Physiology of Nerve Tissue

Exitable Tissue: Physiology of Nerve

• Nerve tissue is excitable tissue

• Chief ions on outer nerve surface are Na, Cl, and HCO3; chief ions on inner surface are K and proteins

• Potassium ions (K+) are concentrated 30 times greater inside than outside the cell, and tend to move outward

• Sodium ions (Na+) are concentrated 15 times greater outside than inside the cell

• Under resting conditions, the nerve membrane is semipermeable

• Permeability to hydrated Na+ is low

• Permeability to hydrated K+ is high (50-100 times greater than Na+)

• K+ diffuses from inside to outside, resulting in a loss of positive ions from inside and an increase in positive charge outside the membrane

• The membrane is impermeable to proteins, keeping a negative charge inside

• Membrane is permeable to Cl- and HCO3-, which diffuse into the inside of the membrane

• Resting membrane potential is -90 mV

Na-K Pump

• Maintains the ionic concentration gradients
• Pumps 3 Na+ ions out of the cell for every 2 K+ ions pumped into the cell, requiring energy (ATP)

Action Potential: Depolarization

• Opening of Na channels
• Slow increase to -50 mV, then rapid increase to +35 mV
• Inflow of Na+, magnitude = 105

Action Potential: Repolarization

• Inactivation of Na+ channels
• Opening of K+ channels
• Rapid decrease to 70% of resting membrane potential
• Slow decrease, returning to original resting membrane potential, followed by hyperpolarization

Action Potential: Phases

• Depolarization: Rapid influx of Na+ ions
• Overshoot: Membrane potential exceeds 0 mV
• Repolarization: Rapid efflux of K+ ions
• After-repolarization: Membrane potential falls below resting potential, then returns to normal

Excitability Changes During Action Potential

• Normal excitability: Nerve can respond to stimulus
• Absolute refractory period (ARP): Nerve cannot respond to any stimulus; coincides with rapid depolarization and upper third of repolarization
• Relative refractory period (RRP): Nerve can be stimulated but requires a strong stimulus; coincides with middle third of hyperpolarization
• Supernormal excitability: Increased excitability; coincides with after repolarization phase

Refractory Periods

• Periods of reduced excitability during an action potential
• Absolute refractory period: No stimulus can trigger another action potential
• Relative refractory period: A larger-than-normal stimulus can trigger another action potential

This quiz explores the physiology of excitable nerve tissue, focusing on the roles of key ions like sodium and potassium. Understand how the Na-K pump maintains ionic gradients and the significance of resting membrane potential. Test your knowledge on nerve membrane dynamics and their physiological implications.