Chapter 5: Membrane Potential and Action Potential

1. Measuring the Membrane Potential

• Outside the membrane: 0 mV (reference)
• Inside the nerve fiber: -70 mV (resting state)
• Only 1/3,000,000 to 1/100,000,000 of total positive charges inside need to be transferred to create this potential
• Reversing the potential from -70 mV to +35 mV requires an equally tiny ion shift, occurring in as little as 1/10,000 of a second

2. Resting Membrane Potential of Neurons (-70 mV)

Active Transport: The Na⁺-K⁺ Pump

• 3 Na⁺ ions outward for every 2 K⁺ ions inward per cycle
• This creates a net negative charge inside (electrogenic effect contributes ~4 mV of the resting potential)
• Energy source: ATP hydrolysis

Ion Concentration Gradients (large nerve fiber)

Origin of the Resting Membrane Potential

1. Potassium diffusion potential: K⁺ leaks out through potassium leak channels (more permeable than Na⁺ at rest), taking positive charge out and creating a negative interior. The Nernst equilibrium potential for K⁺ is about -94 mV.
2. Sodium diffusion potential: Na⁺ leaks inward (opposite direction), partially reducing the negativity. The Nernst equilibrium for Na⁺ is about +61 mV.
3. The Goldman-Hodgkin-Katz equation accounts for the permeabilities of multiple ions simultaneously. At rest, K⁺ permeability is about 100 times greater than Na⁺ permeability, which is why the resting potential (-70 mV) lies much closer to the K⁺ equilibrium potential.
4. The Na⁺-K⁺ pump contributes directly by pumping more positive charges out than in (3 Na⁺ out, 2 K⁺ in), adding an additional ~4 mV of negativity.

3. Neuron Action Potential

Stages of the Action Potential

• A stimulus depolarizes the membrane to the threshold (~-55 mV)
• Voltage-gated Na⁺ channels open suddenly - Na⁺ rushes in
• Membrane potential rises rapidly from -70 mV, overshoots 0 mV, reaches +35 mV
• This is the upstroke (spike) of the action potential

• Within <1 ms, Na⁺ channels begin to inactivate (close via inactivation gates)
• Voltage-gated K⁺ channels open - K⁺ rushes out
• Membrane potential rapidly returns toward -70 mV
• K⁺ channels may stay open briefly longer, causing hyperpolarization (undershoot) to about -80 mV
• When K⁺ channels close, the potential returns to exactly -70 mV

4. Voltage-Gated Sodium and Potassium Channels

Voltage-Gated Sodium Channel

• Has two gates: activation gate (m gate) and inactivation gate (h gate)
• At rest: activation gate CLOSED, inactivation gate OPEN
• On depolarization to threshold: activation gate OPENS rapidly (within 0.01 ms) → Na⁺ floods in
• Within 1/10,000 sec after opening: inactivation gate CLOSES → Na⁺ entry stops
• The channel cannot reopen until the membrane is repolarized (inactivation gate resets)

Voltage-Gated Potassium Channel

• Has only one gate (n gate) - activation gate
• Opens slowly during depolarization (lags behind Na⁺ channel)
• Opens fully as membrane repolarizes → K⁺ rushes out → repolarization accelerates
• Responsible for the undershoot/hyperpolarization (afterhyperpolarization)

Three States of the Na⁺ Channel

1. Resting (closed, activatable): activation gate closed, inactivation gate open
2. Activated (open): both gates open briefly - this is the actual conduction state
3. Inactivated (closed, non-activatable): inactivation gate closed - cannot respond to stimulus

5. Summary of Events During the Action Potential

6. Initiation of the Action Potential

• Threshold: usually -55 mV (about 15 mV above resting -70 mV)
• Positive feedback (Hodgkin cycle): depolarization → Na⁺ channels open → more Na⁺ in → more depolarization → more Na⁺ channels open (explosive, self-sustaining)
• Subthreshold stimuli: do not trigger an action potential; a local depolarization decays passively
• Suprathreshold stimuli: produce a full action potential (all-or-none principle)

