Biyo-Tek Günlüğüm…

Action Potential

Neurons communicate through changes in electrical properties of the plasma membrane that travel from one cell to another.

The inside of the cell is more negatively charged in relation to the outside because of two factors:

1. Large molecules like proteins and nucleic acids that are negatively charged are more abundant inside the cell and cannot diffuse out. These molecules are called fixed anions.
2. There is a higher concentration of sodium ions (Na+) outside the axon and higher concentration of potassium (K+) ions inside the axon.

The ion concentration is maintained by special proteins on the cell membrane such as

• sodium-potassium pump (brings only two potassium ions into the cell for every three sodium ions it pumps out)
• voltage-gated ion channels

In most cells, the permeability of ions through the membrane is constant. The plasma membranes of muscle and neurons, however, are excitable because the permeability of their ion channels can be altered by various stimuli.

Stages of Impulse Transmission

1. Polarization
2. Depolarization (Action potential)
3. Repolarization

Polarization: The difference in electric potential, or voltage, across the plasma membrane of a cell is called its membrane potential. In an unstimulated neuron, this voltage difference is called a resting potential. A cell’s resting membrane potential is about –70 millivolts.

Depolarization: When a nerve or muscle cell is stimulated, sodium channels become more permeable, and Na+ rushes into the cell, down its concentration gradient. This sudden influx of positive charges reduces the negativity on the inside of the cell and causes the cell to depolarize.

Once a particular level of depolarization is reached, a nerve impulse, or action potential is produced. The level of depolarization needed to produce an action potential is called the threshold

Repolarization: Opening the K+ channels allows K+ to diffuse out of the cell, repolarizing the plasma membrane.In many cases, the repolarization carries the membrane potential to a value slightly more negative than the resting potential for a brief period. this is called hyperpolarization.

Properties of action potentials:

1. They follow an all-or-none law
2. Action potentials are always separate events
3. The velocity of conduction is greater if the diameter of the axon is large or if the axon is myelinated

In myelinated neurons, the action potential wav travels along an axon at Nodes of Ranvier and jumps from one node to the other. This is called saltatory conduction.