Properties of Nerve Fibres

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1. Conduction of ap in a Myelinated & Unmyelinated nerve fibre:

Factors The factors that affect the rate of conduction of an action

potential are: 1.Myelination2.Diameter of the nerve fiber

Continuous Conduction in Unmyelinated fibers

Occurs in unmyelinated axons. In this situation, the wave of de- and repolarization simply

travels from one patch of membrane to the next adjacent patch…....

Saltatory Conduction in Myelinated fibres

In a Myelinated Nerve Fibre an Action Potential travels by SALTATORY Conduction, which is in a jumping manner from one Node of Ranvier to the next Node of Ranvier, While in an Unmyelinated Nerve Fibre an Action Potential travels from POINT TO POINT.

The conduction of action potentials down an axon is faster in myelinated axons, in which current leak out of the cell is minimized. The unmyelinated axon has low resistance to current leak because the entire axon membrane is in contact with the extracellular fluid and has ion channels through which current can leak.

Question

Which do you think has a faster rate of AP conduction –

myelinated or unmyelinated axons?

Answer The answer is a Myelinated axon. If you can’t see why, then answer this question: Could you move 100ft faster if you walked heel to toe or if you

bounded in a way that there were 3ft in between your feet with each step?Also, would you be able to move across the computer screen faster if you use the space bar or the TAB key.

Myelination increases speed of nerve impulse conduction Action potentials race along myelinated nerve fibres at rates

of up to 100 metres/second or more, but can barely manage 1 metre/second in many unmyelinated fibres.

Very very important!

Question

Which do you think would conduct an AP faster – an axon

with a large diameter or an axon with a small diameter?

Answer The answer is an axon with a large diameter.

If you can’t see why, then answer this question: Could you move faster if you walked through a hallway that

was 6ft wide or if you walked through a hallway that was 1ft wide?

2. All or none law

ALL OR NONE LAW (also called the All or Nothing Law)

On application of a stimulus, an excitable membrane either responds with a maximal or full-fledged action potential that spreads along the nerve fiber, or it does not respond with an

action potential at all. This property is called the all-or-none law.

(This is in direct proportion to the strength of the stimulus applied.)

e.g: This is similar to firing a gun. Either the trigger is NOT pulled sufficiently to fire the gun (subthreshold stimulus) OR it is pulled hard enough to fire the gun (threshold is reached). Squeezing the trigger harder does not produce a greater explosion, just as pulling the trigger halfway does not cause the gun to fire halfway.

Some Action Potential Questions1. What does it mean when we say an AP is “all or none?”

Can you ever have ½ an AP?

2. How does the concept of threshold relate to the “all or none” notion?3. If all action potentials are the same, how does the neuron transmit information about the strength and duration of the stimulus that started the action potential?4. Will one AP ever be bigger than another?

Why or why not?

- ABSOLUTE REFRACTORY PERIOD- RELATIVE REFRACTORY PERIOD

3. Refractory period:

Refractory Period

Definition:Once an Action Potential has begun, a second

action potential cannot be triggered.

The double gating of Na* channels plays a major role in the phenomenon known as the refractory period. The adjective refractory comes from a Latin word meaning stubborn. The stubbornness of the neuron refers to the fact that once an action potential has begun, a second action potential cannot be triggered for about 2 msec, no matter how large the stimulus. This 2 msec represents the time required for the Na* channel gates to reset to their resting positions. A second action potential cannot occur before the first has finished. Consequently, action potentials moving from trigger zone to axon terminal cannot overlap and cannot travel backward.

ABSOLUTE REFRACTORY PERIODDefinition:

Once an action potential has begun, the time period during which even a suprathreshold stimulus will fail to produce a new action potential is

called the Absolute Refractory period.

During this time the membrane becomes completely refractory (‘stubborn’ or ‘unresponsive’) to any further stimulation.

It is the entire Depolarization phase & most of the Repolarization phase.

Due to Absolute refractory period, one AP must be over before another can be initiated at the same site. APs cannot overlap or be added to one another.

