Muscle Tissue in Human Physiology

Introduction to Muscle Types

Muscle tissue is essential for movement, stability, and vital physiological functions in the human body. There are three primary types of muscle: skeletal muscle, cardiac muscle, and smooth muscle. Each type has distinct structural and functional characteristics.

• Skeletal Muscle: Striated muscle attached to bones; responsible for voluntary movements and posture.

• Cardiac Muscle: Striated muscle found only in the heart; responsible for pumping blood throughout the circulatory system.

• Smooth Muscle: Non-striated muscle found in internal organs and tubes; controls movement of materials within the body.

Example: Skeletal muscles enable walking, cardiac muscle powers the heartbeat, and smooth muscle regulates digestion.

Classification and Structure of Muscle Types

Three Types of Muscle

Muscle types are classified based on their location, control, and microscopic appearance.

• Skeletal Muscle:

  • Striated appearance under the microscope

  • Voluntary control via somatic motor neurons

  • Attached to bones by tendons

• Cardiac Muscle:

  • Striated, branched cells with intercalated disks

  • Involuntary control; responds to autonomic innervation and endocrine modulation

  • Found only in the heart

• Smooth Muscle:

  • Non-striated, spindle-shaped cells

  • Involuntary control; responds to autonomic innervation and endocrine modulation

  • Located in walls of internal organs and blood vessels

Skeletal Muscle Structure and Function

Attachment and Movement

Skeletal muscles are typically attached to bones by tendons, enabling movement of the skeleton. The points of attachment are defined as:

• Origin: The attachment closest to the trunk or to the more stationary bone.

• Insertion: The more distal or more mobile attachment.

Muscle actions are classified as:

• Flexor: Brings bones closer together (e.g., biceps during arm flexion).

• Extensor: Moves bones away from each other (e.g., triceps during arm extension).

Flexor-extensor pairs form antagonistic muscle groups, which move bones in opposite directions.

Antagonistic Muscles

Antagonistic muscle groups are essential for coordinated movement. When one muscle contracts, its antagonist relaxes, allowing controlled motion.

• Example: During a push-up, the biceps (flexor) contract to bend the arm, while the triceps (extensor) contract to straighten it.

Microscopic Anatomy of Muscle Tissue

Cellular Structure

Muscle cells, also called muscle fibers, have specialized structures:

• Sarcolemma: The cell membrane of a muscle fiber.

• Sarcoplasm: The cytoplasm of a muscle fiber.

• Sarcoplasmic Reticulum: Modified endoplasmic reticulum that stores and releases calcium ions.

• Myofibrils: Contractile structures composed of repeating units called sarcomeres.

Satellite cells are stem cells that aid in muscle growth and repair.

Muscle Fiber Organization

• Muscle fibers are bundled into fascicles, surrounded by connective tissue.

• Connective tissue sheaths extend to form tendons, which attach muscle to bone.

Summary Table: Muscle Terminology

Key Concepts

• Striations: Alternating light and dark bands seen in skeletal and cardiac muscle due to the arrangement of actin and myosin filaments.

• Voluntary vs. Involuntary Control: Skeletal muscle is under conscious control, while cardiac and smooth muscle are regulated automatically.

• Antagonistic Muscle Groups: Muscles that produce opposite movements at a joint.

Additional info:

• Further details on the molecular basis of contraction, excitation-contraction coupling, and muscle fiber types are covered in later sections of the chapter.