L11: Propagation of Action Potential In Different Types of Nerve Fibres

Question 1

What happens to action potential after it is initiated?

Answer 1

After action potential is initiated, it propagates along the axon from the region of the initial segment down to the terminal ending, in order to transfer information from one place in the nervous system to the other.

Question 2

What is the function of action potential?

Answer 2

in order to transfer information from one place in the nervous system to the other.

Question 3

What are nerve fibres classified according to?

Answer 3

according to their diameter into:

• Type A
• Type B
• Type C

Question 4

What is the diameter of type A?

Answer 4

the largest diameter (3- 20 microns)

Question 5

What is the speed of type A nerve fibres?

Answer 5

 highest speeds of conduction (for quick actions)

 (15-120 meters/ second).

Question 6

What is An example of Type A nerve fibres?

Answer 6

somatic nerve fibers that transmit motor impulses and deep sensations

Question 7

What is Type A nerve fibres sensitive to?

Answer 7

Most sensitive to pressure (by surrounding tissue for example)(the conduction of impulses in these nerves blocked by pressure).

Question 8

What is the diameter of Group B nerve fibres?

Answer 8

smaller diameters (1.3-3 microns)

Question 9

What is the speed of Group B nerve fibres?

Answer 9

moderate speeds of conduction (3-15 meters/ second)

Question 10

What is the example of Group B nerve fibres?

Answer 10

myelinated preganglionic autonomic nerves

Question 11

What is Group B nerve fibres sensitive to?

Answer 11

Most sensitive to O2 lack.

Question 12

What is the diameter of Group C nerve fibres?

Answer 12

Have the smallest diameter

Question 13

What is the speed of Group c nerve fibres?

Answer 13

slowest speeds of conduction (0.5-3 meters/second) (as they are unmyleinated)

Question 14

What is an example of Group C nerve fibres?

Answer 14

unmyelinated postganglionic autonomic nerves

Question 15

What are Group c nerve fibres sensitive to?

Answer 15

Most sensitive to local anesthetic drugs

Question 16

What are Group A nerve fibres subdevided into?

Answer 16

Subdivided into alpha, beta, gamma and delta nerve fibers.

Question 17

What are the steps of Continous induction in unmyelinated nerve fibres?

Answer 17

 The initial stimulus causes reversal of polarity and action potential at point of stimulation.

 Local circular currents flow between the activated point and neighboring inactive areas

 + ve charges from inactive areas flow into initial area of negativity produced by action potential (AP). This decreases polarity at the inactive areas (electrotonic depolarization) which produces AP on reaching firing level.

 The latter areas, in turn, electrotonically depolarize the membrane in front of it through local circular currents, this sequence of events moves regularly along nerve fiber to its end.

 Therefore, nerve impulse is self-propagated and once it leaves a point, this point will soon repolarize (thus a repolarization wave starts after depolarization wave and is propagated in the same direction).

Question 18

What are the steps of salutatory conduction in myelinated nerve fibres?

Answer 18

 Myelin surrounds the nerve axon and is interrupted at regular intervals at nodes of Ranvier.

 It is an insulator to current flow (in contrast to nodes of Ranvier, which easily permit current flow because of their high permeability to Na+).

 Circular currents also flow in myelinated nerve fibers, but +ve charges jump from inactive nodes to active nodes (bypassing myelin segments because of their insulator effect) .

Question 19

What are the charachteristcs of salutatory induction?

Answer 19

(a) Increasing velocity of conduction.
(b) Conservation of energy (because excitation occurs only in the nodes
and not all over the nerve membrane

Question 20

What does “nerve block” mean?

Answer 20

o It means failure of conduction of nerve impulses from one place to another. o It also means failure of excitability of the nerve fibers i.e. there is no
generation or propagation of nerve impulses.

Question 21

What are the methods of nerve block?

Answer 21

1. Physical methods: -Severe cooling.
2. Mechanical methods:
    Application of pressure on the nerve
    Injury or crushing of the nerve fibers.
3. Chemical methods:
   (a) Ionic changes that decrease nerve excitability: Increased Ca2+ and decreased Na+ or K+ concentration in extracellular fluid.

(b)Local anesthetic drugs (e.g. cocaine and novocaine): These drugs markedly decrease membrane permeability to Na+ (by preventing opening of Na+ channel activation gates), so depolarization process is inhibited and nerve impulses fail to be produced.