Neurons: Cellular and Network Properties – Action Potentials and Signal Conduction

Chapter 8: Neurons – Cellular and Network Properties

Introduction to Neuronal Signaling

Neurons are specialized cells of the nervous system responsible for transmitting electrical and chemical signals. Understanding the mechanisms of action potentials and their conduction is fundamental to the study of neurophysiology.

Voltage-Gated Na+ Channels

Structure and Function

• Voltage-gated Na+ channels are essential for the initiation and propagation of action potentials in neurons.

• These channels have two gates: an activation gate (opens rapidly in response to depolarization) and an inactivation gate (closes more slowly).

Phases of Na+ Channel Activity

1. Resting State: Activation gate is closed; inactivation gate is open. No Na+ movement.

2. Depolarization: Activation gate opens rapidly, allowing Na+ influx.

3. Inactivation: Inactivation gate closes, stopping Na+ entry even if the activation gate remains open.

4. Repolarization: K+ efflux restores membrane potential; both gates reset to their original positions.

Example: During an action potential, the rapid opening of Na+ channels causes the upstroke (depolarization) of the membrane potential.

Action Potential Conduction

Mechanism of Conduction

• Action potentials are conducted along the axon by the spread of positive charge to adjacent sections of the membrane.

• This process is dependent on the sequential opening of voltage-gated Na+ channels along the axon.

Directionality of Conduction

• Local current flow depolarizes the next segment of the axon, triggering a new action potential.

• Backward flow of the action potential is prevented because Na+ channels behind the action potential are inactivated.

Example: In myelinated axons, action potentials jump between nodes of Ranvier (saltatory conduction), increasing conduction speed.

Refractory Periods

Types and Significance

• Absolute refractory period: No new action potential can be generated because Na+ channels are inactivated.

• Relative refractory period: A stronger-than-normal stimulus is required to initiate another action potential, as some K+ channels remain open, making depolarization more difficult.

Graphical Representation

The refractory periods are depicted on a graph of membrane potential versus time, showing the absolute period (due to Na+ channel inactivation) and the relative period (due to continued K+ efflux).

Key Points

• Refractory periods ensure unidirectional propagation of action potentials.

• They set an upper limit on the frequency of action potentials.

Summary Table: Voltage-Gated Na+ Channel States

State	Activation Gate	Inactivation Gate	Na+ Flow
Resting	Closed	Open	No
Activated (Depolarization)	Open	Open	Yes
Inactivated	Open	Closed	No
Reset (Repolarization)	Closed	Open	No

Key Equations

• Nernst Equation (for equilibrium potential of an ion):

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{outside}}{[ion]_{inside}} \right)$

• Ohm's Law (for current flow):

$I = \frac{V}{R}$

Conclusion

Understanding the properties of voltage-gated Na+ channels, the mechanism of action potential conduction, and the significance of refractory periods is essential for comprehending how neurons transmit information rapidly and efficiently throughout the nervous system.