| brain | The complex mass of neural cells and related cells encased in the skull.
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| spinal cord | The nerve fibers in the spinal column.
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| neuron | An individual nerve cell.
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| cell body | The central part of the neuron that includes the nucleus.
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| dendrites | Extensions of the cell body that usually serve as receiving areas for messages from other neurons.
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| axons | Neuron branches that transmit messages to other neurons.
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| nerve | A bundle of long neurons outside the brain and spinal cord.
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| ions | Electrically charged particles.
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| cell membrane | The covering of a neuron or another cell.
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| semipermeable | A surface that allows some, but not all, particles to pass through.
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| polarized | The resting state of a neuron, when more negative ions are inside and more positive ions are outside the cell membrane.
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| depolarization | The process during which positively charged ions flow into the axon, making it less negatively charged inside.
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| all-or-none principle | The law that states that once a neural action potential is produced, its magnitude is always the same.
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| action potential | A brief electrical signal that travels the length of the axon.
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| myelin sheath | The insulating fatty covering wrapped around the axon that speeds the transmission of neural messages.
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| synapse | The space between the axon of one neuron and another neuron.
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| synaptic gap | The small space between two neurons at a synapse.
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| neurotransmitters | Chemical substances, produced by axons, that transmit messages across the synapse.
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| synaptic vesicles | Tiny vessels containing stored quantities of the neurotransmitter substance held in the synaptic knobs of the axon.
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| synaptic knobs | The knoblike tips of axons.
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| receptor sites | Sites on the neuron that receive the neurotransmitter substance.
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| acetylcholine | A neurotransmitter used by somatic neurons that contract the body's large muscles. Acetylcholine also plays a role in memory and is thought to help regulate dreaming.
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| dopamine | A neurotransmitter substance used by neurons in the brain that control large muscle movements and by neurons in pleasure and reward systems in the brain.
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| serotonin | A neurotransmitter used by systems of neurons believed to regulate sleep, dreaming, appetite, anxiety, depression, and the inhibition of violence.
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| norepinephrine | A neurotransmitter believed to be involved in vigilance and attention and released by sympathetic autonomic neurons and the adrenal glands.
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| glutamate | The most widespread excitatory neurotransmitter in the brain.
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| neuropeptides | A large group of neurotransmitters sometimes referred to as neuromodulators, as they appear to broadly influence the action of the other neurotransmitters.
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| central nervous system | The brain and the spinal cord.
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| peripheral nervous system | The network of nerves that branches from the brain and spinal cord to all parts of the body.
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| afferent neurons | Neurons that transmit messages from sense organs to the central nervous system.
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| efferent neurons | Neurons that transmit messages from the central nervous system to organs and muscles.
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| interneuron | Neurons in the central nervous system that connect other neurons.
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| somatic nervous system | The division of the peripheral nervous system that carries messages from the sense organs to the central nervous system and from the central nervous system to the skeletal muscles.
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| autonomic nervous system | The division of the peripheral nervous system that regulates the actions of internal body organs, such as heartbeat.
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| sympathetic nervous system | The division of the autonomic nervous system that prepares the body to respond to psychological or physical stress.
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| parasympathetic nervous system | The division of the autonomic nervous system that promotes bodily maintenance and energy conservation and storage under nonstressful conditions.
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| ganglia | Clusters of cell bodies of neurons outside of the central nervous system.
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| electroencephalogram (EEG) | A recording of the electrical activity of the brain obtained through electrodes placed on the scalp.
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| positron emission tomography (PET) | An imaging technique that reveals the functions of the brain.
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| magnetic resonance imaging (MRI) | An imaging technique using magnetic resonance to obtain detailed views of the brain structure and function.
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| functional MRI | A type of MRI that measures the activity of parts of the brain by measuring the use of oxygen by groups of neurons.
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| hindbrain | The lowest part of the brain, located at the base of the skull.
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| medulla | The swelling just above the spinal cord within the hindbrain responsible for controlling breathing and a variety of reflexes.
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| pons | The part of the hindbrain that is involved in balance, hearing, and some parasympathetic functions.
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| cerebellum | Two rounded structures behind the pons involved in the coordination of muscle movements, learning, and memory.
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| reticular formation | Sets of neurons that project from the medulla and pons downward into the spinal cord to play a role in maintaining muscle tone and cardiac reflexes and upward throughout the cerebral cortex where they influence wakefulness, arousal level, and attention.
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| midbrain | The small area at the top of the hindbrain that serves primarily as a re- flex center for orienting the eyes and ears.
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| forebrain | The parts of the brain, including the thalamus, hypothalamus, and cerebral cortex, that cover the hindbrain and midbrain and fill much of the skull.
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| thalamus | The part of the forebrain that primarily routes sensory messages to appropriate parts of the brain.
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| hypothalamus | The small part of the forebrain involved with motives, emotions, and the functions of the autonomic nervous system.
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| limbic system | A complex brain system, composed of the amygdala, hippocampus, septal area, and cingulate cortex, that works with the hypothalamus in emotional arousal.
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| amygdala | A part of the limbic system that plays a role in emotion.
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| hippocampus | The part of the limbic system that plays a role in emotional arousal and memory.
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| septal area | A part of the limbic system that processes cognitive information in emotion.