BASIS OF AN ABSOLUTE REFRACTORY PERIOD:

During the depolarization phase of AP, the voltage-gated Sodium channels have still NOT reset to their original position. For the Sodium channels to respond to a stimulus, 2 events are important:

Sodium channels be reset to their closed but capable of opening position. i.e: inactivation gates open and activation gates closed.

The Resting membrane potential must be re-established.

Relative Refractory Period

Definition: Following the absolute refractory period is seen a period of short duration during which a second action potential can be

produced, only if the triggering event is a suprathreshold stimulus. This period is called the Relative Refractory Period.

It corresponds to the last half of the Repolarization phase. Basis of a Relative Refractory Period: An action potential can be produced by a suprathreshold

stimulus because of the following reasons:i. By the end of the repolarization phase, some Na channels

have reset while some K channels are also still open. ii.Thus, a greater than normal triggering event (suprathreshold

stimulus) is required to produce an AP.

Absolute VS Relative Refractory Period• Imagine, if you will, a toilet.

• When you pull the handle, water floods the bowl. This event takes a couple of seconds and you cannot stop it in the middle. Once the bowl empties, the flush is complete. Now the upper tank is empty. If you try pulling the handle at this point, nothing happens (absolute refractory). Wait for the upper tank to begin refilling. You can now flush again, but the intensity of the flushes increases as the upper tank refills (relative refractory)

TIME

VM

In this figure, what do the red and blue box represent?

What is the significance of the REFRACTORY PERIOD (both absolute & relative):

1. Refractory periods limit the rate at which signals can be transmitted down a neuron. Only a certain number of Action Potentials can be produced in a nerve fibre because the interval between any 2 action potentials cannot be shorter than the Absolute Refractory Period. It sets an upper limit on the maximum numbers of APs that can be produced in a nerve fibre in a given period of time.

2. There is no fusion or summation of the action potentials. This intermittent, ie. Not continuous conduction of nerve impulses is one of the reasons why a nerve fibre can respond to continuous stimulation for hours without getting tired. Thus, it prevents fatigue in a nerve fibre.

3. The Action Potentials are produced separate from each other. So, a new AP is produced in each part of the nerve fibre. This ensures that the AP does not die out as it is conducted along the membrane.

4. The absolute refractory period also ensures one-way travel of an action potential from cell body to axon terminal by preventing the action potential from traveling backward.

4. Compound action potential:

Compound Action Potential is seen in a “nerve trunk” & NOT a nerve fibre:

An action potential having more than one peak/spike is called a Compound action potential.

CAUSE: A nerve trunk contains many nerve fibres differing widely in their excitability & different speeds of conduction of AP. Multiple peaks are recorded with the AP from fastest conducting nerve fibre first to be recorded followed by the slower ones....

5. Strength-duration curve:

Strength Duration Curve:Strength duration curve represents 2 (two) factors which control

the final strength of the stimulus. These are:i. Voltage or current strength of the stimulus appliedii.Duration of the stimulus

By varying the above 2 factors and plotting the results, a curve is obtained which is called the STRENGTH-DURATION CURVE. (See Mushtaq, vol. 1, ed. 5th , page: 118-119)

It is obvious that a stimulus with a low voltage will have to be applied for a long period of time to reach the threshold level, while high voltage stimulus will need a much shorter duration....

Important definitionsRHEOBASE:

It is the minimum voltage stimulus which when applied for an

adequately prolonged time will produce an AP.

UTILIZATION TIME:The minimum time that a current

equal to rheobase must act to induce an AP is called the

Utilization Time.

CHRONAXIE:It is the minimum duration for

which a stimulus equal to twice the rheobase value has to be applied in

order to start an AP.

Tissues which are more excitable will have a shorter chronaxie and

vice versa...

PROPERTIES OF AN ACTION POTENTIAL:

i. All or none Lawii. Absolute & Relative Refractory periodiii. Compound Action Potentialiv. Strength-Duration Curvev. Conduction through A myelinated nerve fiber (Saltatory conduction) An unmyelinated nerve fiber (Point to Point Conduction)

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