Overview
This lecture explains the process of a muscle twitch, the phases of muscle contraction, and how repeated stimulation affects muscle fiber contraction and force generation.
Muscle Twitch and Contraction Process
- A muscle twitch is a single quick contraction cycle in response to one action potential.
- Stimulation at the neuromuscular junction releases acetylcholine, triggering an action potential in the muscle fiber.
- The action potential travels along the sarcolemma and down t-tubules, causing calcium release from the sarcoplasmic reticulum.
- Calcium binds to troponin, moving tropomyosin and exposing actin sites for myosin cross-bridging.
Phases of a Muscle Twitch
- The latent period is the delay between stimulation and contraction start, involving chemical and electrical events.
- The contraction period starts when cross-bridges form and tension increases.
- The relaxation period begins as calcium returns to the sarcoplasmic reticulum, stopping cross-bridges and reducing force.
Types of Muscle Fibers and Contraction Differences
- Fast twitch fibers contract quickly for short, rapid movements.
- Slow twitch fibers contract more slowly and are specialized for endurance and posture.
- The length and force of contraction depend on fiber type.
Summation and Tetany
- Repeated stimuli before relaxation leads to summation, increasing tension.
- Rapid, repeated stimulation can cause tetany, where the fiber remains fully contracted and tension stays high.
Motor Units and Force Generation
- A motor unit consists of one motor neuron and all the muscle fibers it innervates.
- One muscle fiber is only innervated by one motor neuron, but a motor neuron may control many fibers.
- Increasing force is achieved by recruiting more motor units, activating more muscle fibers.
Key Terms & Definitions
- Twitch — A brief, single contraction and relaxation cycle in a muscle fiber.
- Latent Period — Time between stimulus and beginning of muscle contraction.
- Contraction Period — Phase where muscle fiber develops tension and shortens due to cross-bridge cycling.
- Relaxation Period — Phase when tension decreases as calcium is reabsorbed and cross-bridges detach.
- Summation — Increased muscle tension due to repeated stimulation before full relaxation.
- Tetany (Tetanus) — Sustained contraction when muscle fiber is stimulated rapidly with no relaxation.
- Motor Unit — A motor neuron and all the muscle fibers it innervates.
Action Items / Next Steps
- Prepare for the next lecture on how recruitment of motor units increases muscle contraction force.