Book - Physiology of the Fetus 13
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Windle WF. Physiology of the Fetus. (1940) Saunders, Philadelphia.
1940 Physiology of the Fetus: 1 Introduction | 2 Heart | 3 Circulation | 4 Blood | 5 Respiration | 6 Respiratory Movements | 7 Digestive | 8 Renal - Skin | 9 Muscles | 10 Neural Genesis | 11 Neural Activity | 12 Motor Reactions and Reflexes | 13 Senses | 14 Endocrine | 15 Nutrition and Metabolism | Figures
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Chapter XIII The Fetal Senses
IN discussing sensory mechanisms before birth the be1ief is not implied that the fetus is consciously aware of any sensation either in utero or after removal. Interest lies solely in whether or not the various endiorgans and afkerent neurons can function before birth. This is determinable by observation of the reflex motor effects of stimuli of different kinds applied to the fetus. It is seldom possible to be certain of the nature of neurons stimu1ated. Whether they conduct painful or tactile atkerent impulses, or even whether exteroceptive or proprioceptive, following a given stimulus is usually undeterminable.
Almost everyone who has studied fetal movements has contributed information to this subject, but it is impossible to evaluate and coordinate all the data. We are faced in most in— stances with the task of trying to synthesize experiments performed under good and bad conditions to arrive at some lmowledge of the- subject. No one would thinlc of accepting results in adult physiology of sense org-ans obtained in animals under narcosis and asphyxia Yet that is the lcind of data predominating in respect to the fetus.
It would seem that variations among species as well as in animals of the same species by different investigators are explainable td a very great extent on the basis of experimental methods used to study fetuses. When specimens are examined without using a general anesthetic and when they are exposed very quickly it is found that they are excitable by much milder forms of stimuli than must be used a minute or so later when anoxemia has begun to afkect them.I Furthermore, grossly undetectab1e deterioration of the physio1ogic condition of fetuses after delivery, even when the placental circulation remains intact, aEects some sensory nerves more than others and appears to alter synaptic mechanisms in such a way that motor responses change their Character. These efkects are especially well illustrated in human fetuses at hysterotomy performed under local anesthesia. The human fetus of the third and fourth monthsslies quietly within its crystal clear amnion. A very little pressure, such as follows tapping the membrane lightly, causes it to malce quiclc jerlcy movements of arms, legs and other parts. When the stimulus stops, the movements stop. 0n the other hand, when the placenta is detached and the fetus removed from its membranes it· executes «spontaneous" squirming movements which are more sustained and tonic than those seen at first. Fewer slcin regions are sensitive to the lighter forms of stimulation than was the case in amnio. Excitability diminishes rapidly. Most human fetuses have been studied after removal from the uterus2s Z« 4 and it was only recently that the opportunity arose to observe them at the moment of delivery while the placenta was intact and before the amnion was rupturedP consequently our lcnowledge of sensation in human fetuses is still very incomplete.
The Fetal Skin as a Receptor Organ
(a) Presssure Touch and Pain
Motor function precedes sensibility. 3—8 1n mammalian embryos it is always possible to obtain contractions of slceletal muscles by stimulating them directly before any of the sensory neurons can be activated. spontaneous movements of the chick embryo occur in advance of the reactions which follow stimulating the surface of the body,9- 10 but this is not the case in mammals.
The superlicial epithelium with its underlying mesenchymal connective tissue serves as a receptor organ in early fetal life. Nerves course beneath the epithelium and end under it in primitive free terminations before any reflexes can be elicited in mammalian embryos.11-I2 They appear in the face and forelimbs before they can be seen in the hind limbs and tail. Reflex responses follow stimulating them in cat embryos about 14 mm. long, Ibut the stimuli must be somewhat stronger at that time than later. Mild faradic shoclcs call forth responses before it is possible to obtain them by touching the epithelium with a single hair or a soft brush. A little later, in specimens about 15 mm. or 16 mm. long, touching with a single hair often serves adequately, providing the stimulus is placed directly over a spot supplied by primitive afkerent nerve übers. stimulation with a little brush made up of soft hairs is more often effective than a single punctate stimu1us,7-8-13-I4 because there is greater chance of pressing upon one of the sparse endings with it.
It is relatively easier to lind reflexogenous spots upon the face than upon the limbs. Furthermore, responses to stimulating endings in the forelimb disappear before those from the face, under the influence of progressive anoxemia. Consequently, the failure to elicit fetal movements by punctate stimulation of the epithelium of a limb does not prove the absence of a response-producing mechanism in the limb unless careful consideration is given to experimental conditions.
With further growth the number of nerve endings in the connective tissue beneath the epithelium increases and the Ebers begin to penetrate the epithelium itself. It becomes progressively easier to lind points whose stimulation elicits motor responses. Development proceeds in a cephalo-caudal direction in the body and proximodistally in the limbs. 0n the other band, rising thresholds, either in the endings themselves or in the central nervous System, soon bring about a condition of diminishing excitability of many cutaneous surfaces.15
One is scarcely justiiied in classifying the early sensory func· tions as touch or pain. Strictly spealcing, the fetus experiences no sensation whatsoever; it simply responds automatically, reflexly, in the early part of prenatal life. It is true that the neurons are activated by external environmental changes and may be considered exteroceptive, but there is nothing about their structure and nothing about the response itself which would indicate that some subserve pain and others touch or cutaneous pressure.
