BSC 2085: Human Anatomy and Physiology 1

Lecture on Muscles by Professor Mariah Evans

Introduction to Muscle Contraction

• Muscle contraction is described by the sliding filament theory.
  • Actin (thin filament) and Myosin (thick filament) slide past each other.
• Four Key Actions in muscle contraction:
  1. Excitation: Nerve impulse needed to stimulate skeletal muscle.
  2. Excitation-Contraction Coupling: The nerve impulse (neurotransmitter) interacts with the muscle cell membrane (sarcolemma).
  3. Contraction: The sliding of actin and myosin.
  4. Relaxation: Muscle returns to its relaxed state.

Detailed Steps of Muscle Contraction

Excitation

• Nerve Signal stimulates voltage-gated calcium channels.
• Voltage-gated ion channels open due to changes in charge, allowing ions to flow across the cell membrane.
  • Cations (positive) and Anions (negative) are involved.
• Calcium ions diffuse into the axon terminal, triggering the release of Acetylcholine (ACH).

Excitation-Contraction Coupling

• Acetylcholine binds to receptors on the sarcolemma, opening chemically-gated ion channels.
  • Sodium enters, potassium exits, leading to an end plate potential (localized change in charge).
• Causes adjacent voltage-gated channels to open, allowing more sodium influx, resulting in an action potential.

Muscle Contraction

• Action potential propagates through T-tubules, releasing calcium from the sarcoplasmic reticulum.
• Calcium binds to Troponin, causing the Tropomyosin complex to open and reveal binding sites on actin.
• ATPase enzyme breaks ATP into ADP and Pi, cocking the myosin head.
• Myosin head binds to actin, forming a cross-bridge.
• Power Stroke: Myosin pulls actin past, requiring ATP.
  • Occurs repeatedly as millions of myosin heads and actin sites interact.

Relaxation

• Acetylcholinesterase degrades acetylcholine, closing ion channels.
• ATP is required to transport calcium back into the sarcoplasmic reticulum, allowing the muscle to relax.

Types of Muscle Tissue

1. Skeletal Muscle
   • Voluntary, striated, fast contraction, requires nerve stimulation.
2. Cardiac Muscle
   • Involuntary, striated, can contract without nerve input.
3. Smooth Muscle
   • Involuntary, non-striated, also operates without nerve input.

Muscle Tissue Characteristics

• Excitability: Response to a stimulus.
• Contractility: Ability to shorten (contract).
• Extensibility: Ability to stretch.
• Elasticity: Ability to return to original length.

Functions of Muscles

• Movement, joint stabilization, heat generation, posture maintenance.
• Involvement in organ protection, pupil size regulation, and generating goosebumps.

Anatomical Structure

• Epimysium: Outer muscle covering.
• Perimysium: Surrounds fascicles (bundles of muscle fibers).
• Endomysium: Surrounds individual muscle fibers.

Muscle Fiber Anatomy

• Sarcolemma: Plasma membrane of a muscle fiber.
• Sarcoplasm: Cytoplasm of a muscle cell.
• Sarcoplasmic Reticulum: Stores calcium, involved in excitation-contraction coupling.
• Sarcomere: Functional unit of muscle contraction, contains actin and myosin.

Neuromuscular Junction

• The site where a nerve and muscle fiber meet; crucial for transmitting signals that initiate contraction.

Real-World Applications

• Pesticides: Use acetylcholinesterase inhibitors causing spastic paralysis in insects.
• Tetanus: Bacterial infection causing muscle overstimulation.
• Muscle Relaxants: Use substances like curare to block acetylcholine and relieve spasms.

Importance

• Understanding muscle physiology is critical for comprehending overall body functions and various physiological processes.
• Relevant to digestion, heart function, and neurological processes.

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These notes cover the core concepts of muscle contraction, muscle types, and related anatomy, providing a comprehensive guide for study and review.