Appendix B
Glossary


Absorbed dose:

Mean energy imparted by ionizing radiation to an object per unit mass (units: gray [Gy], rad).

Acquired immunity:

Specific immune response to antigens, pathogens, tumors, and other foreign agents. It requires previous exposure to an agent, and the response is specific to the agent.

Acute effects of radiation:

Effects that occur shortly after exposure to radiation, usually within a week. They result from exposure to radiation at relatively high doses, generally more than 1 Gy, and are usually due to the killing of cells in critical tissues in the body.

Acute phase proteins:

Proteins secreted by the liver into the bloodstream to help the body respond to injury and stress.

Advanced Life Support System:

Current NASA program in bioregenerative and nonbiological life support systems; supersedes the CELSS program.

Affect:

Experience of emotion or feeling.

Affiliative need:

Need to belong to a social group or to be in the presence of others.

Aldosterone:

Hormone secreted by the renal cortex of the kidney and involved in the regulation of fluid and electrolyte balance.

Alveolus:

Smallest unit of the lung, an air sac in which gas exchange occurs.

Amyloplast:

Plant organelle that contains starch; because of its density, it sediments in response to the direction of the gravity vector.



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--> Appendix B Glossary Absorbed dose: Mean energy imparted by ionizing radiation to an object per unit mass (units: gray [Gy], rad). Acquired immunity: Specific immune response to antigens, pathogens, tumors, and other foreign agents. It requires previous exposure to an agent, and the response is specific to the agent. Acute effects of radiation: Effects that occur shortly after exposure to radiation, usually within a week. They result from exposure to radiation at relatively high doses, generally more than 1 Gy, and are usually due to the killing of cells in critical tissues in the body. Acute phase proteins: Proteins secreted by the liver into the bloodstream to help the body respond to injury and stress. Advanced Life Support System: Current NASA program in bioregenerative and nonbiological life support systems; supersedes the CELSS program. Affect: Experience of emotion or feeling. Affiliative need: Need to belong to a social group or to be in the presence of others. Aldosterone: Hormone secreted by the renal cortex of the kidney and involved in the regulation of fluid and electrolyte balance. Alveolus: Smallest unit of the lung, an air sac in which gas exchange occurs. Amyloplast: Plant organelle that contains starch; because of its density, it sediments in response to the direction of the gravity vector.

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--> Anabolic: Denoting a process by which living organisms convert simple substances into the complex materials of living tissue. Anaerobiosis: Life in the absence of oxygen. Anlage: Early stage of tissue development. Anterograde amnesia: Loss of memory of events following trauma or shock. Anteroposterior axis: Imaginary line running from head to tail in an animal. Anthropometrics: Measurement of the dimensions of the human body (e.g., height, weight, length of trunk and limbs, and head and neck diameter). Antibody: Member of a class of proteins called immunoglobulins, found in blood and other secretions and produced by lymphocytes in response to exposure to specific antigens with which they react. Antigen: Substance recognized as foreign by the host immune system and that induces an immune response. Arabidopsis thaliana: Flowering plant of the mustard family used for genetic and developmental studies of plants because of its small size, short generation time, and ease of laboratory culture. Arthropod: Animal of the phylum that includes insects and crustaceans. Ascinus: Ventilatory unit in the lung that includes terminal bronchioles and alveoli. Asthenia: Lack or loss of strength; weakness or debility. Ataxia telangiectasia: Disorder inherited as a recessive trait, characterized by neurological changes, immunological deficiency, increased susceptibility to cancer, and increased cellular radiosensitivity. Autocrine: Denoting a mode of action in which a hormone is secreted from one part of a cell to act on another site within the same cell. Autonomic nervous system: Complex nerve network that connects the central nervous system with the glands and smooth muscles of the body. Autonomic vasomotor: Regulation of vascular peripheral resistance through parasympathetic and sympathetic neural stimulation of arteriole dilation and constriction. Auxin: Plant hormone thought to be involved in control of gravitropism in plants; auxins regulate the rate of cell elongation, as well as a number of other developmental processes, in plants. Basal energy expenditure: Energy expenditure by the body at rest. B cells: Lymphocytes that play a major role in antibody production. Biomechanics: Study of biological principles underlying motion in living organisms. Bion: Russian space capsule that can support animals (e.g., monkeys, rats) and insects in orbit for up to 3 weeks.

