Glossary: Commonly Used Terms Found in Medical Literature — A-C

A- C  •  D - H  •  I - M  •  N - P  •  Q - Z

Abduct: to move or spread away from a position that is at, near, or parallel to the center of a body. Parallel means to extend in the same direction without touching.

Ablation: the act of cutting off any part of the body, or the removal of a growth or damaged tissue. Ablation can also refer to damage induced during a surgical procedure. For example, an ablation may involve surgical cutting (excision); chemical destruction, such as injection of phenol; or the use of high frequency electrical current or radio waves.

Abnormal: Abnormal means not normal. Something that is abnormal differs in any way from the normal state, condition, rule, or structure.

Abulia: 1. A loss or impairment in the ability to perform voluntary actions, show initiative, or make decisions. 2. A decrease in movement, speech, thought, and emotional reactions. Abulia commonly results from damage to both the right and left frontal lobes of the brain. The frontal lobes are large areas of brain tissue located in the front part of the brain.

Acathisia: See Akathisia

Accelerometer: A device used to measure the rate of change in velocity over a specific period of time. Measures the rate or "speed" of the tremor cycle.

Acetylcholine (ACh): a substance in the body that allows messages to travel from one nerve to another. For example, a person who decides to pick up a pen can act on that thought only when the hand receives the message from the brain. This process in normal persons occurs in a fraction of a second. Primary functions of acetylcholine include regulating the delivery of messages from neurons to skeletal muscle fibers, smooth (involuntary) muscle fibers, and effector organs as well as between nerve cells in the brain and spinal cord. Acetylcholine also functions as a vasodilator and triggers certain actions within the parasympathetic nervous system, such as lowering blood pressure and slowing heart and breathing rates.

Acquired: referring to a feature, state, or disease that happens after birth.

Action tremor: a tremor that occurs during the performance of voluntary movements. Such tremors include postural, isometric, kinetic, and intention tremors.

Acute: Referring to symptoms that begin quickly and is intense or sharp, then the symptoms slow after a short time.

Acute confusional state: a central nervous system problem caused by interference with essential biochemical processes. Symptoms may include difficulties with thinking, awareness, memory, and orientation. These may be accompanied by restlessness, apprehension, irritability, and apathy. The condition may be associated with delirium, toxic psychosis (adverse reaction to a large quantity of chemicals – e.g. medication), or acute brain syndrome.

Adaptive deep brain stimulation:  A type of DBS that uses specific feedback from the brain to guide and optimize stimulation settings only when needed, in real time, instead of providing constant stimulation to the brain. 

Agonist: 1. A drug that triggers an action from a cell or another drug or hormone. 2. A muscle whose contraction executes an intended movement.

Agranulocytosis: An acute disease marked by high fever and a sharp drop in circulating granular white blood cells.

Akathisia:  A sense of discomfort or restlessness in a body part.  Typically feels like a need to move or difficulty sitting still.  Can occur as a side effect of medications or a disease process such as Parkinson’s disease.

Akinesia: 1. An abnormal state of physical and mental activity or in ability to move the muscles. 2. Absence of movement or loss of the ability to move such as temporary or prolonged paralysis or "freezing in place."

Akinetic apraxia: the inability to make a quick move without thinking (see apraxia).

Alpha 2-adrenergic agonist: A drug that reduces the activity of the neurotransmitter norepinephrine by stimulating certain receptors within the central nervous system (central presynaptic alpha 2-adrenergic receptors). The release of norepinephrine triggers action within the sympathetic nervous system. This part of the nervous system regulates certain involuntary activities during stress, such as increasing the heart rate, deepening breathing, and raising blood pressure. Norepinephrine also plays a role in regulating mood and emotion. The administration of alpha 2-adrenergic agonists may result in a reduced heart rate and lowered blood pressure and have depressive or sedative effects.

Alzheimer's disease: A progressive degenerative disease of the brain of unknown cause. Alzheimer's disease is characterized by widespread loss of nerve cells, particularly in the outer region of the brain (cerebral cortex), with distinctive neurodegenerative changes (including "senile plaques" and "neurofibrillary tangles") and reduced activity of acetylcholine and other neurotransmitters of the brain. The disease is the most common cause of dementia or progressive deterioration of thought processing and acquired intellectual abilities. Associated symptoms include initial forgetfulness with increasingly severe memory impairment; disorientation and confusion; loss of the ability to recognize familiar people or objects through sensory stimuli (agnosia); and speech disturbances. The disorder may also be characterized by restlessness and agitation; an increasingly impaired ability to conduct purposeful movements; personality disintegration; and symptoms of psychosis, such as the perception of sights, sounds, or other sensations in the absence of external stimuli (hallucinations) and false beliefs of persecution despite evidence to the contrary (paranoid delusions).

