Texturing the brain: secondary and tertiary sulci
As the larger fissures finish forming, shallower grooves begin to form within the major lobes. These shallower groves will form secondary and later tertiary sulci. These valleys between the ridges of the brain provide further differentiation of the cerebral tissue and allow for further specialization of function within the more generalized lobes of the brain. These basic structural features of the brain are fully complete by the time an individual is born.
The major structures of the brain, like the sulci separating the major lobes of the brain and most of the secondary sulci, do not vary between individuals, the more detailed, tertiary sulci, do vary between individuals. (Larsen, 1997, ch. 13) This difference between individuals may account in part for the varied abilities we see in our students.
Neuronal organization and myelination
With approximately 100 billion neurons, a baby's brain would get very messy, and probably become dysfunctional without organization, so shortly before birth the neurons are organized into various groups. Each group of neurons performs a different set of tasks. Myelination effectively insulates neurons and improves their functionality. Neurons become significantly more functional as they are myelinated, so the pattern of myelination mirrors the functionality of various parts of the nervous system during development. This section will include a description of a typical neuron, a review of the brain's organization, and a description of the development of the functional aspects of the nervous system.
Myelination starts with the parts of the brain which appear to be most necessary for life. The life support systems like breathing, heart beat regulation, swallowing and digestion, temperature regulation, and the olfactory, vision and hearing reflexes. The control circuitry for most of these life support systems are all located in the brain stem and are all fairly close to each other. As discussed earlier myelination makes conduction along the axons more efficient, so by myelinating the neurons involved in life support systems early, the infant has a better chance of survival. However, even with the amount of myelination that occurs in a healthy full-term infant, most of these systems are not fully functional at birth. This is evident when one considers an infants inability to regulate it body temperature and the trouble some infants have in swallowing and digesting sustenance.
It is interesting to note that while much of the brain, including most of the cortex is not very functional at birth, two systems, vision and hearing, actually appear to begin functioning before birth. There is some evidence that the fetal brain reacts to light shining through the uterus. At birth, the infant brain immediately begins to react to face-like patterns event before it can distinguish the details of the face. There is also evidence that the fetal brain reacts to sounds beginning at about 26 weeks. The infant brain reacts to speech patterns from birth and appears to recognize speech patterns regardless of the language spoken and regardless of whether the language is the one heard during gestation. (Nelson et al, 2006, Ch. 4) These two brain functions seem to be a bit of an extravagance given that the neonatal brain isn't very good at things like breathing and swallowing, but they make sense in the context of the importance of socialization, speech and facial expression, in human culture.