Biology 12 - Impulse Transmission

Properties of the Neuron Membrane

Cell membranes surrounding neurons consist of phospholipids and membrane proteins, just as any other body cell. The proteins in the membrane play a very important role in transporting ions across the membrane. It is this movement of ions that will ultimately allow an impulse to occur or prevent it from happening.

Facilitated Diffusion

Ions and molecules dissolved in water are constantly in motion. This tends to distribute the ions relatively evenly throughout the solution. Therefore, there is a net movement of ions from regions of high concentration to regions of low concentration through specific protein channels in the cell membrane, which help them cross without using energy.

Protein Channels

Ions do not typically pass through the phospholipid bilayer easily, instead they must move through specific regions in the membrane. For example, if NaCl is dissolved in the fluid on one side of the membrane, sodium and chloride regions will allow the ions to naturally move down the concentration gradient moving from the area of high concentration to low.

Protein Pumps

specialized proteins that require the use of energy in the form of ATP to drive them. ATP is required because the particles are being moved are now going against the concentration gradient, meaning they are moving from low concentration to high concentration. This form of transport is very important in establishing ion concentrations. 

Membrane Potential

Both sides of the cell membrane are bathed in an aqueous solution, in which ions are found in various concentrations. Neuron membranes have an unequal distribution of cations (+) and anions (-) leading to electrical charges between the two sides.

The difference in charges on each side of the membrane creates an electrical potential which is a measurable amount of potential energy.

K+ Ratio (Out:In)

1:20

Na+ Ratio (Out:In)

10:1

Cl- Ratio (Out:In)

11.5:1

Resting Potential

The difference between the electric potential in the intracellular and extracellular fluids of the neuron when NOT conducting an impulse. All cells have their own membrane potential but only neurons and muscle cells can change it. This property is called excitability.  

Action Potential

A sudden, fast propagating change of the membrane potential. Action potentials are nerve impulses. Neurons generate and conduct these signals along their cellular processes in order to transmit messages to target tissues. 

Membrane Potential

The electrical charge difference between the inside and outside of a cell membrane, typically measuring roughly -70 mV for neurons at rest, with the interior being more negative. It is maintained by ion concentration gradients (high inside, high outside) regulated by the sodium-potassium pump. Changes in this potential drive cellular signaling.

The cytoplasm contains negatively charged proteins and macromolecules that cannot leave the cell.

Why is the inside of the neuron negatively charged at rest?

Potassium (K⁺)

Which ion is the membrane most permeable to at rest?

K⁺ diffuses out of the cell through potassium leak (non-gated) channels.

What happens to potassium ions (K⁺) at rest?

Because of its concentration gradient (higher inside than outside).

Why does potassium leave the neuron?

Very low permeability.

How permeable is the membrane to sodium (Na⁺) at rest?

Na⁺ would enter the cell due to both concentration and electrical gradients.

What would happen if sodium leak channels opened?

Very few sodium leak channels are open.

Why doesn’t much sodium enter the neuron at rest?

Moves ions against their concentration gradients.

What does the sodium-potassium pump do?

Pumps 3 Na⁺ out and 2 K⁺ into the cell.

How many ions does the sodium-potassium pump move each cycle?

About −70 mV.

What is the resting membrane potential of a neuron?

A stimulus that causes a change in the membrane.

What triggers the opening of sodium-gated channels in a neuron?

Due to a large concentration gradient and the negative charge inside the membrane.

Why does Na⁺ enter the neuron when channels open?

It depolarizes the membrane, making the inside less negative.

What effect does Na⁺ entering the neuron have on the membrane?

-55 mV

What is the threshold potential needed to trigger an action potential?

An action potential is generated.

What happens when the threshold potential is reached?

About +40 mV

What is the peak membrane potential during depolarization?

The inside of the neuron is positive relative to the outside.

What does it mean when a neuron is depolarized?

A fraction of a millisecond.

How long does the rising phase of depolarization last?

The membrane becomes more negative again.

What occurs during the falling phase of the action potential?

Repolarization

First the sodium channels must become inactive, preventing any more sodium from entering. 

A large number of potassium channels reopen and continue to allow K+ to diffuse out to the cell from high to low. 

Sodium-potassium pumps along the membrane resume the action of moving 3 Na+ out and bring 2 K+ back into the cell.

Hyperpolarization

very brief period, known as the undershoot, where the membrane potential actually exceeds the resting potential.  At this point the membrane  is hyperpolarized because it is actually more negative than at rest. This occurs as voltage gated potassium channels continue to allow potassium to exit the cell. This is resolved as some of the voltage gated K+ channels close again.

Refractory Period

Until the membrane returns to its original resting potential it is unable to conduct another message. 

Saltatory Conduction

This insulation provided by the myelin sheath covering allows the action potential to essentially skip or jump down the length of the axon from node to node, thereby speeding up the conduction of the action potential and consequently the impulse.

All or Nothing Principle

A stimulus must be above a certain threshold level in order for it cause an action potential.  If the stimulus is below this level, then no action potential is generated and there is NO impulse. 

It does not matter how much above the threshold level the stimulus is, it still only generates one action potential.  That impulse will either happen or not there is no in between.  

So, if the neuron will either fire or not, depends on if the critical value required to allow a sufficient level of depolarization is reached.    

The same action potential is generated regardless of the strength of the stimulus. 

The strength of a stimulus is transmitted by changing the frequency of the impulse and/or increasing the number of neurons stimulated.