出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/09/14 14:40:44」(JST)
Primitive reflexes are reflex actions originating in the central nervous system that are exhibited by normal infants, but not neurologically intact adults, in response to particular stimuli. These reflexes are absent due to the development of the frontal lobes as a child transitions normally into child development.[1] These primitive reflexes are also called infantile, infant or newborn reflexes.
Older children and adults with atypical neurology (e.g., people with cerebral palsy) may retain these reflexes and primitive reflexes may reappear in adults. Reappearance may be attributed to certain neurological conditions including, but not limited to, dementia (especially in a rare set of diseases called frontotemporal degenerations), traumatic lesions, and strokes.[2][3] An individual with cerebral palsy and typical intelligence can learn to suppress these reflexes, but the reflex might resurface under certain conditions (i.e., during extreme startle reaction). Reflexes may also be limited to those areas affected by the atypical neurology, (i.e., individuals with cerebral palsy that only affects their legs retaining the Babinski reflex but having normal speech); for those individuals with hemiplegia, the reflex may be seen in the foot on the affected side only.
Primitive reflexes are primarily tested with suspected brain injury for the purpose of assessing frontal lobe functioning. If they are not being suppressed properly they are called frontal release signs. Atypical primitive reflexes are also being researched as potential early warning signs of autistic spectrum disorders.[4]
Primitive reflexes are mediated by extrapyramidal functions, many of which are already present at birth. They are lost as the pyramidal tracts gain functionality with progressive myelination. They may reappear in adults or children with loss of function of the pyramidal system due to a variety of reasons. However with the advent of Amiel Tison method of neurological assessment, the importance of assessment of such reflexes in paediatric population has come down.[5][6][7]
Reflexes vary in utility. Some reflexes hold a survival value, (i.e., the rooting reflex, which helps a breastfed infant find the mother's nipple). Babies display the rooting reflex only when they are hungry and touched by another person, not when they touch themselves. There are a few reflexes that likely assisted in the survival of babies during human evolutionary past (i.e., the Moro reflex). Other reflexes such as sucking and grabbing help establish gratifying interaction between parents and infants. They can encourage a parent to respond with love and affection, and to feed their child more competently. In addition, it helps parents to comfort their infant while allowing the baby to control distress and the amount of stimulation they receive.[8]
This is sometimes referred to as the startle reaction, startle response, startle reflex or embrace reflex. It is more commonly known as the Moro response or Moro reflex after its discoverer, pediatrician Ernst Moro. The Moro reflex is present at birth, peaks in the first month of life, and begins to disappear around 2 months of age. It is likely to occur if the infant's head suddenly shifts position, the temperature changes abruptly, or they are startled by a sudden noise. The legs and head extend while the arms jerk up and out with the palms up and thumbs flexed. Shortly afterward the arms are brought together and the hands clench into fists, and the infant cries loudly.[9] The reflex normally disappears by three to four months of age,[10] though it may last up to six months.[11] Bilateral absence of the reflex may be linked to damage to the infant's central nervous system, while a unilateral absence could mean an injury due to birth trauma (e.g., a fractured clavicle or injury to the brachial plexus). Erb's palsy or some other form of paralysis is also sometimes present in such cases.[10] In human evolutionary history, the Moro reflex may have helped infants cling to the mother while being carried around. If the infant lost its balance, the reflex caused the infant to embrace its mother and regain its hold on the mother's body.[8]
The walking or stepping reflex is present at birth, though infants this young cannot support their own weight. When the soles of their feet touch a flat surface they will attempt to walk by placing one foot in front of the other. This reflex disappears at six weeks due to an increased ratio of leg weight to strength.[12] It reappears as a voluntary behavior around eight months to one year old.
