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

because the sequence has been interrupted

Explanation: Touch stimulation of sensory receptors open the mechanically gated ion channels, but action potentials are not yet initiated because propagation of an action potential requires the opening of voltage-gated Na+ channels.

The spreading of an action potential from the cell body to the axon hillock and eventually the synaptic knobs requires the opening of mechanically and voltage-gated ion channels in sequence. When this sequence is interrupted, the message cannot spread to the sensory regions of the central nervous system thereby causing numbness i.e a loss of sensation in the finger.

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