We sometimes wonder whether the fetus is aware of its environment and consciously responds to it, or whether it is basically a creature of automatic reflexes. Perhaps the best way to decide is to study the development of its nervous system. The drawing on the next page of the developing human brain shows clearly the tremendous growth and fissuring of the cerebrum
The human embryo’ s brain starts to develop extremely early, at about three to four weeks, immediately after the heart begins to beat. It then grows so fast that by the sixth fetal month all the 100 billion or so neurons that are present in our adult brains have formed already. It has been calculated that to make this many neurons the unborn baby would have to produce 250,000 neurons every minute of fetal life (Cowan, 1979).
(Figure available in print form)
Early pregnancy is obviously a crucial period in the development of our brains since only then are our bodies making neurons. This is the worst possible time for the mother to contract rubella, herpes or other viral diseases which are transmitted across the placenta. All of these infections as well as excessive radiation will kill neurons, leaving the individual with a reduced number and possible retardation.
The six month fetus has as many neurons as an adult but the development of the brain is by no means complete yet. There are relatively few pathways in the fetal brain because the neurons have not branched to form many synapses; each neuron when it is fully mature may have up to 10,000 connections. Also, special brain cells called neuroglia cells which appear at the same time as the first neurons, keep multiplying long after the neurons stop and eventually form about half of the brain weight. These cells are important because they help feed the neurons and also coat them with a fatty insulating layer called myelin. Myelinated neurons can carry nerve impulses much faster and with much less expenditure of energy than unmyelinated cells (Morell and Norton, 1980). Myelination is often used as the main criterion in deciding whether a certain part of the nervous system is fully functional or not.
Maturation in the central nervous system proceeds from the tail to the head end, in contrast to most other systems. First the spinal cord begins to exercise control to allow some basic reflex actions to occur. The very earliest reflex that has been observed is the avoidance movement of the head of a seven week embryo when its upper lip was stroked. (This is also when the first muscle cells begin to contract, Crelin, 1973).
The medulla which controls swallowing, coughing, breathing and other vital functions, becomes myelinated at about five months. A fetus as young as eleven weeks has been observed to swallow if its lips were touched but the movements were probably not well coordinated. A thirteen week fetus has irregular chest contractions but real breathing movements do not start until. after five months. All these reflexes are controlled by the medulla.
There would seem to be no need for the fetus to suck its thumb in utero, flex its limbs, hiccup, wrinkle its face and swallow but it is of course practicing for its future life outside the womb. Muscles and organs that do not get used tend to atrophy.
The cerebral hemispheres are still not well myelinated at birth so that the newborn is in many ways a reflex creature like the fetus. Many of these reflexes which are controlled by the spinal cord or midbrain may now have great survival value. For instance, the rooting reflex which causes a baby to turn its head towards a touch on the cheek and at the same time to open its mouth allows the newborn to find its mother’s breast and nipple. The importance of the sucking reflex is obvious: when anything is put in the newborn’s mouth, be it the pediatrician’s finger or the mother’s nipple, it starts sucking vigorously. Swallowing goes along with sucking but often, premature babies are unable to coordinate these two activities.
There is a very common startle reflex among new babies called the Moro response which has no obvious survival value but which may be a holdover from our ape ancestry according to Brazelton (1969). The baby suddenly throws out its arms, pulls up its legs, cries and then brings its arms in again. It does this continually if upset. Brazelton speculates that the baby may feel that it is falling so tries desperately to grab on to something (like its mother of a tree branch, to carry on with the ape analogy).
Newborns can also “walk” if they are held with their feet on a flat surface and can “crawl” if placed on their stomachs with pressure against their feet. All these reflexes persist in the first few months after birth and then usually disappear as learned responses become more important for the baby. In fact, if reflexes, like the Moro response , persist it may be a sign that something is wrong neurologically.
The neonate quickly starts to respond to its new environment in a more conscious fashion in addition to its automatic reactions. Right after birth there is usually a period of wakefulness for the neonate, lasting up to an hour. The baby is very alert at this time despite its recent trauma and may interact with its new mother in an intense fashion. They gaze into each other’s eyes and the mother usually touches her baby all over and talks to it. This process is called bonding.
There are several other things that the newborn is capable of that are probably controlled by its cerebrum. For instance, in the delivery room it can turn its head to follow slowly moving brightly colored objects; although it soon prefers the human face, especially its mother’s, as an object of contemplation. Interestingly enough, the newborn focuses best at about twelve inches, which is the distance a nursing mother holds it from her face. Very soon the new baby produces an occasional smile, completing its conquest of its mother,
The more these responses, controlled by the cerebral cortex, are repeated the more associations are developed in the brain, In former times when unwanted babies were often institutionalized, the lack of stimulation in the wards caused these children to become apathetic and sometimes retarded;their brains simply had no reason to develop.
We humans are unique because we have the largest brains relative to body size of any species, but despite this, when we are born we are almost totally helpless, unlike the newborn of deer or horses which can run after their mothers in a few hours.
The newborn human brain weighs about 350 grams, ten percent of its body weight, so that the head is a very large part of the body. In fact, the circumference of the skull is one centimeter larger than the mother’s fully open cervix, but because there are several separate skull bones that can overlap slightly at their edges, and soft spots called fontanelles, molding of the skull takes place during birth without any danger to the brain.
The newborn’s brain is large but the growth that occurs in the first year is truly phenomenal-a threefold increase from 360 grams to 1000 grams. If this increase had occurred inside the womb before birth, the human race would have been extinct long ago: