Chapter 7: The Muscular System – Structure and Function of Muscle Tissue

Muscle Tissue Overview

Types of Muscle Tissue

Muscle tissue is essential for movement, posture, and various bodily functions. There are three main types of muscle tissue found in the human body:

• Skeletal muscle: Voluntary, striated muscle attached to bones; responsible for body movement.

• Cardiac muscle: Involuntary, striated muscle found only in the heart; responsible for pumping blood.

• Smooth muscle: Involuntary, non-striated muscle found in walls of organs; controls movement of substances within organs.

Skeletal Muscle Functions

Major Functions of Skeletal Muscle

Skeletal muscles perform several critical functions in the body:

• Move the skeleton: Muscles pull on tendons, which then move bones.

• Maintain posture and body position: Continuous muscle contractions stabilize joints and maintain posture.

• Support soft tissues: Muscles of the abdominal wall and pelvic floor support internal organs.

• Guard entrances and exits: Muscles encircle openings of digestive and urinary tracts, controlling passage of materials.

• Maintain body temperature: Muscle contractions generate heat, helping regulate body temperature.

Connective Tissue Organization in Skeletal Muscle

Layers of Connective Tissue

Skeletal muscle is organized and protected by three layers of connective tissue:

• Epimysium: Covers the entire muscle.

• Perimysium: Divides the muscle into fascicles (bundles of muscle fibers); contains blood vessels and nerves.

• Endomysium: Covers each muscle fiber and ties fibers together; contains capillaries and nerve fibers.

Muscle Fiber Structure

Sarcolemma and Transverse Tubules

The sarcolemma is the plasma membrane of a muscle cell, surrounding the sarcoplasm (cytoplasm). It contains openings that lead into a network of tubules called transverse tubules (T tubules), which allow electrical impulses to reach the cell's interior, enabling unified contraction of the entire fiber.

Myofibrils

Myofibrils are composed of bundles of thick and thin myofilaments:

• Actin molecules are found in thin filaments.

• Myosin molecules are found in thick filaments.

• Shortening of myofibrils creates muscle fiber contraction.

Sarcoplasmic Reticulum

The sarcoplasmic reticulum is a specialized smooth endoplasmic reticulum that stores and releases calcium ions, which are essential for muscle contraction.

Sarcomeres: The Functional Unit

Structure and Organization

Sarcomeres are repeating functional units of myofilaments and represent the smallest functional unit of a skeletal muscle fiber. Each myofibril contains about 10,000 sarcomeres arranged end to end.

• Z lines: Mark the boundary at each end of the sarcomere.

Sarcomere Bands

When a muscle fiber is relaxed, the A band contains only thick filaments and includes the M line and light regions on either side.

Thin Filament Structure

• At rest, active sites on actin are covered by strands of tropomyosin.

• Tropomyosin strands are held in position by troponin.

Thick Filament Structure

• Composed of myosin molecules.

• Each myosin molecule has a tail and a globular head.

Summary Table: Connective Tissue Layers in Skeletal Muscle

Layer	Location	Main Function
Epimysium	Surrounds entire muscle	Protects and supports muscle
Perimysium	Surrounds fascicles (bundles of fibers)	Provides pathways for nerves and blood vessels
Endomysium	Surrounds individual muscle fibers	Supports capillaries and nerve fibers

Example: Skeletal Muscle Contraction

When you flex your arm, skeletal muscles contract, pulling on tendons attached to bones, resulting in movement at the joint. This process involves the coordinated action of myofibrils, sarcomeres, and connective tissue layers.

Additional info: The notes above are based on textbook-style lecture slides and are suitable for college-level Anatomy & Physiology students. The content covers the structure and function of muscle tissue, focusing on skeletal muscle organization and its cellular components. More details on contraction mechanisms, energy metabolism, and muscle types are typically included in subsequent sections of the chapter.