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--> Blastocyst: Early stage of a placental mammalian embryo. Blastogenesis: Response of lymphocytes to exposure to an antigen or mitogen that involves their rapid and extensive replication, an indication of the host's ability to carry out an immune response. Bone mineral content: Measure of bone mass based on absorption of x rays by the calcium in the bone. Bone mineral density: Bone mineral content corrected for bone size (bone mineral content/cross-sectional area). Bone morphogenetic proteins: Large family of related molecules with proven role in development, discovered because of their ability to induce bone formation when injected into muscle or skin. Brachiation: Use of upper extremity for grasping and ambulation. Brassica rapa: Rapid cycling flowering plant used for developmental studies because of a short generation time, small stature, and hardy growth habit. Caenorhabditis elegans: Nematode worm used in laboratory studies of genetics and development because of its short life cycle and simple structure. Caloric stimulation: Infusion of cool or warm water (or air) into the external auditory canal to stimulate a horizontal semicircular canal. Cancellous: Spongy, honeycomb-like interior of most bones. Cell cycle: Complete generation of a cell, whose progress is tightly regulated at numerous steps. Cell-killing effect: Cessation of cell division and/or metabolism. Sufficient doses of radiation can kill cells in the body, and this cell death is responsible for most of the acute effects of radiation. Cell-mediated immunity: Specific immunity induced by previous exposure to an antigen or pathogen that can be protective to the host. Cell transformation: Process by which cells in vitro, which have a limited ability to divide, are altered by radiation or chemicals so as to have unlimited potential for division. Chrondrocyte: Cartilage cell. Circadian rhythms: Regular biological cycles of sleep and activity characteristic of each species that synchronize an organism's internal environment with daily events in its surroundings. Cis-acting factor or element: DNA that binds to DNA to regulate gene expression. Clinostat: Apparatus for rotating an object around its longitudinal axis. When this axis is horizontal (horizontal clinostat), the effect is to cancel out the direction of the gravity vector. Although this may mimic weightlessness, it is not identical to it. Clone: Group of identical cells or organisms related by descent from a common ancestor.

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--> Closed Ecological Life Support System: NASA program directed toward developing a plant-based system for spaceflight or extraterrestrial bases, to produce food and recycle waste products such as carbon dioxide. (Now part of the ALS program.) Coleoptile: Sheath that covers the first leaf of a germinating monocot seedling (e.g., corn, oats) and protects the leaf from soil abrasion. Its growth is due primarily to rapid, auxin-regulated cell elongation. Colony stimulating factor: Cytokine that regulates the production of white blood cells from bone marrow. Conserved gene: A gene whose sequence is closely related in widely different types of organisms. Coriolis acceleration: Inertial acceleration that arises when an object moves linearly with respect to a rotating reference frame. Costamere: Protein complex consisting of cytoskeleton, transmembrane glycoproteins, and extracellular matrix that is at the level of Z bands involved in transferring tension from contractile elements to connective tissue and may serve as a mechanosensor for signal transduction. Cupula: Dome-like structure that is part of the sensory receptor apparatus of a semicircular canal. Cyclooxygenase 2: Enzyme that synthesizes prostaglandins and related substances. Cytogenetic: Denoting the association of genes with particular locations on chromosomes. Cytokine: Class of hormones that mediate immune (and other) responses. Cytotoxic T cells: Lymphocytes responsible for specific destruction of pathogens and tumors after previous exposure. Danio rerio: Freshwater fish (zebrafish) used in the study of developmental genetics because of its small size, transparency, rapid growth, and ease of culture. Delayed-type hypersensitivity: Immune reaction observed in skin, characterized by swelling, reddening, and hardness. Deterministic effects: Formerly known as nonstochastic effects, these may appear early or late after irradiation of an organism. Most deterministic effects involve cell killing. Diastolic: The minimal blood pressure that occurs between beats while the heart is relaxed (e.g., for a blood pressure of 120/80, the diastolic pressure is 80). Differential cDNA libraries: Collections of molecularly cloned DNA fragments representing the RNA molecules found in a specified subset of cell types. Differential display: Method for identifying different RNA molecules found in a collection of cells. Differentiation: Change of a cell from a generalized to a specialized type.

