Figure 1. Mechanisms for central pattern generator rhythmicity. (a) Network-based rhythmicity (the leech heartbeat rhythm generator). The network (left) consists of a ring of six neurons interconnected by reciprocally inhibitory synapses (stick-and-ball connectors). Particularly important are the neuron 3 and 4 pairs, each of which forms a half-centre oscillator (see text). The neurons of each pair burst in antiphase (right). The 1 and 2 neurons form a coordinating link between the neuron 3 and neuron 4 half-centres; the network thus produces a two-phase pattern in which the open and closed neurons burst in antiphase (left). Arrows indicate the slow depolarization that allows the off-neuron to escape from inhibition (see text). Modified from Marder and Calabrese (1996). (b) A network (the lobster pyloric network) driven by an endogenous oscillator neuron. Left: The network's neuronal complement and synaptic interconnectivity (inhibitory synapses, stick-and-ball connectors; electrical coupling, resistors; rectifying electrical coupling, diode). The AB neuron is an endogenous oscillator neuron, and under most circumstances is the network's rhythmic driver. AB neuron input induces postinhibitory rebound and plateaux in its follower neurons and, as a consequence of these effects and the interactions among the follower neurons, the network produces a multiphasic rhythmic neural output (right). Left panel modified from Harris-Warrick et al. (1992); right panel from author's laboratory. Neurons not identified in text: VD, Ventricular Dilator; PL, Pyloric Late; PE, Pyloric Early.

Figure 2. Sensory feedback can contribute to central pattern generator activity during normal motor pattern production. (a) Recordings of elevator motor neuron activity during locust flight with (left) and without (right) intact sensory feedback. Cycle period and burst duration increase in the deafferented preparation. (b) Sensory afferent stimulation (black rectangles on bottom trace) in deafferented locusts decreases cycle period and elevator motor neuron burst duration to near normal levels; compare with left panel in (a). (c) A simplified version of the mechanisms underlying the sensory feedback effects. The elevator and depressor interneurons form a half-centre oscillator with a long cycle period and long elevator bursts (top). Wing elevation activates a sensory receptor, the forewing stretch receptor (FSR), which excites, with a delay, the depressors (bottom). This input induces the depressors to fire earlier than they otherwise would, which decreases elevator burst duration and cycle period. Inhibitory synapses are shown with stick-and-ball connectors, excitatory synapses with stick-and-bar connectors. (a) and (b) modified from Wolf and Pearson (1988); (c) modified from Pearson and Ramirez (1990).

Figure 3. Central pattern generator (CPG) modulation. (a) Intrinsic modulation. The Tritonia swimming CPG contains neurons, the DSI neurons, which, in addition to their classical synapses (green connections), release a modulator (serotonin; red arrow) (left). Sustained DSI neuron firing results in increased neuron C2 synaptic strength (middle). In response to strong sensory input (arrow) the network produces a fictive swimming pattern (right); if the serotonergic modulation is blocked pharmacologically, fictive swimming is also blocked (not shown). Inhibitory synapses, stick-and-ball connectors; excitatory synapses, stick-and-triangle connectors; mixed inhibitory–excitatory synapses are shown with a combination. Modified from Katz and Frost (1996). Neurons not identified in text: DRI, dorsal ramp interneuron; VSI, ventral swim interneuron; VFN, ventral flexion neuron; DFN-A, DFN-B, dorsal flexion neurons A and B (b) Single neural networks can produce multiple outputs. The panels show the output of the crab pyloric network in six modulatory conditions. In this species the network is silent in control saline (leftmost panel). Each modulator induces the network to produce a distinctly different output. CCAP, crustacean cardioactive peptide. Modified from Marder and Calabrese (1996). (c) Modulation can switch neurons between neural networks. Top left: A simplified pyloric synaptic connectivity diagram that also shows input from another network, the cardiac sac network (oval), and a sensory input that activates the cardiac sac network and abolishes postinhibitory rebound (PIR) in the VD neuron (leftmost connections; synaptic symbol conventions as in Figures 1 and 2). Top right: When the sensory input is silent, the VD neuron fires with the pyloric network. Bottom: After sensory input activity, the cardiac sac network becomes active (dpon trace), and the VD neuron fires with it. Modified from Hooper and Moulins (1990). Abbreviation not defined in text: IV, Inferior Ventricular.