Section II: Animal, Veterinary and Fishery Sciences 1 98th Indian Science Congress January 3-7, 2011, Chennai I PRESIDENTIAL ADDRESS President : Prof. U. C. Srivastava PRESIDENTIAL ADDRESS Neuronal Projections, Classes and Behaviour: A Futuristic Approach Professor U. C. Srivastava* At the outset, I welcome all of you to the deliberations of the Section: Animal, Veterinary and Fishery Sciences of the 98th session of the Indian Science Congress Association, being held in this historic place of Chennai at SRM University, from January 3 to 7, 2010. I am delighted to have the honour of presiding the Section of Animal, Veterinary and Fishery Sciences; and feel privileged in addressing the galaxy of scientists from far and wide. In particular, I humbly express my reverence and gratitude to my teachers, mentors, research associates and the institutions, for inspiring me all along my professional career. Infact, it is nostalgic to recall the memories of my doctoral research under Prof. D. N. Vishnoi in the Department of Zoology, University of Allahabad, during which I, not only learned many histochemical techniques to investigate changes during transformation periods of insects, but also could produce interesting results, which were published in International journals1-11 giving me the opportunity to work as Post-Doctoral Fellow of Scuola Normale Superiore, Pisa, Italy, with Prof. O. Pompeiano, Director, Institute of Physiology, Pisa University, Pisa. The Post-Doctoral degree in advance studies of Neurophysiology was awarded with honours. Classical experiments indicate that labyrinth and neck reflexes play an important role in the maintenance of posture12. It is known that tilt about the longitudinal axis of the animal evokes vestibulospinal reflexes, whose function is to keep the head in horizontal position both by righting the head on the neck13and by righting the body using the limbs. Experiments have shown that side-down tilt of the whole animal14 or side-down rotation of the head after neck differentiation15 produced contraction, whereas side-up displacement resulted in relaxation of forelimb extensors. The same pattern of EMG responses may also affect the hindlimb extensors during sinusoidal tilt. The changes in posture occurring during animal tilt depend upon stimulation of macular (utricular) receptors. Moreover the responses are apparently mediated by the lateral vestibular nucleus of Deiters (LVN), *Department of Zoology, Allahabad University, Allahabad-211 002. 4 Proc. 98th Indian Science Congress Part II : Presidential Address which exerts an excitatory influence on ipsilateral extensor motoneurons. It appears in particular that most of the LVN neurons recorded in decerebrate cats showed and increase in discharge rate during side-down tilt of the animal and a decrease during side-up tilt. These findings are in agreement with the responses to tilt of the majority of primary afferents from the utriculus, which depend on the morphological polarization of the receptors16. In addition to the labyrinthine input, afferent volleys originating from neck receptors play an important role in the control of posture. In fact side-down rotation of the neck (i.e. side-up rotation of body while the head remains stationary) produces relaxation of limb extensors, whereas side-up rotation of the neck results in opposite posture. The neck input may control the activity of forelimb motoneurons by utilizing propriospinal descending pathways. In fact neck reflexes are still present after high spinalization. Indeed a short latency pathway has been described from C2 ganglion to hindlimb motoneurons and monosynaptic EPSPs in propriospinal neurons terminating on forelimb motoneurons have been recorded following stimulation of C1, C2 and C3 ganglia17. The fact that afferent signals from neck receptors act on fore and hindlimb motoneurons by utilizing propriospinal pathways does not exclude that supraspinal structures, which in turn project to spinal cord, are involved in the cervical control of posture. The lateral vestibular nucleus of Deiters (LVN) is one of these structures, since most of the LVN units showed a decrease in discharge rate during side-up neck rotation18. In addition to vestibulospinal neurons originating from the LVN, reticulospinal neurons may also contribute to the vestibular and cervical control of posture. Indeed, it has been speculated that reticulospinal neurons could be driven by the labyrinthine signals as well as by the neck signals. Moreover, there is evidence that interruption of the vestibulospinal pathways does not suppress the responses of the ipsilateral neck and forelimb extensors18 to vestibular stimulation. It is clear that both vestibular and neck receptors may modify the activity of reticular