Journal of comparative physiology

, Volume 94, Issue 2, pp 121–154 | Cite as

Neuronal control of swimming in the medicinal leech

II. Identification and connections of motor neurons
  • Carol A. Ort
  • William B. KristanJr.
  • Gunther S. Stent


The cell bodies and function of twelve neurons whose impulse pattern is clearly related to that of the swimming rhythm were identified in the segmental ganglion of the leech. These include excitatory and inhibitory motor neurons of the dorsal and ventral longitudinal muscles and the excitatory flattener motor neuron of the dorsoventral muscles. During swimming the membrane potential of these cells oscillates between a depolarized and a hyperpolarized phase. The activity of this ensemble of cells is sufficient to account for the contractile rhythm of the swimming animal. The following connections were found between these motor neurons. Electrotonic junctions link: (1) bilaterally homologous cells; (2) excitors of the dorsal longitudinal muscles; (3) excitors of the ventral longitudinal muscles; (4) inhibitors of both dorsal and ventral longitudinal muscles. The dorsal inhibitors project via an inhibitory pathway to the dorsal excitors, and the ventral inhibitor projects via an inhibitory pathway to the ventral excitors. The membrane potential oscillation of the excitors is at least partly attributable to the phasic inhibitory synaptic input which they receive from the inhibitors. The excitatory shortener motor neuron of the entire longitudinal musculature is maintained in an inactive state during swimming. This control is achieved by rectifying electrotonic junctions linking this neuron to the dorsal and ventral excitors. These junctions allow passage of only depolarizing current from the shortener to the dorsal and ventral excitors and of only hyperpolarizing current in the reverse direction. Furthermore, both dorsal and ventral inhibitors project via inhibitory pathways to the shortener neuron.


Motor Neuron Inhibitory Pathway Medicinal Leech Segmental Ganglion Inhibitory Motor Neuron 
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Copyright information

© Springer-Verlag 1974

Authors and Affiliations

  • Carol A. Ort
    • 1
  • William B. KristanJr.
    • 1
  • Gunther S. Stent
    • 1
  1. 1.Department of Molecular BiologyUniversity of CaliforniaBerkeleyUSA

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