Saccadic premotor neurons in the brainstem: functional neuroanatomy and clinical implications

Abstract

Saccadic eye movements are generated by immediate premotor excitatory (EBN) and inhibitory (IBN) medium-lead burst neurons, long-lead burst neurons (LBN), and omnipause neurons (OPN) in the brainstem. The histological identification of the functional neuron populations is essential for performing any neuropathological analysis of clinical cases with saccadic disorders. This paper describes the localization of some premotor saccadic neurons in human brain and their neurochemistry, based on monkey data. In humans, the EBNs for horizontal saccades (EBN_H) lie in the paramedian pontine reticular formation in the nucleus reticularis pontis caudalis (nrpc) and form a compact group of medium-sized neurons. The medium-sized IBNs lie caudal to the nucleus abducens in the nucleus paragigantocellularis dorsalis (pgd). The EBNs for vertical saccades (EBN_V) are medium-sized neurons in the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) in the rostral mesencephalon. The OPNs are located at the midline of the pontine reticular formation, in the nucleus raphe interpositus, bordered by the EBN area rostrally and the IBN area caudally. The OPNs use glycine as an inhibitory transmitter.

These cell groups (EBN_H, EBN_V, IBN, and OPN) were first identified experimentally in monkeys, and all were shown to express parvalbumin im-munoreactivity. The parvalbumin immunoreactivity was then used as a marker to help identify the homologous neurons in man.

Key Words:

• Burst neurons
• omnipause neurons
• transmitters
• saccadic disorders