Ambulatory: able to walk, hence referring to a person who is not confined to a bed or wheelchair.

Amino acid: Organic compound necessary for forming peptides, a piece of protein, and proteins. Digestion relates the individual amino acids from food. More than 100 are found in nature, but only 22 occur in animals. In humans eight are essential for life: isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

Amniocentesis: A screening or diagnostic procedure during which a sample of amniotic fluid surrounding the developing fetus is withdrawn by means of an ultrasound-guided needle. The amniotic sac is a fluid-filled membrane that surrounds and protects the developing fetus within the uterus. Amniotic fluid contains cells that may be used for DNA analysis, chromosomal testing, and enzyme studies. Such testing may detect certain genetic disorders, metabolic diseases, chromosomal abnormalities, or developmental defects. This procedure is typically performed between the 14th and 18th week of pregnancy.

Amplitude: the "size" or "height" of a tremor; the extend or breadth of a tremor's range.

Analgesic:  A pain reliever.  There are various over the counter as well as prescription analgesics.

Analog: 1. something that is similar in appearance or function to another object. However, the source or final product differ. For example, the eye of a fly and the eye of a human are analogs. 2. A drug or other compound that acts like another substance but has different effects.

Antagonist: 1. A drug that blocks a receptor, preventing stimulation. .) A muscle whose contraction opposes an intended movement.

Antibodies: Specialized proteins that function as an essential part of the immune system. Antibodies are produced by certain white blood cells (B cells) in response to the presence of specific, usually foreign proteins (i.e., antigens), helping the body to neutralize and destroy the invading microorganism, foreign tissue cell, or other antigen in question.

Anticholinergics: Drugs that block the action of acetylcholine, a neurotransmitter whose effects oppose dopamine. By blocking acetylcholine's action, these drugs increase dopamine's ability to control movement.

Anticholinesterase: One of the drugs that inhibits or inactivates acetylcholinesterase (AChE). AChE is an enzyme that inactivates acetylcholine, a neurotransmitter at the junctions of nerve and muscle cells (neuromuscular junctions) that regulates the delivery of messages from neurons to muscle fibers. By inhibiting the activity of AChE, such medications allow high levels of acetylcholine to accumulate, potentially enabling repeated stimulation of nerves at neuromuscular junctions.

Anticonvulsant medication: An agent that prevents or arrests convulsive seizures.

Antigen: Any substance that, as a result of coming into contact with appropriate cells, induces a state of sensitivity and/or immune responsiveness after a latent period and that reacts in a demonstrable way with antibodies and/or immune cells of the sensitized subject. Antigens may be foreign proteins of invading microorganisms (e.g., viruses, bacteria), toxins, or foreign tissue cells (e.g., used in transplantation).

Antioxidants: Agents that inhibit or neutralize potentially harmful compounds known as free radicals. Free radicals are produced during metabolic activity. High levels of free radicals may eventually lead to impaired functioning and destruction of neurons and other bodily cells. Certain antioxidants are thought to neutralize free radicals before cellular damage occurs.

Antiscorbutic vitamin: the same thing as vitamin C.

Aorta: aorta is the largest artery of the body. An artery is a blood vessel that carries blood away from the heart. The aorta is classified as an elastic artery, meaning that it is a large artery that contains many elastic (flexible) layers in the middle of it.

Apoptosis: programmed cell death; deletion of cells by fragmentation into membrane-bound particles; a form of cell death in which certain enzymes activate to degrade DNA within the nucleus, resulting in cellular degeneration and loss.

Apraxia: Loss of the ability to sequence, coordinate, and execute certain purposeful movements and gestures in the absence of motor weakness, paralysis, or sensory impairments. Apraxia is thought to result from damage to the cerebral cortex, such as due to stroke, brain tumors, head injury, or infection. It may also occur as a result of impaired development of the cortex as in certain neurodevelopmental disorders, including Rett syndrome. Apraxia may affect almost any voluntary movements, including those required for proper eye gaze, walking, speaking, or writing.

Archimedes spirals: a relatively simple test used to evaluate tremor severity. During this test, the patient is asked to draw increasingly wider circles on a piece of paper.

