Technique of nerve stimulation


Peripheral nerves are comprised of thousands of nerve fibres; they contain either sensory or motor fibres of the somatic and autonomic nervous systems, but sometimes both in combination. Electrical impulses reaching a nerve, when they exceed a specific threshold stimulus current (rheobase), trigger depolarisation of the neuron membrane, thereby inducing transmission of excitation along the nerve fibres. If the nerve contains motor fibres, the electrical current will induce contractions at the effector muscle. If these stimuli are induced on sensory fibres, they cause paraesthesia in the distribution of the nerve. This underlying principle of electrical nerve stimulation is utilised in peripheral regional anaesthesia.



Nerve fibres can be divided into different classes which are based on myelination, nerve conduction velocity and function. Chronaxie, the duration of the stimulus required at twice the maximum rheobase strength to cause depolarisation of the neuron membrane, is used as a measure of the excitability threshold for different types of nerve tissue, i.e. it indicates the duration of an effective electrical stimulus required to elicit a response. Stimulation of selective motor and sensory fibres occurs when the duration of the stimulus current (pulse duration) is within chronaxie.