The rooting reflex is present at birth and disappears around four months of age, as it gradually comes under voluntary control. The rooting reflex assists in the act of breastfeeding. A newborn infant will turn its head toward anything that strokes its cheek or mouth, searching for the object by moving its head in steadily decreasing arcs until the object is found. After becoming used to responding in this way, (if breastfed, approximately three weeks after birth), the infant will move directly to the object without searching.[13]
The sucking reflex is common to all mammals and is present at birth. It is linked with the rooting reflex and breastfeeding. It causes the child to instinctively suck anything that touches the roof of their mouth, and simulates the way a child naturally eats. There are two stages of the action:
The tonic neck reflex, also known as asymmetric tonic neck reflex or 'fencing posture' is present at one month of age and disappears at around four months. When the child's head is turned to the side, the arm on that side will straighten and the opposite arm will bend (sometimes the motion will be very subtle or slight). If the infant is unable to move out of this position or the reflex continues to be triggered past six months of age, the child may have a disorder of the upper motor neurons. According to researchers, the tonic neck reflex is a precursor to the hand/eye coordination of the infant. It also prepares the infant for voluntary reaching.[8]
The palmar grasp reflex appears at birth and persists until five or six months of age. When an object is placed in the infant's hand and strokes their palm, the fingers will close and they will grasp it with a palmar grasp. To best observe this reflex, on a bed where the child could safely fall onto a pillow, offer the infant two opposing little fingers (as index fingers are typically too large for the infant to grasp), and gradually lift. The grip is strong but unpredictable; though it may be able to support the child's weight, they may also release their grip suddenly and without warning. The reverse motion can be induced by stroking the back or side of the hand.
A plantar reflex is a normal reflex that involves plantar flexion of the foot, which moves toes away from the shin and curls them down. An abnormal plantar reflex (aka Babinski Sign) occurs when upper motor neuron control over the flexion reflex circuit is interrupted. This results in a dorsiflexion of the foot (foot angles towards the shin, big toe curls up). This also occurs in babies under ~1 year, because of low myelination of the corticospinal tracts. As these tracts develop to adult form, the flexion-reflex circuit is inhibited by the descending corticospinal inputs, and the normal plantar reflex develops.[14] Also known as the Babinski reflex, this is a sign of neurological abnormality in adults (e.g., upper motor neuron lesion).[15]
The Galant reflex, also known as Galant's infantile reflex, is present at birth and fades between the ages of four to six months. When the skin along the side of an infant's back is stroked, the infant will swing towards the side that was stroked. If the reflex persists past six months of age, it is a sign of pathology. The reflex is named after the Russian neurologist Johann Susman Galant.[16]
The swimming reflex involves placing an infant face down in a pool of water. The infant will begin to paddle and kick in a swimming motion. The reflex disappears between 4–6 months. Despite the infant displaying a normal response by paddling and kicking, placing them in water can be a very risky procedure. Infants can swallow a large amount of water while performing this task, therefore caregivers should proceed with caution. It is advisable to postpone swimming lessons for infants until they are at least three months old, because infants submerged in water can die from water intoxication.[8]
The Babkin reflex occurs in newborn babies, and describes varying responses to the application of pressure to both palms. Infants may display head flexion, head rotation, opening of the mouth, or a combination of these responses.[17] Smaller, premature infants are more susceptible to the reflex, with an observed occurrence in a child of 26 weeks gestation.[18] It is named after the Russian neurologist Boris Babkin.
This reflex occurs in slightly older infants when the child is held upright and the baby’s body is rotated quickly to face forward (as in falling). The baby will extend their arms forward as if to break a fall, even though this reflex appears long before the baby walks.
As mentioned in the introduction, when primitive reflexes are not being suppressed properly they are generally referred to as frontal release signs (although this may be a misnomer). In addition to the reflexes previously mentioned, they include the palmomental reflex, snout reflex, glabellar reflex or "tap" reflex.
The term high-risk newborns refers to neonates with a significant chance of mortality or morbidity, especially within the first month of being born. High-risk newborns will often show abnormal responses of primitive reflexes, or lack a response entirely. Performance of primitive reflexes in high-risk newborns will often vary in response depending on the reflex (i.e., normal Moro reflex may be present, while the walking reflex is absent or abnormal). Normal performance of primitive reflexes in newborns can be linked to a greater likelihood of having higher Apgar scores, higher birth weight, shorter hospitalization time after birth, and a better overall mental state.
A recent cross-sectional study assessing primitive reflexes in 67 high-risk newborns, used a sample method to evaluate responses of the sucking, Babinski and Moro reflexes. The results of the study showed that the sucking reflex was performed normally most often (63.5%), followed by the Babinski reflex (58.7%), and the Moro reflex (42.9%). The study concluded that high-risk newborns presented more periodic abnormal and absent responses of primitive reflexes, and that each reflex varied in response.[19]
However with the advent of simple and effective methods like Amiel Tison method of neurological assessment, as predictor of neurological sequele in high-risk neonates and infants, the importance of assessment of primitive reflexes is decreasing.[5][6][7]
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| リンク元 | 「歩行反射」 |
| 関連記事 | 「reflex」「walking」 |