Muscle Tissue Overview and Anatomy

Overview of Muscle Tissue

• Nearly half of body’s mass.
• Transforms chemical energy (ATP) into directed mechanical energy, capable of exerting force.
• Investigative focus areas:
• Types of muscle tissue
• Characteristics of muscle tissue
• Muscle functions

Types of Muscle Tissue

• Terminologies:
• Myo, mys, sarco: prefixes related to muscle.
  • Example: sarcoplasm = muscle cell cytoplasm.
• Three Types of Muscle Tissue:
• Skeletal
• Cardiac
• Smooth
• Skeletal and smooth muscle are elongated, called muscle fibers.

Skeletal Muscle

• Package into skeletal muscles (organs) attached to bones and skin.
• Features:
• Longest muscle fibers with striations (stripes).
• Voluntary muscle: consciously controlled.
• Contract rapidly, tire easily, powerful.
• Key Terms: skeletal, striated, voluntary.

Cardiac Muscle

• Located only in the heart, forming most of the heart walls.
• Features:
• Striated and involuntary (not consciously controlled).
• Contracts at steady rate but can be accelerated by the nervous system.
• Key Terms: cardiac, striated, involuntary.

Smooth Muscle

• Found in walls of hollow organs (e.g., stomach, urinary bladder, airways).
• Features:
• Non-striated and involuntary (cannot be consciously controlled).
• Contracts independently of nervous system stimulation.

Comparison of Muscle Types

Characteristics of Muscle Tissue

• All muscles share four main characteristics:
• Excitability: Receives and responds to stimuli.
• Contractility: Shortens forcibly when stimulated.
• Extensibility: Ability to be stretched.
• Elasticity: Recoils to resting length.

Muscle Functions

1. Produce Movement: Responsible for locomotion (e.g., walking, pumping blood).
2. Maintain Posture: Body positioning.
3. Stabilize Joints: Support and strengthen joints.
4. Generate Heat: As muscle contracts, heat is produced.

• Additional functions: protect organs, form valves, control pupil size, cause goosebumps.

Skeletal Muscle Anatomy

• Composed of various tissues, featuring:
• Nerve and blood supply.
• Connective tissue sheaths.
• Muscle attachments.

Nerve and Blood Supply

• Each muscle receives a nerve, artery, and veins:
• Nerves control each muscle fiber.
• Contracting fibers require oxygen and nutrient delivery.
• Waste removal is crucial.

Attachments

• Muscle Attachments:
• Span joints and attach to bones at two points:
  • Insertion: Movable bone attachment.
  • Origin: Immovable or less movable bone attachment.
• Types of Attachments:
  • Direct (fleshy): Epimysium fused to bone or cartilage.
  • Indirect: Connective tissue extends beyond muscle as tendons or aponeuroses.

Connective Tissue Sheaths in Skeletal Muscle

• Epimysium: Covers entire muscle.
• Perimysium: Encloses fascicles (bundles of fibers).
• Endomysium: Surrounds individual muscle fibers.

Muscle Fiber Microanatomy

• Skeletal Muscle Fibers: Long, cylindrical cells containing multiple nuclei.
• Sarcoplasmic Reticulum: Smooth ER tubules surrounding myofibrils, regulating Ca2+ levels.
• T Tubules: Protrusions of sarcolemma into fiber, allowing electrical nerve transmission.

Sliding Filament Model of Contraction

• Mechanism of Muscle Contraction: Cross bridges form between myosin and actin filaments, causing fibers to shorten.
• Key Points of the Model:
• Neither thick nor thin filaments change length, but overlap more during contraction.
• Cross bridges cycle leads to actin pulling towards the center of the sarcomere, resulting in muscle shortening.

Steps for Muscle Contraction

1. Nerve stimulation.
2. Action potential generation in sarcolemma.
3. Propagation of action potential.
4. Rise of intracellular Ca2+ levels.

• Linking Electrical Signals: Steps 2, 3, and 4 are considered excitation-contraction coupling.

Nerve Stimulus and Neuromuscular Junction Events

• Motor neurons stimulate skeletal muscles:
• Each muscle fiber has one junction with a motor neuron.
• Events at Neuromuscular Junction:
• Nerve impulse arrives, causing ACh release.
• ACh binds to receptors on sarcolemma, generating an action potential.
• ACh is broken down by acetylcholinesterase to stop contraction.

Clinical Considerations

• Myasthenia Gravis: Autoimmune condition affecting ACh receptors, causing muscle weakness.
• Rigor Mortis: Postmortem rigidity due to intracellular calcium increase and lack of ATP, resulting in persistent muscle contraction until protein breakdown.