Electrophysiology of Neurons SG-Dr. Poole

Describe the different states that voltage-gated sodium and potassium channels can be found in.

Potassium Voltage Gated Channels only have two states, which are resting and activated states. Resting state is when the activation gate is closed, and the activated state is when it is open. Sodium Voltage Gated Channels have three states. which are resting, inactivated or activated. 1) Resting State: inactivation gate opened and activation gate closed. Sodium ions cannot cross the membrane. 2) Activated State: Both the activation and inactivation gates are opened. Sodium ions can cross the axolemma 3) Inactivated State: Inactivation gate closed, activation gate remains open. Sodium ions can no longer pass through the membrane. The channel returns to the resting state when the action potential is finished.

Describe how an action potential is propagated down the axolemma. a) Can an action potential move in either direction along the axon? Explain your answer b) Does the action potential lose strength as it moves along the axon? c) Can action potential propagation stop halfway down the axon?

1) A local potential depolarizes the axolemma of the trigger zone to threshold 2) Voltage-gated sodium ion channels activate, sodium ions enter, and the axon section depolarizes. The influx of positive charges caused rapid depolarization to about +30 mV 3) Sodium ion channels inactivate and voltage-gated potassium ion channels activate, so sodium ions stop entering and potassium ions exit the axon-repolarization occurs. 4) Sodium ion channels return to their resting state and repolarization continues. 5) The axolemma hyperpolarize before potassium ion channels return to the resting state; after this, the axolemma returns to the resting membrane potential. An action potential cannot move in either direction along the axon. An action potential has an all or nothing where once it has been fired it cannot be stopped or reversed. An action potential does not lose strength as it moves along the axon, and it cannot be stopped halfway down the axon.

What do the terms depolarization, repolarization, and hyperpolarization mean?

Depolarization: The influx of positive charges makes the membrane potential more positive. The cell becomes less polarized as its membrane potential approaches 0 mV Repolarization: Happens when a cell returns to its resting membrane potential Hyperpolarization: When the membrane potential becomes more negative than it is at rest. Potassium cells follow their electrochemical gradient out of the cell, and the loses positive charges.

Describe what happens during hyperpolarization stages of an action potential. a. Describe the state of both voltage-gated sodium and potassium channels. b. What ions are moving and in what direction? c. What is happening to the membrane potential?

During hyperpolarization the voltage gated potassium channels are slow to close and the voltage gated sodium channels are in their resting state. The ions would continue to slowly move into the cell which you make the membrane potential too negative. The membrane potential would dip below the resting threshold value(-55mV) and below the Resting Membrane Potential of -70mV. -Eventually resting membrane potential and ion concentration will be restored by the action of the leak channels and Na+/K+ pump.

Describe what happens during depolarization stages of an action potential. a. Describe the state of both voltage-gated sodium and potassium channels. b. What ions are moving and in what direction? c. What is happening to the membrane potential?

For depolarization the voltage gated sodium channels are open and the potassium channels are closed. The Na+ ions are flowing into the cell. the membrane potential is becoming more positive towards +30mV.

Describe what happens during repolarization stages of an action potential. a. Describe the state of both voltage-gated sodium and potassium channels. b. What ions are moving and in what direction? c. What is happening to the membrane potential?

For repolarization the sodium voltage gated channel is in the inactivated state(inactivation gate is closed) and has the potassium voltage gated channel is open allowing the potassium ions to flow into the cell. This causes the membrane potential to become more negative towards -70mV.

What is the difference between leak and gated channels?

Leak Channels: Are always open and continually allow ions to follow their concentration gradient into or out of the cell. Gated Channels: Are closed at rest, and open only in response to certain stimuli 1) Ligand-Gated Channels: open in response to a certain chemical; binding to the channel 2) Voltage Gated Channels: open or close in response to changes in voltage across the membrane 3) Mechanically Gated Channel: Opens or closes in response to mechanical stimulation such as stretch, pressure,.and vibration.

What are the three fiber types? Where do you find each? Which has the greatest conduction velocity and why?

Type A Fibers: Largest diameter fibers, myelinated (somatic sensory and motor fibers). Type B Fibers: Intermediate in diameter, most are myelinated (autonomic fibers, some sensory fibers). Type C Fibers: Smallest diameter, most are unmyelinated (autonomic fibers, sensory fibers that transmit pain, temperature, and pressure information).

Describe how myelination and fiber diameter influence conduction velocity? Consider the following: a) What is the difference between continuous and saltatory conduction? Which allows for a greater conduction velocity?

A myelinated axon will have a higher conduction velocity since the myelin sheath accelerates the action potential, but a neuron with a larger fiber diameter will also travel at higher conduction velocity than a fiber with a smaller diameter. a) Continuous Conduction: is where the myelin sheath is absent and the action potential does not "jump" from one axon to the next Saltatory Conduction: This is where the myelin sheath is present, and the action potential will jump from one axon to the next, which means that it travels at a greater velocity than the continuous conduction.

Describe how resting membrane potential is generated. What is the value for a neuron?

A typical neuron has a resting membrane potential of about -70mV. The effects of the concentration gradient across the membrane is primarily due to the activity of the Na+/K+ pumps. Potassium tends to diffuse out of the cell, and sodium into the cell. The membrane potential must become more negative, so more of the potassium ions must leave the cell than sodium entering the cell. The membrane is leakier for the potassium ions, so more potassium ions can exit the cell. This causes the cytosol to lose more positive charges than it gains. This results in the membrane potential to become more negative until the value for resting membrane potential is reached.

Will action potentials be generation and propagated in all parts of a neuron? If not, where specifically are they generated then propagated?

Action potentials are generated at the trigger zone which is the junction between the axon hillock and the initial segment of the axon.

How would you describe the relative concentrations of sodium and potassium inside vs. outside a neuron at rest?

The potassium ions tend to diffuse out of the cell and sodium ions tend to diffuse into the cell. At the rest, the membrane potential must become more negative, so there are more potassium ions leaving the cell than sodium ions entering. The concentration for potassium is greater inside the cell and for sodium the concentration is greater outside of the cell.

What is the refractory period? Explain the difference between the absolute and relative refractory periods. Be sure to discuss the state of the voltage-gated sodium channels in your answer.

The refractory period is the time during which the neuron cannot fire another action potential. Absolute Refractory Period: The sodium voltage gated channels are open and the potassium voltage gated channels are activating slowly. The sodium voltage gated channels are then inactivated, and the potassium voltage gated channels become activated, causing the membrane potential to become more negative. There is no second Action Potential until the resting membrane potential is established. Relative Refractory Period: The sodium voltage gated channels are in their resting state, and the potassium voltage gated channels remain activated. A strong stimulus that will generate an Action Potential will cause the neuron to fire again.

What is the threshold value for membrane potential? If this value is reached at the trigger zone what will happen? If it is not reached, what will happen?

The threshold value is -55mV for a membrane potential. The action potential begins when the voltage-gated sodium ion channels in the axolemma of the trigger zone enter the activated state.

What is Membrane Potential?

This is a electrical gradient across the cell membrane.