Lecture Notes: Neuron Function and Action Potentials

Overview of Neuron Structure

• Dendrites: Receive information.
• Cell Body: Processes and integrates info received.
• Axon: Transmits info over long distances within the neuron.
• Axon Terminal: Passes info to the next cell.
• Nerve: A bundle of axons, useful for long-distance info transmission.

Signal Transmission and Action Potentials

• Dendrites receive incoming signals.
• If the incoming stimulation is strong, the neuron initiates an action potential.
  • Neuron "fires" when an action potential occurs.
• Signal transmission relies on the movement of ions (sodium, potassium, chloride).

Ionic Movement and Gradients

• Resting State:

  • Sodium ions: Higher concentration outside the cell.
  • Potassium ions: Higher concentration inside the cell.
  • Establishes a chemical gradient across the membrane.
  • Electrical Gradient: More positive ions outside; creates a charge difference across the membrane.

• Electrochemical Gradient: Combination of chemical and electrical gradients.

• Membrane Potential: Difference in charge across the membrane (approx. -70 millivolts at rest).

Ion Channels and Movement

• Ion Channels:

  • Allow movement of ions across the membrane.
  • Voltage-gated Channels: Open at specific membrane potentials.
  • Ligand-gated Channels: Open when bound by a molecule.
  • Mechanically-gated Channels: Open in response to physical forces.

• Graded Potential occurs when membrane potential changes but is typically small and transient.

Sodium-Potassium Pump

• Uses ATP to move sodium out and potassium in.
• Maintains chemical gradients and contributes to membrane potential.
• Moves 3 sodium ions out, 2 potassium ions in.

Action Potential Process

1. Threshold Trigger: Stimulation raises membrane potential from -70 mV to -55 mV.
2. Depolarization: Sodium channels open; sodium enters, making the membrane more positive (up to +30 mV).
3. Repolarization: Potassium channels open; potassium exits, making the membrane less positive.
4. Hyperpolarization: Membrane potential becomes more negative than resting.

• Refractory Periods:
  • Absolute: No action potentials can fire.
  • Relative: Action potentials require stronger stimuli to fire.

Action Potential Characteristics

• All-or-Nothing: Action potential does not vary in size but in frequency.
• Conduction Velocity: Increased by myelin sheaths (saltatory conduction).
  • Schwann Cells: Form myelin in the peripheral nervous system.
  • Oligodendrocytes: Form myelin in the central nervous system.
  • Nodes of Ranvier: Gaps between myelin sheaths where action potentials jump.

Summary

• At rest, the membrane potential is stable.
• Graded Potentials: Result from sub-threshold stimuli.
• Action Potentials: Occur when stimulus exceeds threshold, causing neuron to fire.