Morphology of rostral medullary neurons with intrinsic pacemaker activity in the rat

Neurons with regular ongoing activity attributable to intrinsic pacemaker properties were recorded in coronal tissue slices within the nucleus reticularis rostroventrolateralis of the rat medulla oblongata (RVL). The cells were injected with horseradish peroxidase or Lucifer yellow and their dendritic and proximal axonal characteristics were investigated (n = 15). These small-to-medium-sized neurons had a simple dendritic arborization (3-6 primary dendrites branching up to 3 times) apparently confined within the limits of nucleus RVL and with limited extension in the rostrocaudal direction. Their axons originated either from the cell body or from a primary dendrite and coursed in a dorsomedial direction without giving rise to local arborizations. It is concluded that RVL pacemaker neurons, presumed to represent a non-adrenergic class of sympathoexcitatory premotor neurons, exhibit characteristics reminiscent of the archetypal 'reticular core' neurons.