neurons, thus having the potential of modifying the posture during the labyrinth and neck reflexes19. The main question was to investigate whether in addition to the excitatory vestibulospinal neurons, reticulospinal neurons originating from the inhibitory area of the medullary reticular formation contribute to the postural adjustments during these reflexes. The responses characteristics of presumably inhibitory reticulospinal neurons to selective stimulation of vestibular receptors; neck receptors, the degree of convergence of both the macular and neck inputs on the inhibitory reticulospinal neurons and the modalities of interaction of the two inputs elicited during costimulation of the corresponding receptors on Section II: Animal, Veterinary and Fishery Sciences 5 the same population of neurons; finally the relation between cell size and response characteristics of reticulospinal neurons to vestibular or neck receptors stimulation were the main questions to be answered. The response characteristics of reticulospinal neurons identified antidromically and histologically located in the inhibitory area of the medullary reticular formation were studied in unanesthetized, decerebrate cats following independent sinusoidal stimulation of macular vestibular and neck receptors20-32. Reticulospinal neurons showing convergence of both macular and neck inputs were studied in detail to find out whether they exert an antagonistic influence on postural mechanisms involving the limb musculature during labyrinth and neck reflexes. In this a selective population of reticulospinal neurons originating from the inhibitory part of medulla and projecting to lumbosacral segment of the spinal cord has been recorded to study the possible correlation between their response characteristics to afferent (labyrinth and neck) inputs22,30and cell size inferred from the conduction velocity of reticulospinal axons. If response characteristics are determined by neuronal properties related to cell size, then they should be invariant in spite of differences in input drive31. On the other hand, if response characteristics depend mainly on synaptic organization such as topography and density of synaptic contacts, then differential control of particular neuronal groups could be observed provided the relevant input system distributed to pooled neurons with different pattern. These observations are related to the results in which the responses of vestibulospinal neurons originating from the lateral vestibular nucleus to sinusoidal tilt of the whole animal have been investigated19,20. The observation that the predominant response pattern of the medullary reticulospinal neurons to roll tilt is just opposite to that of the lateral vestibulospinal neurons suggest that the former neurons are presumably inhibitory, in contrast to the vestibulospinal neurons, which are excitatory on ipsilateral extensor motoneurons. It is postulated, therefore, that during side-down tilt the motoneurons innervating ipsilateral limb extensors are excited by the increased discharge of vestibulospinal neurons but are disinhibited by the reduced discharge of reticulospinal neurons; the opposite would occur during side- up tilt. Reticulospinal neurons located in the inhibitory area of the medullary reticular formation may thus contribute, with excitatory vestibulospinal neurons, to the postural adjustments of hindlimb muscles during the labyrinth reflexes. The response of presumably inhibitory reticulospinal neurons originating from the medial aspect of the medullary reticular formation, together with the response of lateral vestibulospinal neurons to neck input, may, therefore, contribute to the postural adjustments, 6 Proc. 98th Indian Science Congress Part II : Presidential Address as well as to the modulation of the monosynaptic extensor reflexes that occur during the tonic neck reflexes. It is of interest that both reticulospinal as well as lateral vestibulospinal neurons, projecting to lower segments of the spinal cord, also send terminal branches to the cervical enlargements. The two systems could then be involved in the coordinated cervical control of both the fore- and hindlimb musculature32-38. Independently upon the patterns of convergence of neck and macular vestibular inputs within the reticular structures, the responses of medullary reticulospinal neurons to head rotation leading to combined stimulation of both types of receptors closely corresponded to the values obtained by vectorial summation of the responses to the individual inputs. Similar results were also obtained from the lateral vestibular nucleus, the precerebellar lateral reticular nucleus, the Purkinje cell of the cerebellar vermis, the fatigial, interpositus and dentate nuclei and the anterior suprasylvian