Generation and Conduction of Nerve Impulse
- Impulse transmission is electrochemical. It has 3 steps:
1. Maintenance of Resting Membrane Potential
- Neural membrane contains various selectively permeable ion channels.
- In a resting neuron (neuron not conducting impulse), the axonal membrane is more permeable to K+ ions and nearly impermeable to Na+ ions. Also, the membrane is impermeable to negatively charged proteins in axoplasm.
- Therefore, concentration of K+ and –vely charged proteins in axoplasm is high and concentration of Na+ is low.
- The fluid outside the axon contains low concentration of K+ and high concentration of Na+. This forms an ionic or concentration gradient across resting membrane.
- The ionic gradients are maintained by the active transport of ions by the Na-K pump. It transports 3 Na+ outwards for 2 K+ into the cell. As a result, the outer surface becomes positively charged and inner surface becomes negatively charged (i.e, polarized).
- The electrical potential difference across the resting plasma membrane is called as the resting potential.
2. Action Potential
- When a stimulus is applied, the membrane at the site A becomes permeable to Na+. This causes rapid influx of Na+ and reversal of the polarity at that site (outer negative and inner positive). It is called depolarization.
- The electrical potential difference during depolarization across the plasma membrane is called action potential (a nerve impulse).
3. Propagation of Action Potential
- At sites ahead (site B), outer surface is positive and inner surface is negative. As a result, a current flows on the inner surface from site A to site B.
- On the outer surface, current flows from site B to site A to complete the circuit. Hence, the polarity is reversed and action potential is generated at site B. i.e., action potential at site A arrives at site B.
- The sequence is repeated along the axon and the impulse is conducted.
- The rise in permeability to Na+ is extremely short lived. It is quickly followed by a rise in permeability to K+.
- Immediately, K+ diffuses outside the membrane and restores the resting membrane. Thus the fibre becomes ready for further stimulation.
Synaptic Transmission of Impulses
- Synapse is a functional junction between two neurons.
- It is 2 types: Electrical & Chemical.
Electrical Synapses
- In this, the membranes of pre- and post-synaptic neurons are in close proximity. So impulse transmission is similar to the transmission along an axon.
- Impulse transmission is faster than in chemical synapse.
- Electrical synapses are very rare in human system.
Chemical Synapses
- In this, there is a fluid filled space (synaptic cleft) between the presynaptic neuron and postsynaptic neuron.
- The presynaptic regions have swellings called Synaptic knob (buttons). They contain synaptic vesicles filled with neurotransmitters (acetylcholine or adrenaline).
- Impulse transmission through chemical synapse:
- This action potential may be excitatory or inhibitory.
Impulse reaches at axon terminal → synaptic vesicles bind on plasma membrane → release of neurotransmitter → It diffuses across synaptic cleft → combine with receptors on the post synaptic membrane → opening of ion channels allowing entry of ions → generates action potential.