The newborn human infant is an amazingly complex, yet vulnerable, creature. It has the potential to become the next Einstein or Newton, but at the same time, it isn't even very good at breathing and eating yet and can not so much as hold its head up. Even so, within the three years, the human child can walk and talk and do basic math with more proficiency than our most closely related cousins in the animal kingdom. Clearly, much change must take place between these two states, and most of that change originates in the brain.
The neonatal brain grows amazingly fast. The amazing pace of growth generally starts before birth, in the last month of gestation, but it increases after birth. Most of the growth, however, is not the creation of new neurons as one might expect in the brain. Most of the neurons an adult has are present at birth. Instead, the growth is primarily in the form of dendritic and axonal projections and support cells (glial) for the brain. By support cells I mean glial cells such as astrocytes and oligodendrocytes which help the neurons function by insulating their electrical circuitry.
Several of the senses begin to function before birth. As was mentioned earlier, the vision and hearing systems are somewhat functional in the third trimester. Hearing is more completely functional than vision, in that the eye responds to light by closing the pupil. Infants have to develop control the fine muscles of the eye that contract the pupil according to the amount of light entering the eye as well as the muscles which control the curvature of the eye allowing the eye to focus light onto the retina and the muscled which control movement of the eye within the eye socket.
In the third trimester, fetus will instinctively turn towards light, indicating that the brain registers the difference between light and dark. At birth, infants are drawn to faces, particularly faces with eyes. They do not, however, seem to be able to distinguish details of individual faces. This is likely because the eye muscles required for minute focus are not fully developed yet. Additionally, the brain structures for recognizing details in patterns require experience to develop.
The third trimester also ushers in the ability to hear. Because the brain develops gradually, stimuli have varying ability to produce reliable brain responses. Hearing is a good example. By about 26 weeks, a fetus will respond to loud noises some of the time. By 28 weeks, fetal response to sound is more reliable. By 33 weeks, auditory stimuli produce responses in the auditory cortex. In part this may be explained by the fact that much of the brainstem receiving the immediate stimulus only begins to form connections to the cortex at about 26 weeks. As more connections are made, more of the stimulus will be able to be passed on to the cortex. At birth an infant is able to recognize speech patterns. The infant brain reacts differently to human speech, any human speech, whether it is their Mother's language or not, differently than it reacts to other noises. By about 12 months, an infant will react differently to its native language than to a non-native language.
The sense of smell is not possible to measure in utero because the fetus is surrounded by amniotic fluid. However, tests done on preterm infants indicate that there is some response to olfactory stimulation from about 29 weeks. There may be olfactory response before that because the nose opens at about 20 weeks, but preterm infants that are that young generally do not survive out of the uterus. Most infants respond favorably to smells that are familiar, like amniotic fluid and their mother, and to food, like colostrum. This phenomenon explains why infants are soothed by having something nearby, a blanket or piece of clothing that smells like their mother. Infants may also use the sense of smell to find the nipple.
Surprisingly, a prenatal response to pain can actually be elicited. Many years ago studies were performed on therapeutically and spontaneously aborted fetuses. By about 19 weeks fetus withdraw from painful stimuli such as poking. The responses described may actually be spinal reflexes rather than responses elicited from the brain. It is possible that fetal response to pain is limited, however, because of their exposure to several anesthetics produced by the placenta. (Lagercrantz et al, 2009, pg 258) Furthermore, the fetus probably does not experience pain in the way that we do because the somatosensory pathways in its brain are not fully connected until the third trimester. In preterm infants as young as 25 weeks, a response to pain is noted in the somatosensory cortex when blood is drawn. Infants, however, respond to sensations other than simply pain. They are certainly soothed by being held and rocked, which may have more to do with the motion or proximity of a heartbeat. But, they are frequently soothed simply by being touched as well. They respond with crying to cold and to heat.
The tongue develops from two different types of tissue. The outer part of the tongue develops from ectoderm tissue in the pharynx. The internal parts of the tongue develop from muscle precursor tissue cells called myocytes. The derivation from the two different tissue types explains why the tongue, unlike so many other sensory organs in the head, requires two cranial nerves rather than just one. The tongue forms beginning in the 4
week, so presumably as soon as the nerves connecting the ectodermal tissue to the brain are functions, the tongue is able to taste. (Larsen, 1997, pg 349)