Architectonics: is the same thing as cytoarchitecture. Cytoarchitecture is the arrangement, or pattern of arrangement, of cells in a body tissue, organ, or structure. A cell is the smallest, most basic unit of life, that is capable of existing by itself. When groups of cells of the same type come together with non-living material, they are called tissues. An example of cytoarchitecture is the arrangement of nerve cells in the brain, especially the outer, top part of the brain (known as the cerebral cortex). Cytoarchitecture is also known as cytoarchitectonics and architectonics.

Articulation: 1. The act of giving utterance or expression. Commonly used when discussing a patient’s speech production. 2. Distinct connected speech or enunciation. 3. A joining or connecting together loosely so as to allow motion between the parts.

Aspiration pneumonia:  A condition caused when an individual breathes instead of swallows food, saliva, liquids, or other foreign materials that cause a bacterial infection in one’s lungs.

Astereognosis: is an inability to recognize objects by touching them even though the sense of touch and proprioception is intact. Proprioception is the sense of being aware of the position and movements of the body. Astereognosis is caused by damage to the parietal lobe of the brain. The parietal lobe is the middle area of the top part of the brain. There is one parietal lobe on each side of the brain. For people with astereognosis, the damage in the parietal lobe is opposite to the side of the body where they cannot identify objects with touch. Thus, if a person with astereognosis cannot identify objects with his/her left hand, the damage is in the right parietal lobe. If a person with astereognosis cannot identify objects with his/her right hand, the damage is in the left parietal lobe.

Asterixis: involuntary, jerking or flapping movements, especially of the hands. Extending the patient's arm with the wrist bend in a backward position may induce this form of tremor, which may be associated with advanced liver disease.

Astrocyte: A type of glial cell. Glial cells are the connective tissue cells of the central nervous system (CNS), serving as the supportive structure that holds together and protects neurons. Astrocytes are relatively large glial cells with thread-like projections that connect with neurons and small blood vessels (capillaries). These projections form part of the so-called "blood-brain barrier." This barrier slows or prevents the passage of unwanted substances, such as harmful chemicals, infectious agents, etc., from the bloodstream into the brain. Astrocytes also accumulate in areas where nerves have been damaged (astrocytosis), sealing off these areas. An excess of astrocytes in damaged areas of the CNS is known as gliosis.

Atactalia: Loss of the sense of touch.

Ataxia: an inability to coordinate muscle activity during voluntary movement; most often due to disorders of the cerebellum or the posterior columns of the spinal cord; may involve the limbs, head, or trunk.

Ataxia-telangiectasia (AT): A hereditary, progressive disorder that typically becomes apparent in early childhood and is characterized by increasing neurodegenerative changes of the cerebellum, a brain region involved in producing coordinated voluntary movements, sustaining balance, and maintaining proper posture. Associated symptoms typically include delayed motor development; an unsteady, awkward manner of walking; drooling; impaired articulation of speech (dysarthria); dependence on thrusts of the head to achieve proper focusing of the eyes; and involuntary, rapid, jerky eye movements (nystagmus). Affected children may also develop involuntary, "shock-like" muscle spasms (myoclonus); sustained muscle contractions that result in repetitive twisting motions or unusual postures or positions (dystonia); or irregular, jerky, relatively rapid involuntary movements (chorea). AT is also typically associated with permanent widening (dilation) of groups of blood vessels (telangiectasias), particularly in sun-exposed skin regions of the face and the transparent membranes covering the whites of the eyes. In addition, AT is characterized by deficient functioning of the immune system (immunodeficiency), leading to recurrent respiratory and skin infections and an increased risk of certain malignancies (e.g., certain leukemias or lymphomas). The disorder, which is transmitted as an autosomal recessive trait, is thought to result from defective repair of DNA.

Athetosis: a condition in which movements are involuntary, slow, squirming, and continuous. These movements occur during flexing, extending (the opposite of flexing), supination (turning the palm up), and pronation (turning the palm down) of the hands and fingers. There may also be difficulty moving the toes, feet, face, or neck.

Atrophy: a decrease in size or functional activity of a part of the body (or the entire body) due to disease or other factors. Atrophy is usually used to mean that a part of the body is wasting away. Tissues, such as muscles and organs, can experience atrophy. Muscles can experience atrophy as a result of not using them. A good example is when muscles decrease in size as the result of wearing a cast. Disuse can also occur as a result of nerve damage, which makes it difficult or impossible to move a body part. Serious illness can also result in atrophy of muscles because the body needs to use up the reserves of proteins in the muscles for energy.

Atypical: Irregular; not standard or characteristic; not conforming to type.

