Glossary of neurology

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Neuro-anatomy

• adrenal glands: Located on top of each kidney, these two glands are involved in the body’s responseto stress and help regulate growth, blood glucose levels, and the body’s metabolic rate. They receivesignals from the brain and secrete several different hormones in response, including cortisol and adrenaline.

• adrenaline: Also called epinephrine, this hormone is secreted by the adrenal glands in response tostress and other challenges to the body. The release of adrenaline causes a number of changesthroughout the body, including the metabolism of carbohydrates to supply the body’s energy demands.

• allele: One of the variant forms of a gene at a particular location on a chromosome. Differing allelesproduce variation in inherited characteristics such as hair color or blood type. A dominant allele is onewhose physiological function—such as making hair blonde—is manifest even when only a single copy ispresent (among the two copies of each gene that everyone inherits from their parents). A recessive alleleis one that manifests only when two copies are present.

• amino acid: A type of small organic molecule. Amino acids have a variety of biological roles, but arebest known as the “building blocks” of proteins.

• amino acid neurotransmitters: The most prevalent neurotransmitters in the brain, these include gluta-mate and aspartate, which have excitatory actions, and glycine and gamma-amino butyric acid (GABA),which have inhibitory actions.

• amygdala: Part of the brain’s limbic system, this primitive brain structure lies deep in the center of thebrain and is involved in emotional reactions, such as anger, as well as emotionally charged memories. Italso influences behavior such as feeding, sexual interest, and the immediate “fight or flight” stress reac-tion that helps ensure that the body’s needs are met.

• amyloid-beta (Aβ) protein: A naturally occurring protein in brain cells. Large, abnormal clumps of thisprotein form the amyloid plaques that are the hallmark of Alzheimer’s disease. Smaller groupings (oligo-mers) of Aβ seem more toxic to brain cells and are now thought by many researchers to be importantinitiators of the Alzheimer’s disease process.

• amyloid plaque: The sticky, abnormal accumulations of amyloid-beta protein aggregate around neuronsand synapses in the memory and intellectual centers of the brain, in people with Alzheimer’s. These aresometimes referred to as neuritic plaques or senile plaques. While amyloid plaques have long beenconsidered markers of Alzheimer’s, they are also found to some extent in many cognitively normalelderly people. Plaques’ role in Alzheimer’s neurodegeneration remains unclear.

• animal model: A laboratory animal that—through changes in its diet, exposure to toxins, genetic chang-es, or other experimental manipulations—mimics specific signs or symptoms of a human disease. Manyof the most promising advances in treating brain disorders have come from research on animal models.(Italicized terms are defined within this glossary.)

• astrocyte: A star-shaped glial cell that delivers “fuel” to the neurons from the blood, removes wastefrom the neuron, and otherwise modulates the activity of the neuron. Astrocytes also play critical rolesin brain development and the creation of synapses.

• auditory cortex: Part of the brain’s temporal lobe, this region is responsible for hearing. Nerve fibersextending from the inner ear carry nerve impulses generated by sounds into the auditory cortex forinterpretation.

• autonomic nervous system: Part of the central nervous system that controls functions of internalorgans (e.g., blood pressure, respiration, intestinal function, urinary bladder control, perspiration,body temperature). Its actions are mainly involuntary.

• axon: A long, single nerve fiber that transmits messages, via electrochemical impulses, from thebody of the neuron to dendrites of other neurons, or directly to body tissues such as muscles.

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• basal ganglia: A group of structures below the cortex involved in motor, cognitive, and emotionalfunctions.

• basilar artery: Located at the base of the skull, the basilar artery is one of the major vascular compo-nents supplying oxygenated blood to the brain and nervous system.

• biomarkers: A measurable physiological indicator of a biological state or condition. For example,amyloid plaques—as detected on amyloid PET scans, for example—are a biomarker of Alzheimer’sdisease. Biomarkers can be used for both diagnostic and therapeutic purposes.


• brain-computer interface: A device or program that permits direct or indirect collaboration betweenthe brain and a computer system. For example, a device that harnesses brain signals to control ascreen cursor or prosthetic limb.

