Question

Local anesthetics block voltage-gated Na+ channels, but they do not block mechanically gated ion channels. Sensory receptors for touch (and pressure) respond to physical deformation of the receptors, resulting in the opening of specific mechanically gated ion channels. Why does injection of a local anesthetic into a finger still cause a loss of the sensation of touch from the finger?

Answer 1

Local anesthetics inhibit nerve conduction in a reversible manner without altering the nerve. The inhibition appears rapidly and for a longer or shorter duration depending on the products and the concentrations used. The extent of the territory rendered insensitive to pain depends on the modes of administration of the local anesthetic, either at the level of the nerve endings, or at the level of a nervous trunk, for example.

They act at the level of the neuronal membrane by interfering with the process of excitation and conduction. The anesthetic crosses the axon membrane, rich in lipids, in the form of base before taking up a cationic form on the internal face of the neuron where the pH is more acidic.

At this level, there is a blockage of nerve conduction by decreasing the membrane permeability to sodium ions that occurs during the depolarization phase. As the progression of the anesthetic action along the nerve increases, the threshold of excitability increases and the conduction time increases. This is completely blocked from a certain concentration of local anesthetic.

The nerve fibers are unequally sensitive to the action of local anesthetics: they disappear in order: the painful, thermal, tactile sensations.