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--> Distal elongation zone: Region of a root just behind the meristem, in which cells produced in the meristem first begin to elongate as well as increase in width. Dominant negative: Mutant protein that prevents the action of its normal counterpart. Dose: See absorbed dose. Dose-effect (dose-response) model: Mathematical formulation used to predict the magnitude of an effect produced by a given dose of radiation. Dose equivalent: See equivalent dose. Dose rate: Quantity of absorbed dose delivered per unit of time. Dystrophin: Cytoskeletal protein whose absence causes Duchenne muscular dystrophy. Electrogastrogram: Recording of the electrical activity of the gastrointestinal system made with surface electrodes. Electromyographic: Physiological recording of muscle action potentials generated during muscle contractile activity. Electron volt: Unit of energy (equal to 1.6 × 10-19 J) (1 eV is equivalent to the amount of energy gained by an electron passing through a potential difference of 1 V). Embryonic stem cells: Cultured early embryonic mouse cells that, after injection into developing mouse embryos, can be used to produce clones of differentiated cells in a mature mouse, including germ cells. Endochondral: Bone development on a cartilage scaffold. Endocrine: Pertaining to secretions from one organ that have a specific effect on another organ. Endolymph: Fluid within the membranous labyrinth of the inner ear. Endothelial cells: Cells that line blood vessels. Endothelin: Regulatory peptide produced by endothelial cells. Enkephalin: Neuropeptide related to pain control. Enzyme-linked immunosorbent assay: Sensitive and rapid measurement based on the selective binding of antibodies. Epinephrine: Catecholamine hormone produced by adrenal in response to stress. Epiphyses: Extremities of long bones where growth and development occur. Epitope: Portion of an antigen that determines its capacity to combine with its corresponding antibody in an antigen-antibody interaction. Equivalent dose: Absorbed dose averaged over an organ or tissue and weighted for the radiation quality and type of radiation involved. Ethylene: Gaseous hormone produced by plants that adversely affects plant growth.

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--> Excess cancers: Number of individuals in a population who develop cancer over and above the number that would be expected to do so normally. Expansins: Family of cell-wall localized plant proteins involved in loosening cell walls and permitting plant cells to expand. Extensor digitorum longus: Anterior leg muscle that straightens the toes and lifts them and the foot off the ground. Extravasation: Escape of blood from a vessel into the connective tissue. Fast-twitch fibers: Skeletal muscle fibers that contract rapidly and are maximally activated for each muscle fiber action potential. Fibroblast growth factor: Family of regulatory peptides that stimulate cell proliferation. Fibronectin: Large extracellular glycoprotein with binding domains for heparin, cell, and collagen. Flexor: Muscle that acts to decrease the angle of a joint. Flight simulation: Ground-based training or experimental condition designed to simulate specific conditions of long-duration missions, including microgravity, isolation, and confinement. Focal adhesion kinase: Enzyme activated by cell adhesion. Forebrain: Top part of the brain mass, including the cerebrum, corpus callosum, thalamus, and hypothalamus. Fractionation: Delivery of a given total dose of a radiation as several smaller doses, separated by intervals of time. Functional magnetic resonance imaging: Technique for measuring the structural and functional organization of the brain. Galactic cosmic rays: Ionizing radiation originating from the galaxy. Gamma rays: Short-wavelength electromagnetic radiation of nuclear origin with an energy of about 10 keV to 9 MeV. Genetic effects of radiation: Effects that arise from damage to genes in the germ cells of a parent which do not appear in the parent but may be passed on to offspring. Genetic screen: Search through the progeny of a mutagenized population for particular types of mutants. Genome adaptation syndrome: Model of the effects of stress on physical and psychological mechanisms, developed by Hans Selye. Glucocorticoids: Steroid hormones produced by the adrenal cortex (cortisol, hydrocortisone, corticosterone) during stress. Gluconeogenesis: Synthesis of glucose from noncarbohydrate precursors. Golgi tendon organ: Sensory receptor within a muscle tendon that responds to tension.

