Action Potential Propagation The Nervous System Action Potential Propagation

What is an action potential? An action potential is an electrical impulse or signal passed from one neuron to another Electrical properties result from ionic concentration differences across plasma membrane and permeability of membrane Ions are any charged particles found in the cell

Important Ions Cations = Positive Ions Sodium = Na+1 charge Potassium = K+1 charge Calcium = Ca+2 charge Anions = Negative Ions Chlorine = Cl-1 charge Nerve cells are surrounded by a semi-permeable membrane that allows some things to pass through and not others

Membrane Potential Caused when opposite charges are separated by a cell membrane and want to move to balance each other out. More cations on outside of cell than inside of cell so you have a membrane potential

Electrode measures membrane potential of a neuron Axon

Phases of Membrane Potential 1. Resting potential: –70mV (inside of axon more negative than outside) 2. Depolarization: 0 to +30 mV (inside of axon more positive than outside) 3. Repolarization: +30 back to –70mV (inside of axon becoming more negative than outside) 4. Hyperpolarization: -70 mV to –100mV (inside axon way more negative than outside)

Phases of Membrane Potential Hyperpolar-ization

Resting Potential When a neuron is not sending a signal, it is "at rest”. When a neuron is at rest, the inside of the neuron is negative compared to the outside. Although the concentrations of the different ions attempt to balance out on both sides of the membrane, they cannot because the cell membrane allows only some ions to pass through channels (ion channels). The difference in the voltage between the inside and outside of the neuron is measured, you have the resting potential. The resting membrane potential of a neuron is about -70 mV (mV=millivolt) -this means that the inside of the neuron is 70 mV less than the outside.

Resting Potential At rest, there are relatively more sodium ions outside the neuron and more potassium ions inside the neuron. The sodium channels are closed, preventing sodium from entering the cell.

Depolarization The action potential is an explosion of electrical activity that is created by a depolarizing current. This means that some event (a stimulus) causes the resting potential to move toward 0 mV. This event (stimulus) happens when a neurotransmitter binds to receptors on the dendrites. This binding causes sodium channels to open, and sodium begins to enter the cell. When the depolarization reaches about -55 mV a neuron will fire an action potential. This is the threshold stimulus.

Depolarization

All or None Principle If the neuron does not reach this critical threshold level, then no action potential will fire. Also, when the threshold level is reached, an action potential of a fixed sized will always fire...for any given neuron, the size of the action potential is always the same. There are no big or small action potentials in one nerve cell - all action potentials are the same size. Therefore, the neuron either does not reach the threshold or a full action potential is fired this is the "ALL OR NONE" principle.

Repolarization During repolarization, we see a return to the normal resting potential state. The sodium channels begin to close, and in an effort to balance the charges inside and outside the cell, potassium moves out of the cell. When potassium leaves the cell, the membrane potential becomes more negative (returns to -70 mV).

Hyperpolarization During hyperpolarization, the potassium channels continue to stay open, allowing more potassium to leave the cell. This drops the electrical voltage down to -100mV, hyperpolarizing the cell. Once potassium channels close, a return to resting potential occurs.

Hyperpolarization

Action Potential Generated The movement of sodium ions into the cell depolarizes adjacent sites on the axon, triggering the opening of additional channels. The result is a chain reaction that spread across the surface of the membrane like a line of falling dominoes. The action potential continues to move down the axon until it reaches the axon terminal and causes the calcium channel to open. Calcium enters the terminal, binds to the vesicles, and causes the vesicles to be released into the synaptic cleft. The neurotransmitter moves through the synaptic cleft and binds to the next neuron, continuing the propagation of the action potential.

The Nerve Impulse Copyright Pearson Prentice Hall An impulse begins when a neuron is stimulated by another neuron. At the leading edge of an action potential, gates in the sodium channels open, allowing Na+ ions to flow into the cell. This flow of ions causes the action potential to move. Copyright Pearson Prentice Hall

The Nerve Impulse Copyright Pearson Prentice Hall An impulse begins when a neuron is stimulated by another neuron. At the leading edge of an action potential, gates in the sodium channels open, allowing Na+ ions to flow into the cell. This flow of ions causes the action potential to move. Copyright Pearson Prentice Hall

The Nerve Impulse Copyright Pearson Prentice Hall At the trailing edge of an action potential, gates in the potassium channels open, allowing positive ions to flow out and restoring the resting potential of the neuron. Copyright Pearson Prentice Hall

The Synapse Synaptic cleft Copyright Pearson Prentice Hall

Basic Steps to an Excitatory Action Potential 1. Neuron is at resting potential (-70 mV) 2. Neurotransmitters bind to receptors on dendrites 3. Causes Na+ to rush into cell body 4. Membrane potential becomes more positive +30mV 5. Na+ channels close 6. K+ rushes out of cell to balance out differences in membrane potential

Basic Steps to an Excitatory Action Potential (continued) 8. Continues along length of axon 9. Action Potential reaches axon terminal and Ca2+ channel open 7. Membrane potential goes to -100mV but then goes back to -70mV 11. Vesicles with NT inside travel to end of axon terminal 10. Ca2+ enters axon terminal and binds to vesicles 12. Vesicles release NTs into synaptic cleft 13. NT’s travel through synaptic cleft and bind to next neuron