Overview
This lecture explains how skeletal muscles create and control movement, the mechanisms behind muscle contractions, and the functional grouping of muscles.
Structure and Function of Skeletal Muscles
- There are 640 skeletal muscles in the human body, varying in size and function.
- Skeletal muscles perform both powerful and delicate movements.
- Muscles always pull; they never push.
- Each skeletal muscle connects to bones via joints, with movement occurring as the insertion bone is pulled toward the origin.
Functional Groups of Muscles
- Prime movers (agonists) are mainly responsible for a specific movement.
- Antagonists reverse or oppose the movement of the prime mover.
- Synergists assist prime movers with extra force or by stabilizing joints.
- Muscle roles can switch depending on the movement being performed.
Muscle Contraction Mechanisms
- A motor unit consists of a single motor neuron and all the muscle fibers it innervates.
- Large motor units generate big, less precise movements; small units control fine, precise movements.
- An action potential in a motor unit causes a twitch with three phases: latent, contraction, and relaxation.
- Muscle force is graded by frequency of stimulation (temporal summation) and number of motor units activated (recruitment).
Graded Muscle Responses and Fatigue
- Increasing stimulus frequency results in stronger twitches until maximum tension (tetanus) is reached.
- Repeated strong contractions consume ATP, leading to muscle fatigue.
- Recruitment follows the size principle: small units activate first, then larger ones for increased force.
Types of Muscle Contractions
- Isotonic contractions occur when muscle tension changes muscle length (e.g., lifting an object).
- Isometric contractions produce tension without changing muscle length (e.g., trying to lift an immovable object).
Key Terms & Definitions
- Origin — the stationary bone attachment for a muscle.
- Insertion — the movable bone attachment for a muscle.
- Prime mover (agonist) — primary muscle for a specific action.
- Antagonist — muscle that opposes the action of an agonist.
- Synergist — muscle that aids a prime mover or stabilizes a joint.
- Motor unit — a motor neuron and all muscle fibers it controls.
- Twitch — a single contraction-relaxation cycle in a muscle fiber.
- Temporal summation — increased force due to rapid, repeated stimulation.
- Recruitment — increasing force by activating more motor units.
- Isotonic contraction — muscle changes length during contraction.
- Isometric contraction — muscle length stays the same during contraction.
Action Items / Next Steps
- Review functional muscle groups and their roles.
- Practice identifying examples of isotonic and isometric contractions.
- Understand the phases of a muscle twitch and how force is graded.