Nerves and their

impulses

Biology 12

C-11

Nerves and their impulses

Nerves are designed to transmit “electrical” impulses from the dendrites, over the cell body and through the axon.

The impulse will then jump to the next neuron at a synapse or stimulate a gland or muscle.

Action Potential Each phase described in the following graphs is

related to the action of a membrane protein. The initial depolarization (upswing) is a result

of sodium gates opening and sodium ions rushing into the axoplasm.

Action Potential The following repolarization

(downswing) is a result of potassium gates opening and potassium rushing to the outside of the membrane.

During the recovery period, when the neuron can not produce another action potential, the sodium-potassium pump is moving sodium ions out and potassium ions back in to re-establish the resting potential.

The Resting Potential A neuron in the resting state is polarized. There is a

potential difference across the axomembrane of 100 millivolts (mV).

The inside of the neuron (axoplasm) is negatively charged compared to the outside of the axomembrane.

This potential difference is produced by the action of the sodium-potassium pump. To maintain the resting potential of resting neurons, ATP is continually needed to supply energy to the sodium-potassium pumps as sodium ions and potassium ions tend to leak or diffuse back across the axomembrane.

The axoplasm is kept negative relative to the outside of the neuron because the axoplasm contains large organic negative ions.

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The Resting Potential

-65 mV

+40 mV Resting potential Maintained By sodium- Potassium pump

The Action Potential

An action potential is produced by the action of gated channel proteins embedded in the axomembrane.

There are gated protein channels for both sodium ions and potassium ions.

The action potential has three phases; depolarization, repolarization, and a recovery period.

Depolarization

Rapid facilitated diffusion of sodium ions.

Sodium gated channel proteins open and sodium ions rush from the outside of the axomembrane to the inside.

This changes the polarity across the neuron from -65mV (resting potential) to +40mV. The axoplasm is now positively charged compared to the outside of the neuron.

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Depolarization

-65 mV

+40 mV

Sodium gates open, Membrane Depolarizes.upswing

Repolarization

Facilitated diffusion of potassium ions.

Following the movement of sodium into the axoplasm, potassium gated channels open and potassium ions rush to the outside of the axomembrane.

This makes the outside of the membrane positively charged relative to the inside once again.

The potential across the membrane returns from +40mV back to -65mV.

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Repolarization

-65 mV

+40 mV

Potassium gates open, Membrane Repolarizes Down swing

Section Through an Non-

myelinated Axon

Axomembrane

Sodium Ions outside axon

Potassium Ions inside axon

Large Negative Proteins

Recovery/Refractory Period

Active transport of sodium and potassium ions.

During the recovery period following repolarization, a neuron can not generate an action potential.

The sodium-potassium pump is busy re-establishing the resting potential by pumping sodium ions out and potassium ions back in through the axomembrane.

This recovery period also prevents the action potential from moving backwards.

Time in milliseconds

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-65 mV

+40 mV

Refractory period. Gates close. Sodium and potassium pumps re-establish resting potential

Recovery/Refractory Period

Saltatory

Conduction

Schwann cells wrap around the axon with small gaps called Nodes of Ranvier between each cell.

These cells are found along the length of the axon.

They form the myelinated sheath.

These fatty cells act as an electrical insulator to keep neurons isolated from each other and speed up transmission of impulses.

Saltatory Conduction in a

Myelinated Axon

Depolarization and Repolarization can only occur at the nodes of Ranvier.

Impulse will jump from node to node. (saltatory conduction).

Impulse speeds up from 0.5 m/sec. to 200m/sec.