In-Depth Notes on Muscle Structure and Function

Muscle Structure and Function

Muscle Types

• Multinucleate Fibers: Voltag - Long fibers with striated and intercalated discs, involuntary in nature.
• Involuntary Muscle: Controlled by nerves, characterized by sensory and motor neurons.

Muscle Components

• Epimysium: Covering of the whole muscle.
• Perimysium: Surrounds each muscle fascicle.
• Endomysium: Envelops individual muscle fibers.
• Myofilaments: Broken down into actin (thin) and myosin (thick) which form functional units of muscles called sarcomeres.

Neuromuscular Interaction

• Innervation: Muscles are innervated by motor neurons which control contraction through electrical impulses.
• Sensory Neurons: Carry information to the CNS.
• Motor Neurons: Relay commands from CNS to muscles.

Muscle Cell Structure

• Sarcolemma: Membrane covering muscle cells.
• Sarcoplasm: Fluid inside muscle cells.
• Transverse Tubules: Structures that carry action potentials deep into muscle fibers.
• Sarcoplasmic Reticulum: Stores calcium ions used for muscle contraction.

Excitation-Contraction Coupling

• Calcium Ions: Key to enabling muscle contraction, interacting with proteins to expose active sites on actin.
• Troponin and Tropomyosin: Regulatory proteins controlling the binding of myosin to actin.
• Troponin: Binds calcium and shifts tropomyosin to uncover the active sites on actin.
• Cross-Bridge Formation: Myosin heads interact with active sites on actin filaments, forming a bridge that facilitates muscle contraction.

Action Potential and Muscle Contraction

• Action Potential: Electrical impulse that initiates muscle contraction through calcium release.
• Threshold: Minimum stimulus required for muscle contraction.

Muscle Contraction Phases

1. Latent Period: Initial phase with no tension while action potential travels and calcium is released.
2. Contraction Phase: Calcium binds to troponin, allowing cross-bridge formation and tension building.
3. Relaxation Phase: Calcium levels fall, tropomyosin covers active sites, and tension decreases.

Stimulation Patterns

• Single Contraction/Twitch: Lasts about 7-100 ms, resulting from one neural stimulation.
• Treppe: Gradual increase in muscle tension with repeated stimulation after relaxation.
• Wave Summation: Increases tension when new stimuli arrive before relaxation is complete.
• Incomplete Tetanus: Rapid contractions where muscle does not completely relax.
• Complete Tetanus: High stimulation frequency results in continuous contraction without relaxation.

Motor Unit Recruitment

• Motor Unit: A single motor neuron and all muscle fibers it controls.
• Tension Produced: Depends on the number of fibers activated and the frequency of stimulation, leading to coordinated muscle contraction across different motor units.