• brain imaging: Refers to various techniques, such as magnetic resonance imaging (MRI), diffusiontensor imaging, and positron emission tomography (PET), that enable scientists to capture images ofbrain tissue and structure and to reveal what parts of the brain are associated with various behaviorsor activities.

• brain stem: A primitive part of the brain that connects the brain to the spinal cord. The brain stemcontrols functions basic to the survival in animal such as heart rate, breathing, digestive processes,and sleeping.(continued)

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• central nervous system: The brain and spinal cord constitute the central nervous system andare part of the broader nervous system, which also includes the peripheral nervous system.

• central sulcus: The primary groove in the brain’s cerebrum, which separates the frontal lobe inthe front of the brain from the parietal and occipital lobes in the rear of the brain.

• cerebellar artery: The major blood vessel providing oxygenated blood to the cerebellum.

• cerebellum: A brain structure located at the top of the brain stem that coordinates the brain’s instruc-tions for skilled, repetitive movements and helps maintain balance and posture. Recent research alsosuggests the cerebellum may play a role, along with the cerebrum, in some emotional and cognitiveprocesses.

• cerebrum (also called cerebral cortex): The largest brain structure in humans, accounting for abouttwo-thirds of the brain’s mass and positioned over and around most other brain structures. The cere-brum is divided into left and right hemispheres, as well as specific areas called lobes that are associatedwith specialized functions.

• chronic traumatic encephalopathy (CTE): Once known as dementia pugilistica and thought to beconfined largely to former boxers, this progressive degenerative disease, with symptoms includingimpulsivity, memory problems, and depression, affects the brains of individuals who have sufferedrepeated concussions and traumatic brain injuries.

• cognition: A general term that includes thinking, perceiving, recognizing, conceiving, judging, sensing,reasoning, and imagining. Also used as an adjective pertaining to cognition, as in “cognitive processes.”

• cognitive neuroscience: The field of study that investigates the biological mechanisms of cognition.

• computational neuroscience: An interdisciplinary field of study that uses information processing prop-erties and algorithms to further the study of brain function and behavior.

• computed tomography (CT or CAT): An X-ray technique introduced in the early 1970s that enablesscientists to take cross-sectional images of the body and brain. CT uses a series of X-ray beams passedthrough the body to collect information about tissue density, then applies sophisticated computer andmathematical formulas to create an anatomical image from the data.

• connectome: A detailed map, or “wiring diagram,” of the myriad neural connections that make up thebrain and nervous system.

• consciousness: The state of being aware of one’s feelings and what is happening around one;the totality of one’s thoughts, feelings, and impressions.

• corpus callosum: The collection of nerve fibers connecting the two cerebral hemispheres.

• cortex: The outer layer of the cerebrum. Sometimes referred to as the cerebral cortex.

• cortisol: A steroid hormone produced by the adrenal glands that controls how the body uses fat,protein, carbohydrates, and minerals, and helps reduce inflammation. Cortisol is released in the body’sstress response; scientists have found that prolonged exposure to cortisol has damaging effects on the brain.

• CRISPR (clustered regularly-interspaced short palindromic repeats): A relatively precise andreliable DNA editing technique, which is derived from a primitive antiviral system found in bacteria.There is hope that CRISPR technology can be adapted to help treat brain disorders.

• CT scan (also called CAT scan): See computed tomography.

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• deep brain stimulation: A method of treating various neuropsychiatric and neurodegenerative disordersthrough small, controlled electric shocks administered from a special battery-operated neurostimulationimplant. The implant, sometimes called a brain pacemaker, is placed within deep brain regions such asthe globus pallidus or subthalamus.

• default-mode network: The network indicates that the brain remains active even if not involved in aspecific task. So whether asleep or daydreaming, the brain is in an active state.

• delayed discounting: A common cognitive task used to measure impulsivity in individuals. The taskmeasures an individual’s preference for the immediate delivery of a small reward versus a larger rewarddelivered later.

