Voltage and Channels

Voltage

Learning about the electricity of a cell uses lots of specific terminology that is used in a physics class.  Whether the electricity is in the human body or in a car battery, it’s all about the movementA fundamental property of life involving motion of the body or its parts. of ionsCharged atoms or molecules..  Throughout this course, we will frequently discuss sodium(Na⁺): Major ECF cation; important for fluid balance, nerve function. ions. They are really common outside the cell in the IF.  We will also talk about potassium(K⁺): Major ICF cation; essential for muscle and nerve function. ions, which are really common inside the cellsThe basic structural and functional units of life. in the ICF. 

This is not balanced. Inside the cell, there are tons of proteinsLarge molecules made of amino acids with various functions in the body.. Proteins carry, mostly, a negative charge.  So, there is a difference in the charges inside versus outside the cell.  The difference is charges is called voltage.  And when I say charge, I mean like, yes. Imagine you were to literally use math to add up all the negative and positive charges inside versus outside.  Ok, here’s why I don’t like voltage. You have to measure two places to get a reading. 

If we had an electrode inside a cell and outside a cell, we would get a reading of -70 millivolts.  This means that when everything was added up, we had 70 more negative charges than positive charges.  Here, in Human A&P, we call this potential.  Potential is measured in millivolts in cells, and we will learn about a few different types of potentials.  Why do we use this word, potential?  Because the difference in charge, the voltage, has the ability to move ions.  The cell membrane separates these charges from each other. If some channelsProtein passages in the cell membrane that allow specific molecules to pass through. in the cell membrane open, we’d get ion movement.  There is a potential for movement.

The “Resting” Potential

The resting potential is the numerical voltage where cells maintain homeostasisThe maintenance of a stable internal environment in the body.. It reflects the balance with their environment around them. Actually, it’s more accurate to say that cells maintain their resting potential to keep homeostasis with their environment. As we have just learned, voltage is the separation of charges. When a cell is at rest, the cation distribution exists as you see it here. There are many sodium cations in the interstitial fluidThe fluid surrounding cells within tissues. and there are many potassium cations in the intracellular fluidThe fluid inside a cell, primarily composed of cytosol..

The inside of the cell is more negatively charged than the outside. This is because of those negatively charged proteins. Further complicating this is that there are leaky channels in the cell membrane. Sodium channels are open to sodium. They allow it to flow along its concentration gradientA difference in the concentration of a substance across a space. in facilitated diffusionPassive movement of molecules from areas of high to low concentration.. Case sodium would diffuse into the cell. There are also open potassium channels that allow potassium to escape the cell down its concentration gradient. The movement of these two ions disrupts this resting potential. It moves the potential away from the value of negative 70 millivolts. We are going to use this as the optimal resting potential for this class. Luckily there is a protein called the sodium potassium pump.

This pump uses active transport to transport the sodium and potassium cations back to their original position. ATPThe energy currency of cells used for muscle contraction. is required to do this. Also the detail of this diagram shows you one of these sodium potassium pumps moving 3 sodium cation. The other sodium potassium pump shows the pump moving only two potassium cation. This occurs because sodium leaks faster into the cell than potassium leaks out. For every three sodium cations entering the cell there are only two sodium cations leaving the cell. This is why the sodium potassium pump does not make an even exchange of these cations

3 Types of Potentials

There are many types of potentials in the body. The word potential really just means electricity. Actually potential really more accurately means the movement of electricity. No matter what you want the definition to mean a potential is a value of voltage. We will experience and come to know three different types of potentials. Arresting potential, designated as negative 70 millibolts, is the potential or voltage at which cells operate. This occurs when they perform homeostatic processes. Just as we saw with the osmosis lab, a cell that is isotonicA solution with the same solute concentration as the inside of a cell, maintaining equilibrium. to its surroundings will continue to freely exchange. It will do this as needed with its surroundings. A cell at the resting potential will continue to exchange with its surroundings as needed. Player in maintaining the resting potential is that sodium potassium pump.

A local potential which can also be called a graded potential is a stimulus that doesn’t really go anywhere. The best analogy I have for this is like trying to light a lighter that doesn’t light. You get it you get a spark but you don’t really get a flame That’s consistent.  Right now, numerous neuronsThe functional cells of the nervous system that transmit signals. in your brain have local potentials. These produce neither conscious thought nor involuntary output. These are local or graded potentials. You have excited the neuron in some way. However, you have not created enough of a change in voltage. As a result, the electricity cannot move from point A to point B.

An action potentialA rapid, temporary electrical charge that travels along neurons, allowing signal transmission. is something that we have mentioned at various times in this course. Action potential is the movement of electricity down the axon of a neuron. Electricity or potential is the movement of ions. As an action potential moves down the axon of a neuron, sodium and potassium ions exchange. This exchange happens at every location on the axon. This exchange occurs in an incredible magnitude. Right now there are local potentials happening in cell bodies of neurons in your brain. If those local potentials do not generate enough change in voltage, they will not surmount the axon Hillock. Therefore, they will not become an action potential. The local potential must generate a significant voltage change. If it doesn’t, the local potential will die out. As a result, the thought will never be completed. In an A and P glass we use this term propagation to indicate conductionThe transmission of nerve impulses along neurons. in a human tissue. In a physics class, we might call this conduction. This term indicates the movement of electricity from Point A to point B. When you plug in a lamp there is conduction from the outlet to the light. Electricity flows from your wall socket to the light bulb. That is the same thing as an action potential carrying electricity from your brain to the muscle in your big toe. Instead of waking up your big toe, it causes a contraction.

Channels

These types of channels were discussed in a previous chapter and now come back at us with a vengeance. The channel on the far left of this picture is a leak channel. It is always open and incapable of closing. If a substance fits in it, the substance is allowed to move through it. Substances only move through it according to their concentration gradient. In this picture, you can see all of the little blue ions on the top of the cell membrane. Not so many of them are on the bottom of the cell membrane. Therefore, the higher concentration gradient is above that cell membrane. This is why the arrow shows these items moving from their high to their low concentration.

The purple protein channel there is a ligand gated channel. It can also be called a chemically gated channel. This channel is only opened when a neurotransmitterChemicals that transmit signals across synapses. connects with it. In this picture, the neurotransmitter is the blue sphere that is connecting to the ligand gated channel.  This ligand is like a key that opens the channel, but does not move through the channel.  What moves through this channel is usually an ion such as sodium or potassium. Note how the purple ion moves through the Purple Channel. It is also moving from its high concentration gradient to its low concentration gradient.

The green protein here is a bit more complex than what we have previously seen for channels.  This is a voltage gated channel. We encountered it briefly on the axon terminalThe endpoint of an axon where neurotransmitters are stored and released into a synapse. of a motor neuron at the neuromuscular junctionThe connection between a motor neuron and a muscle fiber..  A voltage gated channel opens with a change in the resting potential instead of opening with connection of a ligand.