7. Refractory Periods

Absolute Refractory Period

• During the peak of depolarization and part of repolarization
• No stimulus, however strong, can trigger another action potential
• Reason: Na⁺ inactivation gates are closed and cannot reopen until repolarization

Relative Refractory Period

• After absolute refractory period, during undershoot/hyperpolarization
• A stronger-than-normal stimulus can fire another action potential
• Reason: membrane is more negative than resting, so threshold is harder to reach
• Absolute refractory period in large nerve fibers: ~1/2500 second → maximum firing rate ~2500 impulses/sec

8. Propagation of the Action Potential

• An action potential at one point of the nerve fiber electrically stimulates adjacent membrane segments
• Local current loops form: positive charges flow from the active site to adjacent resting membrane, depolarizing it to threshold → new action potential fires
• In unmyelinated fibers: propagation is continuous and relatively slow
• In myelinated fibers: action potentials jump from one node of Ranvier to the next (saltatory conduction) - much faster and more energy-efficient
• Propagation velocity: up to 120 m/sec in large myelinated fibers; as slow as 0.5 m/sec in unmyelinated C fibers

9. Roles of Other Ions

• Calcium ions: Stabilize the membrane by binding to outer channel proteins, reducing Na⁺ channel permeability. Hypocalcemia causes increased membrane excitability (tetany)
• Chloride: Cl⁻ is passively distributed; its equilibrium potential is close to the resting potential, so it plays a minor role
• Anesthetics: Substances like procaine and tetracaine block Na⁺ activation gates, stabilizing the membrane and preventing action potential generation

10. Energy Metabolism After Action Potentials

• Each action potential causes a tiny loss of the Na⁺/K⁺ gradient (Na⁺ in, K⁺ out)
• This change is so small that 100,000 to 50 million impulses can be conducted before gradients are exhausted
• The Na⁺-K⁺ pump restores gradients using ATP - an active, energy-requiring process
• Pump activity increases by the cube of intracellular Na⁺ concentration (if Na⁺ doubles inside, pump activity increases ~8x)
• Heat production increases proportionally with impulse frequency (measurable as metabolic activity)

11. Plateau Action Potentials

• In some fibers (notably heart muscle), the membrane does not repolarize immediately
• A plateau keeps the potential near +0 mV for 0.2-0.3 seconds
• Mechanism: special Ca²⁺ channels that open slowly and stay open, balanced by delayed K⁺ channel opening
• This extended depolarization drives prolonged cardiac muscle contraction

Chapter 6: Contraction of Skeletal Muscle

1. Physiological Anatomy of Skeletal Muscle

• Skeletal muscle = ~40% of body mass
• Each fiber: 10-80 micrometers diameter, usually extends the entire length of the muscle
• Innervated by a single nerve ending near its midpoint
• Each fiber contains hundreds to thousands of myofibrils
• Each myofibril is made of ~1500 myosin (thick) filaments and ~3000 actin (thin) filaments arranged in parallel

Sarcomere Structure (basic contractile unit)

• Defined by two Z disks (Z membranes)
• A band: dark region containing myosin filaments (and overlapping actin ends)
• I band: light region containing only actin filaments
• H zone: center of A band where actin does not overlap myosin
• M line: center of the sarcomere; connects myosin filaments by protein linkages
• Titin: giant elastic protein (molecular weight ~3 million) anchoring myosin to Z disks, acts as a spring to restore sarcomere length

Sarcoplasm and Sarcoplasmic Reticulum

• Sarcoplasm: cytoplasm of muscle fiber, rich in glycogen, large amounts of myoglobin (oxygen store)
• Sarcoplasmic reticulum (SR): a specialized smooth ER that envelops each myofibril; stores Ca²⁺
• Transverse (T) tubules: extensions of the cell membrane that pass deep into the fiber, carrying electrical signals inward to reach the SR

2. Molecular Characteristics of the Contractile Filaments

Myosin Filament (thick filament)

