Skeletal Muscle Contraction & Relaxation – Comprehensive Video Notes

Key Vocabulary & Structures

• Root words
  • Sarco- → muscle
  • -lemma → membrane/lining
  • Therefore sarcolemma = muscle-cell membrane.
• Sarcolemma – plasma membrane of a muscle fiber.
• Sarcoplasmic Reticulum (SR) – specialized endoplasmic reticulum that stores Ca$^{2+}$ (write “SR $\Rightarrow$ Ca$^{2+}$ storage”).
• T-Tubules (Transverse tubules)
  • Infoldings of sarcolemma that tunnel deep inside the fiber.
  • Function: propagate action potentials (APs) into interior of the cell so all myofibrils are activated simultaneously.
• Sarcomere – repeating contractile unit; contains thick (myosin) & thin (actin) filaments plus tropomyosin & troponin.
• DHP receptor (Dihydropyridine receptor)
  • Voltage-gated ion-channel protein embedded in T-tubule membrane.
  • Acts as a “switch” sensing the AP.
• RYR receptor / channel (Ryanodine receptor)
  • Ca$^{2+}$-release channel in SR membrane.
  • Mechanically coupled to DHP; when DHP changes conformation, RYR opens and Ca$^{2+}$ floods cytosol.
• Mechanical coupling – physical linkage between DHP & RYR; activation of one instantly activates the other (unique to skeletal muscle).
• SERCA pump (Sarco/Endoplasmic Reticulum Ca$^{2+}$-ATPase-1)
  • ATP-driven pump that returns Ca$^{2+}$ to SR during relaxation.
• AChE / Acetylcholinesterase – enzyme in motor end-plate cleft that degrades acetylcholine on a short “timer.”

Cross-Bridge (Sliding Filament) Cycle – “The Dance”

1. Cocked myosin head
   • Myosin hydrolyzed ATP earlier: $\text{ATP} \xrightarrow{H<em>2O} \text{ADP} + P</em>i + \text{Energy}$
   • Energy stored like a spring; head is pulled back, ready but uncomfortable.
2. Ca$^{2+}$ binds troponin ⇒ tropomyosin slides away; actin binding sites exposed.
3. Myosin attaches to actin (cross-bridge formation).
4. Power-stroke
   • Stored energy released; myosin head pivots, pulling actin.
   • ADP + P$_{i}$ released during stroke.
5. Detachment
   • Must bind fresh ATP; ATP binding lowers myosin–actin affinity, so head releases.
6. Re-cocking
   • Myosin ATPase hydrolyzes the new ATP, returning head to strained (cocked) position.
   • Cycle repeats as long as Ca$^{2+}$ is present.

Mnemonic / Kinesthetic demo supplied in lecture: one hand = actin (fist = binding site), other hand = myosin head; students repeatedly run through attach–stroke–detach–recock motions to lock the pattern into memory.

Motor Neuron → Muscle Fiber Signal (Excitation)

(Sometimes asked as “neuromuscular junction / motor end-plate” sequence.)

1. Brain decides to move; AP travels down alpha-motor neuron.
2. At axon terminal (motor end-plate): voltage-gated Ca$^{2+}$ channels open, vesicles release acetylcholine (ACh).
3. ACh diffuses across cleft, binds nicotinic/muscarinic ACh receptors in sarcolemma.
4. Ligand-gated Na$^{+}$ channels open ⇒ end-plate potential; if threshold reached, sarcolemma AP is generated.
5. AP propagates along sarcolemma & dives down adjacent T-tubules.

Excitation-Contraction Coupling (ECC) – Detailed Flowchart

(Use as numbered “steps” many instructors test.)