• dementia: General mental deterioration from a previously normal state of cognitive function due todisease or psychological factors. Alzheimer’s disease is one form of dementia.

• dendrites: Short nerve fibers that project from a nerve cell, generally receiving messages from theaxons of other neurons and relaying them to the cell’s nucleus.(continued)

• depression: A mood or affective disorder characterized by sadness and lack of motivation. Depressionhas been linked to disruptions in one or more of the brain’s neurotransmitter systems, including thoserelated to serotonin and dopamine. Clinical depression is a serious condition that can often be effectivelytreated with medications and/or behavioral therapy.

• Diagnostic and Statistical Manual of Mental Disorders (DSM): The standard classification manualpublished by the American Psychiatric Association to be used by mental health professionals to diag-nose and treat mental disorders.

• diffusion tensor imaging: A brain imaging method that helps visualize the brain’s white matter tracts.

• DNA (deoxyribonucleic acid): The material from which the 46 chromosomes in each cell’s nucleus isformed. DNA contains the codes for the body’s approximately 30,000 genes, governing all aspects ofcell growth and inheritance. DNA has a double-helix structure—two intertwined strands resembling aspiraling ladder.

• dominant gene: A gene that almost always results in a specific physical characteristic, for example adisease, even though the patient’s genome possesses only one copy. With a dominant gene, the chanceof passing on the gene (and therefore the trait or disease) to children is 50-50 in each pregnancy.

• dopamine: A neurotransmitter involved in motivation, learning, pleasure, the control of body movement,and other brain functions. Some addictive drugs greatly increase brain levels of dopamine, leading to aeuphoric “high.” Virtually all addictive substances, from nicotine to alcohol to heroin and crack cocaine,affect the dopamine system in one way or another.

• double helix: The structural arrangement of DNA, which looks something like an immensely long laddertwisted into a helix, or coil. The sides of the “ladder” are formed by a backbone of sugar and phosphatemolecules, and the “rungs” consist of nucleotide bases joined weakly in the middle by hydrogen bonds.

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• electroconvulsive therapy (ECT): A therapeutic treatment for depression and other mental illnessesthat sends small electric currents over the scalp to trigger a brief seizure. It is one of the fastest waysknown to reverse the symptoms of severely depressed individuals.

• endocrine system: A body system composed of several different glands and organs that secretehormones.

• endorphins: Hormones produced by the brain, in response to pain or stress, to blunt the sensation ofpain. Narcotic drugs, such as morphine, imitate the actions of the body’s natural endorphins.(continued)

• enzyme: A protein that facilitates a biochemical reaction. Organisms could not function if they hadno enzymes.

• epigenetics: A subset of genetics that focuses on phenotypic trait variations caused by specificenvironmental factors that influence where, when, and how a gene is expressed.

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• fissure: A groove or indentation observed in the brain. Another word for sulcus.

• frontal lobe: The front of the brain’s cerebrum, beneath the forehead. This area of the brain is associat-ed with higher cognitive processes such as decision-making, reasoning, social cognition, and planning,as well as motor control.

• frontal operculum: The part of the frontal lobe that sits over the insula.

• functional magnetic resonance imaging (fMRI): A brain imaging technology, based on conventionalMRI, that gathers information relating to short-term changes in oxygen uptake by neurons. It typicallyuses this information to depict the brain areas that become more active or less active—and presumablymore or less involved—while a subject in the fMRI scanner performs a cognitive task.

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• gamma-aminobutyric acid (GABA): A neurotransmitter implicated in brain development, muscle con-trol, and reduced stress response.

• gene: The basic unit of inheritance. A gene is a distinct section of DNA in a cell’s chromosome thatencodes a specific working molecule—usually protein or RNA—with some role in brain or body function.Gene defects (genetic mutations) are thought to cause many disorders including brain disorders.

• gene expression: The process by which a gene’s nucleotide sequence is transcribed into the form ofRNA—often as a prelude to being translated into a protein.

• gene mapping: Determining the relative positions of genes on a chromosome and the distancebetween them.

