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.