Neural activity of the superior salivatory nucleus in rats

We recorded the neural activity of the superior salivatory (SS) neurons in brain slice preparations from neonatal rats in vitro and in decerebrate anesthetized rats in vivo. In the in vitro experiment, the SS neurons were retrogradely labeled by the injection of Rhodamine into the chorda-lingual nerve (labeling SS neurons innervating the submandibular and intra-lingual ganglia) or into the anterior part of the tongue. The SS neurons labeled from the nerve were classifiable into two types: Type-I, tonic firing at a frequency of up to 30 Hz; and Type-II, phasic firing at a higher frequency of up to 70 Hz followed by tonic firing at 30-50 Hz. All of the SS neurons labeled from the tongue were Type-II. Since the anterior tongue is a non-glandular area, the type of cells may be involved in vasodilatation. Type-I neurons, which did not innervate the tongue, may be responsible for salivation. In the in vivo experiment, the reflex activity evoked by taste or mechanical stimulation was recorded from the axons of the SS neurons innervating the submandibular ganglia. These fibers also displayed two firing patterns. One was a tonic firing pattern discharging at 5-30 Hz. The other consisted of a transient firing (about 80 Hz) at the beginning of stimulation and then a prolonged firing at 5-40 Hz. The latter firing pattern was similar to that of the Type-II neurons. These findings suggest that the parasympathetic nerves of the salivary glands contain both the secretory- and vasodilator-type of SS neuron.