Anatomy and Physiology Lecture Notes: Nervous System - Action Potentials

Introduction

• Professor Long's Lecture Series: Fourth video in the nervous system series for anatomy and physiology.
• Focus: Understanding how action potentials occur and their propagation in multipolar neurons.

Resting Membrane Potential

• Neurons are at -70 millivolts, due to unequal distribution of ions.
• Sodium-Potassium Pump: Maintains resting potential by pumping ions against their gradient.
  • Sodium ions entering the cell make it more positive.
  • Potassium ions exiting the cell make it more negative.

Ion Channels

• Types: Chemically gated, voltage-gated, mechanically gated.
• Chemically Gated Sodium Channels: Open when neurotransmitter binds, causing depolarization.
• Chemically Gated Potassium Channels: Open to hyperpolarize the cell, moving away from threshold.

Establishing Action Potentials

• Action Potential Diagram drawn for a multipolar neuron.
• Threshold Potential: -60 millivolts, crossing this threshold triggers an action potential.
• Voltage-Gated Channels:
  • Voltage-Gated Sodium Channels: Open at threshold, close at +30 millivolts.
  • Voltage-Gated Potassium Channels: Open at +30 millivolts, close slowly causing hyperpolarization to -90 millivolts.

Process of Action Potential

• Depolarization: Opening of voltage-gated sodium channels causes influx of sodium ions.
• Repolarization: Opening of potassium channels, potassium exits the cell, restoring negative charge.
• Hyperpolarization: Excess potassium leaks out, cell becomes more negative than resting state.

Propagation of Action Potentials

• Once initiated, an action potential spreads down the axon.
• Each segment of the axon undergoes depolarization, propagating the signal forward.

Graded Potentials

• Excitatory Postsynaptic Potentials (EPSPs): Result from chemically gated sodium channels, move the cell toward threshold.
• Inhibitory Postsynaptic Potentials (IPSPs): Result from potassium channels, move the cell away from threshold.

All-or-None Principle

• Any stimulus reaching threshold will cause a full action potential to propagate along the neuron.

Refractory Periods

• Absolute Refractory Period: No new action potential can be initiated.
• Relative Refractory Period: A new action potential can occur if the stimulus is strong enough.

Summary

• Understanding of how neurons communicate via action potentials and synapses.
• Knowledge of how various ion channels contribute to neuron excitability and signal transmission.

This summary provides an overview of the lecture's key points on action potentials and their role in neural communication.