Nerve Impulses: Graded Potentials and Action Potentials Lecture 11:2 PP

Nerve Impulses

Introduction

Nerve impulses are the fundamental means by which neurons communicate within the nervous system. These impulses can be classified into two main types: graded potentials and action potentials. Understanding the properties and mechanisms of each is essential for comprehending neuronal signaling and integration.

Graded Potentials

Definition and Location

• Graded potentials are small, localized changes in membrane potential that occur primarily at the dendrites and cell body of a neuron.

• They are the result of specific ion movements across the neuronal membrane.

Characteristics of Graded Potentials

• Graded potentials are localized and short-lived.

• The signal is graded, meaning it can vary in amplitude (size) and direction (depolarizing or hyperpolarizing).

• They can be summated (added together) to influence the likelihood of generating an action potential.

• Essential for initiating an action potential if the threshold is reached at the axon hillock.

Mechanism: Ion Movements

• Graded potentials are due to the movement of ions such as Na+, K+, and Cl- across the membrane through specific channels.

Ion	Influx	Efflux
Na+	Depolarization	—
K+	—	Hyperpolarization
Cl-	Hyperpolarization	—

Additional info: Influx of Na+ typically causes depolarization, while efflux of K+ or influx of Cl- causes hyperpolarization.

Action Potentials

Definition and Function

• Action potentials are the principal way neurons communicate over long distances.

• They occur on the axon and involve a brief, rapid reversal of the resting membrane potential (RMP).

• All action potentials are identical in a given neuron and do not decrease in amplitude with distance (non-decremental).

• They are capable of self-propagation along the axon.

Phases of the Action Potential

1. Resting State: All voltage-gated Na+ and K+ channels are closed. The membrane potential is at rest (typically around -70 mV).

2. Depolarization: Voltage-gated Na+ channels open, allowing Na+ to enter the cell, causing the membrane potential to become more positive.

3. Repolarization: Na+ channels inactivate, and K+ channels open, allowing K+ to exit the cell, returning the membrane potential toward the resting value.

4. Hyperpolarization: Some K+ channels remain open, causing the membrane potential to become more negative than the resting potential before returning to baseline.

Graphical Representation

• The action potential is typically represented as a sharp spike on a graph of membrane potential versus time, with distinct phases corresponding to the above steps.

Key Properties

• All-or-None Principle: An action potential either occurs fully or not at all, depending on whether the threshold is reached.

• Propagation: Once initiated, the action potential regenerates itself along the axon, moving in one direction due to the refractory period.

• Refractory Periods: The absolute refractory period prevents another action potential from occurring immediately, ensuring unidirectional propagation.

Equations

• Nernst Equation (for equilibrium potential of an ion): $E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{outside}}{[ion]_{inside}} \right)$

• Membrane Potential (Goldman-Hodgkin-Katz equation): $V_m = \frac{RT}{F} \ln \left( \frac{P_{K^+}[K^+]_{out} + P_{Na^+}[Na^+]_{out} + P_{Cl^-}[Cl^-]_{in}}{P_{K^+}[K^+]_{in} + P_{Na^+}[Na^+]_{in} + P_{Cl^-}[Cl^-]_{out}} \right)$

Example

• When a neuron receives a strong enough stimulus at its dendrites, graded potentials summate at the axon hillock. If the threshold is reached, an action potential is triggered and propagates down the axon to communicate with other neurons or effector cells.