Human development requires many years to reach completion. Life begins with the union of two minute cells (sperm and ovum), which then quickly multiply and differentiate. In one respect, the neonate is reasonably well-equipped biologically for survival outside the uterus.
Basic body functions such as circulation, respiration, digestion, elimination and temperature regulation are well developed; in other respects, the neonate is quite immature. Sensory perception is not fully developed and it takes a long time for him to acquire mobility and manipulative skills and to learn how to utilize all his potential abilities.
Flexion dominates the neonate’s postures. When he is picked up and laid in a prone position, the head turns sideways as a result of a protective reflex which prevents suffocation. Moreover, the arms are flexed and the legs are flexed and drown up under the abdomen so that the pelvis is raised. Some movements of the limbs and trunk occur in this position but they occur more freely in supine position because the limbs are less restricted. There is very little head control at this stage and the head flops downward when the baby is lifted into ventral suspension or into a sitting posture.
The neonate shows awareness of both auditory and visual stimuli by quieting if the stimuli are tolerable or by blinking, grimacing, startling and crying if they are stronger. The neonate exhibits a number of reflexes, on which a relatively minor stimulus is sufficient to trigger off an almost instantaneous stereotype response. These primary reflexes demonstrate the activity of the neonate’s central nervous system and they provide the basis for later motor development.
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The nervous system of a newly born baby is well developed and capable of complex activities. This is shown by the facts that neurological control of such a vital function as breathing is well established and that there are many reflex responses, which can be obtained by the appropriate stimulation. This is not the final state because further development of the nervous system occurs rapidly during the early years.
“A reflex action” consists of a prompt, stereotype and often very considerable response to a specific and often quiet minor stimulus. There is no opportunity for variation or choice of action. As the nervous system develops, many of the early reflexes are inhibited and others are modified and incorporated into more complex actions. These changes are necessary because continued activity of the early reflexes would interfere with the development of other actions.
The pattern of reflexes reflects the phylogenetic evolution of the nervous system. Many of the reflexes are evident in other species such as the trunk-righting reflexes, which are also shown by quadrupeds. Some reflexes are considered to be the persisting remnants of vitally important ones in the past. For example, the grasp reflex would serve an essential survival function, when it was necessary for the animal infant to be able to cling to the mother's fur. Others are unique to man, especially those concerned with the attainment and maintenance of the upright posture.
Clinical and developmental significance of reflexes:
Many of the previously described reflexes are of only passing interest and curiosity. Others are more important because they provide useful diagnostic signs as they influence the child’s development; so they can be used therapeutically.
Those reflexes, which are of diagnostic value, indicate an abnormality by their weakness, absence, excessive strength or persistence
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to an inappropriate age. For example, the “ Moro reflex” is particularly a useful diagnostic reflex, as it is present so consistently and is so easily elicited in the newborn period that any variation from the fully normal response by this time is a reliable indication of probable abnormality.
Moreover, its persistence after 6 months of age indicates abnormality.
The most interesting reflexes are those which influence development. For example, the “asymmetrical tonic neck reflex” is normally most evident at 2-3 months of age, when it appears to prepare for the integration of head turning, visual fixation and reaching; and so is probably fundamental to the establishment of visually-directed reaching and eye-hand co-ordination. This reflex affects development favorably.
An example of a reflex which impedes development as a result of its abnormal persistence is seen with respect to the “grasp reflex” .
Manipulative skill is one of man’s great fundamentals which require individual finger action and release but these become possible only as the grasp reflex wanes, so its persistence delays the acquisition of such skill.
Some reflexes are utilized in therapeutic programs for children with cerebral palsy. For example, tactile stimulation in certain areas is used to promote reflex muscle contractions and movement.
Qualifying definitions:
The simplest reflex consists of a prompt but brief stereotype response to an appropriate stimulus. Many of the reflexes elicited in clinical examination are of this nature; others are more complex. Some show a persistence of responses as long as the stimulus lasts; this is called a “tonic effect” . Some reflexes, especially those concerned with posture and movement, trigger off a series of chain responses, which may lead to some form of effective action.
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Changes with age:
The pattern of reflex activity changes with age. Some activity can be seen quite early in fetal life. Thereafter, it increases rapidly both in intensity and complexity. The study of the appearance and evolution of reflex activity from fetal life and infancy onwards is a magic exercise, which provides much insight into the elaboration of the nervous system.
The reflexes fade after serving their purposes or as being superseded by others but they seldom disappear entirely. Normally they persist throughout life without our being aware of them. For example, the continued maintenance of our posture and movement against gravity is possible only as a result of the continued action of reflex postural mechanisms; and we only become aware of this when control is lost.
Reflexes sometimes reappear following cerebral catastrophe in later life.
Production by several stimuli:
In a pure reflex, a specific stimulus is followed by predictable response. The afferent stimulus may be auditory, visual, tactile (touch, pressure and pain), labyrinthine or kinesthetic. Sometimes different stimuli give rise to similar responses. This phenomenon is valuable biologically because it ensures that if one method of elicitation fails, others will still produce the appropriate responses. For example, head righting upon the body may occur as the result of the kinesthetic stimulation from the neck, optic and labyrinthine stimulation. When several different stimuli can produce a particular reflex response, their relative importance varies and each may predominate at different stages of development.
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Terminology confusion:
Unnecessary confusion is produced when the same response is described as different reflexes according to the way it produced. For example, pressure upon the sole of an infant’s foot causes the leg to extend. The essential stimulus comes from the sole of the foot and the response consists of extension of the leg. When this reflex is elicited by holding the infant upright, allowing the sole of the foot to contact a flat surface, the response is a useful anti-gravity supporting reaction. The reflex can also be demonstrated with the child lying down. When the sole of the foot is touched by the examiner’s hand and this hand is withdrawn, the leg extends as if following the stimulating hand (magnet reflex).
These are not different reflexes; the magnet reflex is basically identical with the supporting reaction.
Conclusion:
“Primitive reflexes” appear from the time of fetal development until the time of birth. By the age of 6 months, most primitive reflexes, except those that remain unchanged and persist throughout life are integrated into more complex movements. Once primitive reflexes are integrated, they do not reappear except in response to extreme stress or effort or to CNS damage.
“Genetic subprograms” contain non-learned automatic stereotyped pre-determined responses. Once these genetic subprograms are triggered, they generate a cascade of coordination actions to produce their motor patterns. An activated genetic subprogram can sequentially stabilize the limb joints in time frames, far less than required for reflex arc activity.
These behaviors are the early foundation of the posture-movement dynamic.
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“Integration”
is an organizational process that the brain stem and higher levels can apply for modulation and assimilation of primitive reflexes to attain coordinated motor control. With sensory feedback and reference copy feedback, appropriate facilitation or inhibition can modify motor commands and eventually produce more complex motor responses that supersede the primitive reflexes and genetic subprograms. Integration of primitive reflexes and genetic subprograms usually occurs between 1 and 12 months of age.
“Postural reactions”
are critical to the posture-movement dynamic. Temporary postural reactions appear from birth through approximately the first 21 months of childhood.
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