another non-locomotive function of central pattern generators. for example, larval amphibians accomplish gas exchange largely through rhythmic ventilation of the gills. a study showed that lung ventilation in the tadpole brainstem may be driven by a pacemaker-like mechanism, whereas the respiratory cpg adapts in the adult bullfrog as it matures. thus, cpgs hold a broad range of functions in the vertebrate animal and are widely adaptable and variable with age, environment and behavior. mechanism of rhythmic generators: post-inhibitory rebound rhythmicity in cpg's can also result from time-dependent cellular properties such as adaptation, delayed excitation, and post-inhibitory rebound (pir). pir is an intrinsic property that elicits rhythmic electrical activity by depolarizing the membrane once hyperpolarizing stimulus is gone. it can be produced by several mechanisms including hyperpolarization-activated cation current (ih), low-voltage activated calcium current, or deinactivation of depolarization-activated inward currents. once inhibition has ceased, this period of pir can be explained as the time with increased neuronal excitability. it is the property of many cns neurons that sometimes results in action potential "bursts" following immediately after inhibitory synaptic input."because of this, it has been suggested that pir may contribute to the maintenance of oscillatory activity in neural networks that are characterized by mutual inhibitory connections, like those involved in locomotor behaviors. in addition, pir is often included as an element in computational models of neural networks that involve mutual inhibition" for example, the "pir in crayfish stretch receptor neurons is caused by recovery from adaptation during the course of inhibitory hyperpolarization. one feature of that system is that pir only occurs if the hyperpolarization is imposed on a background of excitation, caused in this case by stretch. they also found that pir can be elicited in the stretch receptor by hyperpolarizing current pulses. this was an important finding because it showed that pir is an intrinsic property of the postsynaptic neuron, related to the membrane potential change associated with inhibition but independent of transmitter receptors or presynaptic properties. the latter conclusion has stood the test of time, marking pir as a robust property of cns neurons in a wide variety of contexts." this cellular property can most easily be seen in the lamprey neural circuit. the swimming movement is produced by alternating neural activity between the left and right side of the body, causing it to bend back and forth while creating oscillating movements. while the lamprey is bent to the left, there is reciprocal inhibition on the right side causing it to relax due to hyperpolarization. immediately after this hyperopolarizing stimulus, the interneurons use post-inhibitory rebound to initiate activity in the right side. depolarization of the membrane causes it to contract while reciprocal inhibition is now applied to the left side. functions in invertebrates cpgs play a similarly critical role in coordinating behaviors in invertebrates, and studying invertebrate cpgs with fewer numbers of neurons has helped establish general principles of cpgs and their organization in the nervous system. one model circuit for studying cpgs is the stomatogastric ganglion in crabs and lobsters, a ~30 neuron circuit containing two cpgs that generate rhythmic motor output for chewing and digesting food. dissection of these circuits has revealed neural mechanisms of cpgs. for example, the pyloric cpg - which controls the contraction and dilation of the pylorus - contains a set of conditional oscillatory neurons and one pacemaker neuron that fires rhythmically when dissected out of the circuit. coordinated rhythmic behaviors like walking, flight and grooming are also controlled by cpgs in some invertebrates. continued research into how cpgs control these behaviors has revealed a nested cpg architecture to control rhythmic behaviors across various timescales. other examples of cpgs in invertebrate animals include a cpg modulating reflexive withdrawal, escape swimming and crawling in the mollusc tritonia, and to control the heartbeat of leeches. central pattern generators play a broad role in all animals and show amazing variability and adaptability in almost all cases. alternate interpretations one theory that reconciles the role of sensory feedback during rhythmic locomotion is to redefine cpgs as “state estimators” as opposed to rhythm generators through this perspective, cpgs are an intrinsic spinal processor that corrects imperfect sensory feedback and adapts central input to this optimized peripheral input. models using this framework are able to accomplish rhythmics behavior as well as fictive locomotion without incorporating independent rhythm generators.