Velar motoneurons of lamprey larvae

Breathing activity continued in isolated brain-velum-gill preparations of larval lampreys,Petromyzon marinus andLampetra aepyptera (Fig. 1). Periodic discharges were recorded extracellularly in the lateral fifth motor nucleus and from the fifth nerve (Fig. 2); these discharges preceded the contractions of the velum. Later in the respiratory cycle, discharges corresponding to the contractions of the branchial muscle were recorded in the tenth motor nucleus. Evidence is presented that discharges from both fifth and tenth nucleus were produced by motoneurons.

Velar motoneurons were identified by physiological criteria. Intracellular stimulation of these cells produced one-to-one movements in ipsilateral velar muscles and one-to-one electromyographio potentials (Fig. 4A). Motoneurons were confirmed as such by antidromic stimulation and collision. Velar motoneurons were located in the lateral part of the fifth motor nucleus (Fig. 3). Branchial motoneurons were also identified in the tenth motor nucleus.

During intracellular recordings velar motoneurons exhibited periodic bursts of excitatory synaptic potentials (Fig. 4B), probably produced by unidentified pacemakers in the vicinity of the fifth motor nucleus.