J. exp. Biol. 112, 337-357 (1984) 337 Printed in Great Britain © The Company of Biologists Limited 1984 HOW DOES THE LAMPREY CENTRAL NERVOUS SYSTEM MAKE THE LAMPREY SWIM? BY STEN GRILLNER AND PETER WALLEN Department of Physiology III, Karolinska Institutet, Lidingbvdgen 1, S-114 33 Stockholm, Stoeden SUMMARY The lamprey spinal cord, in isolation or with the brainstem, can be used in vitro. The motor patterns underlying the swimming movements can be elicited by: (1) a pharmacological activation of a specific type of neuronal receptor (NMDA-receptor), that may in other systems give rise to an un- stable membrane potential, (2) by stimulation of the brainstem or (3) by tactile activation of skin regions left innervated. In the latter case the initia- tion of 'fictive' swimming is partially caused by a release of a transmitter activating NMDA-receptors, as judged by the effect of NMDA-receptor blockers. The central pattern generator (CPG) is strongly influenced by feedback from mechanosensitive elements, which at least partially reside within the spinal cord. The edge cell in the lamprey spinal cord serves as an intraspinal mechanoreceptor. The ability to generate a coordinated motor output is distributed, since spinal cord sections down to 1-5-2 segments can be made to generate alter- nating activity. Motor neurones receive an approximately synchronous al- ternating excitatory and inhibitory drive in each swim cycle and do not appear to be part of the CPG. Motor neurones supplying different parts of the body wall on the same side of a body segment have different morphology with ramifications around different descending axons. The input drive signal during fictive locomotion to motor neurones located close to each other but with different morphological characteristics may differ substanti- ally with regard to the y-relationship (±25 %) and the shape of the oscilla- tion. This implies that even at a segmental level motor neurones may be further subdivided, and furthermore that the ipsilateral network generating the drive signal to ipsilateral motor neurones generates a more complex and individualized output than previously assumed. Motor neurones are not part of the rhythm-generating circuit. The large identifiable interneurones are not required for rhythmic activity to occur although they may be phasically active in the swim cycle. The small segmental interneurones have not yet been completely characterized. Many are phasically active during 'fictive locomotion' and lack an apparent axon. Their phase relationships in relation to the burst patterns vary over the entire swim cycle. INTRODUCTION The major components of the control system for locomotion in higher vertebrates have been identified over the lastfifteenyears (cf. Grillner, 1981). However, a specific Key words: Lamprey, fictive locomotion, feedback.