Fifty years after Massimo Fagioli’s theoretical formulation, the discovery of a new retinal photoreceptor type clarifies how the retina may be able to react to light and trigger neonatal functions including the first breath, during a short time of a few seconds, at birth. In his Human Birth Theory, the author proposes that the light provokes an immediate activation of the brain, via the retina, when the fetus comes into contact with the extrauterine environment. Therefore, according to Fagioli, the beginning of human life consists in this ‘biological reaction to light’, which precedes the first breath by a few seconds, about ‘twenty seconds’. The transition from a situation of complete intrauterine homeostasis to an extraordinary sensory stimulation of the new extrauterine environment implies a dramatic physiological switch that must be rapid and well-orchestrated, in order to ensure the newborn’s survival. Indeed, many aspects of the transformation that occurs at birth are instantaneous and the most impressive of them is the onset of respiration, particularly the newborn’s first breath.
Developmental physiology research has still not fully investigated what triggers the initiation of breathing and other neonatal physiological processes. In fact, there is no consensus on how this physiological transformation occurs. On the other hand, in his theoretical formulation, Fagioli implies that only a very rapid activation of the brain can orchestrate an equally sudden and complex physiological switch. In this regard, it seems very intriguing that recent studies have shown how a brainstem nucleus of the pontine respiratory complex, which is known for its blocking action on the breathing chemoreflex in utero, changes its function abruptly at birth, thus facilitating the first breath and the onset of continuous breathing. However, despite these interesting findings, the authors do not investigate what might determine this functional switch.
In humans, the retina is the only structure that catches and rapidly photo-transduces light, transforming photon energy into electrical signals and conveying this information to the various subcortical processing centers, via the optic nerve. Although it is located outside the skull, the retina is part of the brain; therefore, the retina is the only part of the central nervous system (CNS) that is approachable. Due to these characteristics, the retina is the research model for brain neurophysiological studies and many works have clarified its structure and functions in the last century. The retina is made up of different cell layers: the layer of visual photoreceptors, cones and rods, on the outer side, and the layer of ganglion cells, whose fibers form the optic nerve, on the inner side. Retina’s photoreceptive activity is responsible not only for visual perception, but also for some important functions that are independent from sight and take place without our awareness: the so-called ‘non-visual’ functions. Among them, the most important is the regulation of the hypothalamic pacemaker of circadian rhythms, which influences most of our physiological processes, such as sleep-wake cycles, endocrine production, thermoregulation and heart rate.
In the last century, despite the large number of studies, researchers had the absolute belief that the retina had only two types of photoreceptors, cones and rods, which were responsible for all retinal photoreceptor functions. However, since the early 2000s, after the publication of some important studies, the existence of a third retinal photoreceptor type has been definitively demonstrated. The newly discovered photoreceptors are very different from the traditional ones. In fact, they are a small fraction of ganglion cell layer’s neurons that express a new photopigment, which is called melanopsin and belongs to the same opsin-based family of cones/rods’ photopigments. Melanopsin is very sensitive to light, particularly to blue light, with a peak of activity at a wavelength of 480 nm. Moreover, it is able to catch and photo-transduce even a single photon. The melanopsin-dependent photoreceptors are both photoreceptors and projection neurons. Once activated by light, they produce an electrical signal which is capable of travelling very rapidly, through direct and monosynaptic pathways, to the many subcortical target stations, and from here up to the cortex. Among the many peculiarities that these intrinsically photoreceptive neurons present, a very important specific feature concerns their neurodevelopment. For instance, these cells are the only functioning photoreceptors at birth and in the first weeks of newborn’s life; on the other hand, cones and rods are immature at birth and begin to function approximately after the first month of life.
In conclusion, the discovery of these particular photoreceptive neurons confirms that the retina is photosensitive at birth. Moreover, due to the photochemical and physiological peculiarities of these cells and their diffuse projection to the whole brain, this discovery suggests that they are able to capture light at birth, immediately generating the nerve signal that activates the whole brain, including the respiratory centers which regulate breathing and allow the first breath.
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