1. AP on T-tubule reaches DHP receptors.
2. Voltage change ⇒ DHP undergoes conformational change.
3. Mechanical coupling pulls open RYR channels on SR.
4. Ca$^{2+}$ gushes out of SR into cytosol (sarcoplasm).
5. Ca$^{2+}$ binds troponin-C ⇒ tropomyosin shifts ⇒ binding sites exposed.
6. Run the cross-bridge cycle (#1–6 above) repeatedly ⇒ sarcomere shortens, whole fiber contracts.

Muscle Relaxation Mechanisms

• Central idea: remove Ca$^{2+}$ so troponin loses it ⇒ tropomyosin re-covers sites.
• SERCA pump action
  • Uses ATP to transport Ca$^{2+}$ back into SR against gradient.
  • Equation: $\text{Ca}^{2+}<em>{\text{cyto}} + \text{ATP} \rightarrow \text{Ca}^{2+}</em>{\text{SR}} + \text{ADP} + P_i$
• ACh breakdown
  • AChE hydrolyzes ACh in cleft; without neurotransmitter, motor end-plate can no longer depolarize ⇒ no new T-tubule APs ⇒ Ca$^{2+}$ release stops.
• Once cytosolic Ca$^{2+}$ drops below threshold, myosin cannot bind ⇒ muscle fiber relaxes.

ATP: Source & Roles

• Pick-up for detachment (cross-bridge cycle).
• Hydrolysis to “re-cock” myosin heads.
• Fuel for SERCA pumps during relaxation.
• Generated continuously from mitochondrial oxidative metabolism of glucose, fatty acids, etc.; always assumed to be locally abundant in healthy, well-fed muscle.

Common Q&A / Clarifications

• Where does ATP come from? Primarily mitochondria near myofibrils; secondary phosphagen systems (creatine phosphate) during bursts.
• Is ADP used for anything? No; it is simply a product. Energy was stored in the strained conformation of the myosin head (spring analogy).
• Primary role of Ca$^{2+}$? “Unlocks” the binding sites by binding troponin.
• What causes myosin head detachment? Binding of a fresh ATP molecule.
• How does the head resume cocked position? Hydrolysis of that ATP.
• Why is DHP–RYR coupling unique? Only skeletal muscle uses physical/mechanical coupling; cardiac muscle uses Ca$^{2+}$-induced Ca$^{2+}$ release instead.

Clinical / Real-World Connections

• Muscle Cramps – cross-bridges stuck in attached state due to inadequate ATP to detach (plus electrolyte & SERCA dysfunction elements).
• Tetanus (disease) – blocks AChE action; unchecked ACh ⇒ sustained contraction.
• Delayed-Onset Muscle Soreness (DOMS)
  • Micro-tears in fibers ⇒ inflammation ⇒ repair ⇒ hypertrophy (thicker “rubber-bands”).
  • Not due to “lactic acid build-up.”
• Buffer chemistry myth busting
  • Lactate acts as a buffer: $\text{Lactate} + H^+ \leftrightarrow \text{Lactic Acid}$; soreness during exercise is more correlated with transient $CO_2$ and proton accumulation than lactate.
• Endurance & “mind over body.” Pain ≠ discomfort. Athletes must distinguish healthy discomfort from injurious pain (e.g., rhabdomyolysis risk from pushing through severe pain).

Quick Reference – 10-Step Master List (Exam Flow)

1. Motor neuron AP reaches terminal.
2. ACh released into motor end-plate.
3. ACh binds receptors → sarcolemma AP.
4. AP enters T-tubule.
5. Voltage-gated DHP changes shape.
6. DHP mechanically opens RYR on SR.
7. Ca$^{2+}$ floods cytosol.
8. Ca$^{2+}$ binds troponin; tropomyosin shifts.
9. Cross-bridge cycle repeats (attach–stroke–detach–recock) while Ca$^{2+}$ high.
10. Relaxation: SERCA pumps return Ca$^{2+}$ to SR; AChE removes ACh; binding sites re-blocked, fiber lengthens.

Memorize the sequence, practice the hand dance, and you are “fine on the exam!”