Neural control of swimming in Aplysia brasiliana. I. Innervation of parapodial muscle by pedal ganglion motoneurons.

1. Swimming is an oscillatory locomotor behavior in Aplysia accomplished by rhythmic undulating movements of the parapodia, winglike flaps that cover the dorsum of the body. As part of an analysis of the neural basis of this behavior, we have identified and characterized motoneurons in the pedal ganglia that directly innervate parapodial muscle and fire phasically during fictive swimming. 2. Parapodial musculature is organized into at least eight discrete layers. Fibers of adjacent layers are directed orthogonally. 3. Motoneurons were localized to the middle and rostral portions of the dorsal surface of each pedal ganglion by the use of backfill staining and intracellular dyes. These neurons were defined as motoneurons on the basis of additional physiological evidence for peripheral axons and their ability to cause excitatory junction potentials (EJPs; average amplitude, 2-5 mV) in muscle fibers and discrete contractions of parapodial muscles. Muscle fibers are polyneuronally innervated. Fibers had an average resting potential of -79 mV and no over-shooting action potentials. 4. There are probably at least 50 motoneurons. Their average resting potential was -48 mV, and they do not appear to be directly connected synaptically to one another. One identifiable motoneuron is described in detail. It participates in the opener (downstroke) phase of swimming and causes contraction of one of the described muscle layers. 5. Divalent ion concentrations were altered centrally and peripherally during motoneuron activity to demonstrate that the motoneurons directly innervate muscle fibers. Blockage of EJPs by hexamethonium and the presence of specific anticholinesterase staining in parapodial nerves and muscle fibers strongly suggest that many of the motoneurons are cholinergic. 6. Studies of excitation-contraction coupling showed that single or a few spikes in motoneurons rarely cause an EJP. Bursts of motoneuron spikes produced facilitating EJPs. With approximately 10 spikes in a 1-s motoneuron burst, adequate depolarization occurred in muscle fibers to initiate a small, slow contraction. Increased spike frequency led to greater depolarization, because of EJP summation, and larger contractions. Contraction requires depolarization of the muscle above a threshold, beyond which the force of contraction depends on both the duration and degree of depolarization. 7. Although dozens of motoneurons appear to be involved in the complex control of parapodial movements during swimming, preliminary evidence indicates that these neurons are probably not participating directly in the circuitry of the central pattern generator for swimming, which has been shown by others also to reside in the pedal ganglia.

Neural control of swimming in Aplysia brasiliana. II. Organization of pedal motoneurons and parapodial motor fields.

1. We have examined the locations and functional properties of a large number of motoneurons in the pedal ganglia of Aplysia brasiliana. These neurons control movement of the parapodia and body during swimming. We have grouped the motoneurons into classes based on several criteria, including the topology of the cells and their axons, the properties of their peripheral motor fields, and their phasic activity during an induced swim motor program. 2. A total of 410 motoneurons were analyzed. There are at least 16 distinguishable motor fields in the parapodia, based on the region affected, direction of contraction, and phase of neuronal activity during fictive swimming. 3. Motoneurons for each motor field tend to appear in the same region of the ganglion in different preparations. 4. Most motoneurons have only ipsilateral effects. About 1% cause contralateral contraction, and they project directly to the contralateral parapodium. 5. Three types of motoneuron are described that cause parapodial expansion. 6. Two other groups of motoneurons were found that innervate either the columellar muscle or longitudinal foot muscles. 7. Almost all motoneurons fired rhythmically during fictive swimming, including those controlling foot and columellar muscle.

Neural control of swimming in Aplysia brasiliana. III. Serotonergic modulatory neurons.

1. We describe a group of serotonergic neurons in the pedal ganglia of Aplysia brasiliana and characterize their modulatory effects on the motoneuron input to swimming muscles of the parapodia. Each pedal ganglion contains one cluster of large neurons near its dorsomedial surface that fires in phase with opening (downstroke) of the parapodia; these have been designated parapodial opener-phase (POP) cells. 2. POP cells are large, number 15-20 per ganglion, have peripheral axons in parapodial nerves, have distinctively shaped action potentials, and fire in bursts phasically with motoneurons during the opening, or downstroke portion, of parapodial movement during fictive swimming. Firing individual POP cells with intracellular current indicates that they have no direct detectable effect on muscle, causing neither junction potentials nor contractions. 3. 5,7-Dihydroxytryptamine (5,7-DHT) staining, immunocytochemistry using serotonin (5-HT) antibodies, and direct biochemical measurements revealed that POP cells are serotonergic. Serotonergic nerve endings were also seen in parapodial muscle. 4. Simultaneous intracellular recordings and use of altered divalent concentrations revealed that no detectable direct synaptic interactions exist between POP cells and motor neurons. 5. When POP cells and motoneurons were simultaneously recorded while measuring muscle contractions, it was found that POP cell activity enhances motoneuron-induced tension by 120-900%, averaging around 300%. Variability in the efficacy of individual POP cells suggests that they may influence specific regions or groups of muscle fibers. 6. POP cell activity also significantly increased the rate of relaxation of parapodial muscle contractions, averaging about a 40% reduction in the time required to relax to one-half peak tension. Increased relaxation rate implies a postsynaptic change in muscle behavior. 7. The effectiveness of POP cells to increase contraction tension and relaxation rate was positively correlated with POP cell spike frequency. These effects were slow (seconds) in onset and could persist for a minute or more after cessation of POP firing. Concurrent motoneuron activity is not required for modulation by POP cells. 8. Simultaneous intracellular recording from a POP cell, motoneuron, and muscle fiber revealed that POP cell activity enhanced the amplitude of motoneuron-induced excitatory junction potentials (EJPs). Activity of POP cells did not alter muscle fiber membrane potential, but the experiments left open the possibility that EJP enhancement is presynaptic, postsynaptic, or a combination.(ABSTRACT TRUNCATED AT 400 WORDS)