Do the Motoneurones Constitute a Part of the Spinal Network Generating the Swimming Rhythm in the Lamprey?

In the study of the neural control of rhythmic motor acts, the operation of the central neuronal networks that produce the motor pattern is an area of considerable interest (cf. Delcomyn, 1980; Grillner, 1981). The motor pattern underlying swimming in the lamprey, a primitive vertebrate, can be produced in the isolated spinal cord in vitro (Cohen & Wallen, 1980; Wallen & Williams, 1984). The capacity to generate the rhythmic pattern is distributed throughout the spinal cord, and short pieces of just a few segments may produce a coordinated pattern (Cohen & Wallen, 1980; Grillner & Sigvardt, in Grillner et al. 1982). One question to consider is whether the motoneurones themselves constitute a part of the rhythm-generating circuitry, or merely represent an output stage, phasically driven by signals from an interneuronal network (cf. Wallen, 1982; Russell & Wallen, 1983). Motoneurones could possibly influence other spinal neurones via axon collaterals, gap junctions or even output synapses on the soma or dendrites. While for vertebrate systems the motoneurones are generally assumed to be separate from the pattern-generating network, it has been shown in some invertebrates that motoneurones may be part of the circuitry (Selverston, Russell, Miller & King, 1976; Heitler, 1978; Simmers & Bush, 1983). We have addressed this question for the swim rhythm-generating network of the lamprey spinal cord, by activating motoneurones antidromically by means of electrical stimulation of ventral roots in the in vitro preparation during fictive swimming, i.e. the motor pattern underlying locomotion recorded in the absence of movements. If the motoneurones were part of the generator network, such a stimulation would be expected to modify the rhythmic activity, recorded in nearby ventral roots. The results have been presented in abstract form (Wallen & Lansner, 1983). The lamprey spinal cord is flattened and thin (about 03mm) and survives well under in vitro conditions for several days. Preparations from Ichthyomyzon unicuspis or Lampetra fluviatilis were used with similar results. Animals were anaesthetized with tricaine methane sulphonate (MS-222, Sandoz) in cold tap water. Spinal cordnotochord pieces (11-29 segments long) were dissected out from a level between the last gill opening and the anus and pinned down in a Sylgard dish, containing cooled physiological solution (composition, see Wickelgren, 1977). Fictive swimming activity