Auditory: Pertaining to the sense of hearing or the organs involved in hearing.

Augmentation: A phenomenon that may occur as a result of the use of certain medications (particularly levodopa). Augmentation is characterized by the emergence of worsening symptoms earlier in the day (e.g., early evening, afternoon, or morning). Many people who take levodopa and some who take other dopamingeric agents develop augmentation, especially those who have severe symptoms or are taking high doses of the drug.

Autoimmune: Referring to an immune response against one's own tissues or organs. Autoimmune diseases result from abnormal immune reactions in which the actions of certain white blood cells (T cells) are directed against "self proteins" (autoantigens) or normal tissue components (i.e., cell-mediated immune response)--or in which specialized proteins (antibodies) produced in response to specific, usually foreign proteins (antigens) improperly act against certain of the body's own cells (i.e., antibody-mediated immune response). In certain disorders, the autoimmune process may be primarily directed against one organ, such as the thyroid gland in Hashimoto's thyroiditis or the pancreas in insulin-dependent diabetes mellitus, or may involve multiple organs and bodily systems, such as associated with systemic lupus erythematosus (lupus or SLE).

Autosomal dominant trait: Human traits, including an individual's eye color, hair color, or expression of certain diseases, result from the interaction of one gene inherited from the father and one gene from the mother. In autosomal dominant disorders, the presence of a single copy of a mutated gene may result in the disease. In other words, the mutated gene may dominate or "override" the instructions of the normal gene on the other chromosome, potentially leading to disease expression. Individuals with an autosomal dominant disease trait have a 50 percent risk of transmitting the mutated gene to their children.

Autosomal recessive trait: Human traits, including an individual's hair color, specific blood group, or expression of certain diseases, result from the interaction of one gene inherited from the mother and one from the father. With autosomal recessive disorders, two copies of the disease gene must be inherited in order for an individual to potentially develop the disease. If both the mother and father carry a copy of the disease gene, each child has a 25 percent risk of inheriting the two genes for the disease. There is a 50 percent risk that their children may inherit one copy of the disease gene and be carriers for the disease trait (heterozygous carriers). In addition, there is a 25 percent chance that the parents' offspring will inherit two normal copies of the gene and will not develop the disorder nor be carriers for this disease trait.

Autotherapy: has the following meanings in the field of medicine: 1. Self-treatment. 2. Being cured without any apparent help from an outside source. In other words, it is something that goes away by itself.

Axons: Nerve fibers. Axons are the relatively slender extensions of neurons that transmit nerve impulses away from nerve cell bodies. The ends of the axons or "terminals" release chemical substances known as neurotransmitters, enabling the transmission of nerve impulses to other neurons or effector organs. The whitish, fatty, protein-containing substance called myelin forms an insulating, protective, cylindrical sheath around some axons, serving to increase the speed and efficiency of nerve impulse transmissions.

Azygous: for something to occur as single or unpaired. For example, the spine is an azygous structure because it does not occur in pairs. There is only one spine. The eyes, ears, legs, and arms are examples of structures that are not azygous because they occur in sets of two.

Bacciform: means shaped like a berry.

Baclofen: A mainstay of antispastic drug therapy, baclofen is a GABA agonist that stimulates the GABAB autoreceptor. When stimulated, this autoreceptor inhibits muscle activity. In experimental systems, baclofen inhibits the release of noradrenaline, and acetylcholine from peripheral nerve endings, and acetylcholine, noradrenaline, dopamine, serotonin, and glutamate from CNS neurons.

Balint's syndrome: a condition characterized by optic ataxia (inability to move the hand to an object by using vision), ocular apraxia (inability to voluntarily control the gaze), and simultanagnosia (inability to recognize more than one object shown at the same time).

Ballismus: a type of involuntary movement in which one violently flings or jerks a limb (or limbs) in an uncoordinated manner. These swinging or jerky movements may be sporadic or continuous and, in some patients, restricted to one side of the body (hemiballismus). Ballismus often occurs in association with other abnormal involuntary movements, including athetosis, chorea, and dystonia. It is often associated with damage to the anatomical structure the subthalamic nucleus (an area in the lower part of your brain).

Basal ganglia: Specialized nerve cell clusters of gray matter deep within each cerebral hemisphere and the upper brainstem, including the striate body (caudate and lentiform nuclei) and other cells groups such as the subthalamic nucleus and substantia nigra. The basal ganglia assist in initiating and regulating movement.

