Lecture Notes: Membrane Potentials in Neurons

Introduction

• Discussing resting membrane potentials, graded potentials, and action potentials in neurons.
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1. Resting Membrane Potential

• Definition: Voltage difference across the cell membrane when the cell is at rest.
• Exists in all cells, with a common range of -70 to -90 millivolts (most textbooks support -70mV).
• Key Component: Zooming in on the neuron's cell membrane to understand ion movement.

How Resting Membrane Potential is Established

1. Sodium-Potassium ATPases:
   • Pumps 3 sodium ions out of the cell and 2 potassium ions into the cell.
   • Establishes a concentration gradient and generates a slight negative charge.
2. Leaky Potassium Channels:
   • Always open, allowing potassium to leave the cell passively, increasing negativity inside the cell.
   • The presence of negatively charged anions (like phosphates and proteins) increases negativity as potassium leaves.
3. Leaky Sodium Channels:
   • Allow sodium to move into the cell, which can contribute to positive charge, but less significant than potassium's movement.

Permeability Factors

• The cell is much more permeable to potassium than sodium, contributing to the resting membrane potential being closer to the equilibrium potential of potassium.

2. Graded Potentials

• Purpose: To change resting membrane potential towards or away from threshold.
  • Excitatory Postsynaptic Potential (EPSP): Moves resting potential closer to threshold (e.g., from -70mV to -55mV).
  • Inhibitory Postsynaptic Potential (IPSP): Moves resting potential further away from threshold (e.g., from -70mV to -90mV).

Mechanisms of EPSPs and IPSPs

• EPSP Example:
  • Use of neurotransmitter (e.g., glutamate) to open ligand-gated ion channels, allowing positive ions (like sodium or calcium) to enter, depolarizing the cell.
• IPSP Example:
  • Use of inhibitory neurotransmitter (e.g., GABA) to open channels for negatively charged ions (like chloride) or cause potassium to leave, hyperpolarizing the cell.

Summation

1. Temporal Summation:
   • Multiple signals from one presynaptic neuron in rapid succession.
2. Spatial Summation:
   • Simultaneous signals from multiple presynaptic neurons.

3. Action Potentials

• Triggered when the resting membrane potential reaches threshold (-55mV) due to EPSPs.
• Process:
  1. Depolarization: Voltage-gated sodium channels open at -55mV, allowing sodium ions to rush into the cell, reaching +30mV.
  2. Inactivation: Sodium channels inactivate at +30mV, stopping sodium influx.
  3. Repolarization: Voltage-gated potassium channels open, allowing potassium to exit, returning to resting potential.
  4. Hyperpolarization: The cell may overshoot to around -90mV due to slow closure of potassium channels.

Refractory Periods

• Absolute Refractory Period:
  • No new action potential can occur, regardless of stimulus.
• Relative Refractory Period:
  • A stronger-than-normal stimulus is required to elicit an action potential after hyperpolarization.

Summary of Voltage-Gated Sodium Channels

• At Rest: Activation gate closed, inactivation gate open.
• During Action Potential: Activation gate opens, inactivation gate begins to close at +30mV.
• After Peak: Inactivation gate fully closed, no sodium can enter.

Conclusion

• Recap of the processes of resting membrane potentials, graded potentials, and action potentials in neurons.
• Importance of understanding these mechanisms for further studies in neurophysiology.