• genome: The complete genetic map for an organism. In humans, this includes about 30,000 genes,more than 15,000 of which relate to functions of the brain.

• glia (or glial cells): The supporting cells of the central nervous system. Though probably not involveddirectly in the transmission of nerve signals, glial cells protect and nourish neurons.(continued)

• glioma: A tumor that arises from the brain’s glial tissue.

• glucose: A natural sugar that is carried in the blood and is the principal source of energy for the cells ofthe brain and body. PET imaging techniques measure brain activity by detecting increases in the brain’smetabolism of glucose during specific mental tasks.

• gray matter: The parts of the brain and spinal cord made up primarily of groups of neuron cell bodies(as opposed to white matter, which is composed mainly of myelinated nerve fibers).

• gyrus: The ridges on the brain’s outer surface. Plural is gyri.

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• hemisphere: In brain science, refers to either half of the brain (left or right). The two hemispheres areseparated by a deep groove, or fissure, down the center. Some major, specific brain functions are locat-ed in one or the other hemisphere.

• hippocampus: A primitive brain structure, located deep in the brain, that is involved in memory andlearning.

• hormone: A chemical released by the body’s endocrine glands (including the adrenal glands), as wellas by some tissues. Hormones act on receptors in other parts of the body to influence body functions orbehavior.

• hypothalamus: A small structure located at the base of the brain, where signals from the brain and thebody’s hormonal system interact.

• induced pluripotent stem cell (iPSC): A cell that has been taken from adult tissue and geneticallymodified to behave like an embryonic stem cell, with the ability to develop into any type of cell foundin the body, including nerve cells.

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• insula: Sometimes referred to as the insular cortex, this small region of the cerebrum is founddeep within the lateral sulcus, and is believed to be involved in consciousness, emotion, and bodyhomeostasis.

• interneuronal: Between neurons, as in interneuronal communication.(continued)

• ions: Atoms or small groups of atoms that carry a net electric charge, either positive or negative. When anerve impulse is fired, ions flow through channels in the membrane of a nerve cell, abruptly changing thevoltage across the membrane in that part of the cell. This sets off a chain reaction of similar voltage changesalong the cell’s axon to the synapse, where it causes the release of neurotransmitters into the synaptic cleft.

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• lesion: An injury, area of disease, or surgical incision to body tissue. Much of what has been learnedabout the functions of various brain structures or pathways has resulted from lesion studies, in whichscientists observe the behavior of persons who have suffered injury to a distinct area of the brain oranalyze the behavior resulting from a lesion made in the brain of a laboratory animal.

• limbic system: A group of evolutionarily older brain structures that encircle the top of the brain stem.The limbic structures play complex roles in emotions, instincts, and behavioral drives.

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• magnetic resonance imaging (MRI): A non-invasive imaging technology, often used for brain imaging.An MRI scanner includes intensely powerful magnets, typically 10,000 to 40,000 times as strong as theEarth’s magnetic field. These magnets, combined with coils that send electromagnetic pulses into thescanned tissue, induce radio-frequency signals from individual hydrogen atoms within the tissue. Thescanner records and processes these signals to build up an image of the scanned tissue. MRI scans areable to depict high resolution images of the entire brain, allowing clinicians to determine if the braintissue that is visualized is normal, abnormal, or damaged due to a neurological disorder or trauma. MRItechnology has also been adapted to measure brain activity functional MRI.

• melatonin: A hormone that is secreted by the pineal gland in the brain in response to the daily light-darkcycle, influencing the body’s sleep-wake cycle and possibly affecting sexual development.

• memory: The encoding and storage of information, in a way that allows it to be retrieved later. In thebrain, memory involves integrated systems of neurons in diverse brain areas, each of which handlesindividual memory-related tasks. Memory can be categorized into two distinct types, each with its owncorresponding brain areas. Memory about people, places, and things—that one has experienced directlyor otherwise learned about—is referred to as explicit or declarative memory and seems to be centered inthe hippocampus and temporal lobe. Memory about motor skills and perceptual strategies is known asimplicit, or procedural memory and seems to involve the cerebellum, the amygdala, and specific path-ways related to the particular skill (e.g., riding a bicycle would involve the motor cortex).

