2.3 Action Potentials, Conduction, the Neuromuscular Junction & Muscle Contraction

Why this area matters

The neurophysiology area (17 items) begins with "nerve stimulus and action potential, nerve impulse conduction". Muscle physiology runs through cardiology, anesthesia and pharmacology items. Expect to:

• reason from channel behavior to a clinical sign, such as weakness in hyperkalemia or demyelination;
• tell apart the three classic neuromuscular junction disorders;
• explain why the three muscle types respond differently to calcium-channel blockers, nitrates and anesthetic agents.

Action Potential Phases & Saltatory Conduction

An action potential is an all-or-none, regenerative reversal of membrane polarity in excitable cells (neurons and myocytes).

Phases of the Fast Action Potential (Nerve and Skeletal Muscle)

1. Resting Phase: Membrane resting at $-70\text{ to }-90\text{ mV}$, dominated by $K^+$ leak conductance.
2. Threshold and rapid depolarization (upstroke): When a local graded potential depolarizes the axon initial segment or motor end-plate to threshold (approximately $-55\text{ mV}$), voltage-gated sodium channels ($Nav1$) open rapidly. The outer activation gate ($m$-gate) swings open, producing a massive inward $Na^+$ current ($I_{Na}$) that rapidly drives the membrane toward $+30\text{ to }+40\text{ mV}$ (approaching $E_{Na}$). This positive feedback cycle is termed the Hodgkin cycle.
3. Overshoot and sodium inactivation: Near peak voltage, the inner inactivation gate ($h$-gate, the "ball-and-chain" peptide loop) swings shut, halting $Na^+$ influx.
4. Repolarization: Voltage-gated delayed-rectifier potassium channels ($Kv$) open with a slight kinetic delay. Potassium rapidly exits the cell down its chemical gradient ($I_K$), driving membrane potential back down toward negative values.
5. Afterhyperpolarization (undershoot): Because delayed-rectifier $K^+$ channels close slowly, potassium permeability remains transiently higher than at rest, dipping membrane potential down toward $E_K$ ($-90\text{ mV}$) before restoring baseline RMP.

Refractory Periods

• Absolute Refractory Period (ARP): Extends from the start of the upstroke until repolarization is well under way. During ARP, virtually all voltage-gated $Na^+$ channels are either actively open or locked in the inactivated state ($h$-gate closed). No stimulus, regardless of intensity, can trigger another action potential. This enforces unidirectional propagation and sets an upper limit on firing frequency.
• Relative Refractory Period (RRP): Follows the ARP. A fraction of $Na^+$ channels have reset from the inactivated state to the closed-resting state ($h$-gate open, $m$-gate closed), but delayed-rectifier $K^+$ conductance remains high. An action potential can be triggered, but only by a suprathreshold stimulus, and the resulting spike exhibits a reduced rate of rise and lower peak amplitude.

Determinants of Conduction Velocity & Cable Theory

Conduction velocity along a cylindrical axon depends on its passive electrical cable properties:

• Length Constant (Space Constant, $\lambda$): $\lambda = \sqrt{\frac{r_m}{r_i}}$ Where $r_m$ is membrane resistance and $r_i$ is internal axial cytoplasmic resistance. A larger $\lambda$ means electrotonic current spreads farther along the core before dissipating across the membrane, speeding propagation. Increasing axon diameter lowers $r_i$ substantially, increasing $\lambda$.
• Time Constant ($\tau$): $\tau = r_m \cdot C_m$ Where $C_m$ is membrane capacitance. A lower $\tau$ allows the membrane to charge and discharge more rapidly, accelerating conduction.

Saltatory vs. Continuous Conduction

• Unmyelinated Fibers (e.g., autonomic postganglionic fibers, pain/temperature C fibers): Continuous conduction. Every adjacent patch of axolemma must undergo depolarization and $Nav$ channel activation. Conduction velocity is slow ($0.5\text{--}2.0\text{ m/s}$).
• Myelinated Fibers (e.g., $A\alpha$ motor neurons, $A\beta$ sensory afferents): Wrapped by Schwann cells (peripheral nervous system) or oligodendrocytes (central nervous system) in concentric layers of lipid-rich myelin. Myelin increases membrane resistance ($r_m$) by 100-fold and dramatically decreases membrane capacitance ($C_m$). Current flows almost instantaneously through the low-resistance internodal axoplasm, jumping from one unmyelinated node of Ranvier to the next (saltatory conduction). Voltage-gated $Nav1.6$ channels are densely packed at the nodes ($> 1,000/\mu\text{m}^2$). Conduction velocity reaches $70\text{--}120\text{ m/s}$. In demyelinating neuropathies (Guillain-Barré syndrome, Charcot-Marie-Tooth, Multiple Sclerosis), loss of myelin increases capacitance and causes current leakage across the exposed internodal membrane, resulting in conduction block and severe motor weakness.

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The Neuromuscular Junction: Transmission & Pathologies

The neuromuscular junction (NMJ) is a specialized chemical synapse between an alpha-motor neuron terminal bouton and the motor end-plate of a skeletal muscle fiber.

