Overview
This lecture covers the generation of muscle tension at the individual muscle fiber (cell) level, focusing on the muscle twitch, periods of contraction, and factors affecting tension.
Muscle Twitch and its Phases
- A muscle twitch is the tension produced by one action potential in a muscle fiber.
- The latent period is the time between the stimulus and the start of contraction, when biochemical processes occur but no tension is generated.
- The contraction period is when calcium is present, cross-bridges form, and tension increases.
- The relaxation period is when acetylcholine and calcium levels fall, cross-bridges decrease, and tension declines.
- The refractory period occurs from the start of the latent period to the beginning of contraction, when the muscle cannot respond to another stimulus.
Factors Affecting Muscle Tension
- The amount of tension generated depends on the timing and frequency of stimulation.
- Stimulating the muscle during rising tension increases total tension (wave summation).
- Faster stimulation increases calcium availability and tension; more calcium leads to more cross-bridges.
- The initial length of the muscle fiber affects the number of cross-bridges and tension produced (length-tension relationship).
- Type II muscle fibers and larger motor units can generate more tension.
Tetanus and Summation
- Unfused tetanus occurs with rapid, repeated stimulation that allows partial relaxation between stimuli, creating a wavering tension.
- Fused tetanus happens with very rapid stimulation where no relaxation occurs, producing a sustained maximal contraction.
- Wave summation in muscle cells depends on repeated stimulation and available calcium, distinct from neuron summation.
Length-Tension Relationship
- Optimal tension is produced when muscle fibers are at a mid-range length with maximal overlap of actin and myosin.
- Fully contracted or overly extended fibers generate less tension due to poor cross-bridge formation.
Key Terms & Definitions
- Muscle twitch — A single, brief contraction from one action potential in a muscle fiber.
- Latent period — Delay between stimulus and contraction onset, when preparatory events occur.
- Contraction period — Phase when calcium allows cross-bridge formation and muscle tension increases.
- Relaxation period — Phase when calcium is removed, cross-bridges detach, and tension decreases.
- Refractory period — Time when the muscle fiber cannot respond to another stimulus.
- Wave summation — Increase in muscle tension by repeated stimulation before complete relaxation.
- Unfused tetanus — Partial relaxation between stimuli, resulting in fluctuating tension.
- Fused tetanus — Continuous stimulation without relaxation, resulting in sustained maximal tension.
- Length-tension relationship — Effect of starting muscle length on the amount of tension generated.
Action Items / Next Steps
- Review the steps of excitation-contraction coupling and phases of a muscle twitch.
- Prepare for the next lecture on muscle tension at the organ (whole muscle) level.