Regulation of activity of cardiac vagal motoneurons

The discharge patterns of the axons of cardiac vagal motoneurons responding to carotid baroreceptor activation are described. Step and sinusoidal pressure waves were applied to the right isolated perfused carotid sinus in the cat. Other major baroreceptor inputs were eliminated by nerve section. The threshold for activation of cardiac vagal efferent fibers fell into two groups. The mean threshold for one group of 24 fibers was 117 mm Hg and for the other higher threshold group (3 fibers) was 192 mmHg. The peak frequency of discharge in response to a step rise in carotid pressure was proportional to the value of the pressure but seldom exceeded 35/sec even at high carotid sinus pressure. Adapted frequency during the step rise in pressure was also directly related to the carotid sinus pressure and seldom exceeded 6/sec. The fibers responded to sinusoidal pressure waves with a decrease in the number of spikes per cycle as the sinusoidal frequency increased. The average discharge frequency over 2 minutes remains constant over sinusoidal pressures of 0.03 Hz to 4 Hz. These characteristics of the vagal motoneurons are discussed in relationship to known properties of the baroreceptor afferents, which make up the afferent limb of this reflex pathway.