• ~480,000 molecular weight per myosin molecule
• Each molecule has: 2 heavy chains coiled together (rod portion) + 4 light chains forming the myosin head (cross-bridge)
• ~200 myosin molecules stack to form one thick filament, with cross-bridge heads projecting outward
• The myosin head is the motor unit: it binds actin and hydrolyzes ATP

Actin Filament (thin filament)

• Double-stranded F-actin helix, each strand made of G-actin monomers
• Each G-actin has an active site that binds the myosin cross-bridge
• Two regulatory proteins sit on the actin filament:
  • Tropomyosin: rod-shaped protein that winds around the actin strands and physically blocks the active sites at rest
  • Troponin complex (3 subunits):
    • Troponin I: inhibitory - binds actin and inhibits actin-myosin interaction
    • Troponin T: binds to tropomyosin, anchoring the complex
    • Troponin C: binds Ca²⁺; when it does, the whole complex shifts, uncovering active sites

3. Sliding Filament Mechanism of Contraction

• During contraction, actin filaments slide inward among myosin filaments
• The Z disks are pulled toward each other - the sarcomere shortens
• The A band length stays constant (myosin does not change)
• The I band and H zone narrow (as actin slides in)
• In fully contracted muscle, actin ends from opposite Z disks may overlap each other

4. Cross-Bridge Cycle (Walk-Along Theory)

1. ATP binds to the myosin head → head detaches from actin
2. ATP is hydrolyzed to ADP + Pi → myosin head cocks into a "high-energy" 90° position
3. Myosin head attaches to actin active site (Ca²⁺ must be present to expose the site)
4. Power stroke: Pi is released → myosin head pivots to 45° → pulls actin ~10 nm toward the M line
5. ADP is released
6. A new ATP binds → cycle repeats

• Each cross-bridge cycles 50-100 times/sec during strong contraction
• All cross-bridges in a sarcomere pull simultaneously, each pulling ~10 nm per stroke

5. Excitation-Contraction Coupling

1. An action potential arrives at the motor nerve terminal
2. Acetylcholine (ACh) is released into the neuromuscular junction
3. ACh binds nicotinic receptors on the motor end plate → end-plate potential → muscle fiber action potential
4. The action potential spreads along the sarcolemma and down T-tubules
5. T-tubule depolarization activates dihydropyridine receptors (DHPR) (voltage sensors) on the T-tubule wall
6. DHPR mechanically activates ryanodine receptors (RyR1) on the SR terminal cisternae
7. Ca²⁺ floods out of SR into sarcoplasm (concentration rises from ~10⁻⁷ to ~10⁻⁵ mol/L)
8. Ca²⁺ binds Troponin C → troponin-tropomyosin complex shifts → active sites on actin uncovered
9. Myosin cross-bridges bind actin → power stroke → contraction begins
10. After the stimulus ends, Ca²⁺-ATPase (SERCA pump) on SR actively pumps Ca²⁺ back into SR
11. Ca²⁺ levels fall → tropomyosin re-covers active sites → relaxation

6. Muscle Action Potential

• Resting potential of muscle fiber: approximately -80 mV (slightly more negative than neurons)
• Duration of action potential: ~1-5 ms
• Propagation velocity: 3-5 m/sec
• The action potential in muscle follows the same mechanism as in neurons (Na⁺ channel activation, K⁺ channel repolarization)
• Special feature: after the action potential, Ca²⁺ release from SR lasts 1/50 second (much longer than the electrical event), driving the contraction

7. The Neuromuscular Junction

Structure

• Motor nerve terminal (presynaptic) forms an expanded end plate on the muscle fiber
• Synaptic cleft: ~20-30 nm gap
• Motor end plate (postsynaptic): has deep subneural clefts lined with nicotinic ACh receptors and acetylcholinesterase

