Question

During dental procedures, dentists inject local anesthetics in the nerve pathway supporting the region they are working on. These local anesthetics block Na+ (sodium) channels. Explain in detail the process of a typical action potential, and how local anesthetics prevent the transmission of pain impulses to the brain.

Answer 1

ACTION POTENTIAL:

When acted upon by stimulus strong enough to depolarize membrane potential upto the level of threshold value, it results in action potential.

Resting membrane potential (RMP) of a typical neuron is around -70mV. When an optimum threshold stimulus reaches the neuron, it results in opening of sodium channels.

Sodium channel has two gates: outer 'm' gate or activation gate and inner 'h' gate or inactivation gate.

Resting phase: When activation gate is closed and inactivation gate is open.

Activation phase: Both the gates are open.

Inactivation phase: Inactivation gate is closed and activation gate is closing gradually.

Now upon threshold stimulus, resting phase of sodium channel goes to activated state, which results in influx of sodium ion. This causes depolarization. RMP raches -70mV to -55mV, which is the threshold value. It results in action potential and further depolarization. This action potential propagates along the length of axon and synpase and to the next neuron. After few milliseconds, inactivation gate closes which stops the influx of sodium ion. Thereon, potassium channel opens which causes repolarization.

Now, a local anaesthetic actually prolongs this inctivation phase of sodium channel. So if this phade is prolonged, obviously there will not be any influx of sodium ion as inactivation gate is closed. No action potential would be produced and thereby no pain sensation can be carried to the brain.

Local anaesthetic does this prolongation of inactivated phase of sodium channel by following steps: