Lecture 3 — Muscle Function, Energy & Disorders

Physiology

What is a motor unit?

One motor neuron and all the muscle fibers it innervates.

What type of motor units provide greater precision?

Small motor units.

What is the size principle?

Small motor units are recruited first, followed by larger motor units as greater force is needed.

What is frequency summation?

A stronger contraction produced when another stimulus arrives before the muscle completely relaxes.

What is a single twitch?

One contraction in response to a single action potential.

What is incomplete tetanus?

Repeated stimulation with only partial relaxation between contractions.

What is complete tetanus?

Very high-frequency stimulation producing maximal sustained force with no relaxation.

Why does repeated stimulation increase muscle force?

Cytosolic Ca²⁺ remains elevated because it is not fully returned to the SR between stimuli.

How do Type I, Type IIa, and Type IIx fibers differ?

Type I fibers are slow, aerobic, and fatigue-resistant; Type IIa fibers are fast with mixed metabolism and moderate fatigue resistance; Type IIx fibers are fastest, more anaerobic, and fatigue quickly.

Which muscle fibers are best suited for endurance?

Type I fibers.

Which muscle fibers have many mitochondria and high myoglobin?

Type I fibers.

What is the fastest source of ATP at the beginning of activity?

Stored ATP.

What energy system rapidly regenerates ATP for short, explosive activity?

Creatine phosphate.

What energy system is important for high-intensity activity lasting roughly 10 seconds to 2 minutes?

Anaerobic glycolysis.

What energy system dominates prolonged endurance activity?

Aerobic metabolism/oxidative phosphorylation.

What is the general progression of energy systems?

Stored ATP → creatine phosphate → anaerobic glycolysis → aerobic metabolism.

What does botulinum toxin do?

It decreases ACh release, causing flaccid paralysis.

What does curare do?

It blocks nicotinic ACh receptors, preventing muscle depolarization and causing flaccid paralysis.

What do organophosphates do?

They inhibit acetylcholinesterase, causing excess ACh and continuous stimulation followed by paralysis.

What does succinylcholine do?

It causes persistent depolarization, producing temporary flaccid paralysis.

What is the primary defect in myasthenia gravis?

Autoimmune destruction of postsynaptic nicotinic ACh receptors.

How does weakness change with activity in myasthenia gravis?

Weakness worsens with activity and improves with rest.

What is the primary defect in Lambert-Eaton syndrome?

Autoantibodies attack presynaptic voltage-gated Ca²⁺ channels, reducing ACh release.

How does Lambert-Eaton weakness change with repeated use?

Weakness improves with repeated use.

What is the primary defect in malignant hyperthermia?

A RyR1 mutation causes excessive Ca²⁺ release from the SR after exposure to certain volatile anesthetics or succinylcholine.

What are major findings of malignant hyperthermia?

Severe muscle rigidity, rapidly rising temperature, acidosis, hyperkalemia, and tachycardia.

What drug treats malignant hyperthermia?

Dantrolene.

What is the primary defect in muscular dystrophy?

Dystrophin deficiency causing progressive skeletal muscle degeneration.

What does tetanus toxin do?

It blocks GABA and glycine release from inhibitory interneurons, causing loss of inhibition and sustained muscle contraction.