Book - Physiology of the Fetus 12
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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 XII Fetal Motor Reactions and Reflexes
Many provisions are made during intrauterine like to assure survival akter birth. A number ok these depend upon development of the ketal nervous system. 0ne function ok vital im— portance to all species is respiration and this has been emphasized by separate consideration in another chapter. 0ther kundamental activities involving the somatic motor mechanisms ok the body are sucking and swallowing which are well developed in all mammals at birth, crying which is encountered in most ok them, and locomotion which occurs to a variable extent. ln addition to these instinctive motor reactions, a number ok reflexes which some have thought ok as purposekul or protective have their genesis during prenatal like. The amount of kunctional independence at— tained within the uterus depends upon the degree ok matssdtion reached in the nervous system and varies within wide limits.
Development of Feeding Reactions
Coordinated movements ok suclcing and swallowing are kully kormed in viable premature human inkants, and may be seen in the common laboratory animals considerably before birth. In— deed, suclcing appears to be one ok the kew reactions endowing the tiny newborn opossumKs 2 Feeding reactions may be said to begin with the iirst movements ok the jaw.3- 4 Opening and closing ok the mouth appear -in cat fetuses only about 25 mm. long, roughly comparable with the 9 weelcs human. The tongue can be protruded almost this early but its sides do not. curl until several days later. similar observations have been made in guinea pigss and sheepäs In the latter species, closure ok the jaws is the only response obtained by touching the tongue between the 41st and 49th days ok gestation. Throat movements occurring in briek rhythms follow jaw closure at 49 days. The tongue ok the fetal sheep curls at about 70 days. These early simple activities are the korerunners ok a number ok more complex movements whosekintegration ultimately accomplish suclcing, chewing and biting in the cat, sheep and guinea pig. The early components of the feeding reacti0n have not been thoroughly studied in human fetuses but lip movements which may be related to suclcing have been seen at about 1o weelcs and later.7 8 9
True suclcing is rhythmical in the cat and involves a coordins ation or integration of tongue, lip, jaw and throat movements. Furthermore, alternate forward thrusts of the limbs and side to side head movements enter into the feeding reaction to a remarlcable extent in the newborn kirren. However, the rhythrnical suclcing of the fetus is accomplished without participation of the limbs until late in the fetal period. Coronios4 observed pursing of the lips around the tip of a glass stimulator in specimens 43 mm. long, and at 54 mm. the first deftnite sucking coordinated with head and forelimb movements was encountered. The response continues to improve and appears to be fully developed and vigorous at least one weelc before birth of the Kitten.
The actual contractions of muscles which occur in swallowing are diiIicult to observe and can be seen only in late fetuses. These movements have been reported in sheep of so days gestation and we have seen them in the human fetus at 14 weelcs. Undoubtedly they occur even earlie»1·. When the abdominal wall of a cat fetus 25 to zo mm. long is opened the stomach may be seen to be well lilled with iluid. In specimens a little larger, brought out into the air, bubbles soon appear in this Organ. This early fetal swallowing of the cat is in no way remarlcable when a comparison is made wich ehe aik-hkeekhihg, suekihz 23 day «emhkye" ek khe opossum. »
swallowing of amniotic lluid does not seem to occur under perfectly normal physio1ogic conditions in the fetal guinea pig until about the 42nd day of gestation. This has been demonstrated by roentgenologic studiesso made after injecting thorotrast into the amniotic sac (see chapter VII) . The frequency of swallowing and the amount of iluid talcen into the fetal stomach increase with age. It may be concluded that the feeding mechan —ism, lilce that for respiration, has its genesis in the early part of the fetal period well in advance of the time it is normally called into use.
Development of Posture and Progression
Mammalian locomotion is a complicated act requiring cooperation of many groups of muscles and exquisite integrative development within the Central nervous System. Nevertheless, it too has its genesis in ear1y prenatal life. Establishment of erect posture is prerequisite for walking. It involves the coming into action of righting reflexesz afkerent impulses for these arise in the slcin and muscles of the neclc and body, in the labyrinths and, for higher mammals including the cat, in the retina. Maintenance of erect posture requires the presence of static or postural tonus to establish the proper neurologic balance between flexor and extensor muscle groups in order that the force of gravity be successfully opposed. After these conditions have been met, progression becomes possible by alternately and rhythmically changing the balance in such a way that flexion-extension stepping movements of the limbs are performed. Developmentally, however, the various components of a locomotor reaction are not laid down in utero in the sequence just stated.3
1t is evident that alternation of trunlc movements is fairly well developed in the fetus before there is evidence of a postural mechanism. At the time bilateral flexion of the neclc and upper trunk can be induced in cat embryos less than 20 mm. long, the forelimbs move at the shoulders with the contractions of the trunlc muscles. No rhythmicity manifests itself in these bilateral trunk movements before the 25 to 28 mm. stage. Active, arrhythmic alternation of the forelimbs can be induced in 3o to 35 mm. fetuses with considerable regularityz in fact, crossed ex— tension responses begin to appear at 25 mm.
