Block 1 Notes: Epithelia, Synapses, and Skeletal Muscle

Epithelia

• Epithelia are classified as “leaky” or “tight”.
  • Tight epithelia: high electrical resistance of tight junctions; less permeable to diffusion of ions.
  • Leaky epithelia: lower electrical resistance; relatively more permeable to diffusion of ions.

Nernst Equation

• How to calculate the equilibrium potential of an ion: the Nernst Equation.
  • General form: E{ion} = \frac{RT}{zF} \ln\left(\frac{[ion]{out}}{[ion]_{in}}\right)
  • Common logarithm form at room or physiological temperature (approx. 37°C when using log base 10): E{ion} \approx \frac{61.5}{z}\log{10}\left(\frac{[ion]{out}}{[ion]{in}}\right) \text{ mV}.
• Common log values for quick mental math:
  • \log(100)=2, \quad \log(10)=1, \quad \log(1)=0, \quad \log(0.1)=-1, \quad \log(0.01)=-2.

Membrane Potential: Parallel Batteries Model

• Concept: membrane potential (VM) is determined by multiple ionic conductances acting as parallel batteries.
• Example form (two ions K+ and Na+): VM = \frac{GK EK + G{Na} E{Na}}{GK + G_{Na}}.
  • If given conductances: GK = 4, \quad G{Na} = 5, with typical reversal potentials (e.g., EK \approx -90\ \text{mV}, \quad E{Na} \approx +60\ \text{mV}), then the resting VM would be near a negative value depending on the ratio of conductances. (The slide example lists VM = -65 mV for a particular set of values; use the general formula above and standard ion gradients for actual calculation.)

Chapter 8: Synaptic Transmission and the Neuromuscular Junction

Electrical vs Chemical Synapses

• Electrical synapse:
  • Gap junctions
  • Immediate communication (almost)
  • Reciprocal synapse: bidirectional communication; can be rectifying (unidirectional)
• Chemical synapse:
  • Use neurotransmitters
  • Delayed communication (~1 ms)
  • Rectifying synapse: unidirectional communication

Chemical Synapse: Steps

• Neurotransmitter (NT) synthesis and packaging in vesicles: dense core vesicles vs small clear vesicles.
• An action potential reaches presynaptic terminal.
• Voltage-gated Ca^{2+} channels open; Ca^{2+} influx.
• Increased intracellular Ca^{2+} triggers fusion of synaptic vesicles with presynaptic membrane.
• NTs diffuse across synaptic cleft and bind to receptors on postsynaptic membrane.
• Receptors become activated and stimulate a response in the postsynaptic cell.
• Signal termination:
  • NT breakdown
  • NT reuptake by presynaptic cell
  • Diffusion away from the synapse

Presynaptic Terminal and Synaptic Cleft

• Presynaptic terminal:
  • Motor nerve axon ends at a synaptic bouton.
  • Contains small clear vesicles and large dense core vesicles.
  • H^+-pumps (ATPases) acidify vesicles to load with acetylcholine (ACh).
• Synaptic cleft:
  • Basal lamina: dense meshwork of protein/extracellular matrix that holds membranes together.
• Postsynaptic Motor End Plate:
  • Highly folded postsynaptic membrane: postjunctional folds increase surface area.
  • Clusters of nicotinic ACh receptors (nAChRs) at the top of folds to maximize NT exposure.
  • Clusters allow for local depolarizations (end-plate potentials, EPPs).

SNAREs and Vesicle Fusion

• Key proteins:
  • v-SNARE: on vesicle (e.g., Synaptobrevin/VAMP)
  • t-SNARE: on presynaptic membrane (e.g., Syntaxin-1, SNAP-25)
  • SNARE complex formation brings membranes together.
• Important auxiliary players:
  • Synaptotagmin: Ca^{2+} sensor
  • Rab3: G-protein that traffics vesicles
  • Sec-1/Munc18 (SM proteins): assist SNARE complex formation
• Process: SNARE complex mediates vesicle fusion and NT release

Receptors: Ionotropic vs Metabotropic

• Ionotropic receptors:
  • Ligand-gated ion channels; fast, almost instantaneous responses.
  • Example: Nicotinic ACh receptor (nAChR).
• Metabotropic receptors:
  • G protein-coupled receptors; slower, involve downstream signaling cascades.
  • Example: Muscarinic ACh receptor (mAChR).

