| nervous system | The body's electrochemical communication circuitry.
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| plasticity | The brain's special capacity for change.
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| afferent nerves | Also called sensory nerves; nerves that carry information about the external environment to the brain and spinal cord via sensory receptors.
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| efferent nerves | Also called motor nerves; nerves that carry information out of the brain and spinal cord to other areas of the body.
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| central nervous system (CNS) | The brain and spinal cord.
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| peripheral nervous system (PNS) | The network of nerves that connects the brain and spinal cord to other parts of the body.
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| somatic nervous system | The body system consisting of the sensory nerves, whose function is to convey information from the skin and muscles to the CNS about conditions such as pain and temperature, and the motor nerves, whose function is to tell muscles what to do.
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| autonomic nervous system | The body system that takes messages to and from the body's internal organs, monitoring such processes as breathing, heart rate, and digestion.
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| sympathetic nervous system | The part of the autonomic nervous system that arouses the body.
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| parasympathetic nervous system | The part of the autonomic nervous system that calms the body.
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| neurons | One of two types of cells in the nervous system; neurons are the nerve cells that handle the information-processing function.
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| glial cells | Also called glia; the second of two types of cells in the nervous system; glial cells provide support, nutritional benefits, and other functions and keep neurons running smoothly.
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| cell body | The part of the neuron that contains the nucleus, which directs the manufacture of substances that the neuron needs for growth and maintenance.
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| dendrites | Treelike fibers projecting from a neuron, which receive information and orient it toward the neuron's cell body.
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| axon | The part of the neuron that carries information away from the cell body toward other cells.
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| myelin sheath | A layer of fat cells that encases and insulates most axons.
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| resting potential | The stable, negative charge of an inactive neuron.
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| action potential | The brief wave of positive electrical charge that sweeps down the axon.
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| all-or-nothing principle | The principle that once the electrical impulse reaches a certain level of intensity (its threshold), it fires and moves all the way down the axon without losing any intensity.
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| synapses | Tiny spaces between neurons; the gaps between neurons are referred to as synaptic gaps.
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| neurotransmitters | Chemical substances that are stored in very tiny sacs within the terminal buttons and involved in transmitting information across a synaptic gap to the next neuron.
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| neural networks | Networks of nerve cells that integrate sensory input and motor output.
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| hindbrain | Located at the skull's rear, the lowest portion of the brain, consisting of the medulla, cerebellum, and pons.
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| brain stem | The stemlike brain area that includes much of the hindbrain (it does not include the cerebellum) and the midbrain; it connects with the spinal cord at its lower end and then extends upward to encase the reticular formation in the midbrain.
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| midbrain | Located between the hindbrain and forebrain, an area in which many nerve-fiber systems ascend and descend to connect the higher and lower portions of the brain; in particular, the midbrain relays information between the brain and the eyes and ears.
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| reticular formation | A system in the midbrain comprising a diff use collection of neurons involved in stereotyped patterns of behavior such as walking, sleeping, and turning to attend to a sudden noise.
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| forebrain | The brain's largest division and its most forward part.
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| limbic system | A loosely connected network of structures under the cerebral cortex, important in both memory and emotion. Its two principal structures are the amygdala and the hippocampus.
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| amygdala | An almond-shaped structure within the base of the temporal lobe that is involved in the discrimination of objects that are necessary for the organism's survival, such as appropriate food, mates, and social rivals.
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| hippocampus | The structure in the limbic system that has a special role in the storage of memories.
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| thalamus | The forebrain structure that sits at the top of the brain stem in the brain's central core and serves as an important relay station.
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| basal ganglia | Large neuron clusters located above the thalamus and under the cerebral cortex that work with the cerebellum and the cerebral cortex to control and coordinate voluntary movements.
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| hypothalamus | A small forebrain structure, located just below the thalamus, that monitors three pleasurable activities—eating, drinking, and sex—as well as emotion, stress, and reward.
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| cerebral cortex | Part of the forebrain, the outer layer of the brain, responsible for the most complex mental functions, such as thinking and planning.
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| neocortex | The outermost part of the cerebral cortex, making up 80 percent of the cortex in the human brain.
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| occipital lobes | Structures located at the back of the head that respond to visual stimuli.
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| temporal lobes | Structures in the cerebral cortex that are located just above the ears and are involved in hearing, language processing, and memory.
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| frontal lobes | The portion of the cerebral cortex behind the forehead, involved in personality, intelligence, and the control of voluntary muscles.
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| parietal lobes | Structures at the top and toward the rear of the head that are involved in registering spatial location, attention, and motor control.
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| somatosensory cortex | A region in the cerebral cortex that processes information about body sensations, located at the front of the parietal lobes.
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| motor cortex | A region in the cerebral cortex that processes information about voluntary movement, located just behind the frontal lobes.
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| association cortex | Sometimes called association areas, the region of the cerebral cortex that is the site of the highest intellectual functions, such as thinking and problem solving.
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| corpus callosum | The large bundle of axons that connects the brain's two hemispheres, responsible for relaying information between the two sides.
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| endocrine system | The body system consisting of a set of glands that regulate the activities of certain organs by releasing their chemical products into the bloodstream.
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| glands | Organs or tissues in the body that create chemicals that control many of our bodily functions.
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| hormones | Chemical messengers that are produced by the endocrine glands and carried by the bloodstream to all parts of the body.
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| pituitary gland | A pea-sized gland just beneath the hypothalamus that controls growth and regulates other glands.
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| adrenal glands | Glands at the top of each kidney that are responsible for regulating moods, energy level, and the ability to cope with stress.
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| pancreas | A dual-purpose gland under the stomach that performs both digestive and endocrine functions.
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| ovaries | Sex-related endocrine glands in the uterus that produce hormones related to women's sexual development and reproduction.
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| testes | Sex-related endocrine glands in the scrotum that produce hormones related to men's sexual development and reproduction.
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| stem cells | Unique primitive cells that have the capacity to develop into most types of human cells.
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| chromosomes | In the human cell, threadlike structures that come in 23 pairs, one member of each pair originating from each parent, and that contain the remarkable substance DNA.
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| deoxyribonucleic acid (DNA) | A complex molecule in the cell's chromosomes that carries genetic information.
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| genes | The units of hereditary information, consisting of short segments of chromosomes composed of DNA.
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| dominant-recessive genes principle | The principle that, if one gene of a pair is dominant and one is recessive, the dominant gene overrides the recessive gene. A recessive gene exerts its influence only if both genes of a pair are recessive.
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| genotype | An individual's genetic heritage; his or her actual genetic material.
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| phenotype | An individual's observable characteristics.
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| stress | The responses of individuals to environmental stressors.
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| stressors | Circumstances and events that threaten individuals and tax their coping abilities and that cause physiological changes to ready the body to handle the assault of stress.
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