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SUMMARY1. The effect of systemic hypoxia was tested in anaesthetized, immobilized, thoracotomized and artificially ventilated cats with peripheral chemoreceptor afferents either intact or cut. Extracellular recordings from different types of medullary respiratory neurones and intracellular recordings from stage 2 expiratory neurones were made to determine the hypoxia-induced changes in neuronal discharge patterns and postsynaptic activity as an index for the disturbances of synaptic interaction within the network.2. The general effect of systemic hypoxia was an initial augmentation of respiratory activity followed by a secondary depression. In chemoreceptordenervated animals, secondary depression led to central apnoea.3. The effects of systemic hypoxia were comparable with those of cerebral ischaemia following occlusion of carotid and vertebral arteries.4. In chemoreceptor-denervated animals, all types of medullary respiratory neurones ceased spontaneous action potential discharge during hypoxia.5. Reversal of inhibitory postsynaptic potentials (IPSPs) and/or blockade of IPSPs was seen after 2-3 min of hypoxia.6. During hypoxia, the membrane potential of stage 2 expiratory neurones showed a slight depolarization to -45 to -55 mV and then remained stable.7. The neurone input resistance increased initially and then decreased significantly during central apnoea.8. Rhythmogenesis of respiration was greatly disturbed. This was due to blockade of IPSPs and, in some animals, to more complex disturbances of phase switching from inspiration to expiration.9. Central apnoea occurred while respiratory neurones were still excitable as shown by stimulus-evoked orthodromic and antidromic action potentials.10. The results indicate that the medullary respiratory network is directly affected by energy depletion. There is indication for a neurohumoral mechanism which blocks synaptic interaction between respiratory neurones in chemoreceptorintact animals. MS 9022 D. W. RICHTER AND OTHERS

Studies have shown that in the mammalian neuromuscular system stretch reflexes are localized within individual muscles. Neuromuscular compartmentalization, the partitioning of sensory output from muscles, and the partitioning of segmental pathways to motor nuclei have also been demonstrated. This evidence indicates that individual motor nuclei and the muscles they innervate are not homogeneous functional units. An analysis of the functional significance of reflex localization and partitioning suggests that segmental control mechanisms are based on subdivisions of motor nuclei–muscle complexes. A partitioned organization of segmental control mechanisms could utilize (1) the potential functional diversity of muscle fiber types, (2) the variety of mechanical actions of individual muscles arising from their distributed origins and insertions, and (3) diverse architectural features such as intramuscular variations in pinnation and complex in-series and in-parallel arrangements of muscle fibers. The differentiated activity observed in some muscles during natural movements also calls for localized segmental control mechanisms. Partitioning may also play a role in mechanical interactions between contracting motor units and in increasing the stability of neuromuscular systems. The functional advantages of reflex localization and partitioning suggest they are probably common features of segmental systems, whose organization reflects the structure and function of their associated neuromuscular systems.

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