23.15–23.16: The muscular and skeletal systems enable movement.

The Gentoo penguin is a masterful surfer.

Figure 23.33: Movement: the work done by skeletal muscles.

Running, flying, swimming, crawling, walking, digging, dancing. Animals, with very few exceptions, move. And in vertebrates, movement is generally initiated by cells of the nervous system that stimulate muscle contractions that pull on a rigid skeletal system. In this section, we examine the three types of muscle tissue, how contractions are generated within that tissue, and how input from the nervous system can initiate and control muscle contractions. We also investigate how the size and strength of muscles reflect their use and how physical training can increase muscle strength.

As we saw in Section 20-4, muscle tissue is made up of elongated cells capable of generating force when they contract, and there are three types of muscle tissue.

• 1. Skeletal muscle is attached to bones by connective tissue and is controlled by individual neurons attached to each muscle fiber. It accounts for most of the movement we see in vertebrates and makes up about 40% of human body weight. The muscular system of humans contains about 700 skeletal muscles in all.
• 2. Cardiac muscle causes the heart to pump blood through the body. Not initiated by the nervous system or under conscious control, cardiac muscle contractions and relaxations occur continuously throughout life, with the muscle cells containing more energy-releasing mitochondria than other types of muscle cells.
• 3. Smooth muscle, which also is not under conscious control, surrounds blood vessels and many internal organs. In some cases, smooth muscle contractions are stimulated by the autonomic nervous systems, and in other cases, smooth muscle is able to contract without any stimulation from the nervous system. Compared with the other muscle types, smooth muscle generates slower contractions, which can gradually move blood, food, or other substances through vessels and organs.

Our focus here is on skeletal muscle. To facilitate movement, skeletal muscles are attached to bones by connective tissue. Often, a muscle tapers at each end into tendons, which attach the muscle at the two ends—the “insertion” and the “origin” of the muscle—to bones. The contraction of the muscle then pulls the attachment points closer together and causes a movement of the bone. The biceps muscle of the upper arm, for example, has its origin in the shoulder and its insertion in one of the bones of the forearm. Contraction can rotate the forearm or bring the lower part of the arm closer to the shoulder. Two muscles generally work in opposition, so that one contracts to move a bone in one direction, and the other—the triceps in the upper arm—has its insertion and origin located so that its contraction moves the bone in the opposite direction (FIGURE 23-33).

Figure 23.34: Muscle fibers contract to generate force.

Figure 23.35: Fast-twitch and slow-twitch muscles.

A skeletal muscle is made up of a bundle of fibers. Each fiber is a single cell, but has multiple nuclei and numerous myofibrils, cylindrical organelles that shorten when they contract. A myofibril contains repeating units called sarcomeres, which are where the contraction takes place.

The sarcomeres are composed of large numbers of long filaments, overlapping and parallel to each other. The filaments come in two types: thin filaments made mostly from the protein actin, and thick filaments made mostly from the protein myosin. There may be 100,000 sarcomeres in a biceps muscle cell.

The sarcomere shortens in a four-step process that resembles climbing a rope (FIGURE 23-34).

• 1. Detach. A link between a myosin filament and an actin filament parallel to it is broken as a molecule of ATP binds to the myosin.
• 2. Reach. As the ATP breaks down, energy released alters the shape of the myosin into a higher-energy shape, much like bending a twig—the bent twig is in a higher-energy shape because it can release energy as it snaps back to its original shape.
• 3. Reattach. In its altered shape, the myosin reaches farther down the actin filament, where it reattaches.
• 4. Pull back. The myosin then snaps back to its original shape, pulling the actin filament as it does so and thus shortening the fiber. This last step is considered the “power stroke.”

As the contraction process occurs across numerous sarcomeres in a muscle cell, the entire muscle can shorten. From the perspective of energy, the potential energy of ATP is converted to the kinetic energy of sarcomere shortening, which can do work (for instance, as your arm curls a dumbbell).

When a muscle contracts, the duration between a contraction and a relaxation is called a twitch. Muscle fibers within a muscle vary in how quickly they can twitch, with two general types. Fast-twitch fibers, which tend to function anaerobically, can contract and relax about 10 times faster than slow-twitch fibers, but have relatively weak endurance. The slow-twitch fibers, which are surrounded by rich oxygen-delivering capillary beds and large numbers of oxygen-storing myoglobin molecules, can sustain activity for much longer periods of time. Although most muscles have approximately equal proportions of fast-twitch and slow-twitch fibers, there is some variation among the fibers of different muscles, as well as among different individuals. It has been shown, for example, that world-class sprinters have significantly more fast-twitch fibers in their leg muscles, while world-class long distance runners tend to have more slow-twitch fibers in their leg muscles (FIGURE 23-35).

When muscles are used for high-intensity work against increasing resistance, as in many weight-training programs, muscle fibers increase in size as the fast-twitch fibers become thicker. Endurance training, on the other hand, does not cause an increase in the size of muscle cells.