Chapter 12: Muscle Physiology

Learning Outcomes

This chapter introduces the fundamental concepts of muscle physiology, focusing on the structure, function, and types of muscle tissue. Students will learn to:

• State the functions of muscles

• Draw and describe the arrangement of muscles at the organ and cellular level

• List the steps involved in muscle contraction

• Explain how graded contractions are possible even though all action potentials are identical

• Describe the different types of muscle fibers and their physiological importance

• Compare and contrast skeletal, cardiac, and smooth muscle

I. Skeletal Muscles

Functions of Skeletal Muscles

Skeletal muscles are essential for voluntary movement, posture, and heat production. They are attached to bones and enable locomotion and manipulation of the environment.

• Movement: Skeletal muscles contract to move bones at joints.

• Posture: Maintain body position and stability.

• Heat Production: Muscle contractions generate heat, contributing to thermoregulation.

• Protection: Muscles help protect internal organs by providing padding and support.

Structure of Skeletal Muscle

Skeletal muscle is organized into hierarchical structures, from the whole muscle down to the molecular level.

• Muscle (organ): Surrounded by epimysium.

• Fascicles: Bundles of muscle fibers, surrounded by perimysium.

• Muscle fibers (cells): Surrounded by endomysium.

• Myofibrils: Cylindrical structures within muscle fibers, composed of repeating units called sarcomeres.

• Sarcomere: The functional contractile unit, containing thick (myosin) and thin (actin) filaments.

Key anatomical terms: Periosteum, tendon, fascia, epimysium, perimysium, endomysium, striations, sarcolemma, sarcoplasm, nuclei, filaments, myofibrils.

Example:

The biceps brachii muscle contains multiple fascicles, each made up of many muscle fibers, which in turn contain thousands of myofibrils.

Arrangement of Muscles at the Organ and Cellular Level

Muscle tissue is organized to maximize force generation and efficiency.

• Connective tissue layers: Epimysium (outer), perimysium (middle), endomysium (inner).

• Muscle fiber: Multinucleated, elongated cell containing myofibrils.

• Myofibrils: Composed of sarcomeres, the repeating contractile units.

Additional info:

Each sarcomere is defined by Z-lines and contains overlapping thick and thin filaments responsible for contraction.

Key Terms and Definitions

• Epimysium: Connective tissue sheath surrounding the entire muscle.

• Perimysium: Connective tissue surrounding fascicles.

• Endomysium: Connective tissue surrounding individual muscle fibers.

• Sarcolemma: Plasma membrane of a muscle fiber.

• Sarcoplasm: Cytoplasm of a muscle fiber.

• Myofibril: Contractile element within muscle fiber, composed of sarcomeres.

• Sarcomere: The basic functional unit of muscle contraction.

Example:

During muscle contraction, the sarcomeres within myofibrils shorten, pulling on the connective tissue layers and ultimately moving the bone.

Summary Table: Structure of Skeletal Muscle

Additional info:

Connective tissue layers provide structural support and transmit force generated by muscle fibers to tendons and bones.