Nerve 2 Flashcards by Wadhha Alajmi

Describe the resting membrane potential

At rest :

Inside cell : negative
Outside cell: positive

Membrane potential about -70 mV

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Nerve signals are transmitted by ___________________.

Action potentials

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What are action potentials

Rapid changes in the membrane potential that spread rapidly along the nerve fiber membrane

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What causes the changes in the membrane potential?

The movement of ions in and out of the cell

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An action potential has three phases

Depolarization
Overshoot
Repolarization

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What is meant by ion gating in axons?

We know that the changes in membrane potential are caused by changes in the net flow of ions through ion channels in the membrane.

These gates can open or close the ion channels in response to particular stimuli

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When ion channels are closed , plasma membrane is _________ permeable to an ion.

Less

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When ion channels are open , plasma membrane is _________ permeable to an ion.

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At what reading is the resting state

-70 mV

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What channel is closed during the resting state

Voltage-gated Na and K channels

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Depolarization is characterized by what

Entry/infux of sodium (Na)

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Steps of depolarization

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During depolarization, we said the membrane potential depolarizes from -70 mV to +30 mV. Does the membrane potential reach more than +30mV?

NO because these voltage-gated Na channels open for a LIMITED time.

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Describe the voltage-gated sodium channel during
- resting (-70 mV)
- activated (-70 to +35 mV)
- Inactivated (+35 to -70 mV)

Resting :
- activation gate: closed
- inactivation gate: open

Activated:
- activation gate: open
- inactivation gate: open

Inactivated:
- activation gate: open
- inactivation gate: closed

NOTE: the selectivity filter

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Steps of repolarization

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True or false
Voltage-gated sodium Na channel has 3 states

True

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True or false
Voltage-gated potassium K+ channel has 3 states

False

2 states only (resting and slow activation)

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What is afterhyperpolarization ?

What channels are closed or open?

More negative than resting membrane potential

Na channels are CLOSED
K channels are still OPEN

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The length of time that Na and K channels stay open is ______________ of the strength of the depolarization stimulus

Independent

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The change from -70 mV to +30 mV and back to -70 mV lasts only about ______ msec

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3 msec

The image of action potential on an oscilloscope screen looks like what shape?

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Spike

NOTE: action potentials, therefore are called SPIKE POTENTIALS

What is threshold?

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The minimum stimulus required for generation of action potential

Do the voltage-regulated gates open, when the depolarization is below threshold value?

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NO, threshold must be reached in order to generate an action potential and open gates.

What happens once the stimulus reaches threshold?

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Action potential is produced

Comment on the relationship between stimulus and response (action, potential generation)

Weak stimulus < threshold = no response Strong stimulus > threshold = response

Action potentials are all-or-none. What does this mean ?

A stimulus normally either produces a full-sized action potential or fails to produce one. The amplitude of action potential is constant, regardless of the strength of the stimulus ; therefore, a stronger stimulus cannot produce an action potential of greater amplitude.

True or false Stronger stimulus can produce an action potential of greater amplitude

False The amplitude of action potential is constant, regardless of the strength of the stimulus

How can the nerve transmit different strengths of stimuli ?

(See picture) 1. Weak stimulus - no AP 2. Strong stimulus - few AP 3. Stronger stimulus - many AP

When is the cell membrane refractory ?

When it is producing an action potential

During refractory periods, the cell membrane is in either one of two situations. Explain

It is either : - completely unable to fire an action potential OR - requires a much stronger stimulation than usual

There are two types of refractory periods.

Absolute Relative

Absolute refractory period

The time during which the membrane cannot respond to any other stimulus NO MATTER HOW STRONG IT IS Therefore, a SECOND action potential is NOT possible

During absolute refractory period , a second action potential is not possible as the membrane cannot respond to any stimulus. Why??

Large fraction of Na channels are inactivated = cannot be reopened until the membrane is repolarized “Ball and chain” effect

Explain the “ball and chain” action during absolute refractory period

First we know that the : - Na voltage-gated ion channel is closed at resting membrane potential, and it opens in response to a threshold level of depolarization where ions required for action potential diffuse. SO… After a brief period, the channel is inactivated by the "ball and chain" portion of a polypeptide chain. This inactivation ends after a fixed period when the ball leaves the mouth of the channel. Hence, no action potential is produced/ no response to any stimuli.

Relative refractory period

The time during which an axon can be stimulated with sufficiently very strong stimulus. So, the cell CAN fire a second action potential, BUT a stimulus stronger than normal is required.

Why are we able to fire an action potential with a strong stimulus during the relative refractory period BUT not during the absolute refractory period ?

Because during the relative refractory period: - some Na channels are still inactivated - enough Na channels in the closed state - K+ channels are open

Explain the local ionic disturbance at the end of an action potential.

The Na+/K+ pump reset the chemical equilibrium at the end of the action potential by pumping 3 Na+ out of the cell and 2K+ in the cell, restoring the resting ionic distribution.

Explain the conduction of action potential in an UNMYELINATED axon

Conduction of action potential in a MYELINATED axon

Myelin sheath provides ______________ for the axon , preventing the movement of Na and K+ through the membrane

Insulation

The myelin sheath has interruptions called

Nodes of Ranvier

Na+ channels are __________________________ at the nodes but they are almost _____________ in the regions of axon membranes between the nodes

Highly concentrated Absent

What is meant by Saltatory conduction Can it occur in unmyelinated axons?

Action potential only occurs at nodes of Ranvier (Saltatory conduction describes the way an electrical impulse skips from node to node down the full length of an axon) NO, only in myelinated axons

Compare the conduction rate of myelinated vs unmyelinated axons

Myelinated : fast (100 m/sec ) Unmyelinated: slow (1 m/sec)

What is the relationship between conduction velocity and nerve diameters?

As the diameter DECREASES, the conduction velocity DECREASES. Therefore, the relationship is DIRECTLY PROPORTIONAL

True or false Multiple sclerosis (MS) is a demyelinating autoimmune disease. What occurs in multiple sclerosis??

True - Myelin is destroyed by immune cells - Slow or no transmission of action potential - Difficulty in performing routine tasks

What are local anesthetics?

Drugs that REVERSIBLY bind to Na+ channels in the axon membrane, preventing these channels from opening ( NO DEPOLARIZATION) - Block the ability of sensory axons to produce action potentials - Reduce membrane excitability = nerve impulses fail to pass along the anesthetized nerves

Examples of local anesthetics

Lidocaine Procaine

Nerve 2 Flashcards

(48 cards)