Signaling in the Nervous System

Nerve Impulse

Nerve impulses are generated by the movement of ions across the neuron's membrane, which changes the electrical potential of the neuron.

1. Resting Membrane Potential

A neuron at rest has a resting membrane potential of about –70 mV (inside relative to outside).
This is due to the uneven distribution of ions:
- High K⁺ (potassium) inside, low outside.
- High Na⁺ (sodium) outside, low inside.
The sodium-potassium pump actively transports 3 Na⁺ out and 2 K⁺ in, maintaining this gradient.
The membrane is more permeable to K⁺ at rest, contributing to the negative potential.

2. Stimulus and Threshold

A stimulus (mechanical, chemical, or electrical) causes ion channels to open.
If the stimulus is strong enough to depolarize the membrane to a threshold (~–55 mV):
- Voltage-gated Na⁺ channels open, allowing Na⁺ to rush into the cell.

3. Depolarization

Influx of Na⁺ makes the inside of the neuron less negative (more positive).
The membrane potential may rise to around +30 mV.
This rapid reversal of polarity is the action potential.

4. Repolarization

After a brief delay, Na⁺ channels close and K⁺ channels open.
K⁺ flows out of the cell, restoring the negative inside.
The neuron becomes more negative again.

5. Hyperpolarization

K⁺ channels stay open slightly longer, causing the membrane potential to drop below –70 mV temporarily.
This is called hyperpolarization.

6. Refractory Period

The neuron cannot fire another action potential immediately.
This ensures one-way propagation of the impulse.

7. Propagation

The depolarization of one segment of the axon triggers the next segment to depolarize.
The action potential travels down the axon without losing strength.
In myelinated axons, the impulse jumps between Nodes of Ranvier (saltatory conduction), making it faster.

Key Concept:
Nerve impulses are generated by voltage-gated ion channels responding to a stimulus, creating a rapid wave of depolarization and repolarization along the neuron.

++++++++++++++++++++++++

This protein molecule, Na, K-ATPase, extrudes Na+ from the intracellular compartment, moving it to the extracellular space,

It imports K+ from the extracellular space, carrying it across the membrane into the cell.

The cell membrane of the neuron is formed by a lipid bilayer and is generally impermeable to charged particles.

The double layer of phospholipids is hydrophobic. Charged ions are hydrophilic and as a result attract water molecules. This allows the neuronal cell membrane to separate charges across its surface to maintain the electrochemical gradient.

However, to create and use the energy stored in the electrochemical gradient, structures must exist to allow for the passage of ions across this membrane. Ion channels, formed by transmembrane spanning proteins, serve that specific function within the neuron. The basic structure consists of transmembrane proteins with carbohydrate groups attached to their surface and a central pore-forming region to allow for the passage of ions. This pore-forming region spans the entirety of the membrane and is generally made up of two or more subunits.

Ion channels must also be selective for specific charged particles. One method by which channels select for specific ions is by size. Although the diameter of a potassium ion (K+) is larger than the diameter of a sodium ion (Na+), the Na+ ions demonstrate a stronger electrostatic attraction for water molecules. Thus, in a solution the Na+ ion has a larger shell of water than K+ ions. Channels can therefore select for K+ ions based upon the size differential in a solution. Other types of channels are selective for specific ions based upon the ion’s electrical affinity to charged portions of the channel. The attraction between an ion and the channel must be sufficiently strong enough to overcome the hydrostatic attraction of the ion. Once the shell of water surrounding the ion is shed, the ion can diffuse through the channel.

The flow of ions through a channel is passive and governed by the electrochemical gradient. Some ion channels are highly selective for a specific anion or cation, while others are more indiscriminate. Ion channels also open and close based upon the needs of the neuron. This change in state requires a conformational change of the proteins that form the channel, a process called gating.