Axon: The part of the neuron that transmits messages to other neurons, muscles, or glands of the body.

Brain: An oblong organ with folds, furrows, and a fissure or groove down the center. This groove separates the brain into two hemispheres. A baby's brain triples its weight in the first few years of life and quadruples in size by the time a child becomes an adult. This gain is not due to addition of new brain cells (most of which form before birth), but rather, to the massive growth, branching, and increased connections among individual brain cells.

Brainstem: The lower part of the brain that includes the medulla, pons, midbrain, and cranial nerves. It directs our reflexes, vital signs, and automatic control of the eye, face, and head. It also helps us interpret basic sensations involving taste, hearing, vision, and balance.

Brain Imaging: This process involves taking pictures of the brain. The most recent technologies can take images of areas that are especially active while the brain is processing specific types of information, for example, sound or visual input. Three examples of the technology used in imaging are called Positron Emission Tomography (PET), functional Magnetic Resonance Imaging (FMRI), and computerized tomography (CAT scans). Analysis of the electroencephalogram (EEG) is another way to study brain activity.

Central Nervous System (CNS): Term used to refer to the brain and spinal cord together.

Cerebellum: This part of the brain helps to control our posture, balance, coordination, and rhythmic movements. It is also important in certain aspects of learning and memory.

Cerebral Cortex: The cortex is the "highest" region of the brain, both in location and function--that is, it controls the lower, older, more automatic parts of the nervous system. The prefrontal cortex, located behind the forehead, is linked with making decisions and judgments, as well as modulating emotional tone.

Cerebral Hemispheres: This term refers to two halves of the human brain. In most adults, the Left cerebral hemisphere controls voluntary movement of the right hand, arm, and leg; coordinates learned movement patterns (such as playing a piano or writing) for the whole body; enables us to express and comprehend language; processes abstract symbols for reading, writing, and mathematical computation; and allows us to see things that are to our right side. The Right cerebral hemisphere for most adults manages nonverbal processes such as paying attention, recognizing visual patterns (pictures) or sound patterns (tunes), and giving us our bearings in space. It is also important for hand-eye coordination and emotional processing. The two hemispheres of our brain are in constant communication with each other through a massive fiber bundle kown as the corpus callosum. The functions of the two hemispheres are less clearly differentiated in infants than in adults. There seems to be progressively greater lateralization (distinction between the abilities of the two hemispheres) with age, from birth to puberty. However, in most people, language processing is supported mainly by the left hemisphere even at birth.

Dendrites: These neuron branches are the major receptive surface of the neuron, receiving and processing signals from other neurons (brain cells).

Electroencephalogram (EEG): A measurement of the brain's electrical activity. The EEG is derived from sensors pasted in various spots on the scalp, which react to the activity of neurons in a particular region of the brain (e.g., beneath the forehead, temples, etc.).

Exuberant Period: The phase in which a child's number of synapses exceeds the number present in adulthood. Although babies are born with relatively few synapses in the cerebral cortex, the number surpasses adult levels in just the first year of life, remains elevated throughout most of childhood, and finally declines to adult levels by the onset of puberty. This decline is a result of synaptic pruning.

Hippocampus: This part of the brain is important in processing and storing long-term memories. Doctors link most cases of amnesia (the inability to remember new facts or events) to problems with the hippocampus.

Lobes: The four areas of the brain that are demarcated by major fissures (grooves in the surface of the brain). The occipital lobes, located in the back of the brain, are primarily responsible for vision. The frontal lobes are involved in movement, complex judgment, emotional regulation, problem solving, decisions, planning, and creativity. The parietal lobes, in the upper left and right sides of the brain, are involved in higher sensory-motor coordination and language functions. Temporal lobes located above and behind the ears on the left and right sides of the brain are involved in memory, hearing, language, and emotion.

Myelin: A butter-like insulation [greasy coating] around the nerves [axons]. Myelin enables nerves to quickly conduct electrical impulses from one brain area to another. Without myelin, the nerves work sluggishly and inefficiently.

Myelination: The process by which myelin is laid down on the nerves of the brain. At birth, most of the brain lacks myelin, which is an important reason why newborns are so comparatively helpless. As different parts of the brain myelinate, the function of that particular area (e.g., vision, movement, language) makes sharp gains.

Neural Plate: The earliest form of brain tissue, present in the developing embryo for just a few days in the third week after conception.

Neural Tube: Origin of a baby's entire brain and spinal cord, which forms during the fourth week after conception. First, the neural plate elongates into a neural groove. Then the groove zippers shut in two directions, beginning in the middle of the embryo and progressing both towards the head and towards the "tail" end of the embryo.

Neurons: Brain cells that store and send information. There are about 25 types of neurons in a human brain, but they all consist of a cell body (which produces energy and makes chemicals to sustain the neuron); dendrites (tree-like branches that receive signals from other neurons); and an axon (a very long, branching cable that communicates signals to other neurons).

Neurotransmitter: A chemical that carries signals between brain cells. Neurotransmitters are released from packets located at the end of one neuron (the presynaptic cell). They then diffuse across the synapse, where they bind to special molecules called receptors, located on the outer surface of the next neuron in the chain (the postsynaptic cell). This binding stimulates electrical activity in the postsynaptic cell, which in turn releases neurotransmitters from its synaptic terminals. This chain of electrical-chemical activity carries signals from one brain area to another, such as from sensory areas (eyes, ears, skin, etc.) to the brain, and from the brain to the muscles. Some familiar neurotransmitters are acetylcholine, serotonin, dopamine, and glutamate.

Plasticity: The brain's ability to change as a result of experience or injury. The term derives from the original meaning of "plastic" as "a moldable or pliable material." The human brain remains plastic throughout life (which is how we are able to learn new facts or skills at any age), but it is massively more so in early life. Scientists believe that harmful behaviors or neglect in early life can affect the brain, leading to lifelong problems. A healthy and caring environment, however, can create opportunities for the child to develop to his or her full potential.

Pruning: A process in brain development whereby unused synapses (connections among brain cells), are shed. Between about 2 and 10 years of age, a child's brain has about twice as many connections as an adult's brain. During the pruning phase, a child's experience and environment decide which synapses will be shed and which will be preserved.

Synapses: These are connections between neurons through which nerve impulses travel. In the womb and during the first years of life, many more synapses form than a baby will use as an adolescent and adult. In time, unused synapses wither and die off. Those that survive grow stronger from being stimulated. The largest number of synapses are present between about one and ten years of age. In later life, the rate of production of new synapses seems to be about equal to the rate at which synapses are pruned, or perhaps slightly slower (so that there is a net loss of synapses as we age). Those that remain presumably process information more efficiently. .