Nicotinic Acetylcholine Receptors (nAChR)

• Location: motor endplate of muscle fiber.
• Structure: pentameric ionotropic Cys-loop receptor family (includes GABA, Glycine, 5-HT3 channels).
• Function: binds two ACh molecules cooperatively to open the channel.
• Result: Na^+ influx (excitatory) -> depolarization of motor endplate; triggers local depolarizations within junctional folds; voltage-gated Na^+ channels open and action potential can be triggered if enough EPPs sum.

End Plate Potential (EPP) and Mini EPP

• EPP: change in postsynaptic membrane potential due to NT binding to postsynaptic receptors; Na^+ influx leads to depolarization; ligand-gated channels (AChR) cause graded, distance-decaying potentials; not all-or-none; does not propagate, but can depolarize adjacent fibers.
• Mini EPP: change in postsynaptic membrane potential due to release of a single vesicle of NT; spontaneous, small, EPP-shaped depolarization.
• Action Potential (AP): Na^+ influx via voltage-gated Na^+ channels leading to an all-or-none AP.

Myasthenia Gravis (MG)

• Autoimmune condition: antibodies target and block acetylcholine receptors at the NMJ.
• Symptoms: muscle weakness, ptosis, diplopia, dysphagia, slurred speech, fatigue that worsens with use.

Chapter 9: Cellular Physiology of Skeletal Muscle

Muscle Types

• SKELETAL: striated, voluntary motion, enables breathing and venous return; all three muscle types exist in the body.
• SMOOTH: non-striated, involuntary, lines organs.
• CARDIAC: striated, involuntary, heart muscle.
• There are three muscle types in the body.

Hierarchy of Skeletal Muscle Structure

• Muscle cell (fiber) is multinucleated; surrounded by sarcolemma.
• Myofibril: repeating units of sarcomeres.
• Fascicle: bundle of muscle fibers; muscle consists of fascicles.
• Sarcomere: fundamental contractile unit, from Z-disc to Z-disc.

The Sarcomere and Filaments

• Thick filament: myosin complex; myosin heads bind actin and hydrolyze ATP to drive contraction.
• Thin filament: actin complex (F-actin); tropomyosin covers myosin binding sites on actin; troponin complex (TnT, TnC, TnI).
• Z-disc: anchors actin of adjacent sarcomeres.
• M-line: center of sarcomere; anchors myosin.
• Titin: runs along myosin from Z-line to M-line; provides elasticity and structural stability.
• Nebulin: runs along actin, from Z-disc; helps set actin length and stability.
• In contraction: I-band and H-zone shorten; A-band remains constant (sliding filament model). HI goes by (I and H shorten, A stays fixed).

The Contractile Unit: Motor Unit

• Motor unit: a single alpha motor neuron and all the muscle fibers it innervates.
• A motor neuron pool contains multiple motor units; the number of innervations can vary within a pool.

Cross-Bridge Cycle (Muscle Contraction)

• Key steps (simplified sequence):
  1) ATP binds to the myosin head → myosin detaches from actin (dissociation).
  2) ATP hydrolysis to ADP + Pi → myosin head pivots to a cocked, high-energy state.
  3) Weak cross-bridge formation: myosin binds to actin at a new site.
  4) Pi release → transition to strong cross-bridge state; cross-bridge firmly binds to actin.
  5) Power stroke: conformational change moves actin past myosin, shortening the sarcomere; filament slides; tension generated.
  6) ADP release → myosin remains attached (post-power-stroke state) until another ATP binds.
  7) ATP binds again → dissociation; cycle restarts.
• States (as shown in diagrams): Resting/relaxed state → Cocked state → Weak cross-bridge → Strong cross-bridge (power stroke) → Post-power-stroke (ADP-bound) → Released when ATP binds → Rigor state if no ATP is present.

Excitation-Contraction Coupling (ECC) in Skeletal Muscle

• Calcium-induced Ca^{2+} release (CICR): action potential travels down T-tubule; activates DHP receptor (L-type Ca^{2+} channel).
• Mechanical coupling: DHP receptor physically activates ryanodine receptor (RYR1) on the sarcoplasmic reticulum (SR).
• SR releases Ca^{2+} into cytosol.
• Ca^{2+} binds troponin C (TnC), triggering a conformational change that moves tropomyosin away from actin binding sites, exposing myosin-binding sites and enabling cross-bridge cycling.
• Termination of contraction:
  • Reduced nerve stimulation.
  • Sequestration of cytosolic Ca^{2+} back into SR and removal from cytosol.
  • SERCA pumps drive Ca^{2+} back into SR (ATP hydrolysis fuels the H^+/Ca^{2+} exchange).
  • NCX (Na^+/Ca^{2+} exchanger) exports Ca^{2+} at the plasma membrane (often 3 Na^+ in for 1 Ca^{2+} out).
  • Mitochondria can also take up Ca^{2+}.

