Figure 3.

Schematic of a hypothesis describing a portion of the spinal neuronal network responsible for the production of the turtle rostral scratch as described in the modular Unit-Burst-Generator (UBG) hypothesis. Only ipsilateral hip and knee UBGs are shown; only a subset of the possible synaptic connections are included in the sketch. EINs: excitatory interneurons; IINs: inhibitory interneurons; MNs: motor neurons. Reciprocal inhibition between agonist and antagonist UBGs at each degree of freedom is a fundamental characteristic of organization of this network. Active neurons shown in black; quiet neurons shown in light gray. (A) During normal rostral scratch, all hip and knee UBGs are rhythmically active. (B) During hip-extensor deletion variation of rostral scratch, the hip-flexor, the knee-flexor, and the knee-extensor UBGs are rhythmically active. Neurons in the hip-extensor UBG are quiet. This schematic emphasizes that neurons in the hip-flexor UBG are rhythmic even when neurons in the hip-extensor UBG are quiet. From¹⁸ and used with permission of and copyright 2008 by Elsevier B.V.