4 The Muscular System

Muscle tissue and its roles

Muscles produce movement, maintain posture, stabilize joints, move substances through the body, and generate heat. The body has three types of muscle tissue: skeletal, cardiac, and smooth. They differ in where they are found, how their cells appear, and whether their contractions are generally voluntary or involuntary.

The three types of muscle

Skeletal muscle is usually attached to bones by tendons. Its long, striated fibers are generally under voluntary control; skeletal muscles also help maintain posture and produce heat. A skeletal muscle is an organ made up of muscle fibers, connective tissue, blood vessels, and nerves.

Cardiac muscle forms the wall of the heart. Its striated cells contract involuntarily to pump blood. Smooth muscle is nonstriated and is found in the walls of hollow organs and blood vessels. It contracts involuntarily, for example, to move food through the digestive tract or regulate the diameter of blood vessels.

Skeletal muscle structure

Connective-tissue layers surround individual muscle fibers, bundles of fibers, and the whole muscle. These layers continue into tendons that attach muscle to bone. Inside each fiber, threadlike myofibrils contain repeating sarcomeres, the basic contractile units. Each sarcomere contains thin actin filaments and thick myosin filaments.

Major skeletal muscle groups

Skeletal muscles are commonly grouped by the parts of the body they act on. Axial muscles act mainly on the head, neck, and trunk; appendicular muscles move the limbs and their girdles.

Examples across major regions include:

• Head and neck: The masseter closes the jaw; the sternocleidomastoid turns or flexes the head.

• Shoulder and chest: The deltoid raises the arm; the pectoralis major draws the arm toward the body and across the chest.

• Back: The trapezius and latissimus dorsi move the shoulder blades and arms; the erector spinae extends and stabilizes the spine.

• Upper arm: The biceps brachii flexes the elbow, while the triceps brachii extends it.

• Abdomen and thorax: The rectus abdominis and obliques flex or rotate the trunk; the diaphragm and intercostals assist breathing.

• Hip and thigh: Gluteal muscles extend or abduct the hip; the quadriceps femoris extends the knee, while the hamstrings flex it.

• Lower leg: The gastrocnemius points the foot downward; the tibialis anterior lifts it upward.

How skeletal muscle contracts

A motor neuron signals a muscle fiber at the neuromuscular junction, releasing the chemical messenger acetylcholine. This triggers an electrical signal along the muscle-fiber membrane and into the fiber’s interior. The signal causes the sarcoplasmic reticulum to release calcium ions.

Calcium exposes binding sites on actin, allowing myosin heads to attach and pull the thin filaments inward. Repeated attachment and pulling make actin and myosin slide past one another, shortening sarcomeres and generating force. The filaments themselves do not become shorter. ATP supplies energy for the cross-bridge cycle and helps myosin detach from actin. When signaling stops and calcium returns to storage, the fiber relaxes.

Recruitment and muscle force

A muscle fiber contracts when stimulated, but the force produced by a whole muscle depends in part on how many fibers are recruited and how frequently they are stimulated. A motor unit consists of one motor neuron and the muscle fibers it controls; recruiting more motor units generally increases force.

Muscle attachments and joint movements

Skeletal muscles generate force by pulling. Most cross a joint and attach to bones on either side; when a muscle shortens, it pulls one attachment toward the other and changes the joint’s position. The more stable attachment is often called the origin, and the more movable attachment is often called the insertion. Their roles can vary with the movement being performed.

Common joint movements include flexion, which bends a body part and usually decreases the angle between bones, and extension, which straightens it and usually increases that angle. Abduction moves a body part away from the body’s midline; adduction moves it toward the midline. Rotation turns a bone around its axis. Bending the elbow is flexion, while straightening it is extension.

How muscles work together

Muscles cooperate in functional groups rather than acting alone. The agonist, or prime mover, produces most of a particular movement. The antagonist opposes or controls that movement, while synergists assist the agonist or help steady a joint. For example, during an elbow curl, the biceps brachii is a prime mover for elbow flexion and the triceps brachii acts as an antagonist. During controlled lowering of the forearm, the biceps remains active as it lengthens to control the motion.

Types of muscle contraction

A contraction describes muscle activity, not necessarily visible movement. In a concentric contraction, the active muscle shortens, as when lifting a weight. In an eccentric contraction, it lengthens while resisting a load, as when lowering a weight slowly. In an isometric contraction, the muscle develops tension without a noticeable change in length, as when holding a weight still. Muscles also stabilize joints and maintain posture through sustained or repeated activity.