<P> The first modern evidence of the central pattern generator was produced by isolating the locust nervous system and showing that it could produce a rhythmic output in isolation resembling that of the locust in flight . This was discovered by Wilson in 1961 . Since that time, evidence has arisen for the presence of central pattern generators in vertebrate animals, starting with work on the cat in the 1960s by Elzbieta Jankowska in Gothenburg, who provided the first evidence for a spinal cord CPG . This section addresses the role of the central pattern generator in locomotion for the lamprey and humans . </P> <P> The lamprey has been used as a model for vertebrate CPGs because, while its nervous system has a vertebrate organization, it shares many positive characteristics with invertebrates . When removed from the lamprey, the intact spinal cord can survive for days in vitro . It also has very few neurons and can be easily stimulated to produce a fictive swimming motion indicative of a central pattern generator . As early as 1983, Ayers, Carpenter, Currie and Kinch proposed that there was a CPG responsible for most undulating movements in the lamprey including swimming forward and backward, burrowing in the mud and crawling on a solid surface, that although not surprisingly did not match the activity in the intact animal, nevertheless provided the basic locomotor output . The different movements have been found to be altered by neuromodulators, including serotonin in a study by Harris - Warrick and Cohen in 1985 and tachykinin in a study by Parker et al. in 1998 . The lamprey model of CPG for locomotion has been important to the study of CPGs . Although Sten Grillner claims that the locomotor network is characterised, a claim that has seemingly been uncritically accepted by the spinal cord locomotor network field, there are in fact many missing details and Grillner cannot provide the evidence he uses to support his claims (Parker 2006). A general scheme of the lamprey CPG is now being used in the creation of artificial CPGs . For example, Ijspeert and Kodjabachian used Ekeberg's model for the lamprey to create artificial CPGs and simulate swimming movements in a lamprey - like substrate using controllers based on a SGOCE encoding . Essentially, these are the first steps toward the use of CPGs to code for locomotion in robots . The vertebrate model of CPG has been also developed with both Hodgkin - Huxley formalism, its variants and control system approaches . For example, Yakovenko and colleagues have developed a simple mathematical model that describes basic principles proposed by T.G. Brown with integrate - to - threshold units organized with mutually inhibitory connections . This model is sufficient to describe complex properties of behavior, such as different regimes of the extensor - and flexor - dominant locomotion observed during electrical stimulation of the mesencephalic locomotor region (MLR), MLR - induced fictive locomotion . </P> <P> Connections between the CPGs controlling each limb influence the coordination between the limbs and hence the gaits in quadrupedal and possibly also bipedal animals . Left right coordination is mediated by commissural and fore - hind by long - projecting propiospinal interneurons . The balance of the left - right alternation (mediated genetically identified V0d and V0v neuron classes) to left - synchronization promoting commissural interneurons (potentially mediated V3 neurons) determines whether walk and trot (alternating gaits) or gallop and bound (synchronous gaits) are expressed . This balance changes with increasing speed, potentially because of connection from the MLR to commissural interneurons, and causes speed dependent gait transitions characteristic for quadrupedal animals . The walk to trot transition potentially occurs because of the stronger decrease of extension than flexion phase durations with increasing locomotor speed, which leads to progressively increasing overlap between the diagonal limbs up until diagonal synchronization (trot). </P> <P> Central pattern generators also contribute to locomotion in higher animals and humans . In 1994, Calancie, et al. claimed to have witnessed the "first well - defined example of a central rhythm generator for stepping in the adult human ." The subject was a 37 - year - old male who suffered an injury to the cervical spinal cord 17 years prior . After initial total paralysis below the neck, the subject eventually regained some movement of the arms and fingers and limited movement in the lower limbs . He had not recovered sufficiently to support his own weight . After 17 years, the subject found that when lying supine and extending his hips, his lower extremities underwent step - like movements for as long as he remained lying down . "The movements (i) involved alternating flexion and extension of his hips, knees, and ankles; (ii) were smooth and rhythmic; (iii) were forceful enough that the subject soon became uncomfortable due to excessive muscle' tightness' and an elevated body temperature; and (iv) could not be stopped by voluntary effort ." After extensive study of the subject, the experimenters concluded that "these data represent the clearest evidence to date that such a (CPG) network does exist in man ." Four years later, in 1998, Dimitrijevic, et al. showed that the human lumbar pattern generating networks can be activated by drive to large - diameter sensory afferents of the posterior roots . When tonic electrical stimulation is applied to these fibers in motor complete spinal cord injured individuals (i.e., individuals in whom the spinal cord is functionally isolated from the brain) rhythmic, locomotor - like movement of the lower limbs can be elicited . These measurements were performed in supine position, thus minimizing peripheral feedback . Subsequent studies showed that these lumbar locomotor centers can form a large variety of rhythmic movements by combining and distributing stereotypical patterns to the numerous lower limb muscles . </P>

Which of the following structures does not participate in the production of sound