Table: Comparison of Muscle Properties (9-3)

• Mechanism of excitation
  • SKELETAL: Neuromuscular transmission releases ACh, activating nicotinic ACh receptors.
  • CARDIAC: Pacemaker depolarization spreads electrotonically via gap junctions.
  • SMOOTH: Agonist-activated receptors; electrical coupling; pacemaker potentials; slow waves.
• Electrical activity of muscle cell
  • Skeletal: Action potential spikes; brief twitch duration.
  • Cardiac: Action potential plateaus; longer twitch duration.
  • Smooth: Slow, graded membrane potential changes; sustained contractions.
• Ca^{2+} sensor
  • Skeletal: Troponin C (TnC).
  • Cardiac: Troponin C (TnC).
  • Smooth: Calmodulin (CaM).
• EC coupling
  • Skeletal: Cav1.1/L-type Ca^{2+} channel in T-tubule and RYR1 on SR; CICR triggers Ca^{2+} release.
  • Cardiac: Cav1.2 (L-type) triggers Ca^{2+}-induced Ca^{2+} release via RYR2 on SR.
  • Smooth: Ca^{2+} entry via Cav1.2, with Ca^{2+} release via IP3 receptors or ryanodine receptors; multiple pathways.
• Terminator of contraction
  • Skeletal: Breakdown of ACh by acetylcholinesterase (AChE).
  • Cardiac: SR Ca^{2+} uptake by SERCA; other termination mechanisms for Ca^{2+} homeostasis.
  • Smooth: SR Ca^{2+} uptake; MLCP activity governs relaxation.
• Twitch duration
  • Skeletal: 20–200 ms.
  • Cardiac: 200–400 ms.
  • Smooth: 200 ms to sustained.
• Regulation of calcium entry
  • Skeletal: Oxidative and glycolytic metabolism; regulation via troponin complex.
  • Cardiac: Oxidative metabolism predominates; Ca^{2+} handling via L-type channels and SR release.
  • Smooth: Ca^{2+} entry via Ca^{2+} channels and Ca^{2+} release via IP3 receptors and related signaling pathways.
• Other notes:
  • MLCP vs MLCK control; latch state in smooth muscle; metabolic differences among muscle types.

Types of Muscle Fibers

• Three fiber types in human skeletal muscle:
  • Type 1: Slow-twitch (I) – high mitochondrial content; oxidative metabolism; high fatigue resistance; low Vmax; high specific endurance.
  • Type 2a: Fast-twitch (IIa) – high/moderate mitochondria; combination oxidative/glycolytic metabolism; high Vmax; high/ moderate fatigue resistance.
  • Type 2x (or IIx/IId): Fast-twitch (IIx) – glycolytic; high Vmax; lower fatigue resistance; low mitochondria.
• Myosin ATPase activity:
  • Type I: lowest
  • Type IIa: high
  • Type IIx: highest
• Mitochondria: Type I high; Type IIa high; Type IIx relatively few
• Color (myoglobin): Type I red; Type IIa red; Type IIx white
• Metabolism: Type I oxidative; Type IIa oxidative; Type IIx glycolytic
• Specific tension and Vmax: Type I low to moderate tension; Type IIx highest Vmax and often high tension; Type IIa intermediate
• Myoglobin provides O2 transport; red fibers have more myoglobin; white fibers have less

Three Muscle Fiber Types: Functional Summary

• Slow-twitch fibers (Type I):
  • High fatigue resistance
  • Suited for endurance and posture
  • Oxidative metabolism with many mitochondria
• Fast-twitch fibers (Type IIa, IIx):
  • Higher force and power potential
  • Type IIa is more oxidative than IIx; IIx is more glycolytic
  • Higher Vmax and greater contraction speed; prone to quicker fatigue

Maximal Shortening Velocity and Power

• Maximal shortening velocity varies by fiber type: Type I < Type IIa < Type IIx
• Maximal specific force and power output:
  • Specific force (force per area) and maximal power depend on fiber type; Type II fibers typically exhibit greater power at high shortening velocities than Type I.

