Action Potentials

Overview of Neurons

• Neurons are specialized cells forming the nervous system.

• Composed of three main parts:

  • Dendrites: Branching structures that receive signals from other neurons.

  • Soma (Cell Body): Contains organelles, including the nucleus.

  • Axon: Long projection wrapped in myelin that transmits signals.

Signal Reception and Transmission

• Dendrites receive signals via neurotransmitters.

• Neurotransmitter binding to receptors opens ion channels:

  • Allows charged ions to flow, creating an electrical signal.

• If combined effects of multiple dendrites sufficiently change the cell's charge, it leads to an action potential.

Electrical Properties of Neurons

• Neurons maintain a resting membrane potential of approximately -65 mV.

• This potential results from differing ion concentrations:

  • Outside the cell: High concentrations of Na+, Cl-, Ca2+.

  • Inside the cell: High concentrations of K+ and negatively charged anions (A-).

• The net negative charge is essential for generating action potentials.

Ligand-Gated Ion Channels

• When neurotransmitters bind to receptors:

  • Ligand-gated ion channels open, allowing specific ions to enter or exit.

  • Example: Sodium channels open, causing depolarization (less negative charge).

Excitatory and Inhibitory Postsynaptic Potentials

• Net influx of positive charge results in Excitatory Postsynaptic Potential (EPSP).

• Influx of negative charge through chloride channels results in Inhibitory Postsynaptic Potential (IPSP).

• Individual EPSPs or IPSPs cause small potential changes.

• Sufficient EPSPs can depolarize to threshold (~-55 mV), triggering action potential.

Action Potential Generation

• Action potential triggered at the axon hillock:

  • Voltage-gated sodium channels respond to membrane threshold.

  • Sodium rushes in, causing rapid depolarization (up to +40 mV).

  • Inactivation of sodium channels occurs post-depolarization, preventing further sodium influx.

Potassium Channel and Repolarization

• Voltage-gated potassium channels open after sodium influx:

  • Potassium ions exit, repolarizing the membrane.

• Sodium-potassium pump helps restore original ion concentration by:

• Moving 3 Na+ out and 2 K+ into the cell.

• Absolute Refractory Period: Sodium channels inactivated; prevents fast re-firing of action potentials.

• Relative Refractory Period: Sodium channels closed, but can be opened by strong stimuli.

Summary of Action Potential Phases

• Resting potential: -65 mV.

• Threshold reached (EPSPs): -55 mV.

• Peak depolarization: +40 mV.

• Initiation of repolarization phase via potassium outflow.

• Hyperpolarization before returning to resting state.

Myelination and Conduction Speed

• Myelin, produced by glial cells (Schwann cells and oligodendrocytes), facilitates faster signal propagation.

• Action potentials jump via saltatory conduction from node of Ranvier to node.

• The mechanism involves sodium rushing and displacing other sodium ions, creating a wave effect.

Conclusion

• Neuron action potentials result from dendritic signal reception leading to membrane depolarization and rapid electrical transmission along the axon through a combination of ion channel activity and myelination.