ACh Release and Recycling

• ACh is packaged in ~40-nm vesicles (formed in the spinal cord motoneuron soma, transported to terminals)
• Each vesicle contains ~10,000 molecules of ACh
• ~300,000 vesicles are stored in one nerve terminal
• Arrival of action potential opens voltage-gated Ca²⁺ channels in the terminal → Ca²⁺ increases 100-fold → ~125 vesicles rupture and release ACh
• ACh diffuses across the cleft → binds nicotinic receptors → opens cation channels → depolarization (end-plate potential)
• Acetylcholinesterase splits ACh into acetate + choline within 5-10 ms
• Choline is actively reabsorbed and recycled into new ACh
• Vesicles are recycled within ~20 seconds via clathrin-coated pits

Safety Factor

• Each nerve impulse normally produces an end-plate potential ~3-4x the threshold needed to fire a muscle action potential - this large safety factor ensures reliable transmission

8. Pharmacology of the Neuromuscular Junction

9. Mechanics of Muscle Contraction

Summation of Contractions

• A single action potential produces a brief muscle twitch
• If a second stimulus arrives before the first twitch ends, the twitches summate (add) - greater force
• Wave summation: at high frequency, twitches merge into a tetanus (smooth, sustained maximal contraction)
• Summation occurs because: Ca²⁺ does not have time to be pumped back into SR before the next action potential arrives

Isometric vs. Isotonic Contraction

• Isometric: muscle generates force but does not shorten (e.g., holding a heavy weight still); length constant, tension increases
• Isotonic: muscle shortens while tension remains constant (e.g., lifting a weight through a range of motion)

Length-Tension Relationship

• Maximum contraction force is generated at the optimal resting length (full functional muscle length, ~2.0-2.2 µm sarcomere length)
• At this length, maximum actin-myosin overlap exists
• At shorter lengths: actin filaments from opposite sides overlap and interfere → reduced force
• At longer lengths (over-stretch): fewer cross-bridges can form (actin pulled away from myosin) → reduced force

10. Fast vs. Slow Muscle Fibers

• Examples of slow-fiber-dominant muscles: soleus (postural)
• Examples of fast-fiber-dominant muscles: anterior tibialis, ocular muscles

11. Contraction Durations

12. Energy Sources for Muscle Contraction

1. ATP (immediate energy, only 4-5 mmol/L in muscle) - lasts seconds
2. Creatine phosphate (phosphocreatine) - rapidly regenerates ATP via creatine kinase; lasts 5-8 seconds of maximal effort
3. Glycolysis (anaerobic) - from glycogen/glucose; produces ATP quickly but inefficiently; lasts ~1 minute; produces lactic acid
4. Oxidative phosphorylation (aerobic) - from glucose, fatty acids, amino acids; efficient but slow; sustains prolonged activity

13. Muscle Fatigue

• Defined as inability to maintain a given force output despite continued stimulation
• Causes:
  • Depletion of glycogen and creatine phosphate stores
  • Lactic acid accumulation (lowers pH, inhibits enzyme activity)
  • Intracellular K⁺ changes (altered membrane potentials)
  • Reduced Ca²⁺ release from SR over time
  • Neuromuscular junction fatigue (rarely the limiting factor)

14. Muscle Remodeling

Hypertrophy

• Increased mass from increased number of actin and myosin filaments per fiber (fiber hypertrophy)
• Requires loaded contractions; only a few strong contractions per day can cause significant hypertrophy within weeks
• Protein synthesis increases; muscle enzymes, mitochondria, and vascular supply also increase

Atrophy

• Occurs with disuse, immobilization, denervation, or prolonged illness (cachexia)
• Progressive loss of filaments and fiber cross-section
• Denervation atrophy is most severe - without a nerve supply, muscle eventually replaced by fibrous and fatty tissue

15. Lever Systems and Body Mechanics

• Muscles act through lever systems at joints
• Three classes of levers exist; most body movements use Class III (effort between fulcrum and load)
• Agonist-antagonist coactivation: virtually all movements involve simultaneous contraction of both agonist and antagonist muscles, controlled by CNS - this provides fine position control
• Muscle force depends on: fiber cross-section, number of fibers, sarcomere length, rate of stimulation, and temperature

Quick-Reference Summary Table