Neurons and Action Potentials

Structure of Neurons

• Neurons: Form the nervous system.
• Three main parts:
  • Dendrites: Receive signals from other neurons.
  • Cell Body: Contains the main organelles including the nucleus.
  • Axon: Wrapped in a fatty substance called myelin for protection.

Signal Reception and Transmission

• Neurotransmitters: Chemical signals received by dendrites.
  • Bind to receptors on the dendrites.
  • Open ion channels, converting chemical signals into electrical signals.
• Action Potential: Triggered if the combined effect of many dendrites changes the overall charge enough.
  • Transmits the signal within the cell.
  • Travels through the axon at speeds up to 100 meters/second.

Ionic Basis of Electrical Charge

• Ion Concentrations: Differ inside and outside the cell.
  • Outside: More Na+ (sodium), Cl- (chloride), Ca2+ (calcium).
  • Inside: More K+ (potassium), A- (negatively charged anions).
• Resting Membrane Potential: About -65 millivolts.
  • Relatively negative net charge compared to the external environment.

Depolarization and Excitatory Postsynaptic Potentials (EPSPs)

• Ligand-Gated Ion Channels: Open in response to neurotransmitters.
  • For example, Na+ channels allow sodium to enter the cell, decreasing the negative charge (depolarization).
• EPSPs: Net influx of positive charge due to neurotransmitters.

Inhibitory Postsynaptic Potentials (IPSPs)

• Cl- Ion Channels: Allow chloride ions to enter, causing a net influx of negative charge.
  • Leads to hyperpolarization (more negative charge).

Threshold Value and Action Potential

• Threshold Value: Usually about -55 millivolts.
  • Activates voltage-gated Na+ channels at the axon hillock.
  • Starts a chain reaction along the axon.
• Firing of the Neuron: When the action potential propagates along the axon.
  • The membrane potential can reach +40 millivolts.

Repolarization and Refractory Periods

• Inactivation: Sodium channels stop allowing sodium in after depolarization.
• Potassium Channels: Open slowly, allowing K+ to exit the cell.
  • Helps repolarize the cell.
• Sodium-Potassium Pump: Transfers three Na+ ions out and two K+ ions in.
• Refractory Periods:
  • Absolute: Sodium channels are inactivated.
  • Relative: Sodium channels return to a closed state, but strong stimulation is needed due to open potassium channels.

Role of Myelin

• Myelin Sheathing: Provided by glial cells (Schwann cells or oligodendrocytes).
  • Prevents ion flow in myelinated regions.
  • Nodes of Ranvier: Where ions can flow.
• Saltatory Conduction: Rapid movement of charge, making the action potential appear to "jump" from node to node.

Quick Summary

• Action potentials in neurons are triggered by EPSPs leading to rapid depolarization and electrical propagation along the axon.