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

Chapter 8: Neurons – Cellular and Network Properties

Introduction to Neurons and the Nervous System

Neurons are the fundamental units of the nervous system, responsible for receiving, processing, and transmitting electrical and chemical signals. Understanding their cellular properties and network behaviors is essential for grasping how the nervous system functions.

Voltage-Gated Na+ Channels

Voltage-gated sodium (Na+) channels are crucial for the initiation and propagation of action potentials in neurons.

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

• Resting State: At resting membrane potential, the activation gate is closed, preventing Na+ entry.

• Depolarization: When a stimulus depolarizes the membrane, the activation gate opens, allowing Na+ to enter the cell, causing further depolarization.

• Inactivation: Shortly after opening, the inactivation gate closes, stopping Na+ influx.

• Resetting: During repolarization (as K+ leaves the cell), both gates return to their original positions, ready for the next action potential.

Example: Figure 8.10 illustrates the sequence of gate openings and closings during an action potential.

Action Potential Conduction

Action potentials are electrical impulses that travel along the axon, allowing rapid communication within the nervous system.

• Local Current Flow: Positive charge spreads to adjacent sections of the axon, depolarizing them and triggering new action potentials.

• Unidirectional Propagation: The action potential moves in one direction because Na+ channels behind the impulse are inactivated (refractory period).

• Dependence: Conduction relies on the presence and function of voltage-gated Na+ channels.

Example: Figure 8.13 shows how depolarization spreads along the axon.

Refractory Periods

The refractory period is a brief time after an action potential during which a neuron cannot fire another action potential or is less likely to do so.

• 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 voltage-gated K+ channels are open, making depolarization more difficult.

Graphical Representation: The refractory periods are depicted as distinct phases on a membrane potential vs. time graph (see Figure 8.12).

Key Terms and Definitions

• Action Potential (AP): A rapid, temporary change in membrane potential that travels along the axon.

• Depolarization: The process by which the membrane potential becomes less negative (more positive).

• Repolarization: The return of the membrane potential to a negative value after depolarization.

• Voltage-Gated Channels: Ion channels that open or close in response to changes in membrane potential.

• Refractory Period: The time during which a neuron is unable or less able to fire another action potential.

Summary Table: Properties of Voltage-Gated Na+ Channels

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

Relevant Equations

• Nernst Equation (for equilibrium potential):

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

• Ohm's Law (for current flow):

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

Applications

• Understanding action potentials is fundamental for studying neural communication, muscle contraction, and many physiological processes.

• Dysfunction in voltage-gated channels can lead to neurological diseases and disorders.

Additional info: The notes above are based on standard content from college-level Anatomy & Physiology courses, focusing on neuronal action potentials and their conduction mechanisms.