Lecture on Neuronal Ion Flow and Membrane Potentials

Resting State and Ion Flow

• Neurons have a baseline ion flow through non-gated leak channels.
• Proper neuronal function relies on ion flow through other channels which we will discuss later.
• Ion flow changes membrane potential in response to stimuli.

Postsynaptic Potentials

General Concept

• Changes in membrane potential moving the cell from resting state.
• Measured in dendrites and cell bodies.
• Stimuli open ion channels, causing brief ion flow.

Types of Stimuli

• Neurotransmitters from presynaptic neurons.
• Changes in extracellular environment (e.g., temperature changes, sensory stimuli).

Excitatory Postsynaptic Potential (EPSP)

• Cause: Opening of sodium channels.
• Effect: Sodium influx depolarizes the cell, making membrane potential more positive.
  • Moves membrane potential toward 0 mV (less polarized).
• Outcome: Increases likelihood of firing an action potential.
• Typically brief; returns to resting potential without further stimuli.

Inhibitory Postsynaptic Potential (IPSP)

• Cause: Opening of chloride channels.
• Effect: Chloride influx hyperpolarizes the cell, making membrane potential more negative.
  • Moves membrane potential away from 0 mV (more polarized).
• Outcome: Decreases likelihood of firing an action potential.
• Typically brief; returns to resting potential without further stimuli.

Membrane Potential Influences

• Resting Membrane Potential: Example given at -60mV.
• Equilibrium Potential of Chloride: -65mV.
• Movement of Chloride:
  • At -60mV, chloride moves into the cell.
  • At -65mV, no net movement of chloride.
  • Below -65mV, chloride exits the cell, initially depolarizing but overall inhibitory.

Summation of Postsynaptic Potentials

Excitatory Summation

• EPSPs can summate temporally (repeated stimulations) or spatially (multiple simultaneous stimulations).
• Causes stronger depolarization.
• Neurons have a threshold potential for action potentials.

Combined Excitatory and Inhibitory Summation

• EPSPs and IPSPs can summate together.
• Inhibitory inputs can decrease or prevent depolarization depending on strength.
• Sodium (positive depolarizing) and chloride (negative hyperpolarizing) channel activities oppose each other:
  • Sodium aims for +60mV potential.
  • Chloride aims for -65mV potential.

Key Takeaways

• Opening sodium channels is excitatory.
• Opening chloride channels is inhibitory.
• Summation of stimuli affects the likelihood and threshold for action potentials.