class="enumerated" style="list-style-type:decimal">Extracellular recording techniques have been used to study nerve impulses in single sensory nerve terminals in guinea-pig cornea isolated in vitro.Nerve impulses occurred spontaneously and were evoked by electrical stimulation of the ciliary nerves.The nerve impulses were identified as originating in polymodal receptors, mechano-receptors or ‘cold’ receptors. All three types are believed to be nociceptors.Tetrodotoxin (TTX, 1 μm) blocked nerve impulses evoked by electrical stimulation of the ciliary nerves. However, ongoing and/or naturally evoked nerve impulses persisted in the presence of TTX in all three types of receptors. Lignocaine (lidocaine; 1 mm) blocked all electrical activity.TTX-resistant sodium channels therefore play a major role in generating the action potentials that signal pain to the brain.
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