Postnatal development of postganglionic parasympathetic neurones in the heart of the albino rat.

Electrical stimulation of the sinoatrial node region of isolated atria in medium containing physostigmine (0.1 micrograms/ml) produces a negative chronotropic effect whose intensity and duration depend mainly on the amount of acetylcholine released from postganglionic parasympathetic fibres endings. This technique was used to study functional maturation of the given neurones during postnatal development of albino rats. Preparations from animals of different ages were stimulated with 2-second bursts of rectangular pulses (frequency 50 Hz, pulse duration 0.02 ms, voltage 22.5--27.5 V) and frequency changes of the preparation were registered by recording extracellular action potentials. At 10 days the negative chronotropic effect is very weak and at 15 days it is only slightly stronger, but at 18 days it is almost the same as in adult animals. At 24 and 34 days the reaction is somewhat stronger than in adulthood. It can be concluded from these observations that functional maturation of the postganglionic parasympathetic neurones innervating the sinoatrial node in albino rats occurs between the 10th and 20th day of postnatal life.