Sequence of Neuromuscular Transmission

1. Presynaptic Action Potential & Calcium Entry: A motor nerve action potential depolarizes the presynaptic terminal, opening voltage-gated P/Q-type $Ca^{2+}$ channels.
2. Vesicle Docking & Fusion (SNARE Complex): Inward $Ca^{2+}$ flux binds the calcium sensor synaptotagmin-1, which triggers immediate assembly of the core SNARE (Soluble N-ethylmaleimide-sensitive factor Attachment protein REceptor) complex:
   • Synaptobrevin (also known as VAMP, Vesicle-Associated Membrane Protein) on the synaptic vesicle.
   • Syntaxin-1 and SNAP-25 (Synaptosomal-Associated Protein 25) on the presynaptic plasma membrane.
   • Tight zippering of the SNARE pins drives quantal exocytosis of acetylcholine (ACh) into the $20\text{--}50\text{ nm}$ synaptic cleft (roughly 5,000 to 10,000 ACh molecules per quantum/vesicle).
3. Postsynaptic Activation: ACh diffuses across the cleft and binds to the two $\alpha$ subunits of the pentameric nicotinic acetylcholine receptor ($nAChR$) located on the crests of junctional folds. The adult muscle receptor has an $(\alpha_1)_2\beta_1\epsilon\delta$ subunit stoichiometry. Binding opens a nonselective cation channel permeable to both $Na^+$ and $K^+$.
4. End-Plate Potential (EPP): Because the resting muscle potential is $-90\text{ mV}$ (far from $E_{Na} = +65\text{ mV}$ and close to $E_K = -90\text{ mV}$), inward $Na^+$ current through the open $nAChR$ channels vastly exceeds outward $K^+$ current. This produces a local, graded depolarization known as the End-Plate Potential (EPP). In healthy tissue, the EPP amplitude ($40\text{--}50\text{ mV}$) far exceeds the threshold required to activate voltage-gated $Nav$ channels in the perijunctional muscle membrane (high safety factor).
5. Signal Termination: Acetylcholine is rapidly hydrolyzed by acetylcholinesterase (AChE) anchored to the synaptic basal lamina into acetate and choline within sub-milliseconds. Choline is recaptured by the presynaptic high-affinity choline transporter 1 (CHT1) to synthesize new ACh via choline acetyltransferase (ChAT).

Neuromuscular Transmission Disorders

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Excitation-Contraction (EC) Coupling in Muscle

Excitation-contraction (EC) coupling converts an electrical membrane action potential into mechanical cross-bridge cycling and force generation. The molecular transducers vary fundamentally across skeletal, cardiac, and smooth muscle.

Skeletal Muscle EC Coupling

1. Sarcolemmal Invaginations: The muscle action potential propagates across the sarcolemma and plunges deep into the myofiber along Transverse Tubules (T-tubules). T-tubules flank the terminal cisternae of the sarcoplasmic reticulum at the A-I band junction, forming triads (one central T-tubule and two adjacent SR cisternae).
2. Voltage-sensor (mechanical) coupling: Depolarization triggers a conformational change in the Dihydropyridine Receptor (DHPR / Cav1.1), an L-type voltage-gated calcium channel located on the T-tubule membrane. In skeletal muscle, DHPR acts strictly as an electromechanical voltage sensor; extracellular $Ca^{2+}$ influx is not required.
3. Ryanodine Receptor Activation: Conformational shift of the cytoplasmic loop of DHPR physically pulls open the Ryanodine Receptor 1 (RyR1) foot-protein channel on the adjacent terminal cisternae.
4. Intracellular Calcium Surge: Massive quantities of $Ca^{2+}$ stored within the SR bound to the low-affinity buffer protein calsequestrin rush into the myoplasm, driving free $[Ca^{2+}]$ from $10^{-7}\text{ mol/L}$ to $10^{-5}\text{ mol/L}$.
5. Troponin Complex Activation: Calcium binds to Troponin C ($TnC$). This induces a conformational change across the heterotrimeric troponin complex:
   • Troponin I ($TnI$) releases its inhibitory clamp on actin.
   • Troponin T ($TnT$) shifts tropomyosin deeper into the groove of the F-actin helix, exposing the previously sterically blocked myosin-binding sites on the actin monomers.
6. Cross-Bridge Cycle & Power Stroke:
   • Rigor State: Myosin head is tightly bound to actin in the absence of ATP (rigor mortis occurs after death due to complete ATP depletion).
   • Detachment: A new molecule of ATP binds the nucleotide pocket of the myosin head, causing immediate allosteric dissociation from actin.
   • Cocking (Hydrolysis): The intrinsic myosin ATPase hydrolyzes ATP to ADP and inorganic phosphate ($P_i$), both remaining bound to the head. This rotates the myosin lever arm into a "cocked", high-energy 90-degree conformation.
   • Cross-Bridge Attachment: The cocked myosin head binds weakly to an exposed actin monomer.
   • Power Stroke: Release of inorganic phosphate ($P_i$) triggers the mechanical power stroke. The myosin lever arm rotates to a 45-degree angle, pulling the thin actin filament approximately $10\text{ nm}$ toward the center of the sarcomere (M-line). Sarcomere shortening occurs: the I-band and H-zone narrow, while the A-band length remains constant.
   • ADP Release: ADP dissociates from the myosin head, returning it to the high-affinity rigor state awaiting another ATP.
7. Relaxation: Sarcoplasmic reticulum $Ca^{2+}$-ATPase (SERCA1) pumps free cytoplasmic $Ca^{2+}$ back into the SR lumen against a concentration gradient, allowing tropomyosin to slide back over the actin binding sites.

