Neuronal Signaling

Introduction

The purpose of the nervous system is to transfer information from the PNS to the CNS, process the information in the CNS, and send back information to the PNS. This transfer of information from the external environment, through neurons, and back again to the external environment is known as neuronal signaling.

A summary of neuronal signaling:

Membrane Potential

Example where the concentration of an ion is 100% on one side of the membrane (blue line) and 0% on the other. This imbalance is maintained because the membrane is impermeable to that ion.

Here the ions have diffused across the membrane because it has become permeable to that ion (dotted black line) and the concentration on either side is 50/50. Equilibrium has been reached.

 

Here two kinds of ions are displayed. The membrane is impermeable to both.

This membrane is permeable to only one kind of ion, but is impermeable to the other. The member is therefore selectively permeable to one kind of ion. Here the selectivity is based on size, however, other facts may influence permeability.

  • Electrostatic Pressure

Neuronís Resting State

In its resting state, an electrical gradient is maintained across the neuronís membrane, thereby creating a resting membrane potential. This section explains how this is maintained.

 

Neuronal Stimulation

A number of factors contribute to a neuronís stimulation, which causes a change in the neural membraneís permeability

Passive Potential

The moment a neuronís membrane is affected by some stimuli, the following happens:

Action Potential

As long as the stimuli does not cause the membrane potential to reach -50 mV, only a passive current that diminishes with time and distance is generated through the neuron. However, if the stimuli is enlarged or additional stimuli is provided to the cell, a depolarization of more than 15 to 20 mV may occur (this is possible because the current is proportional to the size of the simuli).

A. Depolarization

If a passive potential depolarizes the membrane to about -50 mV, all Na+ voltage-gated channels are opened:

B. Absolute Refractory Period

The moment the membrane potential hits +50 mV, all the Na+ voltage-gated channels are closed and the K+ voltage-gated channels are opened:

C. Relative Refractory Period

After the absolute refractory period, there is a period when both Na+ and K+ voltage-gated channels remain closed:

Sections A, B, and C above are depicted graphically in the diagram below:

Comparison Between Passive Potential and Active Potential

Description

Passive Potential

Active Potential

Amplitude

Graded with stimulus intensity

Always the same size

Stimulation

Requires very little

Requires a 15-20 mV change

Summation

Adds the stimuli strengths

Only one potential at a time

Spread

Decay with distance

Actively regenerated

Duration

As long as the stimulus

Constant duration

Main channels used

Non-gated channels

Voltage-gated channels

Passive Conduction

Both passive potentials and active potentials propagate current in the intracellular fluid of the neuron.

Active Conduction

Because the action potential's depolarization is localized, it is not able to conduct the current signal very far. Axons therefore provide a method, called active conduction, to maintain the current with undiminished intensity by way of repeated action potentials. There are two forms of active conduction:

Unmyelinated Axons

Myelinated Axons