It is the opinion of some investigators that both pain and touch are differentiated in late fetal life. Very little difkerence could be observed in cat fetuses between responses elicited by coarse but innocuous stimuli and ones which produced demonstrable trauma until after the 45th day of gestationJ Even at full term, pain, touch and pressure are not well differentiated Raney and carmichaels have dealt with the question of localization to tactual stimuli in relation to the genesis of space perception in the rat. They found greater speciiicity of response as the time of birth approached.
(b) Temperature Sensitivity
Only one attempt has been made to study the eifect of tempera— ture stimulation systematically throughout the entire fetal periodss Physiological saIine solutions of different temperatures were applied in drops upon six representative cutaneous areas of guinea pig fetuses and the motor responses were recorded with a motion—picture camera. The parts stimulated were vibrissae area, ear, shoulder, rump, forepaw and hind paw. Control tests were made with the solution at body temperature. Responses were obtained throughout most of the fetal period but the warmer or cooler the solution the greater their number. Cold solutions appeared to be a little nsore effective than warm during the early part of the motile period. sensitivity increased with age to some extent but the growth of hair modified the effectiveness of stimuIation in the older specimens Furthermore, there »was evidence of sensitivity spreading from cephalic to caudal and from pro-c— imal to distaI parts as development progressed. Fig. 65 illustrates the relative eEectiveness of solutions of Various temperatures used in three age groups of fetuses.
Fig 65. Temperature sensitivity in guinea pig fetuses. (carmichae1 s: Lehnen J. Genetic Psycholsp Vol. so, 1g37.)
Proprioceptive Function in the Fetus
It is quite possible that afferent nerves of the deeper fetal cis— sues such as muscles and joints become functionaI very early. Some of the first responses of mammglian embryos may result from their activation. Movements ok the primitive limbs can be induced by bending the limbs or by tapping on them. They can lilcewise be obtained without touching the embryos at all by tapping lightly upon the fluid filled amniotic sac containing a specimen, but one does not know what nerves are being stimulated. since akkerent neurons are present in the connective tissue just underneath the epithelium, it is just as likely that the response is due to exteroceptive as to proprioceptive stimulation.
Nevertheless there are many observations suggesting that primitive endings in the muscle are capable ok being stimulated by the middle ok the gestation period or a little latet. The stretch— ing ok the ketus upon opening the amniotic vesicle and thus changing the pressure upon the specimen is a case in point. 0ther observations on the development ok muscle tonus and the tonic neclc and body righting rellexes leave no doubt that proprioception is present, and well kormed, considerably bekore birth.
In newborn rats whose spinal cords were sectioned completely during intrauterine like it was often very diklicult to determine by physiologic tests that nerve pathways had been interr"upted. The animals responded to stimulation ok points below the level ok section much as did their unoperated litter mates.I7 It was suggested that in these very immature animals reflex movements below the lesion were responsible kor stimulating proprioceptive endings in muscles above, setting up proprioceptive reilex movements ok which the rats were aware and in this way acquainting the rats, as it were, with what was happening in a part ok the body from which no direct messages could be received. The spread ok activity krequently seen in much less mature mammalian ketuses ok other species suggests a mechanism ok a similar sort. Ik proprioceptive kunction plays a part in the responses ok the early human ketus it is certain that it does not require highly specialized neuromuscular spindles because these structures do not appear until about the third month.
Function ok the vestibular mechanism begins rather late in ketal like ok the can« In other species it may be present relatively earlier, as seems to« be thes case in the sheepJs However, caution must be exercised in attempting to determine its presence, kor righting reflexes and eye movements can be induced by stimulating other receptors such as those of the neck and body. The righting reiiexes have beens discussed in the preceding chapter.
Olfactory Gustatory and Visceral Senses
The olfactory apparatus is made ready during prenatal life, but it is doubtful if adequate stimuli are ever present in utero where no air comes into contact with the nasal recept0rs. In the newborn and prematurely born human infant, various observers have obtained evidence of olfactory function.20s 21 It has been pointed out, however, that common chemical receptors of the trigeminal nerve are readily stimulated by strongly aromatic materials and they must not be confused with true olfactory phenomena.
Taste has been demonstrated at birth in man as well as in lower animals, but it is doubtful if dikkerentiation between sour, salt and bitter is very well formed. The day old lcitten can distinguish between millc and a mixture of millc and sodium chloride.22 The experiments of De snooks who injected saccharin solution into the amniotic sac of women sukkering from polyhydramnios, seem to indicate that the fetus responded to the sweet taste and swallowed unusually large quantities of the amniotic fluid.
