Figure 2.

Conservative evolution of axial locomotion. (a) The segmental CPG for swimming in lampreys, tadpoles and zebrafish larvae is organized as a half-centre oscillator. The left and right sides are mutually inhibitory through commissural interneurons (CIN). Excitatory and inhibitory interneurons (EIN and IIN) participate in motor pattern generation. EIN synapses on motor neurons (MN) that cause muscle contraction. Each neuron in the diagram represents large pools of heterogeneous neurons. Triangles are excitatory synapses and circles are inhibitory. CPG based on ref. [16]. (b) The lamprey swims with a sinusoidal movement. The left and right sides of the body alternately flex. (c) Salamanders walk using an alternating axial muscle gait with left and right sides in alternation.