Figure 2: Components of the rhythm generating layer and use of deletions to separate rhythm and pattern.

A. The rhythm generating layer (green box) contains flexor rhythm generating neurons (blue box) and extensor rhythm generating neurons (red box) on both sides of the spinal cord (midline denoted by dashed line). Flexor rhythm generating neurons (blue) likely include Hb9, Shox2 non-V2a, and other unidentified populations. Extensor rhythm generating neurons (red) include other Shox2 non-V2a and unidentified neurons. The rhythm generating populations are coordinated by populations of excitatory and inhibitory neurons in between them. Several of the coordinating populations have been identified and hypothesized [6,9,22,67]. Excitatory connections from heterogeneous populations are shown with arrows. Other excitatory connections are denoted with triangles and inhibitory connections with circles. Connectivity structure is based on [8]. B. Neuronal activity during spontaneous deletions has been used to categorize neurons as belonging to the rhythm generating layer or patterning formation layer. All neurons in the rhythm generating layer (green box), including rhythm generating neurons (blue and red) and neurons coordinating left-right and flexor-extensor rhythm generating cores, should maintain activity during deletions. Coordinating neurons (green) categorized in this way include dI6, V2a type I, and unspecified commissural interneurons, CINs [5,17]. There are interconnections between core rhythm generating neurons and other neurons in this layer (bidirectional arrow). The core rhythm generating neurons drive the neurons in the pattern formation layer (yellow box). Neurons in the pattern formation layer (yellow) do not maintain activity during deletions. Patterning neurons categorized by lack of activity include V2a type II (which may be Shox2 V2a) and dI6 neurons [5,9,17]. Both rhythm and pattern layers likely contain several other populations. Motor neurons (purple) are not active during deletions [5,15].