PLE High-Yield Point (Malignant Hyperthermia): An autosomal dominant mutation in the $RYR1$ gene predisposes patients to uncontrolled, massive sarcoplasmic $Ca^{2+}$ release when exposed to volatile halogenated inhalational anesthetics (halothane, isoflurane, sevoflurane) or the depolarizing muscle relaxant succinylcholine. Sustained hypermetabolism, generalized muscle rigidity, hyperkalemia, extreme hyperthermia, and metabolic acidosis ensue. The immediate antidote of choice is dantrolene, a specific RyR1 antagonist that blocks SR calcium release.

Cardiac Muscle EC Coupling

• Calcium-Induced Calcium Release (CICR): Unlike skeletal muscle, cardiac myocytes require extracellular calcium influx. The cardiac action potential plateau (Phase 2) opens L-type $Ca^{2+}$ channels (Cav1.2 / DHPR), allowing inward trigger $Ca^{2+}$ current ($I_{Ca,L}$). This trigger $Ca^{2+}$ binds to Ryanodine Receptor 2 (RyR2) on the SR, opening the channel to release a much larger wave of calcium into the cytoplasm (CICR).
• Modulation by Phospholamban (PLB): Cardiac SERCA2a is tonically inhibited by unphosphorylated phospholamban. Beta-1 adrenergic stimulation (epinephrine, norepinephrine) activates adenylyl cyclase $\rightarrow$ cyclic AMP $\rightarrow$ Protein Kinase A (PKA). PKA phosphorylates phospholamban, relieving its inhibitory clamp on SERCA2a. This dramatically accelerates SR $Ca^{2+}$ reuptake, speeding myocardial relaxation (positive lusitropy) while increasing SR calcium load for the next beat (positive inotropy).

Smooth Muscle EC Coupling

Smooth muscle possesses no troponin, no T-tubules, and no striations or organized sarcomeres. Instead, actin and myosin filaments are tethered to cytoskeletal dense bodies (analogous to Z-lines) and the plasma membrane.

1. Calcium Entry: Cytosolic calcium rises through voltage-gated Cav1.2 channels, receptor-operated channels, or $IP_3$-mediated release from sparse SR stores following $G_q$-coupled receptor activation.
2. Calmodulin Activation: $Ca^{2+}$ binds to the ubiquitous intracellular calcium-binding protein calmodulin ($4\ Ca^{2+}$ ions per calmodulin).
3. MLCK Phosphorylation: The $Ca^{2+}$-calmodulin complex binds and activates Myosin Light-Chain Kinase (MLCK).
4. Cross-Bridge Formation: Activated MLCK phosphorylates the 20-kDa regulatory light chain of the myosin head. Phosphorylation activates myosin ATPase, allowing it to bind actin and initiate cross-bridge cycling.
5. Relaxation via MLCP: Dephosphorylation of the regulatory light chain by Myosin Light-Chain Phosphatase (MLCP) terminates contraction.
6. The Nitric Oxide / cGMP Vasodilatory Pathway: Nitric oxide (NO) diffuses from endothelial cells into vascular smooth muscle, activating soluble guanylyl cyclase $\rightarrow$ cyclic GMP $\rightarrow$ Protein Kinase G (PKG). PKG phosphorylates and activates MLCP, while inhibiting Cav channels and SERCA re-uptake, promoting profound smooth muscle relaxation and arteriolar vasodilation.
7. Latch-State Phenomenon: When myosin is dephosphorylated by MLCP while still attached to actin, the cross-bridge detaches extremely slowly. This allows vascular and visceral smooth muscle to maintain sustained, tonic wall tension for hours with minimal ongoing ATP expenditure.

Systematic Comparison of Muscle Types

Motor units and muscle fiber types

A motor unit is one alpha motor neuron plus all the muscle fibers it supplies. Units are recruited from small to large (Henneman's size principle), so fine movements use small units first.

• Type I (slow-twitch, red) fibers are oxidative, rich in myoglobin and mitochondria, and fatigue-resistant. Postural muscles such as the soleus are rich in them.
• Type II (fast-twitch, white) fibers are glycolytic, fast and fatigable.

Force also depends on starting length: the length-tension relationship peaks at the sarcomere length that gives optimal actin-myosin overlap. The same principle underlies the Frank-Starling law of the heart.