Let’s break down how local and general anesthetics interfere with nerve impulse transmission:

Local Anesthetics:

Local anesthetics, by blocking voltage-gated Na+ channels, primarily impair the initiation and propagation of action potentials. Here’s the process:

1. Resting State: In a neuron at rest, voltage-gated Na+ channels are closed. The inside of the cell is negatively charged relative to the outside.

2. Depolarization: When a stimulus reaches the neuron, it causes some Na+ channels to open. If enough Na+ ions enter the cell, the membrane potential reaches a threshold.

3. Action Potential: At the threshold, a large number of voltage-gated Na+ channels open rapidly. This allows a massive influx of Na+ ions, making the inside of the cell positively charged (depolarization). This rapid depolarization is the action potential.

4. Propagation: The action potential triggers a wave of depolarization that travels down the axon. This happens because the opening of Na+ channels at one location triggers the opening of Na+ channels in the adjacent region.

5. Repolarization: After the Na+ channels open, they quickly inactivate. Voltage-gated K+ channels open, allowing K+ ions to flow out of the cell, restoring the negative charge inside (repolarization).

How Local Anesthetics Interfere: Local anesthetics bind to voltage-gated Na+ channels and prevent them from opening. If these channels cannot open, Na+ ions cannot rush into the cell, and an action potential cannot be generated or propagated. Therefore, the nerve cannot transmit the signal of pain or other sensation to the brain. The nerve is essentially “numb.”

General Anesthetics:

General anesthetics work through a more complex and less completely understood mechanism. They are thought to enhance the activity of inhibitory neurotransmitters, often by activating GABA (gamma-aminobutyric acid) receptors, which are ligand-gated chloride (Cl-) channels.

1. GABA Activation: GABA is the primary inhibitory neurotransmitter in the central nervous system. When GABA binds to its receptors, it opens Cl- channels.

2. Increased Cl- Permeability: The opening of Cl- channels allows Cl- ions to flow into the neuron. This influx of negatively charged Cl- ions hyperpolarizes the neuron, making it even more negative inside than it is at rest.

3. Inhibition of Neuronal Activity: This hyperpolarization makes it much harder for the neuron to reach the threshold for an action potential. The neuron is less excitable, and the transmission of nerve impulses is suppressed.

How General Anesthetics Interfere: By enhancing inhibitory neurotransmission, general anesthetics effectively dampen overall neuronal activity in the central nervous system. This leads to a loss of consciousness, sensation, and muscle relaxation, which are necessary for surgical procedures. The precise mechanisms by which general anesthetics produce these effects are still being researched, but the potentiation of GABAergic inhibition is considered a significant factor.

In Summary:

• Local anesthetics: Block Na+ channels, preventing action potential initiation and propagation in specific nerves.
• General anesthetics: Enhance inhibitory neurotransmission (primarily through GABA receptors and Cl- channels), suppressing overall neuronal activity in the central nervous system.