Lecture Notes: Neuron Potentials

Introduction

• Overview of resting membrane potentials, graded potentials, and action potentials in neurons.
• Reminders to engage with the content (like, subscribe, comment).

Resting Membrane Potential

• Definition: Voltage difference across the cell membrane when the cell is at rest.
• Exists in all cells, not only neurons.
• Typical Voltage Range:
  • Average: -70 mV
  • Generally between -70 mV to -90 mV.

Mechanisms for Establishing Resting Membrane Potential

1. Sodium-Potassium ATPases:

   • Pumps 3 sodium ions out and 2 potassium ions into the cell.
   • Makes the inside of the cell slightly negative.
   • Establishes concentration gradients for sodium and potassium.

2. Leaky Potassium Channels:

   • Always open, allow potassium to move passively.
   • Higher concentration of potassium inside the cell leads to potassium moving out, making the inside negative.
   • Can cause voltage to reach near -90 mV.

3. Leaky Sodium Channels:

   • Allow sodium to enter the cell, but much less permeable compared to potassium.
   • Slightly moves the voltage to -70 mV due to limited sodium influx.

Nernst Potential Calculation

• Used to calculate equilibrium potentials for different ions (sodium, potassium).
• Nernst Equation for Potassium:
  • E(K) = 61.5/z * log([K+] outside / [K+] inside)
• Example: For potassium, usually results in -90 mV.*

Graded Potentials

• Purpose: Change resting membrane potential towards threshold.
• Types of Graded Potentials:
  • EPSP (Excitatory Postsynaptic Potential):
    • Brings membrane potential closer to threshold (-55 mV).
    • Caused by neurotransmitters (e.g., glutamate) opening ligand-gated channels allowing cations (Na+, Ca2+) in.
  • IPSP (Inhibitory Postsynaptic Potential):
    • Moves membrane potential away from threshold, causing hyperpolarization.
    • Caused by neurotransmitters (e.g., GABA) opening channels for chloride ions (Cl-) to enter or potassium ions (K+) to leave.

Summation of Potentials

• Temporal Summation: Repeated stimulation from one presynaptic neuron.
• Spatial Summation: Simultaneous stimulation from multiple presynaptic neurons.

Action Potentials

• Initiated when membrane reaches threshold (-55 mV).
• Phases of Action Potential:
  1. Depolarization:
     • Voltage-gated sodium channels open, Na+ rushes in, leading to positive spike (up to +30 mV).
     • Activation and inactivation gates of sodium channels involved.
  2. Repolarization:
     • Voltage-gated potassium channels open, K+ exits, returning membrane potential toward resting state.
  3. Hyperpolarization:
     • Membrane potential drops below resting (-90 mV) due to prolonged potassium efflux.

Refractory Periods

• Absolute Refractory Period: Period immediately after action potential when no new action potential can be generated regardless of stimulus.
• Relative Refractory Period: Following hyperpolarization when a stronger stimulus is required to initiate another action potential.

Summary

• Understanding the processes behind resting membrane potentials, graded potentials, and action potentials allows better comprehension of neuronal activity.
• Importance of ions and channels in creating and propagating these potentials.