Regarding other visceral aiferent stimulation, nothing is known. 0ne can speculate that the normally occurring intestinal movements stimulate afkerent neurons. Perhaps the active swallowing of amniotic Auid by fetuses in the last months of gestation is reflexly controlled by» such a mechanism. Vigorous «hunger" contractions of the stomach are found in prematurely delivered mammals.24
Hearing and Vision
Hearing has been considered to be imperfect «at birth but seems to improve within a short time after the amniotic fluid and secretions drain from the middle ear.25 Prematurely delivered infants show evidence of audition a little while after birth. Some investigators hold that the respiratory changes observed in the human infant concomitant with the production of sounds signify a functional auditory mechanism.
Attempts were made by Peiper26 to observe changes in intra— uterine activity associated with Hund. »Alt.hough it was difficult to rule out stimulation of the fetus by the mother’s own responses, the evidence suggests that strong sounds may have initiated reflex movements of the fetus near term. 0thers have confirmed these results« We have found that very slight tapping upon the amnion at the time of hysterotomy under local anesthesia results in similar quiclc fetal movements even at a much earlier time in prenatal life) such stimuli need not be thought of as sound producing. It is possible that the strong sounds, especially in the case of tapping upon a metal bath tub in which the pregnant woman 1ay, were not in themselves the cause of the fetal responses, but that pressure was transmitted to the fetus as in our own ex— periments. 0n the other hand, sontag and Wallace have presented good evidence that the human fetus does react in utero to sound producing stimuli app1ied externally to the mother’s abdomen. The fetal responses (movements) became more marked as term approachedks Electrical responses have been re— corded from the fetal cochlea after auditory stimulation in guinea pigs of 52 days gestation.29 This was the earliest period at which this species reHd overtly to such stimuli.
Although there can be no stimulation by 1ight before birtln it is probable that the visual mechanism is functional t«o an imperfect degree in late prenatal life. 0bservations in premature infants indicate that some sort of differentiation of light and dark is present. Pupillary responses to strong light can be obtained in late fetal life.
1. Windle, W. F. sc R. F. Becken 194o. Arch. Neur. sc Psychiat., 43: ge.
2. Minkowslci. M. 1938. Abderhaldecks Handb. biol. Arbeitsmeth., Abt. V, Teil 5 B: Zu.
. Bolaflio, M. sc G. Artom. 1924. Arch. di sei. Biol., H: 457.
. Hoolcen D. 1936. Yale biol. sc Med., s: 579.
. Fit2gerald, E. sc W. F. Windle, unpublished observations
. Preyer, W. 1885. spedielle Physiologie des Embryo. Gnaden, Leipzig.
7. Windle, W. F. sc A. M. Griiiiw xgsn J. comp. Neun, se: 149.
. Raney, E. T. sc L. carrnichaeL 1934. J. Genetic Psychol., 45: Z.
9. 0rr, D. W. sc W. F. Windle. 1934. J. comp. Neur., 6o: 271.
10. Kuo, Z. Y. 1932 J. Exp. Zool» Hi: 395.
11. Windle, W. F. 1937. Proc. soc. Exp. Ziel. sc. Med., 36: 64o.
12. Windle, W. F. sc E. Fitzgerald. 1937. comp. Neun, 67: 493.
13. coronios, J. D. 1933. Genetic PsychoL Monog., I4: 283.
14. carmichaeh L. 1933. In c. Murchisocks Handb. child Psychol., and ed., clarlc Univ. Press, Worcester.
15. Barcroft, J. s- D. H. Bau-on. 1939. J. comp. Neun, 7o: 477.
cartnichaeh L. s- G. F. J. Lehnen 1937. J. Genetic Psychol., so: 217. Hoolten D. s- J. s. Nicholas 193o. J. Comp. Neun, so: 413.
. cuajunco, F. 1927. contn Emb., 19: 45.
Winde, W. F. s- M. W. Fish. Igzn J. comp. Neun, 54: 85.
Peipetx A. 1928. Ergebw inn. Med. Kinderhlh 33: 5o4.
Frau, K. c» A. K. Nelson s- K. H. sun. 193o. The Behavior of the
Newborn Inkanr. Ohio state Univ. studies, No. to.
Pkalkmanm C. 1936. J. Genetic Psychol., 49: Si.
De snoo, K. 1937. Monatschtz Geburtsh. Gyn., 1o5: As.
carlson, A. J. s- I-I. Ginsburg. Ists. Am. J. Physiol» 38: 29.
Peterson, F. s- I.. I-I. Rainezu 191o. Bu11. N. Y. Lyingsin I-Iosp., 7: 99.
26. Peipen A. 1925. Monatschtn Kind-Ethik» sey: 236.
27. Forbes, H. S. & H. B. Forbes. 1927. J. Gott-F. Psychol., 7: 353.
28. Sontag, L. W. and R. Wallace I935 Child Developmeny 6: 253.
29. Rawdonismith A. F» L. carmichael and B. We1Iman. 1938 J. Exp. Psychol., 23: 531.
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