• metabolize: To break down or build up biochemical elements in the body, effecting a change in bodytissue. Brain cells metabolize glucose, a blood sugar, to derive energy for transmitting nerve impulses.(continued)

• microbiota: The community of various microorganisms found in the digestive tract. Scientists are nowlearning that microbes found in the microbiota can influence brain development and behavior.

• microglia: A small, specialized glial cell that operates as the first line of immune defense in the centralnervous system.

• minimally conscious state: A disorder of consciousness, often caused by stroke, head injury, or loss ofblood flow to the brain, in which an individual maintains partial conscious awareness.

• molecular biology: The study of the structure and function of cells at the molecular level and how thesemolecules influence behavior and disease processes. Molecular biology emerged as a scientific disci-pline only in the 1970s, with advances in laboratory technologies for isolating and characterizing DNA,RNA, proteins, and other types of biological molecule.

• motor cortex: The part of the brain’s cerebrum, just to the front of the central sulcus in the frontal lobe,that is involved in movement and muscle coordination. Scientists have identified specific spots in themotor cortex that control movement in specific parts of the body, the so-called “motor map.”

• MRI: See magnetic resonance imaging and/or functional magnetic resonance imaging.

• mutation: A permanent structural alteration to DNA that alters its previous nucleotide sequence. In mostcases, DNA changes either have no effect or cause harm, but occasionally a mutation improves anorganism’s chance of surviving and procreating.

• myelin: The fatty substance that sheathes most nerve cell axons, helping to insulate and protect thenerve fiber and effectively speeding up the transmission of nerve impulses.

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• narcotic: A synthetic chemical compound that mimics the action of the body’s naturalendorphins—hormones secreted to counteract pain. Narcotic drugs have a valid and useful role in themanagement of pain but may lead to physical dependence in susceptible individuals if used for long periods.

• neuroeconomics: An interdisciplinary field of study that uses neuroscientific research to help explainhuman decision-making behavior.

• neurodegenerative diseases: Diseases characterized by the progressive deterioration and death of nervecells (neurodegeneration), typically originating in one area of the brain and spreading to other connectedareas. Neurodegenerative diseases include amyotrophic lateral sclerosis (also known as Lou Gehrig’sdisease), Huntington’s disease, Alzheimer’s disease, frontotemporal degeneration, and Parkinson’s disease.(continued)

• neuroeducation: Sometimes referred to as educational neuroscience, this collaborative,interdisciplinary field of study uses findings in cognitive neuroscience to inform teaching and othereducational practices.

• neuroethics: An interdisciplinary field of study that addresses the ethical issues of our increased abilityto understand and change the brain. Privacy, life extension, cloning, and many other issues are includedin this ongoing social-scientific debate.

• neurogenesis: The production of new, maturing neurons by neural stem and progenitor cells. Rapidand widespread neurogenesis obviously occurs in the fetal brain in humans and other animals. Neuro-scientists long believed that neurogenesis essentially does not occur in the adult human brain. Howev-er, over the past two decades, research has shown that it does in fact occur in the dentate gyrus of thehippocampus and possibly other brain regions. This “adult neurogenesis” appears to be vital fornormal learning and memory, and may help protect the brain against stress and depression. Neuralstem cells, which can produce new, “young” neurons and glial cells, also may be used widely somedayto treat brain disorders, particularly neurodegenerative diseases that otherwise deplete the neuronalpopulation.

• neuroimmunology: A complex field in biomedical research, which focuses on the brain, the immunesystem, and their interactions. Neuroimmunology holds the potential for conquering ills as diverse asspinal cord injury, multiple sclerosis, and bodily reactions to pathogens, both naturally occurring andintentionally inflicted.

• neuron: Nerve cell. The basic unit of the central nervous system, the neuron is responsible for thetransmission of nerve impulses. Unlike any other cell in the body, a neuron consists of a central cellbody as well as several threadlike “arms” called axons and dendrites, which transmit nerve impulses.Scientists estimate that there are approximately 100 billion neurons in the brain.

