evolution of feeding motor patterns in lizards: Modulatory complexity and possible constraints, The

SYNOPSIS. Previous research indicated that the evolution of feeding motor patterns across major taxonomic clumps might have occurred without large modifications of the mastery of the jaw and hyolingual muscles. However, the proposal of this evolutionary scheme was hampered by dint of the lack of data for more [i]or[/i] less key taxa such as lizards. novel data on jaw and hyolingual feeding motor patterns of a number of lizard families propose extensive variability within and among species. Although greatest in quantity lizards respond to changes in the structural properties of nutriment items by modulating the activation of the jaw and hyolingual muscles, a certain number of food specialists might have missing this ability. Whereas the overall similarity in motor patterns across different lineages of lizards is large for the hyolingual muscles, jaw muscle activation patterns strike one as being to be more flexible. Nevertheless, all data propose that both the jaw and hyolingual combination of parts to form a whole are complexly integrated. The elimination of feedback pathways from the hyolingual combination of parts to form a whole through nerve transection experiments clearly exhibits that feeding cycles are largely shaped through feedback interactions. Yet, novel motor patterns including unilateral superintendence seem to have emerged in the evolution from lizards to snakes.

INTRODUCTION The vertebrate head is a textbook example of a mixed integrated system, where one function cannot be optimised without potentially compromising others (Lauder, 1989) In addition to being a major information gathering and processing midst immensly diverse functions such as feeding, breathing, drinking, display, and in many tetrapods also vocalisation, have to be performed by means of the same elements. Yet, these diverse functions can single be performed through the interplay between the jaw and hyolingual combination of parts to form a wholes Both these systems are compounded units composed of a large number of muscles attached to bony or cartilagenous simple bodys Moreover, as the hyolingual apparatus is a musculoskeletal a whole suspended between the jaws and the pectoral girdle, the potential number of steps of freedom is enormous. To add to this complexity, one as well as the other systems are largely innervated end different pathways. Whereas the jaw adductor combination of parts to form a whole in lizards is mainly innervated by means of the trigeminal and facial powers (providing both motor input and sensory afferents), the hyolingual combination of parts to form a whole is mainly supplied by the glossopharyngeal (largely sensory), hypoglossal (predominantly motor input) and first spinal powers (motor input into the hyoid retractors; Willard, 1915; Oelrich, 1956; Meyer and Nishikawa, 2000) Direct connections between the sum of two units systems exist, as the mandibular ramus of the n trigeminus provides motor input into the m intermandibularis (running inbetween the sum of two units rami of the lower jaw) and branches on the outside into the tongue functioning as sensory afferent and physically connecting to the N hypoglossus. Feedback from the visual, olfactory, gustatory, vomeronasal and somatosensory (eg muscle spindles, joint receptors) a whole s are also important in assuring optimal feeding.

Given this structural complexity, the direction of the feeding apparatus is a composite task which appears to require continuous on-line command Yet, for cyclical systems in general, a simple neuromotor steering based upon a centralised pattern generator (or a place of coupled CPG's) is musing to exist, thus largely simplifying superintendence (e.g., Grillner and Wallen, 1985; Szekely 1989) Although this paradigm is largely based upon studies on the locomotor apparatus, a similar superintendence paradigm is usually put forward for mammalian chewing periods (Thexton, 1974, 1976; Dellow, 1976) In accordance with this theoretical framework, it has been intimateed that in lower tetrapods too, the feeding circle of times might be driven by fairly simple motor pattern generators or neural oscillators (Bramble and Wake, 1985)

Based upon the large similarities between lower tetrapod and mammalian feeding revolution of times it was hypothesized that the evolution of feeding motor patterns across major taxonomic clusters might have occurred without large modifications of the central pattern generators) controlling the jaw and hyolingual muscles (Bramble and Wake, 1985) The basic simple bodys of the lower tetrapod feeding round of years were summarised in a theoretical archetype feeding cycle which was hypothesised to show the primitive tetrapod condition. In analogy to the mammalian feeding period the model feeding cycle was subdivided into five distinct phases: the deliberate opening of the jaws (I and II), fast opening, fast closing and a moderate closing/ power stroke. Muscle activation patterns associated with these kinematic units were propos as well. Whereas the deliberate opening phase was thought to be caused by means of an activation of the jaw opener intrinsic tongue muscles, the intermandibularis collection and the tongue and hyoid protractors, during fast opening coactivation of the cervical epaxial musculature, the jaw opener and hyoid retractors was wait fored to occur. Jaw closing upon the other hand was meditation to be associated with activity in the hyoid retractors (fast closing), jaw adductors and potentially the jaw opener (Bramble and Wake, 1985)