Muscle Tissue and Physiology: Structure, Types, and Organization

Unit 10: Muscle Tissue and Physiology

Introduction to the Muscular System: Muscle Tissue

The muscular system is composed of specialized tissues that enable movement, maintain posture, and support vital bodily functions. Muscle tissue is classified based on structure, location, and function.

• Muscle tissue is one of the four primary tissue types in the body, alongside epithelial, connective, and nervous tissues.

• There are three main types of muscle tissue: skeletal, cardiac, and smooth.

Types of Muscle Tissue

Each muscle tissue type has unique structural and functional characteristics.

Type of Muscle Tissue	Structure	Location	Voluntary/Involuntary	Function
Skeletal	Long, cylindrical, multinucleated cells; striated	Mostly attached to skeleton	Voluntary	Produces movement of the body
Cardiac	Short, branched cells; single nucleus; striated; intercalated discs	Heart	Involuntary	Produces beating of the heart
Smooth	Thin, smooth muscle cells; single nucleus; non-striated	Walls of hollow organs (e.g., stomach, intestines, blood vessels)	Involuntary	Changes diameter of hollow organs; causes hair to stand erect; adjusts shape of lens and pupil of the eye

Skeletal Muscle Anatomy

Skeletal muscles are highly organized structures composed of bundles of muscle fibers, each surrounded by connective tissue layers. This organization allows for efficient force generation and transmission.

• Epimysium: The outermost connective tissue layer that surrounds the entire muscle and binds it to tendons.

• Perimysium: Connective tissue that surrounds groups of muscle fibers, forming bundles called fascicles.

• Fascicles: Bundles of muscle fibers (cells) grouped together within the muscle.

• Endomysium: Thin connective tissue layer that surrounds each individual muscle fiber (cell).

Diagram Explanation: The provided diagram shows the hierarchical organization of skeletal muscle, from the whole muscle down to individual muscle fibers, highlighting the connective tissue layers.

Microscopic Structure of Skeletal Muscle

At the microscopic level, skeletal muscle fibers contain specialized structures that enable contraction.

• Sarcolemma: The plasma membrane of a muscle fiber.

• Sarcoplasm: The cytoplasm of a muscle fiber, containing organelles and myofibrils.

• Myofibrils: Long, cylindrical organelles composed of repeating units called sarcomeres, which are the functional units of muscle contraction.

• Sarcomere: The segment of a myofibril between two Z-discs; contains thick (myosin) and thin (actin) filaments responsible for striations and contraction.

• Striations: Alternating dark (A bands) and light (I bands) regions seen in skeletal and cardiac muscle due to the arrangement of myofilaments.

• Multinucleate: Skeletal muscle fibers contain multiple nuclei, a result of their development from the fusion of myoblasts.

• Sarcoplasmic Reticulum (SR): Specialized endoplasmic reticulum that stores and releases calcium ions ($Ca^{2+}$) necessary for muscle contraction.

• Transverse (T) Tubules: Invaginations of the sarcolemma that transmit action potentials into the muscle fiber.

• Triad: A structure formed by a T-tubule flanked by two terminal cisternae of the SR, essential for excitation-contraction coupling.

Key Terms and Definitions

• Muscle Fiber: Another term for a muscle cell, especially in skeletal muscle.

• Myofilaments: Protein filaments (actin and myosin) that make up myofibrils and are responsible for muscle contraction.

• Excitation-Contraction Coupling: The physiological process linking the electrical signal (excitation) to muscle contraction.

Summary Table: Comparison of Muscle Tissue Types

Feature	Skeletal Muscle	Cardiac Muscle	Smooth Muscle
Striations	Present	Present	Absent
Number of Nuclei	Multiple	Single	Single
Control	Voluntary	Involuntary	Involuntary
Location	Attached to bones	Heart	Walls of hollow organs

Example: Functional Significance

• Skeletal muscle enables voluntary movements such as walking and lifting objects.

• Cardiac muscle contracts rhythmically to pump blood throughout the body.

• Smooth muscle regulates the diameter of blood vessels and moves food through the digestive tract.

Additional info: The notes above expand on the provided slides by defining key terms, clarifying the organization of muscle tissue, and summarizing the main differences between muscle types for comprehensive exam preparation.