The bilateral trunk flexions, synchronized more or less pas— sively with the limbs, are forerunners of squirming movements. Early in development the resemblance between squirming and aquatic locomotion is rather strilcing At birth the kitten utilizes side to side movements in its crawling-search reaction. The reflex crossed extension following active flexopwithdrawal of one forelimb foreshadows the act of steppi·ng. squirming and stepping are distinct from one another until the cat fetus has reached a length of about 50 mm., at which time the first integrated activity vaguely resembling the act of crawling can be seen. synchronized stepping movements of all fourjegs are not encountered before 8o mm.,I1 and even at birth the hind limbs are imperfectly coordinated with the forelimbsU Rhythmicity of forelimb stepping movements improves as the time of birth approaches. Kittens 95 to ioo mm. long delivered two weelcs prematurely manage to crawl very credibly.
Development of Progression difkers according to species in respect to late stages of developmenh but there is a surprising amount of similarity at Erst. 1t requires no great imagination to see a resemblance between the side to side head movements and coordinated forelimb activities by means of which the opossum «embryo" reaches the mother’s pouch at birth and the side to side necl(, trunlc and forelimb movements of the so mm. cat fetus. similar reactions have been observed in rat13- «« and guinea pig fetuses.5 The records are very incomplete in the human, but we have seen Hexor withdrawal of one leg accompanied by crossed extension of the opposite leg in 4o mm. specimens Rhythmicity of stepping has not been reported until much later in fetal life and even at birth behavior of a locomotor type is most ineffectual.
Fig. 63.-Record of muscle tonus in a sheep fetus at 144 days gestation delivered at caesarean Section with placental circulation intact. In the Erst, third and fifth records, the fetus lay quietly in a warm saline bath. In the second and fourth records it was lifted into the air and impulses began to flow to the muscle-s. (Bak croft: Irish Jour. Med. Sci. 1935.)
Development of postural tonus has not been studied thorough1y. 1t is probable that muscle tone as we know it in the adult is not present at all in utero under nor-mal physiologic conditions. The fact that rhythmical respiratory movements can occur toward the end of gestation without aspiration of a signiftcant amount of amniotic fluid demonstrates that the fetal chest is not held tonically in an elevated position which it must assume after birthLE The absence of afferent stimulation in utero is unquestionably an important factor in maintaining tonus at a low level. As illus— trated in Fig. 63, it was found that action potentials from fetal muscles appear when a fetus is lifted out of its warni saline bath and disappear again when it is returned to the bathJC The relation of anoxemia to thresholds of nervous activity and to tonus has been discussed in the preceding chapter.
How early in prenatal life postural tonus can be induced ex— perimentally is not deiinitely known, but an indication of it may be seen in cat fetuses about 50 mm. long. It was noticed that re » lease from their membranes was followed by straightening of the back and extension of all limbs in such a way that they appeared to stretch. Full term birth posture, i.e., extension of the head and forelimbs," seem to be induced in part by a similar release of tension when the membranes burst. A factor in development of muscle tonus may be observed in the sustained fetal movements during anoxemia. sustained extensor movements are sometimes so marked in cat fetuses only 30 mm. long that they resemble the decerebrate condition.
Decerebration of cat fetuses results in hyperextension of the limbs during the last three weeks of prenatal life.3- IS 0ne or two weeks before birth deiinite decerebrate rigidity appears in the forelegs of specimens in which the brain has been cut through from the rostral border of the mesencephalon to the rostral border of the pons. When the level of the transection passes farther for— ward, leaving the region of the red« nucleus intact, decerebrate rigidity fails to appearRss IV Postural tonus can be called forth in the human fetus by decerebration20 and there too it seems to be more especially related to the midbrain and lower centers than to the higher parts of the nervous System. Decerebrate rigidity in the rabbit after birth is illustrated in« Fig. 64.
The mechanism by which the righting reaction evolves likewise has its development in the early fetus, but the actual accomplishment of righting has not been observed until after postural tonus and alternate stepping movements can be induced.21 Cat fetuses 75 to 100 mm. long try to hold their heads up and turn their jaws parallel to the ground when they are placed upon a Hat surface. The general impression gained is that they could right themselves save for the weakness of their muscles. Between 1oo and 1 Io mm., they are actually able to right the head in respect to the surface on which they 1ie, but they are completely disoriented when placed in warm water beyond their depth. No evidence of vestibular function was obtained in cat fetuses less than iio mm. in lengtlr Neither rotation of the specimens nor destruction of one or both labyrinths experimentally had any efkect before this time. It was concluded that the vestibular righting reflex appears about the 54th day of prenata1 life in the cat and that a body righting reklex precedes it by at least four days. The latter is activated by akferent impulses from the skin and deep tissues of the body and necl(. Visual impulses play no part in righting reactions until Some time after birth of the kitten whose eyes remain closed for several days. ln the newborn kitten the vestibular righting reflex is still incompletely developed.22 Development of a body righting reaction before the vestibular mechanism begins to function has been conHrmed in other species. In the sheep, which is more mature than the cat at birth and has a longer gestation period, righting is accomplished relatively earlier.3 Orientation in respect to gravity, seen in the opossum at the time of birth, which is long before the vestibular mechanism is functional, is an interesting related phenomenon.29
Fig. 64. Decerebrate rigidity in a young rabbiL The midbrain was sectioned through the rostral border of the superior colliculus and rostral third of the pons.