• neuroscience: The study of brains and nervous systems, including their structure, function, anddisorders. Neuroscience as an organized discipline gained great prominence in the latter part of thelast century.

• neurotransmitter: A chemical that acts as a messenger between neurons and is released into the synap-tic cleft when a nerve impulse reaches the end of an axon. Several dozen neurotransmitters have beenidentified in the brain so far, each with specific, often complex roles in brain function and human behavior.

• nurture: A popular term for the influence of environmental factors on human development such as theexperiences one is exposed to in early life. The term is often used in the context of “nature versus nurture,”which relates to the interplay of “nature” (genetic or inherited, predetermined influences) and environmen-tal, or experiential, forces.(continued)

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• occipital lobe: A part of the brain’s cerebrum, located at the rear of the brain, above the cerebellum.The occipital lobe is primarily concerned with vision and encompasses the visual cortex.

• olfactory: Pertaining to the sense of smell. When stimulated by an odor, olfactory receptor cells in thenose send nerve impulses to the brain’s olfactory bulbs, which in turn transmit the impulses to olfactorycenters in the brain for interpretation.

• opiate: A synthetic (e.g., Demerol, Fentanyl) or plant-derived (e.g., opium, heroin, morphine) compoundthat binds and activates opioid receptors on certain neurons. Opiates typically but not always have pain-re-lieving, anxiety-reducing, and even euphoria-inducing effects, and are generally considered addictive.

• opioid: An artificially derived drug or chemical that acts on the nervous system in a similar manner toopiates, influencing the “pleasure pathways” of the dopamine system by locking on to specialized opioidreceptors in certain neurons.

• opioid receptors (e.g., mu, delta, kappa): A class of receptors found on neurons in the brain, spinalcord, and digestive tract. Opioid receptors are involved in numerous functions, including pain control,mood, digestion, and breathing.

• optogenetics: An innovative neuroscientific technique that uses light to turn genetically modifiedneurons on and off at will, in live animals.

• oxytocin: Sometimes referred to as the “cuddle chemical,” this hormone can work as a neurotransmitterin the brain and has been linked to social attachment and parental care.

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• pain receptors: Specialized nerve fibers in the skin and on the surfaces of internal organs, which detectpainful stimuli and send signals to the brain.

• parietal lobe: The area of the brain’s cerebrum located just behind the central sulcus. It is concernedprimarily with the reception and processing of sensory information from the body and is also involved in mapinterpretation and spatial orientation (recognizing one’s position in space vis-a-vis other objects or places).

• peripheral nervous system: The nervous system outside the brain and spinal cord.

• persistent vegetative state: A disorder of consciousness, often following severe brain trauma, in which anindividual has not even minimal conscious awareness. The condition can be transient, marking a stage inrecovery, or permanent.

• PET: See positron emission tomography.

• pharmacotherapy: The use of pharmaceutical drugs for therapeutic purposes.

• pituitary gland: An endocrine organ at the base of the brain that is closely linked with the hypothalamus.The pituitary gland is composed of two lobes and secretes a number of hormones that regulate the activityof the other endocrine organs in the body.

• plasticity: In neuroscience, refers to the brain’s capacity to change and adapt in response to developmen-tal forces, learning processes, injury, or aging.

• positron emission tomography (PET): An imaging technique, often used in brain imaging. For aPET scan of the brain, a radioactive “marker” that emits, or releases, positrons (parts of an atomthat release gamma radiation) is injected into the bloodstream. Detectors outside of the head cansense these “positron emissions,” which are then reconstructed using sophisticated computer pro-grams to create “tomographs,” or computer images. Since blood flow and metabolism increase inbrain regions at work, those areas have higher concentrations of the marker, and researchers areable to see which brain regions are activated during certain tasks or exposure to sensory stimuli.Ligands can be added to a PET scan in order to detect pathological entities such as amyloid ortau deposits.

• postsynaptic cell: The neuron on the receiving end of a nerve impulse transmitted from anotherneuron.

