Mode of action
Local anesthetics prevent information from being transmitted from its site of action to the brain or spinal cord. This means that pain is produced locally, but cannot be perceived by the brain. This is caused by an indirect blockage of the superficial nerves, or rather the outer membrane of the nerve fiber and the sodium channels located there, which can perceive pain and transmit the information about the pain to the brain.
Thus, action potentials cannot be formed sufficiently and the transmission of excitation is stopped. Primarily, this prevents the transmission of pain. These so-called “pain nerves” are essential because they ensure that when our hand is lying on a hotplate, we quickly withdraw it in pain.
Thus, the pain nerves (so-called fast C-fibers) protect our hand from burning. However, local anesthetics only want to switch off this effect for a short time. To do this, they have to stop the flow of information from the superficial (peripheral) nerve (neuron) to the brain.
In order to understand the exact mechanism of action of the local anaesthetic, one must first understand how pain can occur. Our nerve fibers consist of different parts. The transmission of pain takes place via axons, the mechanism of action of which can be compared to a telephone cable.
There are different channels in the axons. For the transmission of pain, the aforementioned sodium channel is important, which allows sodium ions to pass. These sodium ions could be compared to the dial button on a telephone.
As soon as the sodium ions enter the axon, depolarization occurs and the pain can be transmitted from its point of origin to the brain. Here the information is processed and the patient feels the pain. To make the whole thing understandable again with the telephone example: We press the dial button (sodium ions flow into the axon) and through our telephone cord the information that we want to call (that we feel pain) is passed on to the telephone (the brain), only now can we call our call partner (only now do we feel pain).
A local anesthetic now blocks the sodium channel. It would be as if the dial button on the phone is broken. Due to the blockade, the information can no longer be passed on. In our telephone example this would mean that we can speak into the receiver, but our telephone partner does not hear anything. In humans this means that our skin can be injured by a doctor’s cut, but we cannot perceive the pain because our sodium channel is blocked and the information cannot reach our brain.
