Properties of Nerve Fibres

Dr. Ayisha Qureshi

Professor & HoD

Department of Physiology

Objectives

By the end of the lecture, the student should be able to:

• Enlist the properties of Action Potential.

• Explain the role of diameter and myelination of nerve fiber in conduction velocity of Action Potential.

• Define Refractory Period.

• Differentiate between a Absolute and Relative Refractory Period.

• Explain the All or None Law.

• Define chronaxie, rheobase and utilization time.

PROPERTIES OF AN ACTION POTENTIAL

1. Propagation of an AP

2. Conduction/ Conductivity in nerve fibers

3. All or none Law.

4. Refractory Period: Absolute & Relative Refractory period.

5. Compound Action Potential.

6. Strength duration curve

1. PROPAGATION OF AN ACTION

POTENTIAL

Does the action potential become weak (decremental) as it travels down the

nerve fiber?

Does the action potential become weak (decremental) as it travels down the nerve fiber?

• NO, the action potential does NOT become weak as it travels down thenerve fiber.

• In fact, the AP does NOT travel down the nerve fiber but triggers a NEW APin every new part of the membrane. It is like a “wave” at a stadium. Eachsection of spectators stands up (the rising phase of an action potential),then sits down (the falling phase) in sequence one after another as thewave moves around the stadium.

• The wave, not individual spectators, travels around the stadium.

• Thus, the last action potential at the end of the axon is identical to the original one, no matter how long the axon is.

• In this way, action potentials can serve as long-distance signals without becoming weak or distorted or decremental.

2. CONDUCTION/ CONDUCTIVITY IN A NERVE FIBER

The nerve impulse is conducted along the nerve fiber. Conductivity shows the following characteristics:

• Impulse is propagated along the nerve in both directions, however, under normal conditions, it is conducted along one direction only.

• The nerve impulse is conducted with a definite speed which is called conduction velocity. The velocity depends upon the diameter of the nerve fiber and also on the presence or absence of myelination.

Which do you think has a faster rate of AP conduction – myelinated or unmyelinated axons?

Conduction of AP in a Myelinated & Unmyelinated nerve fibre:

Continuous Conduction in Unmyelinated fibers

• Point to Point Continuous conduction 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.

Saltatory conduction is faster because the current leak 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.

Myelination increases speed of nerve impulse conduction

• Action potentials race along myelinated nerve fibres at rates of up to 100 metres/second or more, while in unmyelinated fibers they have a speed of 1 metre/second only.

Very, very important!

Which do you think would conduct an AP faster: an axon with a large diameter or an axon with a small diameter?

Conduction of AP in a large & small diameter nerve fiber:

Ans: Axon with a larger diameter

3. 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 notrespond 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 (sub-threshold 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 Questions

1. Can you ever have ½ an AP?

Ans: No

2. Will one AP ever be bigger than another?

Ans: No. All APs are identical.

3. What is the significance of All or None law for CNS function?

Ans: This is important as it helps the nervous system to discriminate between important and unimportant events. Stimuli that are too weak to bring the membrane to threshold DO NOT fire an AP and therefore do not clutter up the nervous system by transmitting insignificant signals.

4. 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. 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?

Ans: The answer lies not in the amplitude of the action potential but in the frequency of action potential (number of

action potentials per second).

Because all action potentials are the same, the CNS differentiates between the strength of different stimuli by

the frequency of the action potentials. The more the number of action potentials, the stronger the stimulus.

Similarly, the duration can be known by the simple fact: as long as the action potentials are firing, the stimulus is still

there.

- ABSOLUTE REFRACTORY PERIOD- RELATIVE REFRACTORY PERIOD

4. Refractory period:Once an Action Potential has begun, a second action potential cannot be started. It is of 2 main types:

ABSOLUTE REFRACTORY PERIOD

Definition: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 (‘unresponsive’) to any further stimulation.

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

ADVANTAGE: Due to Absolute refractory period, one AP must be over before another can be initiated at the same site. A second action potential cannot occur before the first has finished, thus,

action potentials moving from trigger zone to axon terminal CANNOT overlap and CANNOT travel backward.

• 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, 2events are important:

1. Sodium channels be reset to their resting position. i.e: inactivation gates open and activation gates closed.

2. The Resting membrane potential must be re-established.

Relative Refractory Period

Definition:

During an action potential, there is a short duration of time during which a second action potential

CAN be produced, 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 asuprathreshold stimulus because of the followingreasons:

1. By the end of repolarization phase, some Na channels have reset. These Na channels will respond to a larger than normal stimulus.

2. Thus, a greater than normal triggering event (suprathreshold stimulus) is required to produce an AP.

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. 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. It prevents fatigue in a nerve fibre. 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.

3. 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.

5. 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....

6. STRENGTH DURATION CURVE

The strength-duration curve is a plot of the threshold current (I) versus pulse duration (d) required to stimulate excitable tissue. Strength-duration curves are useful in studies where the current required is changed when the pulse duration is changed.

• Rheobase: It is the minimum voltage value of the stimulus which when applied for an adequately prolonged time will give rise to an action potential.

• Utilization time: It is the time needed by the threshold stimulus (rheobase) to give a response.

• Chronaxie: It is the minimum duration for which a stimulus equal to twice the Rheobase value has to be applied in order for it to be effective in obtaining a response.

(Chronaxie is a measure of excitability: Tissues which are more excitable will have a shorter

chronaxie and vice versa.)

PROPERTIES OF AN ACTION POTENTIAL

1. Propagation of an AP

2. Conduction/ Conductivity in nerve fibers

3. All or none Law.

4. Refractory Period: Absolute & Relative Refractory period.

5. Compound Action Potential.

6. Strength duration curve