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Transmission of nerve impulses

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Introduction

Transmission of nerve impulses A nerve impulse is the sum total of the physical and chemical reactions that take place in the propagation of a wave of physiological activity along a nerve fiber. Nerve impulses travel more slowly that electrical current. The physiological make up of a resting neurone is just like that of most other cells. The difference in composition between the outside and inside the nerve cell is able to use it generate an electric impulse for transmission. At resting stage The composition of the external medium contains Na+ and Cl- more that the inside. At the same time the concentration of the K+ inside the cell is higher that that of it outside. The anions inside the cell produced during metabolism being negatively charged, counter balance the K+ and give the inside of the cell a negative charge. The outside and inside of the cell are separated by a selectively permeable cell membrane. Na+ are kept outside and their inward diffusion is prevented due to low permeability. ...read more.

Middle

Thus depolarization of the nest segment of the axoplasm takes place. There is therefore a series of local currents propagated along the axon. At recovery stage In the meantime the depolarized area begins to recover because the Na+ begins to diffuse out due to the activity of the Na+/K+ pump. Thus the impulse passes along the axon in the form of a wave. The whole lasts only for about 1-2 ms. Once the resting potential is restored another impulse can be transmitted. The wave of excitation causes two phases in the action potential and is said to be biphasic. During the period between repolarisation and depolarization nerve impulses cannot pass. This is known as the absolute refractory period. This is followed by the relative refractory period lasting for about 4 to 8 ms during which time the nerve reverts to its original excitability. The lowest strength of stimulus required to produce an action potential is known as the threshold of stimulus. ...read more.

Conclusion

This change in resting potential opens more Na+ gated channels allowing excess Na+ to rush into the nerve cell. 2. The excess Na+ in the cytoplasm makes the potential difference to reach up to +40 mV. The Na+ gated channels open for only about 5.5 ms before shutting down. 3. These gated channels close and become inactive. This allows the K+ gated channels to open and K+ moves outside. This outward flow of K+ leads to the rapid fall of potential towards the resting potential level. 4. During the refractive period (0.5 to 2 ms) the membrane is unable to react to addition stimulus. This has two phases: a) Absolute refractive period The charge is briefly reduced below the resting potential of -70mV due to excess loss of K+ b) Relative refractive period During this short period the inactivated Na+ gated channels return to their original closed but active state and the K+ gated channels close, preparing the system to work again. The biological significance of the refractive period is to limit the number of action potential that can be generated per second. ...read more.

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