Muscle Tension, Fiber Types, Motor Units & Types of Contractions

Muscle twitch

Response of a muscle fiber to a single action potential in its motor neuron; represents the smallest possible muscle contraction and is generally observed in laboratory conditions rather than whole muscles in the body.

Myogram

Recording of muscle tension over time that can be used to examine the phases of a muscle twitch.

Three phases of a muscle twitch

Latent period → contraction period → relaxation period.

Latent period

Brief period of about 1–2 ms between the stimulus and development of tension; the action potential spreads along the sarcolemma and excitation-contraction coupling occurs.

Contraction period

Period when crossbridge cycling occurs and muscle tension increases.

Relaxation period

Period when cytosolic Ca2+ decreases, crossbridge cycling ends, and muscle tension returns toward resting levels; usually lasts about 10–100 ms.

Refractory period

Brief period of about 5 ms during which the muscle fiber cannot respond normally to another stimulus.

Wave summation

Increase in muscle tension that occurs when another stimulus arrives before the muscle has completely relaxed; Ca2+ has not been completely pumped back into the SR, allowing more crossbridge cycling.

Effect of increasing stimulation frequency on muscle tension

As stimulation frequency increases, the muscle has less time to relax between stimuli and produces greater tension.

Unfused (incomplete) tetanus

Rapid stimulation, around 50 stimuli per second, in which the muscle only partially relaxes between contractions, producing a sustained but pulsating tension.

Fused (complete) tetanus

Very rapid stimulation, generally greater than 80–100 stimuli per second, in which no relaxation occurs between contractions and tension remains sustained.

Why does tetanus produce high muscle tension?

Frequent stimulation keeps cytosolic Ca2+ elevated, allowing continuous crossbridge cycling with little or no relaxation.

Tetanus disease

Disease caused by a toxin from Clostridium tetani that excessively stimulates skeletal muscles and causes tetanic muscle spasms; vaccination can prevent the disease.

Botulism

Toxic condition caused by Clostridium botulinum toxin, which blocks ACh release at the neuromuscular junction and prevents skeletal muscle contraction; severe cases can cause respiratory failure.

Botox

Very small doses of botulinum toxin used therapeutically or cosmetically to reduce muscle contraction by blocking ACh release.

Length-tension relationship

The amount of tension a muscle fiber can produce depends on the length of its sarcomeres before contraction because sarcomere length determines how many crossbridges can form.

Optimal sarcomere length

Length at which thick and thin filaments have enough overlap to form the maximum useful number of crossbridges while still having room to slide; produces about 100% maximum tension.

Sarcomere too short

Excessive filament overlap leaves insufficient room for further sliding, reducing the amount of tension that can be generated.

Sarcomere too stretched

There is too little overlap between thick and thin filaments, so fewer crossbridges can form and tension decreases.

Fast-twitch vs. slow-twitch fibers

Classified partly by myosin ATPase activity; fast-twitch fibers have high myosin ATPase activity and contract rapidly, while slow-twitch fibers have low myosin ATPase activity and contract more slowly.

Type I fibers (slow oxidative)

Small-to-intermediate diameter, slow contraction and less force, highly fatigue resistant, oxidative metabolism,