Benign myoclonus of infancy: A neurologic condition that typically becomes apparent by approximately 4 months of age. Affected infants and children experience relatively short, mild episodes of myoclonus, or sudden, involuntary, "shock-like" spasms of muscles, particularly of the head, neck, trunk, and arms. Development is typically normal, and myoclonus usually ceases by age 2 or 3.

Benzodiazepines: A class of medications that act upon the central nervous system to reduce communication between certain neurons, lowering the level of activity in the brain. Benzodiazepines are effective in reducing anxiety, stress, or agitation; promoting sleep; alleviating restlessness; and relaxing muscles.

Beta-adrenergic receptor: a specialized molecular structure on the surface membrane of a neuron that selectively receives the neurotransmitter norepinephrine. Reception of this neurotransmitter causes changes in the neuron that increase its likelihood of "firing" or sending its own signal to other neurons. The activities of norepinephrine affect that part of the nervous system involved in the control of some involuntary body functions, such as blood pressure regulation, etc. (sympathetic nervous system).

Bifurcate: a descriptive term for an object that is forked or has two branches. Bifurcate can be used as a verb when an object is split into two branches. An example of something in the body that can be bifurcated are blood vessels.

Bilateral: 1. Relating to, or having, two sides. 2. Appearing or occurring on two sides; affecting both sides of the body. An example would be bilateral hearing loss, in which the person has difficulty hearing in both ears. 3. Affecting both organs, if the organs are paired.

Biopsy: 1. The process of removing living tissue or cells from organs or other body parts of patients for examination under a microscope or in a culture to help make a diagnosis, follow the course of a disease, or estimate a prognosis. A culture is an artificial way to grow cells or tissues in the laboratory. Most biopsies are minor procedures and do not require the area where the sample is being taken from to be numbed with medication. Some biopsies are more serious and/or complicated and require that the appropriate body part be numbed. 2. A sample obtained by using the above procedure. This definition of biopsy is more accurately known as a "biopsy specimen."

Biosynthesize: To form or produce (during normal physiologic functions) a chemical compound in the body.

Biphasic: having two distinct stages or phases.

Botulinum toxin (BTX): Any of a group of toxins, designated as A through G, produced by Clostridium botulinum bacteria. Localized injection of minute amounts of commercially prepared BTX may help to relax an overactive muscle by blocking the release of acetylcholine, a neurotransmitter responsible for the activation of muscle contractions. BTX-A is currently the only form (i.e., serotype) of botulinum toxin approved for clinical use. (BTX-A [BOTOX®] is produced by Allergan, Inc. and used in the United States and many other countries. Outside the U.S., it is available as Dysport® from Ipsen, Ltd.) It was originally introduced in the 1970s for the treatment of misalignment of the eyes (strabismus) and involuntary contraction of eyelid muscles (blepharospasm) associated with dystonia or facial nerve disorders. BTX-A is now increasingly being used as a therapeutic option for selected patients with other disorders characterized by severely increased muscle activity (hyperactivity), such as tremor, other focal dystonias, and spasticity. BTX-B is currently under investigation (by Athena Neurosciences, Inc.) for patients with cervical dystonia.

Bradykinesia: The gradual loss of spontaneous movement; slowness of voluntary movements.

Brainstem: The region of the brain consisting of the medulla oblongata, pons, and midbrain. The brainstem primarily contains white matter interspersed with some gray matter. This area of the brain serves as a two-way conduction path, conveying nerve impulses between other brain regions and the spinal cord. In addition, most of the 12 pairs of cranial nerves from the brain arise from the brainstem, regulating breathing, digestion, heartbeat, blood pressure, pupil size, swallowing, and other basic functions.

Bruxism: Involuntary grinding, clenching, or gnashing of the teeth, particularly during sleep or times of stress. Without appropriate protection, such as the use of night guards that cover the teeth, severe dental problems may result. Bruxism may also be a feature of certain neurologic movement disorders, including dystonia of the jaw, mouth, and lower face (oromandibular dystonia [OMD]), Rett syndrome, or tardive dyskinesia.

Carbonic anhydrase inhibitors: A class of medications that inhibits activity of the enzyme carbonic anhydrase. This zinc-containing enzyme is present in red blood cells and the renal tubules. These tubules form part of the filtering units of the kidneys. Carbonic anhydrase activity serves to accelerate the transfer of carbon dioxide from tissue to the blood and on to the lungs. By restricting carbonic anhydrase activity, these agents decrease hydrogen ion concentrations in the renal tubules, increasing excretion of sodium, potassium, bicarbonate, and water. Certain carbonic anhydrase inhibitors are prescribed to promote urinary production and excretion (i.e., as diuretics) or to lower fluid pressure within the eyes for those with glaucoma. They are also used to treat other conditions including epilepsy or certain neurologic movement disorders.

Carnitine: A natural substance found in skeletal and cardiac muscle and the liver. Carnitine serves to transport fatty acids across mitochondrial membranes, thereby playing an important role in energy production and the metabolism of fatty acids.

Caudal: 1. Pertaining to or being a tail. 2. A position towards the bottom, such as the lower part of the spine.

Caudate nuclei: One of the 3 major substructures that, together with the globus pallidus and putamen, form the basal ganglia. The caudate nuclei and putamen, which are relatively similar structurally and functionally, are collectively known as the striatum. Specialized clusters of nerve cells or nuclei within the caudate receive input from certain regions of the cerebral cortex. This information is processed and then relayed (by way of the thalamus) to areas of the brain responsible for controlling complex motor functions. The caudate nuclei are specifically thought to process and transmit cognitive information that influences the initiation of complex motor activities.

Cell: the smallest, most basic unit of life, which is capable of existing by itself. Cells carry out the chemical processes that are necessary for life to exist. They use energy and reproduce themselves. The bodies of living organisms are made up of cells. Some organisms are made up of only one cell. Human beings, on the other hand, are much more complex and are made up of billions of cells.

Central nervous system (CNS): The brain and spinal cord. The CNS, which receives sensory impulses from and sends motor impulses to the peripheral nervous system (i.e., nerves outside the CNS), plays an essential role in the coordination and control of the entire body.

Central oscillators: nerve cells that discharge signals that result in alternating fluctuations of electrical impulses along certain tracts of the central nervous system.

Cerebellar gait: a wide based type of gait (walking), in which one changes direction or falls forward, backwards, or to the side. The person with cerebellar gait walks in a staggered, unsteady, and irregular manner. People with cerebellar gait have difficulty turning. The feet are faced outward and the person comes down first on the heel and then on the toes. As the name implies, cerebellar gait is caused by a lesion(s) in the cerebellum (a part in the back of brain responsible for movement and coordination) or nerve pathways to and from the cerebellum. Cerebellar gait is also known as ataxic gait. Cerebellar comes from the Latin word "cerebellum" meaning "little brain," and the Old Norse word, "geta" or "gata," meaning "a way."

Cerebellum: A two-lobed region of the brain located behind the brainstem. The cerebellum receives messages concerning balance, posture, muscle tone, and muscle contraction or extension. Working in coordination with the basal ganglia and thalamus, the cerebellum integrates, adjusts, and refines messages transmitted to muscle groups from the cerebral cortex (i.e., motor cortex). Thus, the cerebellum plays an essential role in producing smooth, coordinated voluntary movements; maintaining proper posture; and sustaining balance.

Cerebral cortex: The outer region of the brain's cerebral hemispheres. Comprised of gray matter, the cerebral cortex contains several deep folds (gyri) and grooves (sulci or fissures). Two sulci divide the surfaces of both cerebral hemispheres into four distinct lobes that are named for overlying bones of the skull. These include the frontal, temporal, occipital, and parietal lobes. The cerebral hemispheres are joined by a thick band of nerve fibers known as the corpus callosum. The cerebral cortex is responsible for integrating higher mental functioning and conscious thought, sensations, and general movements.

Cerebrospinal fluid (CSF): The fluid that flows through and protects the 4 cavities (ventricles) of the brain, the spinal cord's central canal, and the space (known as the subarachnoid space) between the middle and inner layers of the membrane (meninges) enclosing the brain and spinal cord. Laboratory analysis of CSF, usually obtained via lumbar puncture, may help to diagnose central nervous system infections, certain tumors, or particular neurologic disorders. During lumbar puncture, CSF is removed from the spinal canal via a hollow needle inserted between certain bones of the spinal column within the lower back (i.e., usually the third and fourth lumbar vertebrae).

Cerebrum: originally referred to the largest portion of the brain, including practically all parts within the skull except the medulla, pons, and cerebellum. It now usually refers to the cerebral cortex and basal ganglia.

Chemodenervation: Interruption of a nerve impulse pathway via administration of a chemical substance, such as botulinum toxin (BTX). For example, intramuscular injections of BTX produce local relaxation of treated muscles by inhibiting the release of acetylcholine, a neurotransmitter that is present at the junctions of nerve and muscle cells and that regulates the delivery of messages from neurons to muscle fibers.

Chorea: Jerky, irregular, relatively rapid involuntary movement that primarily involves muscles of the face or extremities. Choreic movements are relatively simple and discrete or highly complex in nature. Although involuntary and purposeless, these movements are sometimes incorporated into deliberate movement patterns. When several choreic movements are present, they often appear relatively slow, writhing, or sinuous, resembling athetosis. Chorea may occur in association with certain neurodegenerative diseases, including Wilson's disease and Huntington's disease, or systemic disorders, such as lupus. In addition, chorea is a dominant feature in Sydenham's chorea (which occurs in childhood and adolecents) or may result from the use of certain medications, such as particular anticonvulsant or antipsychotic agents.

Chorionic villus sampling (CVS): A screening and diagnostic procedure performed during which tissue samples are obtained from a portion of the placenta using a specially guided needle via ultrasound. The placenta is the organ attached to the lining of the uterus that links the blood supplies of the developing fetus and the mother. The tissue sample is obtained from the layer from which the chorionic villi develop. Blood from the fetus flows through the umbilical cord to the placenta and enters minute blood vessels arranged in multiple "thread-like" projections or chorionic villi surrounded by maternal blood. Tissue samples obtained during chorionic villus sampling are analyzed to detect certain genetic or chromosomal abnormalities.

Chromosomes: The thread-like structures within the nuclei of cells comprised of DNA. Deoxyribonucleic acid or DNA carries genetic information involved in directing cellular activities, thus controlling bodily growth and functioning and determining the expression of inherited traits. The chromosomes contain thousands of hereditary units known as genes or segments of DNA molecules. The nuclei of all human cells--except the reproductive (i.e., egg and sperm) cells--normally contain 46 chromosomes, with one of each pair from the father and one from the mother. The chromosomal pairs are numbered from 1 through 22, as well as a 23rd pair that includes one X chromosome from the mother and an X or a Y chromosome from the father. In females, the 23rd pair consists of two X chromosomes, whereas males have one X and one Y chromosome. All chromosomes have a short arm known as "p" and a long arm known as "q." Both chromosomal arms are subdivided into numbered bands.

Chronic obstructive pulmonary disease (COPD): a general term for diseases that are characterized by long-term or permanent narrowing of small airways (known as bronchi) connected to the lungs.

Circadian: A rhythm of biological functions occurring in a 24-hour periodic cycle (e.g., sleeping, eating, etc.).

Clonus: movements characterized by alternate contractions and relaxations of a muscle, occurring in rapid succession. Clonus is frequently observed in conditions such as spasticity and certain seizure disorders.

Co-contraction: The simultaneous contraction of agonist and antagonist muscles.

Cogwheel phenomenon: rhythmic brief increase in resistance during passive movement about a joint.

Comorbid disease: A disease that coexists with another disease.

Computerized tomography (CT) imaging or scanning: An advanced diagnostic scanning technique during which cross-sectional images of tissues and organs are produced by passing x-rays through the body at various angles. In some cases, a contrast medium, which is opaque on x-rays, may be injected intravenously to produce enhanced images of certain tissues, organs, or blood vessels.

COMT inhibitors: Drugs that block catechol-O-methyltransferase, an enzyme that breaks down dopamine. COMT inhibitors include entacapone (Comtan®) and tolcapone (Tasmar®).

Confabulation: when someone unintentionally tries to cover up gaps in memories by fabricating experiences or situations through making up incorrect and bizarre responses to questions. These responses can sometimes appear detailed, fluent, and plausible. However, the responses are often unrelated to the original topic of conversation. Furthermore, people who confabulate do not seem to care about distorting the facts in the face of questions. Since confabulation is unintentional, it is not the same as lying.

Congenital: Existing at birth.

Contractures: Fixed resistance to passive stretching of certain muscles due to shortening or wasting (atrophy) of muscle fibers or the development of scar tissue (fibrosis) over joints.

Contralateral: the opposite side. For example, during brain surgery for essential tremor, if the surgery is performed on the right side of the brain, the left side of the body will be affected.

Controlled-release formulation: A form of a drug (e.g., L-dopa) that is absorbed slowly by the digestive system, prolonging the duration of effect of each dose.

Conventional neuroleptics: The older classes of antipsychotic medications that are not as commonly used.

Corticobasal degeneration (CBD): A slowly progressive disorder characterized by neurodegenerative changes of certain brain regions, including the cerebral cortex (particularly the frontal and parietal lobes) and parts of the basal ganglia. Most patients initially develop symptoms in their 60s or 70s. Primary findings may include stiffness (rigidity); slowness of movement (bradykinesia); loss of the ability to coordinate and execute certain purposeful movements of the arms or legs (limb apraxia); the sensation that a limb is not one's own ("alien limb phenomenon"); and other sensory abnormalities. Affected individuals may also develop slurred, labored speech (dysarthria); dystonia; and irregular, involuntary, "shock-like" contractions of certain muscle groups, particularly of the hands and forearms, that may be provoked or aggravated by voluntary movement and certain external stimuli (action and reflex myoclonus).

Corticospinal: Referring to or connecting the outer region of the brain (cerebral cortex) and the spinal cord.

Corticosteroid agents: Synthetic medications similar to corticosteroid hormones, which are naturally produced by the outer regions of the adrenal glands (adrenal cortex). Corticosteroid agents may be prescribed to treat inflammatory conditions; as long-term therapy to suppress the immune system (immunosuppressive therapy) in order to prevent rejection of a transplanted organ; as hormone replacement therapy for those with insufficient levels of natural corticosteroid hormones; or as therapy for other conditions. High dose, long-term corticosteroid therapy may result in various adverse effects, including an increased susceptibility to infection; osteoporosis, a bone disorder characterized by a progressive loss of bone mass; high blood pressure (hypertension); tissue swelling (edema); or retarded bone growth in children.

Cranial: Of or from the cranium or skull.

Cranial nerve nuclei: Specialized groups of nerve cells (nuclei) that give rise to and convey or receive impulses from sensory and motor constituents of the cranial nerves, which are the 12 pairs of nerves that emerge from the brain. These nerve pairs convey sensory impulses for various functions including taste, smell, hearing, and vision; motor impulses involved in controlling eye movements, chewing, swallowing, facial expressions, etc.; and impulses for transmission to certain organs and glands for regulation of various involuntary or autonomic activities.

Cranial neuropathy: Disease or damage of a cranial nerve or nerves. The cranial nerves are the 12 nerve pairs that arise directly from the brain and are involved in conveying impulses for various functions including smell, hearing, vision, and taste; pupil contraction; eye movements; facial expressions; movements of the tongue, head, and shoulders; etc. Cranial neuropathy may result in associated muscle weakness; abnormal sensations, such as numbness, tingling, or pain; or other findings. Specific symptoms depend upon the specific nerve(s) affected.

Creutzfeldt-Jakob disease (CJD): A rare, degenerative, life-threatening brain disorder characterized by severe, progressive dementia; visual disturbances; muscle weakness; and abnormal involuntary movements, such as sudden, brief, "shock-like" muscle spasms (myoclonus), tremor, and relatively slow writhing motions that appear to flow into one another (athetosis). Although CJD usually appears to occur sporadically, about 10 percent of cases are familial, potentially suggesting a hereditary predisposition to the disease. In rare cases, CJD may also result from exposure to contaminated surgical instruments during brain surgery and was reported in the past due to therapy with pituitary-derived human growth hormone. In addition, a variant form of CJD (V-CJD) has been reported primarily in the United Kingdom; V-CJD has been potentially linked to consumption of beef from cows with bovine spongiform encephalopathy or BSE (so-called "mad cow disease"). Evidence suggests that CJD may be caused by mutations in the human prion* protein gene or contamination with abnormal prion protein. (*The term "prion" was named for "protein infectious agent.") Changes in the prion protein appear to lead to distinctive neurodegenerative abnormalities, i.e., relatively small, round, "sponge-like" (spongiform) cavities or gaps in certain brain regions. CJD and BSE belong to a group of related neurodegenerative disorders categorized as "transmissible spongiform encephalopathies."

Crossreact: Refers to the interaction of an antibody with an antigen that did not specifically or originally trigger its production. For example, in an abnormal autoimmune reaction, an antibody that was initially formed in response to an invading bacterium may inappropriately react against certain of the body's own tissues that contain some of the same amino acid sequences as within the microorganism.

Cytoarchitecture: the arrangement, or pattern of arrangement, of cells in a body tissue, organ, or structure. A cell is the smallest, most basic unit of life, that is capable of existing by itself. When groups of cells of the same type come together with living or non-living material, they are called tissues. An example of cytoarchitecture is the arrangement of nerve cells in the brain, especially the outer, top part of the brain (known as the cerebral cortex). Cytoarchitecture is also known as cytoarchitectonics and architectonics.

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