Modulatory control of buccal musculature by a serotonergic neuron (metacerebral cell) in Aplysia.

Abstract

I. We have studied the central and peripheral functions of the metacerebral cell (MCC) ofAplysia. This research concentrated on the role of the MCC in controlling the ARC muscle, which is representative of a group of muscles that effect the biting and swallowing response of Aplysia. 2. Firing of the MCC produced a slowly rising and slowly decaying excitatory synaptic potential in motoneurons of the ARC muscle and other muscles of the buccal mass. The excitatory synaptic potential was not sufficient to fire motoneurons unless they received another source of depolarization. 3. In isolated buccal ganglia that exhibit periodic burst output, firing of the MCC at low, physiological rates, increased the frequency of occurrence of the burst output. Firing of the MCC was never effective in eliciting sustained burst output from nonbursting ganglia. 4. Although the MCC sends axonal branches to nerves innervating the accessory radula closer (ARC) and other muscles, firing of the MCC produced neither an increase nor decrease of the resting tension of the ARC muscle. Furthermore, firing of the ARC was never associated with any direct depolarizing or hyperpolarizing responses in muscle fibers. 5. Brief firing of the MCC enhanced the muscle contraction produced by firing of ARC motoneurons. The enhancement of contraction was maximum approximately 30 s after termination of a brief burst of MCC spikes (e.g., 30 spikes at 5/s), and the process then gradually decayed over a period of a minute or more. Physiological rates of MCC firing (see below) also enhanced contraction of ARC muscles. 6. The above data suggested that the MCCs might be involved in feeding behavior. To explore this hypothesis, chronic extracellular recordings from a MCC were obtained while ani-