STUDIES ON THE EXTRINSIC AND INTRINSIC NERVE MECHANISMS OF THE HEART

Abstract

Evidence is presented that in the turtle the extrinsic vagus fibers responsible for the negative inotropic effect are both myelinated and non-myelinated. The fibers for chronotropic effects are mostly non-myelinated, a few are myelinated. All fibers having inotropic and chronotropic effects have axons with slow properties characteristic of autonomic fibers in general. In the cat the extrinsic vagus chronotropic fibers are mostly myelinated, a few are non-myelinated. The inotropic fibers are inferred to be chiefly myelinated. Both the myelinated and non-myelinated fibers concerned have properties typical of autonomic fibers in general. Inotropic and chronotropic fibers are considered to be functionally distinct. Nicotine sulphate (1/2-2% in Ringer''s soln.) applied to the excised median cardiac nerve of Limulus polyphemus causes first an increase in the frequency of the rhythmic volley discharges and then some shortening and lowering of the potential complex. Further depression results in a decrease in frequency of the volley discharges, further shortening and lowering of the potential complex. The heart of Limulus is stopped by nicotine when the activity of its nerve cells ceases. The blocking effect of nicotine on the synapses intervening between the extrinsic and intrinsic nerves of the Limulus heart is similar to its blocking effect on other synapses of the peripheral sympathetic nervous system of vertebrates. There is no evidence to indicate that the intrinsic nerve mechanism of Limulus contains synapses of the type associated with the extrinsic to intrinsic nerve relations. The intrinsic nerve mechanism of Limulus functions in this respect like the central nervous system of vertebrates. The myoneural junction is not blocked by nicotine in the period of time required to block an ordinary nerve synapse. It is similar to the time required to block a nerve axon of comparable size. The effect of nicotine on the turtle heart seems comparable to its effect on the heart of Limulus. By analogy from evidence derived from Limulus studies, the extrinsic nerves of the turtle are considered to act through an intrinsic nerve mechanism and not directly on muscle. On recovery from nicotine depression it is possible to obtain by right vagus stimulation a slowing of the sinus and atrium of the turtle with no evident inotropic depression of either structure. The significance of the findings in these turtle and Limulus studies in their relation to the problem of determining the nature of the pace-maker mechanism of the heart is discussed.