Resting Membrane Potentials, Graded Potentials, and Action Potentials in Neurons

Introduction

• Topics Covered: Resting Membrane Potentials, Graded Potentials, Action Potentials
• Platforms: Instagram, Facebook, Patreon

Resting Membrane Potential (RMP)

• Definition: Voltage difference across the cell membrane when the cell is at rest.
• Existence: Present in all cells, not just neurons.
• Value: Typically ranges from -70mV to -90mV; average textbook value is -70mV.
• Visualization: Zooming in on a neuron’s axon, cell body, and axon terminal.

Mechanisms Leading to RMP

1. Sodium-Potassium ATPases:

   • Pumps 3 Na⁺ ions out and 2 K⁺ ions into the cell.
   • Generates a small negative charge inside the cell.
   • Establishes the concentration gradients for Na⁺ and K⁺.

2. Leaky Potassium Channels:

   • Always open channels that allow K⁺ to move passively.
   • K⁺ moves out, leaving anions behind, making the cell more negative.
   • If unchecked, would make the cell reach around -90mV.

3. Leaky Sodium Channels:

   • These allow Na⁺ to flow back into the cell, counteracting some of the negativity.
   • The cell is more permeable to K⁺ than to Na⁺.
   • This permeability difference sets the RMP close to -70mV.

Nernst Potential

• Equation Use: When ion concentration gradient equals electrostatic gradient.
• Formula:
  • For K⁺: E_K = 61.5 / z * log([K⁺ outside] / [K⁺ inside])
  • For Na⁺: E_Na = 61.5 / z * log([Na⁺ outside] / [Na⁺ inside])
• Example Values:
  • K⁺: ~ -90mV
  • Na⁺: ~ +70mV

Graded Potentials

• Purpose: To move the RMP closer or further away from the action potential threshold.
• Key Terms: Excitatory Postsynaptic Potential (EPSP) and Inhibitory Postsynaptic Potential (IPSP):
  • EPSP: Makes the neuron more likely to fire (e.g., via Na⁺ influx).
  • IPSP: Makes the neuron less likely to fire (e.g., via Cl⁻ influx or K⁺ efflux).

Mechanisms

1. EPSPs:

   • Generated by excitatory neurotransmitters (e.g., glutamate) binding to ligand-gated ion channels, allowing cations like Na⁺ or Ca²⁺ to flow in.

2. IPSPs:

   • Generated by inhibitory neurotransmitters (e.g., GABA) causing Cl⁻ influx or K⁺ efflux.

Summation

• Temporal Summation: One presynaptic neuron repeatedly fires to accumulate EPSPs.
• Spatial Summation: Multiple presynaptic neurons fire simultaneously to accumulate EPSPs.
• Goal: Have more EPSPs than IPSPs to reach the action potential threshold.

Action Potentials

Mechanism

1. Threshold Potential: -55mV

   • Opens voltage-gated sodium channels at the axon hillock.
   • Na⁺ influx depolarizes the cell to +30mV.

2. Axon Propagation:

   • Depolarization moves down the axon via sequential activation of Na⁺ channels.

3. Calcium Channels:

   • Voltage-gated Ca²⁺ channels open at +30mV.
   • Ca²⁺ influx facilitates neurotransmitter release at the axon terminal.

Repolarization

• Achieved by opening voltage-gated K⁺ channels, allowing K⁺ efflux.
• The cell hyperpolarizes to -90mV before returning to -70mV.

Refractory Periods

1. Absolute Refractory Period:

   • No new action potential can be initiated.
   • Occurs from the peak of the action potential back to resting membrane potential.

2. Relative Refractory Period:

   • A stronger-than-normal stimulus is required to initiate another action potential.
   • Occurs during hyperpolarization back to resting membrane potential.

Summary Graphical Representation

• Resting Membrane Potential: -70mV
• Threshold Potential: -55mV
• Depolarization: +30mV (Na⁺ influx)
• Repolarization: -90mV (K⁺ efflux)
• Return to RMP: Assisted by sodium-potassium ATPases, leaky channels.
• Absolute and Relative Refractory Periods explained through ionic mechanisms and voltage channel configurations.

Conclusion

• Final Recap: Transition from resting membrane potential, to graded potentials (EPSPs and IPSPs), to the generation and propagation of action potentials.
• Closing: Importance of understanding these processes for neuron function.