In all animals in which fetal studies have been made, three components essential for locomotion—righting, postural tonus and alternate synchronous limb movements—have been developed by the time of birth. All three are present in some form well before the end of the gestation in most species, thus providing a factor of safety against the danger of premature interruption of intrauterine life.
Development Of Eye Reflexes
We have considered the prenatal development of motor mechanisms for respiration, feeding, posture and locomotion in some detail. 0ther equally interesting fetal activities could be studied profttably, but at the present time little is known about them. some observations on movements of the eyes and eyelids have been made but they are incomplete.
The eyeballs move behind closed lids in the fetuses of several species. such movements can be seen at the middle of the gestation period in guinea pigss when the face is stimulated in the neighborhood of the eyes. somewhat later, postural changes elicit eye movements. Reactions to light appear during the third quarter of prenatal life. In the cat, eye movements in response to vestibular stimulation were not obtained before birth.« Most l(ittens 5 to 7 days old showed ocular nystagmus during and after stimulation of the labyrinthine afferents by rotation.
Contraction of the orbicularis oculi muscle can be elicited in cat and guinea pig fetuses at about the middle of the gestation period. The palpebral reflexes for protection of the eye have their genesis in these movements and are well developed in late fetal life, as can be determined by opening the eye experimentally and stimulating the cornea. Contraction of the human orbicularis oculi was seen at about 12 weelcs (4o mm.) when the eye region was touched.
Very little is known about the early development of light reflexes. The iris of guinea pig fetuses contracts in response to light early in the last third of prenatal life. The visual mechanism of this animal is more advanced than that of the cat or man at birth.
Development Of Palmar And Plantar Reflexes
Several investigators 7,25, 26 have studied the movements of the digits of the hand which ente·r into the prehension reaction of the fetus. These constitute the palmar or grasp reflex. HoolcerV observed Hexion of the iingers but not the thumb when the palm of a human fetus of 11 weelcs was touched, and it may occur even earlier. At 12 weelcs the. fetus formed a true list by flexing the thumb and fingers. Later, around 16 weelcs, when the iingers were held in a flexed posture stimulation of the palm brought about tightening of the iingerss Thumb movements were never as marked as those of the iingers and apposition of the thumb did not occur until after birth. Eikective sustained gripping of objects with the lingers began to manifest itself around 18 weelcs but even at 25 weelcs it was not strong. The grasp reflex appears to have two componentst iinger closure and gripping.
The genesis of the human plantar reflex has interested many because of its practical importance in neurologic diagnosis. Although several have studied it in prenatal life,25-29 Minlcows slci’s7s S« 30 observations have been the most complete. spontaneous dorsal ilexion of the great toe characterized the early fetal period. No responses to stimulating the sole of the foot were obtained before Io weelcsz at this time one specimen exhibited a plantar flexion of the foot right after delivery. This was of very brief duration and when it was no longer elicitable, due to progressive asphyxiation of the specimen, only direct muscle responses could be obtained.
In fetuses of 11 to 15 weeks gestation, stimulating the sole of the foot sometimes produced dorsal flexion of the foot or of the great toe with spreading of the others. These responses, which constitute the Babinski phenornenon, were obtainable only when local anesthesia had been used and in the first few minutes of the observations. General anesthesia and progkessive asphyxia led to an inversion of the response, in which the same type of stimulus brought about plantar instead of dorsal Hexion. section of the cervica1 spinal cord or brain stem did not alter the plantar re— flexesz they could be observed for longer periods before they changed from the dorsal to the ventral type of response. There was only slight indication that higher centers participated in the dorsal form of plantar reflext before— 6 months gestation.
Minlcowski 8 named the earIy period up to i6o mm. C. H. length the neuromuscular stagez the period between 160 and i8o mm. he called the spinal stagez the period between 190 and 270 mm., the tegmento-spinal stagez and the remainder up to birth, the pal1ido—cerebello-tegmento-spinal stage. These divisions were set somewhat arbitrarily, for the number of observations was limited and the physiologic conditions of specimens varied to a great extent. The phenomenon of inversion of the response has been discussed in chapter XI. The subject of plantar responses of infants and young children has been thoroughly reviewed recemiy by Richakds and Irwin.31
Throughout the literature, mention has been made of many other reactions and reflexes not only in experimental animals but also in human fetusesF For the most part, the course of development of the slcin, tendon, deep neclc and other reflexes has not been followed completelsy and these activities do not Iend themselves to signiücant discussion for this reason. Here are Helds which future investigations may be expected to explore very proütably.
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27. Krabbe, K. «1912. Rest. Neuro1.. 24: 434.
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29. Hooker, D. 1939. Atlas ok Early Human Fetal Behavior (Pkivate1y Printed).
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31. Richards T. W. Z: O. c. Irwin. 1935. Unisn Iowa stud. child Welkare, u (No. 1): I.
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