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| cingulate cortex | A part of the limbic system lying in the cerebral cortex that processes cognitive information in emotion.
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| cerebral cortex | The largest structure in the forebrain, controlling conscious experience and intelligence and being involved with the somatic nervous system.
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| frontal lobes | The part of the cerebral cortex in the front of the skull involved in planning, organization, thinking, decision making, memory, voluntary motor movements, and speech.
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| Broca's area | An area of the frontal lobe of the left cerebral hemisphere that plays a role in speaking language.
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| stroke | A rupture or blockage of a blood vessel in the brain that interrupts blood flow and often results in the destruction of a part of the brain.
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| expressive aphasia | An impairment of the ability to generate spoken language, but not in the comprehension of language.
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| parietal lobes | The part of the cerebral cortex that is located behind the frontal lobes at the top of the skull and that contains the somatosensory area.
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| somatosensory area | The strip of parietal cortex running parallel to the motor area of the frontal lobes that plays a role in body senses.
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| temporal lobes | The part of the cerebral cortex that extends back from the area of the temples beneath the frontal and parietal lobes and that contains areas involved in the sense of hearing and understanding language.
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| Wernicke's area | The language area of the cortex that plays an essential role in understanding spoken language.
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| Wernicke's aphasia | A form of aphasia in which persons can speak fluently (but nonsensically) and cannot make sense out of language spoken to them by others.
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| occipital lobes | The part of the cerebral cortex, located at the base of the back of the head, that plays an essential role in the processing of sensory information from the eyes.
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| association areas | Areas within each lobe of the cerebral cortex believed to play general rather than specific roles.
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| cerebral hemispheres | The two main parts of the cerebral cortex, divided into left and right hemispheres.
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| corpus callosum | The major neural structure connecting the left and right cerebral hemispheres.
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| plasticity | The ability of parts of the brain, particularly the cerebral cortex, to acquire new functions that partly or completely replace the functions of a damaged part of the brain.
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| neural pruning | The normal process of selective loss of gray matter in the brain over time, which is thought to improve the efficiency of neural systems by eliminating unnecessary cells.
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| neurogenesis | The hypothesized growth of new neurons in adult mammals.
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| endocrine system | The system of glands that secretes hormones.
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| glands | Structures in the body that secrete substances.
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| hormones | Chemical substances, produced by endocrine glands, that influence internal organs.
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| pituitary gland | The body's master gland, located near the bottom of the brain, whose secretions help regulate the activity of the other glands in the endocrine system.
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| adrenal glands | Two glands on the kidneys that are involved in physical and emotional arousal.
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| epinephrine | A hormone produced by the adrenal glands.
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| norepinephrine | A hormone produced by the adrenal glands.
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| cortisol | A stress hormone produced by the adrenal glands.
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| islets of Langerhans | Endocrine cells in the pancreas that regulate the level of sugar in the blood.
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| pancreas | The organ near the stomach that contains the islets of Langerhans.
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| glucagon | A hormone produced by the islets of Langerhans that causes the liver to release sugar into the bloodstream.
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| insulin | A hormone produced by the islets of Langerhans that reduces the amount of sugar in the bloodstream.
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| ovaries | Female endocrine glands that secrete sex-related hormones and produce ova, or eggs.
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| testes | Male endocrine glands that secrete sex-related hormones and produce sperm cells.
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| gonads | The glands that produce sex cells and hormones important in sexual arousal and that contribute to the development of secondary sex characteristics.
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| estrogen | A female sex hormone.
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| testosterone | A male sex hormone.
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| thyroid gland | The gland below the voice box that regulates metabolism.
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| metabolism | The process through which the body uses energy.
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| thyroxin | A hormone produced by the thyroid that is necessary for proper mental development in children and helps determind weight and level of activity in adults.
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| cretinism | A type of mental retardation in children caused by a deficiency of thyroxin.
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| parathyroid glands | Four glands embedded in the thyroid that produce parathormone.
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| parathormone | A hormone that regulates ion levels in neurons and controls excitability of the nervous system.
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| pineal gland | The endocrine gland that is largely responsible for the regulation of biological rhythms.
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| chromosomes | Strands of DNA (deoxyribonucleic acid) in cells.
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| DNA (deoxyribonucleic acid) | Structures in cells in the form of two curved rails of a ladder type connected at intervals by base pairs of adenine, thymine, guanine, and cystine that code genetic information.
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| genes | Segments of chromosomes made up of sequences of base pairs of adenine, thymine, guanine, and cystine that are the basic biological units of inheritance because they contain all the coded genetic information needed to influence some aspect of a structure or function of the body.
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| gamete | A sex cell, which contains 23 chromosomes instead of the normal 46.
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| fertilization | The uniting of sperm and ovum, which produces a zygote.
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| zygote | The stable cell resulting from fertilization; in humans, it has 46 chromosomes --23 from the sperm and 23 from the ovum.
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| dominant gene | The gene that produces a trait in an individual even when paired with a recessive gene.
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| recessive gene | The gene that produces a trait in an individual only when the same recessive gene has been inherited from both parents.
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| Down syndrome | An abnormality caused by the presence of an additional 21st chromosome.
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| monozygotic twins | Twins formed from a single ovum; they are identical in appearance because they have the same genetic structure.
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| dizygotic twins | Twins formed from the fertilization of two ova by two sperm.
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