[Pain in the fetus: neurobiological, psychophysiological and behavioral aspects.].
Until a short time ago, the view prevailed worldwide that children were less sensitive to pain than adults, and such operations as circumcision were performed in babies without adequate anesthesia or analgesia. This view is now considered a misconception, as psychophysiological and behavioral studies show that even neonates have a well-functioning nociceptive system. Nociception generally refers to the neural and sensory aspects of pain, which do not necessarily include conscious experience. There is no discontinuity in the development of the nervous system during birth, and therefore it can be concluded that the fetus is also responsive to noxious stimuli. The question arises as to the stage of ontogeny of the human at which nociceptive behavior begins. Literature on the fetal nervous system reveals that the first signs of somatosensory system function occur at week 7 of gestation and at week 22 the synaptic connection from the nervous periphery to the somatosensory cortex is becoming established. During this period, motor behavior matures, from stereotyped reflexes to spontaneously generated complex motor patterns reminiscent of the repertory of voluntary movement. From week 22 onward the electroencephalogram (EEG) shows increasingly more varied patterns, and sleep-wake states can be discerned after week 30 of gestation. Somatosensory evoked cortial potentials have been recorded from gestational week 28 onward. Substance P, a neuropeptide associated with pain in the adult nervous system, is present in the fetal spinal cord as early as week 12 of gestation, while the antinociceptive opioid peptide enkephalin does not appear until week 24. From week 15 onward, opioid peptides such as beta-endorphin appear in the pituitary; their release becomes sensitive to environmental stimuli from about week 20 onward, which can be considered the onset of pituitary stress responses. In particular, parturition and abortion induced the release of opioid peptides. Studies of conditioned behavior show that the fetus has the ability to learn. It has been hypothesized that the fetus and neonate possess a procedural memory, which is not transferred to the language-based memory of later phases of life. Learning and memory are the most essential elements for the construct of "consciousness." Therefore, a primitive type or level of consciousness might exist in the fetus. Thus, a considerable range of sensorimotor function, including memory, develops during fetal life. Anatomical, physiological and behavioral data suggest that the nociceptive system is included in this development. Although we cannot be sure at present whether the fetus consciously experiences pain, beyond the protective nociceptive behavioral responses, anesthesia should be used for invasive procedures to protect the fetus and its nervous systems.
Zimmermann
M
M
Abteilung für Physiologie des Zentralnervensystems, II. Physiologisches Institut der Universität Heidelberg, Im Neuenheimer Feld 326, W-6900, Heidelberg, Bundesrupublik Deutschland.