Principles of Bioelectricity

Key Concepts The cell membrane is composed of a phospholipid bilayer The cell membrane may have transport channels (made of protein) embedded in its structure These transport channels may be for facilitated diffusion (allow specific materials to leak through) or for active transport (requires ATP to move materials) If materials diffuse, they spread out evenly If materials are actively transported, they can be moved all to one side of a membrane

Electrically Active Tissue Muscle and Nervous tissues are electrically active This means the tissues can be stimulated by electrical current The reason they are electrically active is because they use ion transport to create a message system The rapid, long-distance messages that are sent are called action potentials Action potentials are NOT electricity like in circuits (fast-moving electrons), nor are they chemicals!

Membrane Potential If the charges are equal on each side of a cell membrane, then the cell membrane is said to have no electrical potential (or no voltage) If instead there is a net positive charge on one side of the membrane, that side has a positive voltage (and the other side has a negative voltage) If this is accomplished with ions, then the ions have concentration gradients from one side to the other of the membrane

Active Transport In muscle and nerve cells, an ion gradient is created by a sodium-potassium pump 1ATP = 3 Sodium out, 2 Potassium in For every 1 ATP, the cell builds up a +1 charge on the outside of the membrane Additionally, there are some potassium leak channels that allow some potassium ions to diffuse back Creates an additional +1 on the outside This creates a voltage across the cell membrane

Resting Membrane Potential Neurons have a resting membrane potential (RMP) of -70 mV Different muscle fibers have different RMPs but all are negative Maintained by sodium-potassium pump and potassium leak channels The RMP can be used to send signals! Much like pulling a rope taut, energy must be expended to maintain the readiness of the message-carrying device These signals are called action potentials

Gated Ion Channels Action potentials are created by the following gated ion channels: Chemically gated ion channels and Voltage-gated ion channels (b) Voltage-gated ion channels open and close in response to changes in membrane voltage. Na + ClosedOpen Receptor (a) Chemically (ligand) gated ion channels open when the appropriate neurotransmitter binds to the receptor, allowing (in this case) simultaneous movement of Na + and K +. Na + K+K+ K+K+ Neurotransmitter chemical attached to receptor Chemical binds ClosedOpen Membrane voltage changes

Ion Channels Suppose a cell is at -70mV internal voltage because there is much more sodium outside the cell than inside the cell Also only slightly more potassium inside the cell than outside What would happen if a sodium channel were to open? What would happen if a potassium channel were to open?

Chemically Gated Ion Channel Suppose a chemically gated sodium channel were opened by a chemical like acetylcholine What would happen to the voltage? What would acetylcholine’s effect be on the target cell? How long would this go on for? Remember that some channels are voltage- gated Meaning they open at a threshold voltage

Voltage-Gated Ion Channel In nerves and many muscles, the voltage-gated sodium channels open at -55mV Inactivate soon so they are only open for a short time Sodium rushes into the cell due to diffusion and due to electrical force of positive voltage Causes depolarization Voltage-gated potassium channels also open at -55 mV Open slower than sodium channels Cause repolarization Entire process spreads throughout the cell very quickly

Action Potential Steps

Electrical signaling Muscles and nerves use action potentials to send signals long distances in a matter of milliseconds Signal does not degrade as long as entire cell has enough sodium and potassium in its fluids

See you Wednesday! Or maybe at Quidditch? :3