Fatigue and Exercise Physiology

• Exercise fatigue defined as a decline in muscle power output.
• Fatigue mechanisms depend on exercise intensity:
  • Heavy, very heavy, severe exercise (1–10 minutes): multifactorial causes including decreased Ca^{2+} release from SR and accumulation of Pi, H^+, and free radicals which impair cross-bridge cycling and Ca^{2+} sensitivity.
  • Key metabolites contributing to fatigue: increases in Pi, H^+, and free radicals. H^+ binds to Ca^{2+} binding sites on troponin, preventing Ca^{2+} binding; Pi and radicals modify cross-bridge heads, reducing cross-bridge formation.
  • Moderate exercise (>60 minutes): fatigue due to radical accumulation and glycogen depletion; Pi and H^+ contribute less; glycogen depletion reduces TCA cycle intermediates and ATP production via oxidative phosphorylation; radicals still reduce cross-bridge formation.

Exercise Types and Muscle Actions

• Types of muscle actions:
  • Concentric: muscle contracts with force greater than resistance and shortens.
  • Eccentric: muscle contracts with force less than resistance and lengthens.
  • Isometric: muscle contracts without changing length.

Motor Unit and Innervation Ratios

• Innervation ratios for fine motor control (e.g., muscles of the fingers) are small (few fibers per motor neuron) to allow precise control; coarse control uses larger innervation ratios.
• Question examples (from practice):
  • Muscles of fingers: small innervation ratio; fine motor control. Answer: 3. Small; fine

Skeletal Muscle Organization: Quick Facts

• Notable banding and zone changes during contraction:
  • I-band and H-zone shorten; A-band remains constant.
  • The arrangement of actin (thin filament) and myosin (thick filament): actin is in the I-band and near the Z-disk; myosin spans the A-band; the H-zone contains only myosin when relaxed.
• Sarcomere architecture:
  • Z-discs anchor actin filaments from adjacent sarcomeres.
  • Titin extends from Z-line to M-line; provides elasticity and alignment.
  • Nebulin aligns with actin along the thin filament; contributes to stability and actin length.

Practice Questions and Quick Answers

• Which type of synapse allows for greater flexibility in modulating signals?
  • Answer: Chemical synapse (B).
• What effect do neurotransmitters binding to postsynaptic receptors typically have on the postsynaptic neuron?
  • Answer: They change the membrane potential of the postsynaptic neuron (B).
• What role does calcium play in vesicle fusion at the synapse?
  • Answer: It activates the SNARE complex to promote vesicle fusion (B).
• How are nicotinic acetylcholine receptors activated when acetylcholine binds?
  • Answer: It directly opens an ion channel, allowing sodium and other ions to flow through (C).
• What triggers the release of acetylcholine from the motor neuron’s synaptic vesicles?
  • Answer: An influx of calcium ions into the motor neuron (C).
• What is the primary function of acetylcholinesterase at the NMJ?
  • Answer: To degrade acetylcholine and terminate its action (C).
• Which statement best describes the action of the nicotinic ACh receptor when acetylcholine binds?
  • Answer: It directly opens an ion channel, allowing sodium and other ions to flow (C).
• Innervation ratios for finger muscles indicate they use:
  • Answer: Small; fine (3).
• Which muscle type(s) are not striated?
  • Answer: Smooth (3).
• During contraction, which bands or zones in the sarcomere decrease in width?
  • Answer: I-band and H-zone (E).
• During the cross-bridge cycle, which step is associated with breaking of the cross-bridge and release from the rigor state?
  • Answer: ATP binding to myosin (3).
• Role of SERCA pump and NCX in skeletal muscles?
  • Answer: SERCA actively transports Ca^{2+} into the SR, while NCX exports Ca^{2+} out of the cell (C).

Equations to Remember (Summary)

• Nernst Equation: E{ion} = \frac{RT}{zF} \ln\left(\frac{[ion]{out}}{[ion]_{in}}\right)
• Approximate Nernst at physiological temperature: E{ion} \approx \frac{61.5}{z}\log{10}\left(\frac{[ion]{out}}{[ion]{in}}\right)\text{ mV}
• Simple membrane potential with two conductances: VM = \frac{GK EK + G{Na} E{Na}}{GK + G_{Na}}.
• Sarcomere regions and bands (conceptual): I-band and H-zone shorten on contraction; A-band constant.
• Cross-bridge cycle (sequence in brief): ATP binding → hydrolysis → cocked state → weak cross-bridge → Pi release → strong cross-bridge (power stroke) → ADP release → rigor/relaxation when new ATP binds.