• Post-traumatic stress disorder (PTSD): A mental disorder that develops in response to a traumaticevent such as combat, sexual assault, terrorism, or abuse. Symptoms can include mood disturbances,hyperarousal, memory flashbacks, sleep problems, anxiety, and depression.

• prefrontal cortex: The area of the cerebrum located in the forward part of the frontal lobe, which medi-ates many of the higher cognitive processes such as planning, reasoning, and “social cognition”—acomplex skill involving the ability to assess social situations in light of previous experience and personalknowledge, and interact appropriately with others. The prefrontal cortex is thought to be the most recent-ly evolved area of the brain.

• premotor cortex: The area of the cerebrum located between the prefrontal cortex and the motor cortex, inthe frontal lobe. It is involved in the planning and execution of movements.

• presynaptic cell: In synaptic transmission, the neuron that sends a nerve impulse across the synapticcleft to another neuron.(continued)

• prion: A protein aggregate that can propagate itself, inducing the formation of new aggregatesfrom individual copies of the protein it encounters. Prions have the potential to spread within thebody and brain, and even from one organism to another—“infectiously,” like a virus. The first prionsdescribed were hardy aggregates of PrP, the prion protein. They are responsible for a set of rapid,fatal and potentially transmissible neurodegenerative diseases including Creutzfeldt-Jakob diseaseand bovine spongiform encepalopathy (“mad cow disease”). Many researchers now argue thatprotein aggregates in other neurodegenerative diseases, such as the Aβ and tau aggregates ofAlzheimer’s, have such similar properties that they also deserve to be called prions. In someorganisms, such as yeast, certain proteins apparently evolved to function normally in a prion-likeform.

• protein folding: The process by which the chain of amino acids that make up a protein assumes itsfunctional shape. The protein aggregation that occurs in some neurodegenerative disorders is thoughtto be triggered when proteins “misfold.”

• psychiatry: A medical specialty dealing with the diagnosis and treatment of mental disorders. (Contrastwith psychology).

• psychoactive drug: A broad term for any drug that acts on the brain and noticeably alters one’s mentalstate such as by elevating mood or alertness, or reducing inhibitions. Psychoactive pharmaceuticals canhelp control the symptoms of some neurological and psychiatric disorders. Many “recreational drugs” arealso psychoactive drugs.

• psychological dependence: In the science of addiction, refers to the mood- and motivation-relatedfactors that sustain addiction (such as craving a cigarette after a meal), as opposed to the “physicaldependence” that manifests when a person attempts to kick the habit (e.g., tremors, racing pulse). Brainscientists now understand that psychological factors are central to addictive disorders and are often themost difficult to treat. (Also see dependence.)

• psychology: An academic or scientific field of study concerned with the behavior of humans andanimals and related mental processes. (Contrast with psychiatry.)

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• receptors: Molecules on the surfaces of neurons whose structures precisely match those of chemicalmessengers (such as neurotransmitters or hormones) released during synaptic transmission. The chem-icals attach themselves to the receptors, in lock-and-key fashion, to activate the receiving cell structure(usually a dendrite or cell body).

• recessive: A genetic trait or disease that appears only in patients who have received two copiesof a mutant gene, one from each parent.

• resting state: The state of the brain when it is not consciously engaged in an explicit task. Brainimaging techniques such as fMRI can be used to measure the residual activity that occurs in thisstate. Scientists are currently using resting state data to help map the connectome, for example.

• reward/reinforcement brain network: Also known as the mesolimbic circuit, this important network ofbrain regions is implicated in risk and reward processing, as well as learning. It primarily uses dopaminefor signaling.

• reuptake: A process by which released neurotransmitters are absorbed for subsequent re-use.

• ribonucleic acid (RNA): A chemical similar to a single strand of DNA. The sugar is ribose, not deoxyri-bose, hence RNA. In RNA, the letter U, which stands for uracil, is substituted for T in the genetic code.RNA delivers DNA’s genetic message to the cytoplasm of a cell, where proteins are made.

