STRUCTURES OF THE BRAIN

The cerebral cortex, also called the neocortex, is the outer covering of the brain. The brain has two hemispheres, and each hemisphere has four major lobes that can be further subdivided into many specialty areas. Further division also occurs, including cortical and subcortical structures, such as the hypothalamus. Figure 1-1 and Figure 1-2 present the basic structural areas of the brain.

FIGURE 1-2 Mapping of the brain, including thought, movement, and sensory functions.

The four lobes are frontal, occipital, parietal, and temporal.

At 7 months of gestation, convolutions begin to form in the outer layer of the brain to accommodate for the restricted space; this formation is one of the factors responsible for the eventual power of the brain. These convolutions allow the cortex to increase its surface area without an increase in skull size. Gyri—bumps, bulges, or convolutions—and sulci, shallow grooves or fissures between convolutions, form anatomical landmarks that can be identified on MRI scans, which helps demarcate areas of the cortex associated with different functions (M. Martone, personal communication, July 15, 2006).

The motor cortex is located at the junction of the frontal and parietal lobes, below the area where a set of headphones might sit across the top of the head. The motor cortex plays a critical role in controlling muscle movements, as do the cerebellum and basal ganglia, a compilation of associated subcortical cell groups in the forebrain. All movement areas of the body have a corresponding area in the motor cortex. The right side controls muscles on the left side of the body, and the left side controls muscles on the right side.

The premotor cortex is dedicated to initiation and sequencing of movements and is part of the frontal lobe.

The somatosensory cortex, a strip across the top of each hemisphere, is adjacent and posterior to the motor cortex and is associated with the processing of incoming sensory stimulation: pain sensation, touch, pressure, temperature, and proprioception (body position in space).

The angular gyrus is located on the margins of the temporal, parietal, and occipital lobes, up and behind Wernicke’s area. This area is associated with vision, spatial skills, and language and is believed to act as a bridge between the language process and visual word recognition(meaning).

The brain is basically hollow. Fluid-filled canals and caverns inside the brain make up the ventricular system (VS). The fluid is called cerebrospinal fluid (CSF), and it fills the lateral, third, and fourth ventricles and the cerebral aqueduct. This is of importance to the nurse because of the incidence of hydrocephalus in children caused by a blockage in the ventricular system.

During the first 7 months of gestation, neurons generate at an awesome speed and migrate to designated places; approximately 50% are pruned away before birth. This is a normal process called apoptosis, or programmed cell death (Diamond and Hopson, 1998; Bear, Connors, and Paradiso, 2007). This usually is thought to contribute to the efficiency of the brain. Unfinished apoptosis may contribute to the dramatic abilities of autistic savants and may also be one factor affecting their other deficits (Carter, 1999). Conversely, overpruning may lead to the cognitive impairment in a child with Down syndrome. This pruning of cells and connections occurs from 7 months gestation and continues throughout childhood and adolescence (Diamond and Hopson, 1998; Huttenlocher and Dabholkar, 1997).

Language areas for general processing are located in the left hemisphere in approximately 96% of the population. There are two major language areas:

OTHER AREAS OF THE BRAIN

The thalamus lies beneath the cortex and acts as a relay station that transmits incoming sensory signals, such as pain, to the appropriate part of the cortex for further processing. All sensory systems, except the sense of smell, send signals to the thalamus.

The cortex controls the hypothalamus, which is a subcortical brain structure that lies below the thalamus. In turn, the hypothalamus controls the pituitary gland, which is often referred to as the head gland, because it controls the other glands of the body. The hypothalamus, together with the pituitary gland, is involved in activities vital to survival, such as the maintenance of blood glucose levels, body temperature, body rhythms, heart rate, thirst, activity and rest, sexual desire, and reproductive and menstrual cycles. The hypothalamus works to maintain homeostasis, the “set point” in the body, by regulating the balance between the sympathetic and parasympathetic branches of the autonomic nervous system.

The hippocampus, a structure shaped like a ram’s horn, is located deep within the temporal lobe near the amygdala and hypothalamus. It is associated with conscious long-term memory, spatial ability, learning, and motivation. Damage to the hippocampus causes severe memory problems.

The amygdala consists of two almond-shaped structures located at the anterior end of each hippocampus. They coordinate autonomic and endocrine responses with states of emotions, such as anger or fear, and emotional behavior, and they are involved in the regulation and storage of emotional memory. The nucleus accumbens is a small, hook-like structure linked to pleasure, cravings, and reward.

