Chapter Summary

Internal Affairs (205.0K)

Introduction to the Muscular System

1. The contraction of skeletal muscle fibers results in body motion, heat production, and the maintenance of posture and body support.
2. The four basic properties characteristic of all muscle tissue are irritability, contractility, extensibility, and elasticity.
3. Axial muscles include facial muscles, neck muscles, and trunk muscles. Appendicular muscles include those that act on the girdles and those that move the segments of the appendages.

Structure of Skeletal Muscles

1. The origin of a muscle is the more stationary attachment. The insertion is the more movable attachment.
2. Individual muscle fibers are covered by endomysium. Muscle bundles, called fasciculi, are covered by perimysium. The entire muscle is covered by epimysium.
3. Synergistic muscles work together to promote a particular movement. Muscles that oppose or reverse the actions of other muscles are antagonists.
4. Muscles may be classified according to fiber arrangement as parallel, convergent, pennate, or sphincteral.
5. Motor neurons conduct nerve impulses to the muscle fiber, stimulating it to contract. Sensory neurons conduct nerve impulses away from the muscle fiber to the central nervous system.

Skeletal Muscle Fibers and Types of Muscle Contraction

1. Each skeletal muscle fiber is a multinucleated, striated cell. It contains a large number of long, threadlike myofibrils and is enclosed by a cell membrane called a sarcolemma.
1. Myofibrils have alternating A and I bands. Each I band is bisected by a Z line, and the subunit between two Z lines is called the sarcomere.
2. Extending through the sarcoplasm are a network of membranous channels called the sarcoplasmic reticulum and a system of transverse tubules (T tubules).

2. During muscle contraction, shortening of the sarcomeres is produced by sliding of the thin (actin) myofilaments over and between the thick (myosin) myofilaments.
1. The actin on each side of the A bands is pulled toward the center.
2. The H bands thus appear to be shorter as more actin overlaps the myosin.
3. The I bands also appear to be shorter as adjacent A bands are pulled closer together.
4. The A bands stay the same length because the myofilaments (both thick and thin) do not shorten during muscle contraction.

3. When a muscle exerts tension without shortening, the contraction is termed isometric; when shortening does occur, the contraction is isotonic.
4. The neuromuscular junction is the area consisting of the motor end plate and the sarcolemma of a muscle fiber. In response to a nerve impulse, the synaptic vesicles of the axon terminal secrete a neurotransmitter, which diffuses across the neuromuscular cleft of the neuromuscular junction and stimulates the muscle fiber.
5. A motor unit consists of a motor neuron and the muscle fibers it innervates.
1. Where fine control is needed, each motor neuron innervates relatively few muscle fibers. Where strength is more important than precision, each motor unit innervates a large number of muscle fibers.
2. The neurons of small motor units have relatively small cell bodies and tend to be easily excited. Those of large motor units have larger cell bodies and are less easily excited.

Naming of Muscles

1. Skeletal muscles are named on the basis of shape, location, attachment, orientation of fibers, relative position, and function.
2. Most muscles are paired; that is, the right side of the body is a mirror image of the left.

Muscles of the Axial Skeleton

The muscles of the axial skeleton include those responsible for facial expression, mastication, eye movement, tongue movement, neck movement, and respiration, and those of the abdominal wall, the pelvic outlet, and the vertebral column. They are summarized in tables 9.3 through 9.10.

Muscles of the Appendicular Skeleton

The muscles of the appendicular skeleton include those of the pectoral girdle, humerus, forearm, wrist, hand, and fingers, and those of the pelvic girdle, thigh, leg, ankle, foot, and toes. They are summarized in tables 9.11 through 9.19.