Overview
This lecture explains how skeletal muscles generate movement, the principles behind muscle contraction, and the mechanisms for regulating force and movement precision.
Skeletal Muscles Overview
- Skeletal muscles, numbering 640, vary in size and function, from the largest gluteus maximus to the tiny stapedius.
- Muscles perform both powerful and subtle tasks, enabling a wide range of movements.
How Muscles Move Bones
- Muscles never push; they always pull, moving bones by contracting toward their origin points.
- The insertion is the moving attachment of a muscle, while the origin is the fixed or less movable attachment.
- All skeletal movement relies on the pulling action of muscles drawing insertions closer to origins.
Muscle Functional Groups
- Prime movers (agonists): Main muscles responsible for producing specific movements.
- Antagonists: Oppose or reverse the movement, providing control and balance.
- Synergists: Assist prime movers by adding force or stabilizing joints.
Motor Units and Graded Responses
- A motor unit consists of a motor neuron and all the muscle fibers it innervates.
- Large motor units control powerful movements; small units provide fine motor control.
- A muscle twitch has three phases: latent period (stimulus arrival), contraction (fiber shortens), and relaxation (fiber returns to rest).
- Graded muscle responses arise from varying the frequency and strength of stimulation.
Summation, Tetanus, and Recruitment
- Temporal summation: Increased firing frequency enhances contraction strength, leading to fused contractions (tetanus) at high rates.
- Recruitment: Increasing the number of active motor units raises muscle force.
- The "size principle" activates smaller units first, then larger ones as more force is needed.
Types of Muscle Contraction
- Isotonic contraction: Muscle changes length to move a load (e.g., lifting a mug).
- Isometric contraction: Muscle develops tension without changing length (e.g., trying to lift an immovable object).
Key Terms & Definitions
- Origin — The attachment of a muscle to the stationary bone.
- Insertion — The attachment of a muscle to the movable bone.
- Prime mover (agonist) — Main muscle responsible for movement.
- Antagonist — Muscle that opposes the prime mover.
- Synergist — Muscle that aids the prime mover.
- Motor unit — One motor neuron and all associated muscle fibers.
- Twitch — Single quick contraction and relaxation of muscle fibers.
- Temporal summation — Increased contraction force from rapid stimulation.
- Tetanus — Sustained muscle contraction from rapid stimuli.
- Recruitment — Activating additional motor units to increase force.
- Isotonic contraction — Muscle changes length during contraction.
- Isometric contraction — Muscle maintains length during contraction.
Action Items / Next Steps
- Review the sliding filament model and phases of muscle twitch.
- Practice identifying muscle roles (agonist, antagonist, synergist) in different movements.
- Study isotonic vs. isometric contractions and provide movement examples for each.