The Action Potential: Generation and Conduction of the Nerve Impulse

At rest the neurons is:

polarized

Graded Potentials

small, temporary voltage fluctuations that are restricted to the vicinity on the neuron where ion concentrations change

Hyperpolarzation

due to an efflux of K+ (or Cl-), making the extracellular side of the membrane more positive (inside more negative)

Depolarization

due to an influx of Na+ through Na+ channels, making the extracellular side of the membrane more negative (inside more positive)

The Action Potential

• also known as a spike or nerve impulse discharge

• neurons fire

• ONLY seen in axons

• begins at the spike-initiation zone (axon hillock in a typical neuron and sensory nerve ending in sensory neurons)

Voltage-gated Ion Channels

opened/closed by changes in membrane voltage

The three important classes of Voltage-Gated Ion Channels

1. Na+ channels (more outside than inside)

2. K+ channels (more inside than outside)

3. Ca2+ channels (more outside than inside)

Membrane Potential

separation of charges across the membrane

Current

• movement of ions across the membrane through ion channels

• cause the membrane to become positive or negative depending on which ions move

Distribution of Ions Across the Plasma Membrane due to Ion Channels

• Na+ and K+ can passively diffuse across the membrane through specific protein channels

• At resting membrane potential, the membrane is much more permeable to K+ than to Na+

At resting membrane potential, the membrane is much more permeable to K+ than to Na+. Why?

Because the cell has much more channels open for passive K+ traffic than for passive Na+ traffic

Action Potential

• The electrical signal used for neuronal communication

• active response of the neuron to a depolarizing input

• transient (last 1-2 milliseconds) change in membrane voltage from negative resting potential to positive voltages

• responsible for long-range transmission of information with the nervous system (so all information is conveyed exactly how it came in)

• all or nothing (we either get one or we don’t)

• every compontent of the action potential is due to the functioning of voltage-gated ion channels

Components of Action Potential

1. Threshold: going to have an action potential

2. Upstroke: rising phase

3. Downstroke: getting the membrane potential back down to resting'

4. Afterhyperpolarization: having a hyperpolarization

5. Absolute Refractory Period: impossible for the neuron to generate another action potential

6. Relative Refractory Period: can generate another action potential but requires a larger than normal stimulus

Action Potential is caused by:

ion currents flowing through voltage-gated channels

Which Ion channels are involved in the action potential process?

• Sodium

• Potassium

Hodgkin-Huxley Experiment

• showed that depolarization of the neural membrane results in:

1. Activation of voltage-gated Na+ channels

2. Subsequent inactivation of Na+ channels

3. Delayed Activation of K+ channels

What currents underlie the Action Potential?

1. Threshold: Na+ influx

2. Upstroke: Na+ influx

*Action Potential*
3. Downstroke: Na+ off, K+ on

4. Afterhyperpolarization: K+ efflux

Deactivation

Passive recovery at depolarization offset

Inactivation

voltage dependent reduction in current before offset

Potassium channels have only one gate (2 confirmations):

They open when depolarized and stay open until neurons get back to resting potential

Sodium channels have 2 gates (3 confirmations): Activation and Inactivation

The inactivation gate results in sodium channels “closing” even though the neuron is still depolarized

Absolute Refractory Period

• caused by sodium channel inactivation

• occurs when voltage-gated sodium channels are inactivated

• it is IMPOSSIBLE to generate another action potential

• ensures one-way propagation of the action potential

Relative Refractory Period

• after the absolute refractory period

• time after an Action Potential when enough Na channels have recovered from incativation to trigger an action potential, but K efflux is still active and the cell is hyperpolarized, therefore more stimulus is needed to reach threshold

• the rate of firing is related to the intensity of stimulation

Refractory Periods

responsible for two important characteristics of neural activity

A neuron codes for the intensity of a stimulus by:

increasing or decreasing the frequency of action potentials

Properties of Action Potential

• Initiated at the axon hillock

• Threshold potential

• All or none

• Non-decremental

• Refractory Periods

• Very rapid

Myelinated Nerves

• Large nerve fibers are wrapped in myelin which is formed by Schwann cells (PNS) or oligodendrocytes (CNS)

Voltage gated channels are restricted mainly to breaks in the myelin called:

Nodes of Ranvier

Saltatory Conduction

• Myelin restricts current flow to the Nodes of Ranvier where current flows easily through the channels

• Due to high resistance internodal regions, current jumps (saltare) from node to node

• Myelinated axons conduct faster and can fire at higher frequencies than non-myelinated fibers

You are recording from 2 different axons and want to determine which of them is myelinated and which is unmyelinated. Which characteristic would you expect only the myelinated axon to have?

The presence of Nodes of Ranvier and faster conduction of action potential

Multiple Sclerosis

• An autoimmune disorder in which the body attacks and destroy the myelin sheath

• impaired AP propagation