Final answer:
Lidocaine causes numbness and acts as an effective local anesthetic by blocking voltage-gated sodium channels, preventing the initiation and propagation of action potentials which are essential for transmitting pain signals to the brain.
Step-by-step explanation:
Lidocaine is a local anesthetic that works by blocking voltage-gated sodium channels on nerve cells (neurons). When these channels are blocked, sodium ions (Na+) cannot enter the neuron, preventing the depolarization needed to trigger an action potential. Action potentials are necessary for the transmission of electrical signals along a neuron. If these signals cannot be transmitted, then the sensation of pain, which relies on these signals reaching the brain, is inhibited, leading to numbness in the area where lidocaine was applied.
Neurons transmit information through action potentials, which are initiated when the membrane potential of the neuron reaches a threshold level, typically around -55mV. The depolarization phase is marked by the rapid influx of sodium ions when voltage-gated Na+ channels open. These channels then become inactivated shortly after opening, leading to the repolarization phase which is dominated by the outflux of potassium ions (K+) through voltage-gated K+ channels.
The effect of lidocaine as a sodium channel blocker makes it an effective local anesthetic because it directly interrupts the propagation of action potentials along neurons that would normally transmit pain signals. Without the initiation and propagation of these action potentials, the perception of pain is effectively reduced or eliminated in the area where lidocaine has been applied, thus explaining the absence of pain when lidocaine is used during medical procedures.