An Hypothesis on the Bulbospinal Locomotor Column

Abstract

Two properties of the spinal cord allow it to control, swimming of such animals as a lamprey, a dogfish, an eel or a frog embryo: the distributed capacity of cyclic generation (perhaps intrinsic for each halfsegment) and the ability to organize metachronal wave by local interaction of adjacent generators (Grillner, 1981). Neither one nor the other property is privilege of vertebrates. Indeed, metachronal wave is observed during swimming in movements of the comb plates of the ctenophore (Sleigh, 1968), in muscular activity of the leech (Weeks, Kristan, 1978; Weeks, 1981), in swimmeret movements of the crayfish (Stein, 1976), during locomotion of Multipeda (Smolyaninov, Karpovich, 1975). Unique feature of vertebrates (and some related animals) is arrangement of their axial nervous system in form of the nerve tube which is continuous in its intrinsic organization although has metameous inputs and outputs. Correspondingly, in this paper only mechanisms of locomotor control in vertebrates are considered. In particular, similarity of mechanisms employed by the axial nervous system to control both swimming of animals without appendages and terrestrial locomotion of tetrapods is discussed. In quadrupedal animals, capability of cyclic generation is restricted to enlargements of the spinal cord, interaction of multiple generators of one “half-enlargements” (Szekely, Czeh, 1971) is used to form the functional unit — synergy of a stepping limb, and there are additional oligosynaptic connections for interlimb coordination.