The impulses produced by sensory nerve‐endings

THE present investigation deals with the analysis of the impulses set up in cutaneous nerves by painful stimuli and compares the pain discharge with that of other receptors. The older view that pain resulted from excessive stimulation of any cutaneous receptor has been largely abandoned in favour of the idea of special pain receptors and conductors. The specific character of " pain " suggests that these receptors and conductors may differ considerably from those of the other types of cutaneous sensation. For instance, the "protopathic and epicritic" hypothesis of Head and Rivers demands that the nerve fibres concerned with pain must belong to a system phylogenetically distinct from those concerned with touch and the histological work of R ans o n has added the view that the pain fibres may be non-medullated. Again, the pain receptor probably differs from many others in the absence of a capsule surrounding the axon termination and for this reason it has been suggested that the discharge of impulses from the pain endings may have some characteristic frequency or grouping of its own. These and similar possibilities can evidently be tested by recording the impulses produced in the afferent nerve fibres by a painful stimulus. Preliminary observations (1) with the capillary electrometer and amplifier showed that it was possible to record the action currents set up in afferent nerves when the skin was pinched or pricked. The next step was to secure that not more than a few nerve fibres should be in action together so that each impulse might be recorded without interference. Apart from the use of nerves with very few fibres two general methods may be applied in any research of this character, namely (1) to restrict the stimulus to a very small area so that only a few receptors are stimulated; (2) to destroy or put out of action all but a few of the receptors or nerve fibres which will be affected by the stimulus. The former method is suited to the present case since a prick with a very fine needle will produce sensation which is clearly painful, but will scarcely stimulate