Projection of the hepatic vagal nerve in the medulla oblongata.

Afferent neurons in the solitary tract nucleus (NTS) which innervate the liver were identified by electrical stimulation of the hepatic branch of the vagus. This nerve branch projects predominantly into the left side of the medulla. A collision test was employed to discriminate the afferent unit discharges from the efferent ones. These afferent neurons are characterized by no cancellation of the evoked unit discharges by the collision. Latencies were widely scattered and lay between 73.2 and 222.5 ms. A variable latency in each response was always recognized. Excitability curves obtained by applications of conditioning and test stimuli revealed that there exist two different types of units; one is characterized by a higher safety factor for synaptic transmission, while the other shows a lower safety factor than the former. It is concluded that the neurons in the NTS activated by the hepatic vagal afferent nerve locate predominantly in the left side of the medulla and also these nerve fibers are mostly non-medullated fibers in which conduction velocities are slower than 1 m/s. The variable latency recognized in each neuron and low safety factors for synaptic transmission recognized in some neurons indicate integrative action within the NTS as well as a relay of the afferent signals from the liver.