5E · Module 3 Review: Skeletal Muscle

Practice this module: MCQ · Flashcards · Problems, Module 3

Structure¶

Whole muscle → fascicles → fibers → myofibrils → filaments. Fibers are among the largest cells (about 100 µm by up to 30 cm).
Striations: regions with thick filaments (A band) alternate with regions without (I band).
Actin offers the binding site; myosin binds it; tropomyosin covers it; troponin moves tropomyosin when Ca²⁺ binds troponin C.
Titin and nebulin: both form filament cores and anchor at the Z disc; titin is thick-filament and coiled, nebulin thin-filament.
SR stores Ca²⁺. T tubules carry the AP inward because diffusion time ∝ distance squared.

Somatic (A-alpha) motor neuron → ACh → nicotinic receptors on the motor end plate → EPSP → AP on sarcolemma. AChE ends it.
End plate potential: ligand-gated, graded, no threshold, no refractory period. Sarcolemmal AP: voltage-gated, all-or-none, threshold, refractory.
Nicotinic channel passes Na⁺ and K⁺, but |−90 − 65| = 155 against |−90 + 95| = 5, so Na⁺ current is about 31× larger.
Lambert-Eaton: presynaptic Ca²⁺ channels. Myasthenia gravis: postsynaptic ACh receptors. AChE inhibitors (pyridostigmine) help when ACh release is low.

DHPR opens the RyR (Ca²⁺ release channel). SERCA is the Ca²⁺ pump. Ca²⁺ is required in the sarcoplasm.
Know the A·M table: A·M (bound, nothing attached), A + M·ATP (ATP, detached), A + M*·ADP·P_i (cocked, detached), A·M*·ADP (cocked, bound, ADP), A·M·ADP (stroke done).
No ATP → no detachment → rigor mortis.
“Not a relaxation step”: DHPRs opening RyR channels.

Fastest: creatine phosphate. Most efficient: fatty acid oxidation (90–140 ATP).
Type I: oxidative, dark red, fatigue resistant, recruited first. Type IIa: both pathways, medium red. Type IIx: glycolytic, pale, most powerful, fatigues fastest.
Fast-twitch makes 2–3× the power of slow twitch, not the same power.

Force rises with optimal length, higher frequency, and more motor units. Size principle from I = CV.
Wave summation: the next stimulus lands on residual Ca²⁺ and tension. Fusion Index = relaxed/contracted × 100; 99%+ counts as fused.
Concentric (force > load), eccentric (force < load), isometric (force = load). Bicep curl up, down, paused.
Maximal velocity at zero load. Power = force × velocity, peak near one third of max force.