muscle contracts by about 30 percent. The tension developed during this contraction depends on the total number of myofibrils the muscle contains. Convergent Muscles A convergent muscle has muscle fascicles that extend over a broad area and converge on a common attachment site. The muscle may pull on a tendon, an aponeurosis, or a slender band of collagen fibers known as a raphe . The muscle fibers spread out, like a fan with a tendon at the apex. A convergent muscle is versatile because the stimulation of different portions of the muscle can change the direction of pull. However, the muscle fibers of a convergent muscle are different from those of a parallel muscle because when the convergent muscle contracts, its muscle fibers do not pull as hard on the attachment site. This occurs because, instead of pulling in the same direction, convergent muscle fibers pull in different directions. Pennate Muscles In a pennate muscle, the fascicles form a common angle with the tendon. Contracting pennate muscles pull at an angle and do not move their tendons as far as parallel muscles do. However, a pennate muscle produces more tension and contains more muscle fibers—and thus more myofibrils—than does a parallel muscle of the same size. When all of the muscle fibers are on the same side of the tendon, the pennate muscle is referred to as unipennate . A pennate muscle that has fibers on both sides of the tendon is called bipennate . If the tendon branches within a pennate muscle, the muscle is said to be multipennate . Circular Muscles In a circular muscle, or sphincter , the fascicles lie concentrically around an opening. The diameter of the opening decreases with muscle contraction. The entrances and exits of internal passageways, such as the digestive and urinary tracts, have circular muscles controlling their movement.

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