Nerve impulse transmission

Dr. K. Rama Rao
Govt. Degree College
TEKKALI; Srikakulam Dt. A. P
Phone: [phone]

1. The   transmission of a nerve impulse along a neuron from one end to the other occurs as a result of electrical changes across the membrane of the neuron. The membrane of an unstimulated neuron is polarized that is, there is a difference in electrical charge between the outside and inside of the membrane.

2. Transmission of Nerve Impulses at an Electrical Synapse At electrical synapse there is continuity between the presynaptic and postsynaptic neurons. The continuity is provided by the gap junction between the two neurons.

3. The gap junctions are small protein tubular structures that allow free movement of ions between the two neurons. Because of this, the action potential reaching the presynaptic terminal produces potential change in the post-synaptic neuron.

4. In electrical syn­apse there is minimal synaptic delay because of the direct flow of electrical current from one neuron into the other through gap junction. Thus impulse trans­mission across an electrical synapse is always faster than that across a chemical synapse.

5. At an electrical synapse, the transfer of an impulse occurs by purely electrical means without involving any chemical (neurotransmitter). However, electrical synapses are relatively rare. It is found in the cardiac muscle fibers, smooth muscle fibers of intestine and the epithelial cells of lens. Most impulse transmission across the synapse between neurons takes place at the chemical synapses.

6. 2. Transmission of Nerve Impulse at a Chemical Synapse The process of chemical transmission across synapses was discovered by Henry Dale (1936). The physiological importance of synapse for the transmission of nerve impulses was established by McLennan in 1963.

7. Description of the mechanism of synaptic transmission When an impulse arrives at a presynaptic knob, calcium ions from the synaptic cleft enter the cytoplasm of the presynaptic knob. (ii) The calcium ions move on synaptic vesicles to the surface of the knob.

8. The synaptic vesicles are fused with the presynaptic membrane and get ruptured ( exocytosis ) to discharge neurotransmitters into the synaptic cleft.

9. (iii) The synaptic vesicles then return to the cytoplasm of the synaptic knob where they are refilled with neurotransmitter. (iv) The neurotransmitter of the synaptic cleft binds with protein receptor molecules on the post synaptic membrane. The opening channels in the membrane and allowing sodium ions to enter the cell .

10. This causes the depolarization and generation of action potential in the post­synaptic membrane. Thus the impulse is transferred to the next neuron.

11. (v) Having produced a change in the permeability of the postsynaptic membrane the neurotransmitter is immediately lost from the synaptic cleft. In the case of cholinergic synapses, acetylcholine ( ACh ) is hydrolysed by an enzyme acetyl cholinesterase ( AChE ) which is present in high concentration at the synapse.

12. (vi) The products of the hydrolysis are acetic acid and choline which are reabsorbed into the synaptic knob where they are resynthesized into acetylcholine, using energy from ATP.

13. Neurotransmitters Neurotransmitters are chemicals released from a presynaptic neuron that interact with specific receptor sites of a postsynaptic neuron. At least thirty chemicals thought to have the capacity to act as neurotransmitters.

14. Acetylcholine The neurotransmitter acetylcholine is released at all neuromuscular junctions between motor neurons and skeletal muscle cells. The enzyme acetyl cholinesterase is present on the membrane of the muscle cell or the postsynaptic neuron, where it breaks down acetylcholine into acetate and choline and terminates the action of the transmitter .

15. Norepinephrine Another transmitter, norepinephrine (formerly called noradrenalin) is secreted by some neurons of the sympathetic neural system and also by some neurons of the central neural system. Norepinephrine is usually inactivated by the action of an enzyme monoamine oxidase .

16. Gamma amino-butyric acid (GABA) : Gamma amino-butyric acid is released by syn­aptic knobs of the fibres of some intemeurons in central neural system. It inhibits postsyn­aptic regeneration of action potential, hence it is called inhibitory neurotransmitter. Other neurotransmitters are dopamine (DA), serotonin, glycine , histamine, glutamic acid, aspirate, substrate P and nitric oxide (NO).