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Nerve Impulse Propagation

1. What is meant by nerve impulse?

Ans. it may be defined as potential change where nerve cell is excited by adequate stimulus resulting rapid changes in trans membrane potential lasting for a few milliseconds. It is a brief period of negativity occurs at a point of surface of the axon.

2. What are the three successive stages of action potential?

Ans. The three stages are-

(a) Polarisation- At resting phase there is potential difference between the outside and inside of the cell with outside +ve in respect to inside. The resting membrane potential in large nerve fibres is approximately -90 mv. In small nerve fibres as well as in many neurons of the central nervous system is -40 to -60 mv.

(b) Depolarization- At this time, the membrane suddenly becomes very permeable to sodium ion and a state of -90 mv is lost with the potential rising to the positive direction.

(c) Repolarization-At this stage, rapid diffusion of K+ ions to the exterior the normal negative resting membrane potential is reestablished.

3. What is the cause of generation of action potential?

Ans. During the time of generation of action potential the membrane suddenly becomes very permeable to sodium ions allowing tremendous number of Na+ ions to flow to the interior of the axon. It has been found out that due to conformational change in the activation gate Na+ channel opens and Na+ ions literally pour in ward. As a result negativity results outside the channel.

4. What do you mean by “overshoot” of membrane?

Ans. During depolarisation state, the normal “polarized” state of – 90 mv is lost, with the potential rising rapidly in the positive direction. In large nerve fibres, the membrane potential actually overshoots beyond the zero level and becomes somewhat positive. But in small fibres and nervous of central neurons system the potential merely approaches the zero level and does not overshoot to the positive state.

5. What are the absolute and relative refractory period?

Ans. During a short period after an action potential no stimulus of whatever the strength is sufficient to evoke a second action potential. This phase is known as absolute refractory period. This is followed by a relative refractory period. This is followed by a relative refractory period. When an unusually large threshold of stimulus is needed to get spike of diminished amplitude.

6. What do you mean by negative and positive after potential?

Ans. The recovery phase of the action potential at the end process is characterized by the slower fall and is known as after-depolarization or negative after potential. After reaching the previous resting level the membrane potential becomes even more negative than the resting potential for a few milliseconds after the action potential is over. This is known as after hyperpolarization or positive afterpotential. Strangely enough the positive afterpotential is misnomer which is actually even more negative than the resting potential.

7. What is Spike Potential?

Ans. The sharp rise and rapid fall of membrane potential are the spike potential of the axon. The slower fall at the end of the process is the after-depolarization.

spike-potential

8. What is meant by “All or None” law in nerve impulse?

Ans. If an axon is arranged for recording, it is possible to determine the minimal intensity of stimulating current or threshold intensity that will just produce an impulse. Further increase in the intensity of a stimulus produce no change in action potential. The action potential is therefore “all or none” in characters and obey the law.

9. What do you mean by rheobase and chronaxie?

Ans. Classically, the magnitude of the current just sufficient to excite a given nerve is called the rheobase and the wice for which it must be applied is the utilization time. Another measurement is the chronaxie i.e., the length of time a current of twice rheobasic intensity is applied to produce a response.

10. What is meant by fast channels and slow channels?

Ans. In addition to Volatage-gated Na+ and K+ channels there are also calcium channels. Such channels are very slow to become activated requiring 10-20 times as long for activation as the sodium channels. Therefore, they are called slow channels. Sodium channels are called fast channels.

11. Why do you mean by monophasic and biphasic action potential?

Ans. Monophasic action potential is recorded in voltameter by putting micropipette electrodes inside and outside of the fiber. Such potential shows various components of the action potential such as firing level, overshoot, spike potential, after-depolarization and after-hyperpolarization.

Biphasic action potential is recorded by placing the electrodes on the outside of fibres. This shows a biphasic curve starting from 0 mv to -50 mv again rising +50 mv and finishing to 0 mv.

12. What do you mean by saltatory conduction of nerve impulse?

Ans. The jumping of depolarization from noe to noe in myelinated axons is known as saltatory conduction. It is a rapid process and such axons conduct upto 50 times faster than the fastest unmylinated fibres.

13. What are the functional characteristics of the Na+- K+ pump?

Ans. All cell membranes of the body have a powerful sodium-potassium pump and that this continually pumps sodium to the outside of the fibre and potassium to inside. This is an electrogenic pump because more positive charges are pumped to the outside than to the inside (3 Na+ ions to the outside for each 2 K+ ions to the inside), leaving a net deficit of ‘+’ ve ions on the inside. This cause a negative charge inside the cell membrane.

14. What do you mean by ion channels?

Ans. Most channel proteins in the plasma membrane connect cytosol to the cell exterior and necessarily have narrow, highly selective pores. These proteins are concerned specifically with inorganic ion transport and so are referred to as ion channel. Such channels are not continously open. Instead, they have gates which open briefly and then close again. Change in the voltage across the membrane cause ion channels to open and hence they are also known as voltage gated channels. Na+ and K+ ions have such channels. They play a critical part in maintaining the membrane potential.

15. What are the leak channels?

Ans. K+ channels are open even in an unstimulated or “resting” cell and are hence sometimes called K+ leak channels.

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