Muscular System
High-Yield Summary
- 3 muscle types: skeletal (voluntary, striated, multinucleated), smooth (involuntary, nonstriated, uninucleated, tonus, calmodulin-regulated), cardiac (involuntary, striated, intercalated discs/gap junctions, strong myogenic activity).
- Sarcomere = region between two Z-lines; contains thick filaments (myosin) and thin filaments (actin + troponin + tropomyosin). A-band = thick filament length (constant); I-band = thin filament only; H-zone = central thick-only zone; M-line = center anchor.
- Hierarchy: sarcomeres → myofibrils → muscle fiber (myocyte) → whole muscle.
- Contraction cascade: motor neuron AP → ACh release → nicotinic ACh receptors open → Na+ influx depolarizes sarcolemma → muscle AP travels down T-tubules → Ca2+ released from sarcoplasmic reticulum → Ca2+ binds troponin → tropomyosin shifts off binding sites → cross-bridge cycling.
- Cross-bridge cycle: (1) myosin head energized (ATP hydrolyzed to ADP+Pi, cocked), (2) binds exposed actin, (3) power stroke pulls actin toward M-line (releases ADP+Pi), (4) new ATP binds → myosin detaches → re-cocks. No ATP = no detachment = rigor mortis.
- Sliding filament model: filaments don't shorten, they slide. Z-lines move closer, I-band and H-zone shrink, A-band stays constant.
- Stimulation pattern: single stimulus → twitch (latent, contraction, relaxation phases) → repeated stimuli before relaxation completes → summation → very high frequency → tetanus (fused, sustained, max force).
Skeletal vs. Smooth vs. Cardiac Muscle
| Skeletal | Smooth / Cardiac |
|---|---|
| Voluntary, somatic nervous system | Smooth: involuntary, autonomic. Cardiac: involuntary, autonomic |
| Striated, multinucleated | Smooth: nonstriated, uninucleated. Cardiac: striated, mostly uninucleated |
| No myogenic activity — needs neural input | Smooth: tonus + myogenic activity, Ca2+ via calmodulin. Cardiac: strong myogenic activity, intercalated discs + gap junctions, SA node → AV node → bundle of His → Purkinje fibers |
Key Terms
- Sarcomere
- The contractile unit between two Z-lines, containing organized actin and myosin filaments.
- Sarcolemma
- The muscle fiber's plasma membrane; propagates the action potential that triggers contraction.
- Sarcoplasmic reticulum
- Specialized smooth ER wrapped around each myofibril; stores and releases Ca2+.
- T-tubules
- Inward extensions of the sarcolemma that carry the action potential deep into the fiber, next to the SR.
- Tonus
- A baseline partial contraction sustained by smooth muscle, important for regulating blood vessel tone.
- Oxygen debt
- The gap between oxygen needed to restore resting state and oxygen currently available during/after intense activity.
Cross-Bridge Cycle
- 1Energized state — myosin head has already hydrolyzed ATP → ADP + Pi, held in high-energy cocked position
- 2Cross-bridge formation — Ca2+ has exposed actin's binding sites; energized myosin head attaches to actin
- 3Power stroke — myosin head pivots toward M-line, pulling thin filament inward; ADP + Pi released, sarcomere shortens
- 4Detachment and re-cocking — new ATP binds myosin → releases actin → ATP hydrolyzed again → head re-cocked. No ATP = heads stay stuck (rigor mortis)
Red (Slow-Twitch) vs. White (Fast-Twitch) Fibers
| Red Fibers | White Fibers |
|---|---|
| High myoglobin, more mitochondria | Low myoglobin, fewer mitochondria |
| Oxidative metabolism, high fatigue resistance | Glycolytic metabolism, low fatigue resistance |
| Posture, endurance activities | Rapid, powerful contractions |
Common MCAT Trap
- A-band never changes length during contraction — it reflects myosin (thick filament) length, which is fixed. Only the I-band and H-zone shrink.
- ATP is required for myosin to DETACH from actin, not just to power the stroke — no ATP after death means no detachment, which is why rigor mortis occurs.
- Smooth muscle regulates contraction via calmodulin, not troponin (troponin is a skeletal/cardiac muscle mechanism).
- Summation increases tension not because individual twitches get stronger (twitches are all-or-nothing) but because Ca2+ stays elevated and cross-bridge cycling overlaps.
Quick Recall
What ultimately triggers tropomyosin to move off the myosin-binding sites?
Why does rigor mortis occur after death?
What structural feature lets cardiac muscle contract as one coordinated unit?
What's the difference between summation and tetanus?