A proposed relationship between increases in the number of olfactory receptor neurons, convergence ratio and sensitivity in the developing rat.

In the rat neonate, as in other altricial young, olfaction, in contrast to vision and hearing, is functional at birth, being critically important in feeding, growth and other aspects of infant-mother interaction. Yet, olfactory sensitivity, i.e., the ability to detect odors at low levels, is poor in the newborn, improving dramatically in the first few postnatal weeks. To find a neural explanation for this phenomenon at the level of peripheral olfactory system, we present quantitative light microscopic data which reveal that during the suckling period of postnatal development the surface area of the olfactory receptor sheet and the total number of olfactory receptor neurons increase by about 8- and 12-fold respectively, being about 15 mm2 and 1.0 million on each nasal half of the newborn. Since the number of mitral cells, the principal relay neurons of the olfactory bulb, is already established at birth, at about 40,000 per olfactory bulb, it may be estimated that the convergence ratio of the olfactory neurons to mitral cells increases by more than 10-fold in the suckling period. We propose that the increased number of primary sensory afferent units and the higher convergence upon the central relay cells enhances the physiological capacities of the olfactory afferent pathway, increasing the opportunity for spatial summation and facilitation. The latter changes may lead to reduced olfactory thresholds and improved sensitivity with development. The relative contribution of these peripheral changes in enhancement of olfactory sensitivity during growth is discussed in the light of our knowledge on the developing olfactory system.