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

Muscle Tissue Overview

Types of Muscle Tissue

Muscle tissue is specialized for contraction and is essential for movement and support in the human body. There are three primary types of muscle tissue, each with distinct structure and function.

• 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 internal organs; responsible for movements such as peristalsis.

Skeletal Muscle Functions

Major Roles of Skeletal Muscle

Skeletal muscles perform several vital functions in the body:

• Move the skeleton: Muscles pull on tendons, which then move bones, enabling locomotion and manipulation.

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

• Support soft tissues: Muscles form the abdominal wall and pelvic floor, supporting internal organs.

• Guard entrances and exits: Skeletal muscles encircle openings of the digestive and urinary tracts, controlling entry and exit.

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

Connective Tissue Organization in Skeletal Muscle

Structural Layers

Skeletal muscle is organized into several connective tissue layers that provide structure and support:

• Epimysium: Covers the entire muscle, providing an outer protective layer.

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

• Endomysium: Surrounds each individual muscle fiber, tying fibers together and containing capillaries and nerve fibers.

Microscopic Structure of Skeletal Muscle

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 transmit electrical impulses from the surface to the cell's interior, ensuring coordinated contraction.

Myofibrils

Myofibrils are cylindrical structures within muscle fibers, composed of bundles of myofilaments:

• Thin filaments: Contain actin molecules.

• Thick filaments: Contain myosin molecules.

• The shortening of myofibrils results in muscle fiber contraction.

Sarcoplasmic Reticulum

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

Sarcomeres

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

Sarcomere Lines and Bands

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

• A band: Contains thick filaments (myosin) and includes the M line and lighter regions on either side.

Filament Structure

• Thin filaments: Active sites on actin are covered by strands of tropomyosin, which are held in place by troponin proteins.

• Thick filaments: Composed of myosin molecules, each with a tail and a globular head.

Summary Table: Connective Tissue Layers in Skeletal Muscle

Layer	Location	Function
Epimysium	Surrounds entire muscle	Protects and supports muscle as a whole
Perimysium	Surrounds fascicles (bundles of fibers)	Contains blood vessels and nerves
Endomysium	Surrounds individual muscle fibers	Binds fibers together; contains capillaries and nerves

Additional info: The images referenced in the slides (e.g., Figure 7-1 and 7-2) visually depict the hierarchical organization of muscle tissue from whole muscle to myofibril, and the arrangement of sarcomeres within myofibrils.