Lecture 3 - Voltage Dependant Membrane Permerability

What causes membrane permeability to change during action potential?

A change in membrane potential.

What happens to permeability at a specific membrane potential?

It greatly increases

What is the purpoltase of the votalge clamp method?

to study ion permeability at different membrane potentials

What does the voltage clamp method allow researchers to do?

Clamp the membrane potential at a chosen voltage

What does the voltage clamp measure?

The current need to maintain the membrane at a given voltage.

What does the measured current in a voltage clamp represent?

The current flowing across the membrane

Who developled/ daped the voltage clamp method used in these experiments?

Hodgkin and Huxley in 1952.

What happened when Hodgkin and Huxley hyperpolarized the membrane to -130mV?

A near-instantaneous capacitive current occurred.

What happens when the membrane is depolarized to 0mV?

A capacitive current occurs, followed by an inward current and then a delayed outward current

What did the voltage clamp experiments prove about membrane permeability?

Membrane permeability is voltage-dependent.

What two current patterns were seen as the membrane potential increased?

An early inward current and a later outward current

What type of relationship did the early inward current show?

A U-shaped relationship with membrane potential

What happened to the late outward current as the membrane potential increased?

It increased progressively

At what membrane potential did the early inward current become zero?

About +52mV

Why did +52mV suggest that the early inward current was caused by Na+?

Because the Na+ equilibrium potential is about +55mV

What happened to the early current when external Na+ was removed?

It reversed direction and became an outward current

What did removing external Na+ show about the early current?

It was caused by Na+ entering the neuron

Did removing external Na+ affect the late outward current?

No

What ion was identified as causing the late outward current?

K+ leaving the neuron

What blockers were used to confirm the roles of Na+ and K+?

Tetrodotoxin blocked Na+ current, and tetraethylammonium blocked K⁺ current.

What did Hodgkin and Huxley assume caused ion currents?

Changes in membrane conductance.

How is membrane conductance related to membrane permeability?

They are very similar concepts

What happens to Na+ current shortly after it reaches it peak?

it quickly declines

Why does Na⁺ current decline after its peak?

Because Na+ permeability undergoes time-dependent inactivation

Does K+ current show the same rapid inactivation as Na+ current?

No

How does K+ conductance differ from Na+ conductance?

K+ conductance activates more slowly.

Why does Na⁺ current occur before K⁺ current?

K⁺ conductance is delayed by several milliseconds.

How does depolarization affect Na⁺ and K⁺ conductance?

Both increase as the neuron becomes more depolarized

What initiates an action potential?

A selective increase in Na+ conductance

What happens when Na+ conductance increases?

Na⁺ enters the neuron and depolarizes the membrane.

Why does depolarization begin to fall after the peak of the action potential?

The electrochemical force on Na⁺ decreases, and Na⁺ permeability inactivates.

What happens to K+ conductance during depolarization?

It slowly increases

What happens when K+ conductance increases?

K+ leaves the cell

What effect does K+ leaving the cell have on membrane potential?

it repolarizes the membrane

Why does the membrane become hyperpolarized after repolarization?

K+ conductance remains temporarily higher than at rest

What happens to K⁺ permeability during hyperpolarization?

It inactivates

What happens after K+ permeability inactivates?

The resting membrane potential is restored

Why is the action potential described as regenerative?

Depolarization causes more Na⁺ channels to open, producing further depolarization.

What type of feedback occurs early in the action potential?

positive feedback

What type of feedback occurs later in the action potential?

Negative feedback

Why do neurons need active mechanisms to propagate action potentials?

Because neurons are poor passive conductors of electricity

What happens as an action potential travels along an axon?

Voltage-sensitive Na⁺ channels open in nearby regions.

What happens when voltage-sensitive Na⁺ channels open?

Na+ enters the neuron and depolarizes that region

How is the action potential moved along the axon?

Depolarization opens Na⁺ channels in the next region of the What is membrane.

What is the refractory period?

a period when a recently activated membrane region cannot be immediately reactivated

What causes the refractory period?

Delayed K+ channel activation and Na+ channel inactivation

Why is the refractory period important for action potential propagation?

It prevents the recently activated membrane from firing again right away.

How does the refractory period affect the direction of the action potential?

it prevents the action potential form reversing direction

n what direction does an action potential normally travel along an axon?

Forward, into membrane regions that have not recently been activated

What is the main role of Na⁺ channels during action potential propagation?

They depolarize successive regions of the axon.

What is conduction velocity?

The measurable rate at which an action potential travels along an axon

What are two ways conduction velocity can be increased?

Increasing axon diamater and insulating the axon with myelin

Why do larger axons conduct action potentials faster?

they have lower internal resistance to passive flow

What is the most effective way to increase conduction velocity?

Insulating the axonal membrane to reduce current leakage

What cells produce myelin around axons in the CNS?

Oligodendrocytes

What is the myelin sheath?

Multiple layers of glial membrane wrapped around an axon

How does myelin increase conduction velocity?

It reduces current leakage from the axon

How fast can conduction occur in myelinated axons?

Up to about 120 m/s.

What are Nodes of Ranvier?

Gaps in the myelin sheath where voltage-gated Na+ channels are located W

What is saltatory propagation?

the action potential appears to jump form one Node of Ranvier to the next

Why are voltage-gated Na⁺ channels concentrated at the Nodes of Ranvier?

They regenerate the action potential as it travels along the axon.

What is multiple sclerosis?

a CNS disease involving demyelination and inflamation of axons

What age range does MS generally appear in?

About 20-40 years old

How can symptoms of multiple sclerosis behave over time?

they can appear abruptly and then subside after days or weeks

What determines the symptoms seen in multiple sclerosis?

The location of the affected regions in the CNS.

What are some possible symptoms of multiple sclerosis?

Vision problems, motor weakness or paralysis, abnormal sensations, and dizziness.

What causes the loss of myelin in multiple sclerosis?

The exact cause is unclear, but MS is considered an autoimmune disease.