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• senses: The physiological inputs that provide critical information for perception and behavior from theoutside world. The five classic senses are: sight, hearing, taste, touch, and smell.

• serotonin: A neurotransmitter believed to play many roles, including, but not limited to, temperatureregulation, sensory perception, and the onset of sleep. Neurons using serotonin as a transmitter arefound in the brain and in the gut. A number of antidepressant drugs are targeted to brain serotoninsystems.

• social neuroscience: The field of study investigating the biological systems underlying social process-es and behavior.

• sonogenetics: A novel investigative approach that turns genetically modified neurons on and off usingultrasonic waves.

• sono-stimulation: The activation of neural networks using ultrasound.

• spinal cord: The “other half” of the central nervous system (with the brain). The spinal cord is acable that descends from the brain stem to the lower back. It consists of an inner core of gray mattersurroundedby white matter.

• stem cells: Undifferentiated cells that can grow into heart cells, kidney cells, or other cells of the body.Originally thought to be found only in embryos, stem cells in the brain have unexpectedly been discov-ered in adults. Researchers have shown on research animals that stem cells can be transplanted intovarious regions of the brain, where they develop into both neurons and glia.

• subgenual cortex: The region of the frontal lobes below the genu of the corpus callosum implicated inmood states.

• sulcus: The shallower grooves on the brain’s cerebrum (deeper grooves are called fissures). Plural is sulci.

• synapse: The junction where an axon approaches another neuron or its extension (a dendrite); thepoint at which nerve-to-nerve communication occurs. Nerve impulses traveling down the axon reach thesynapse and release neurotransmitters into the synaptic cleft, the tiny gap between neurons.

• synaptic transmission: The process of cell-to-cell communication in the central nervous system,whereby one neuron sends a chemical signal across the synaptic cleft to another neuron.

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• tau protein: A type of protein abundantly found in neurons. When this protein is not adequately clearedfrom the brain, it can form tangles that are a key pathology of several neurodegenerative disordersincluding frontotemporal degeneration, CTE, and Alzheimer’s disease.

• telomere: The protective cap found at the end of a chromosome. Research studies suggest that thesecaps may be shortened in neurodegenerative disorders.

• temporal lobes: The parts of the cerebrum that are located on either side of the head, roughlybeneath the temples in humans. These areas are involved in hearing, language, memory storage,and emotion.

• thalamus: A brain structure located at the top of the brain stem, the thalamus acts as a two-way relaystation, sorting, processing, and directing signals from the spinal cord and mid-brain structures to thecerebrum, and from the cerebrum down.

• transcranial electrical stimulation (tDCS and tACS): A non-invasive procedure that applies electricalstimulation to the scalp to increase or decrease neural signaling.The two main types are direct currentstimulation (tDCS) and alternating current stimulation (tACS). They are used for therapeutic purposesas well as to study cognitive processing.

• transcranial magnetic stimulation (TMS): A non-invasive procedure that uses magnetic fields, appliedover the scalp, to stimulate changes in neural processing. It is used as a treatment for depression aswell as a research method to investigate cognitive processes.

• traumatic brain injury (TBI): An injury to the brain acquired when the head is violently shook, struck, orpierced by an object. Moderate to severe TBI causes permanent impairments in brain function. Symp-toms of mild TBI may include headache, dizziness, attention problems, or issues with behavior andmood.

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V

• vagus nerve stimulation: A treatment for epilepsy that involves a small implant that electrically stimu-lates the vagus nerve, which runs from the brainstem to the abdomen.

• vertebral arteries: The major arteries of the neck, which merge to form the basilar artery.

• vestibular: Refers to the sense of balance. Many people with hearing loss also have some degree ofbalance difficulties, since the vestibular (or balance) system and the auditory (or hearing) systems areso closely related.

• visual cortex: The area of the cerebrum that is specialized for vision. It lies primarily in the occipital lobeat the rear of the brain, and is connected to the eyes by the optic nerves.

W

• white matter: Brain or spinal cord tissue consisting primarily of the myelin-covered axons that extendfrom nerve cell bodies in the gray matter of the central nervous system

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