The ventral tegmental area (VTA), located at the front of the brainstem, relays messages of pleasure from their nerve cells to nerve cells in the nucleus accumbens through neurotransmitters, chemicals involved in transmission of nerve impulses between synapses, such as dopamine. This pleasure circuit also includes the prefrontal cortex and is called the mesolimbic dopamine system, which is associated with acts of pleasure—such as eating and sexual activity—and addiction.

To understand the brain, scientists delineate the parts by shape and function and name the structures and areas; however, the purpose of particular structures of the brain, or which areas are involved in any given brain activity, are not always clearly understood.

OTHER FETAL DEVELOPMENT

Until approximately 6 weeks, the embryo is basically gender neutral. Genetic predetermination and the absence or presence of male hormones begins the sexual design for development. At 6 weeks, a male embryo begins to convert gonadal lumps into testes that produce testosterone; at 8 weeks, external genitalia are present; and at 12 weeks, a female embryo begins to develop ovaries. At 25 weeks, the fetus weighs approximately 1 pound and responds to the maternal voice and to light and smells, which suggests that learning occurs in utero.

Human deoxyribonucleic acid (DNA) contains approximately 30,000 to 40,000 genes that determine characteristic development—such as eye color, hair color, and body size—and they predispose an individual to certain diseases, disorders, or traits, such as temperament. Genetic endowment, along with the environment, plays a decisive role in development. Both nature and nurture are required for the neurodevelopment of the mature, efficient human brain.

The intrauterine environment can influence development in a positive or negative way. Factors such as early and significant exposure to alcohol, nicotine, cocaine, or poor nutrition and stress all play a role in fetal development.

Newborn: Birth to 1 Month

Brain Findings

Basic connectivity of the newborn brain has been established, although it does not function the same as an adult brain. After birth, the infant demonstrates utilization of networks of neurons by moving arms and legs, opening and closing eyes, and by crying and breathing. The brain’s early developmental task is to continue forming and reinforcing some connections and pruning others. This process is a function of genetic predisposition and experience with the outside environment. If experience does not occur during a critical period, synaptic connections do not develop, and the potential ability is gone. For example, cataracts impair visual acuity, and if they are not detected and surgically removed by 2 years of age, visual acuity does not develop, and the synapses in the visual cortex are pruned away and the child is forever blind.

A. Brain weight at birth is approximately 400 g.

B. At birth, the cortex is approximately 1 mm thick—about one half the thickness of an adult brain.

C. Synapses form in the newborn’s brain at the rate of approximately 3 billion per second at given times.

D. Each neuron can be connected to as many as 15,000 other neurons and, with some, many more thousands of synapses are made.

E. The brain uses about 20% oxygen; and in the cerebral cortex, from birth to about 4 years of age, glucose usage is twice that of an adult’s brain.

F. The brain’s capacity is not fixed at birth and is most plastic during infancy. The more complex areas of the brain, such as the cortex, are more plastic; the life sustaining areas of the brain, such as the brainstem, are less plastic.

G. The brain does not grow uniformly but has peaks that correspond to the learning of various skills.

H. The brain constitutes approximately 12% of body weight at birth and approximately 2% to 3% of body weight in adulthood.

I. Brain growth is reflected in the head circumference; growth charts (see Appendix B) indicate that brain growth occurs more rapidly in the first year of life than at any subsequent age. The brain reaches one half its eventual size by 12 months.

J. Neurons are still being produced postnatally, and the neurons are increasing in size and complexity (M. Martone, personal communication, July 15, 2006). A significant part of the increase in brain weight observed at this time can be attributed to the development of glial cells, primarily those that produce myelin.

K. The newborn’s head is approximately one fourth of the total body length (see Head and Body Proportions in Figure 1-3 on page 8).

FIGURE 1-3 Head and body proportions from prenatal development to 25 years.

INFANCY: 2 TO 12 MONTHS

TODDLER: 1 TO 3 YEARS

Brain Findings

The number of neurons remains relatively stable while neurons are undergoing vigorous growth and remodeling during the first 3 years. The brain’s neuroplasticity is reflected by change in response to experience. Prime times for growth occur in specific brain areas at different times. For example, optimal development of the brain systems responsible for attachment is dependent upon the positive interaction between caregiver and infant during the first year of life. Unhealthy early bonding of caregiver and infant can lead to a fragile, neurological, and emotional foundation for later development of relationships (Perry, 2002).

The brain can compensate or alter itself after insults dependent on timing, intensive intervention, and the nature of the insult. The plasticity of the young brain is more acute than the adult brain.

Animal research supports the fact that more synaptic growth occurs when living in enriched environments versus impoverished environments, which can lead to thinning of neuronal networks (Diamond and Hopson, 1998). Brain functions remain dynamic through practice and use, although current knowledge does not prove that more stimulation will necessarily create smarter children with denser brain systems (Nelson, 2000).

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