Skeletal Muscle Notes
Skeletal Muscle
Learning Objectives
- Understand the function of the actomyosin system and its importance for movement.
- Know the main structural differences between skeletal, heart, and smooth muscle.
- Comprehend how contraction in the three muscle types is controlled.
- Appreciate proprioception and know the main sensory cells involved (muscle spindles, Golgi tendon organs).
Skeletal vs Smooth vs Cardiac Muscle
| Feature | Skeletal | Smooth | Cardiac |
|---|---|---|---|
| Striations | Striated, tubular fibers | Non-striated, spindle-shaped | Striated, branched |
| Nuclei | Multinucleated | Uninucleated | Uninucleated |
| Control | Voluntary | Involuntary | Involuntary |
Skeletal Muscle Structure
Hierarchical Organization:
- Muscle
- Fascicles
- Myofibers/Myocytes
- Myofibrils
- Sarcomeres (basic motor unit).
Myofibers:
- Multinucleated with peripheral nuclei.
- Striated due to sarcomere arrangement within myofibrils.
Skeletal Muscle: Sarcomere
- Components:
- Z disc: Boundary of the sarcomere.
- M line: Middle of the sarcomere.
- I band: Region containing only thin filaments (actin).
- A band: Region containing thick filaments (myosin) and overlapping thin filaments.
- H zone: Region within the A band containing only thick filaments.
- Thick filament: Myosin.
- Thin filament: Actin.
- Sarcolemma: Muscle cell membrane.
- Sarcoplasmic reticulum: Stores calcium.
- Transverse tubule: (T-tubule) Helps transmit action potential.
- Mitochondrion: Provides energy for muscle contraction.
Skeletal Muscle: Sarcomere Organization
- Z Disk: Defines the boundaries of a sarcomere.
- M Line: Located in the middle of the sarcomere.
- I Band: Contains only thin filaments (actin).
- A Band: Contains thick filaments (myosin) and overlapping thin filaments.
- Sarcolemma: The cell membrane of a muscle fiber.
- Intermediate Filaments: E.g., Desmin. Provides structural support.
Skeletal Muscle: Connective Tissue
- Endomysium:
- Loose connective tissue.
- Surrounds each muscle fiber.
- Binds to the basement membrane of the muscle fiber.
- Perimysium:
- Mixed connective tissue (loose and dense).
- Separates muscle fibers into groups called fascicles.
- Epimysium:
- Mostly loose connective tissue.
- Connects the muscle body to surrounding fascia.
- Fascia: Connective tissue layer surrounding muscles and other organs.
Skeletal Muscle: Motor Unit
- Motor Unit (MU): A motor neuron and all the muscle fibers it innervates.
- Innervation:
- Each muscle fiber is innervated by only one synapse, located near the middle of the fiber.
- Components:
- MEP: Motor End Plate.
- Axon Terminals: Where the motor neuron connects to the muscle fiber.
- Neuromuscular Junction: The synapse between a motor neuron and a muscle fiber.
Contraction: Summary
- Muscle fibers organized into fascicles contain thousands of myofibrils.
- Sarcomeres within myofibrils contain myosin and actin filaments.
- Motor neurons send action potentials to neuromuscular junctions, releasing Acetylcholine (ACh).
- ACh causes depolarization at the motor end plate.
- Action potential travels along the sarcolemma and through T-tubules to activate calcium channels in the sarcoplasmic reticulum.
- Increased intracellular calcium levels cause a change in the troponin complex, allowing myosin to bind to actin.
- Myosin pulls actin filaments closer using ATP, resulting in cross-bridge cycling and contraction.
Skeletal Muscle: Proprioception
- (Alpha) α-motor neurons: Control contraction of extrafusal/regular fibers.
- (Gamma) γ-motor neurons: Control contraction of intrafusal/spindle fibers.
- Intrafusal (spindle) muscle fibers: Located within the muscle spindle.