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--> Granulopoiesis: Formation of neutrophils and other white blood cells. Gravitaxis: Swimming of an organism in a particular direction in response to the direction of the gravity vector. Gravitropism: Orientation of plant stems and roots in response to the force of gravity. Gray (Gy): SI unit of absorbed dose, equal to the energy transferred by ionizing radiation to a mass of matter corresponding to 1 joule/kg (equals 100 rads). Growth factor: A protein that signals cells to divide or differentiate. G-threshold: Minimum value of gravity that elicits a response in an organism. Haptic: Relating to or based on sense of touch (i.e., arising from grasping, manipulating, or touching with the hand, or from contact with the body surface). Helix-loop-helix superfamily: Family of proteins that dimerize with each other in order to bind to DNA and activate gene expression. Helper T cells: Lymphocytes responsible for assisting B cells in making specific antibodies and helping cytotoxic T cells to kill their targets. Hemopoiesis: Generation of blood cells in the bone marrow. Hindlimb immobilization: Reduction of mechanical load and function in the hindlimb of an experimental animal model. Hindlimb unloading: Simulation of microgravity unloading of muscles in an animal model by employing various harnessing strategies to elevate the animal and prevent its hindlimbs from weight bearing on the floor. Hippocampus: Part of the brain that is essential to the transfer of information from short-term to long-term memory. Histogenesis: Process of formation of a tissue, with its characteristic cell types, from a progenitor cell population. Histomorphometry: Computer-aided quantitative histology. Homeobox: Domain found in a number of proteins that bind DNA and thereby regulate different genes; many of these proteins are necessary for critical events in animal development. Homologous recombination: Replacement of a gene or segment of DNA with the corresponding gene or segment from another source. H-reflex: Hoffman's reflex; reflexive contraction of a muscle elicited by electrical activation of muscle afferent fibers. Humoral factors: Hormones and other substances delivered via the circulation. Humoral immunity: Specific antibody-mediated immunity that is most effective against extracellular pathogens. Hypercortisolinemia: Excess of cortisol. Hypermetabolism: Abnormally increased utilization of material by the body.

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--> Hypocotyl: Portion of a plant stem between the root and the first pair of leaves (i.e., the cotyledons). Hypophysectomized: Describing an animal whose pituitary has been removed. Hypothalamic-pituitary-adrenal axis: Neuroendocrine axis that interacts with the immune response. Mediates response to stress and other factors which could have a profound effect on immune responses and resistance. HZE particles: Heavy (high-atomic-number), high-energy particles (e.g., carbon or iron nuclei) in cosmic rays, with an energy range of about 102 to 103 MeV per nucleon. Immunoglobulin: Serum glycoprotein that can bind specific molecules. Innate immunity: Nonspecific immunity that is always present and constitutes the first defense against pathogens. Insulin-like growth factor: Proteins related to insulin that influence the survival, metabolism, and proliferation of cells. Integrins: Dimeric cell surface receptors that mediate cell-cell and cell-matrix interaction and are involved in attachment, migration, and signaling. Interferons: Cytokines that play a fundamental role in regulation of immune response. Interleukins: Cytokines that play a major regulatory role in immune response. Interstitium: Extracellular connective tissue between cells. Intronic enhancer: DNA sequence that activates transcription from a promoter by serving as a specific binding site for gene regulatory proteins. Ionizing radiation: Radiation that can penetrate and deposit its energy at random within cells and tissues by ejecting electrons from atoms, thereby ionizing them. Ischemic necrosis: Cell and tissue death resulting from severe reduction of blood flow. Knockout mice: Mice from which certain genes have been removed. Knockout mutation: Deletion or mutation that results in the inability to manufacture the protein product of the mutant gene. Labyrinthectomy: Destruction of the labyrinth of the inner ear. Laminin-2 (merosin): Member of the family of basement membrane proteins present in striated muscle. Latent period: Time between exposure to an agent and expression of a disease. Latent viral epitope: Inactive virus to which carriers will demonstrate an immune response. Linear energy transfer: Average amount of energy lost per unit of particle track length. Low-LET radiation is characterized by light charged particles such as electrons. High-LET radiation is characterized by heavy charged particles such as alpha particles and heavy nuclei. Lymphocyte: Type of white blood cell responsible for immunological function.

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--> Macrophage: White blood cell that can nonspecifically engulf pathogens and destroy them. Mechanostat: Hypothetical mechanism (receptor) that senses mechanical strain and maintains bone mass accordingly. Mitogen: Substance that induces cell division. Molecular cascade: Group of proteins that act sequentially in the same process. Muscle spindle: Sensory receptor within a muscle that is sensitive to its length and rate of change in length; spindle sensitivity is controlled by gamma motoneurons of the spinal cord innervating striated muscle fibers. Musculovenous pumping: Compressive squeezing of blood from intramuscular veins by contracting skeletal muscle fibers. Myocyte: Differentiated skeletal, cardiac, or smooth muscle cell. MyoD family: Closely related myogenic proteins that can activate the expression of muscle-specific genes. Myopathy: Degenerative disease of muscle thought to originate from a primary defect in muscle tissue. Natural killer cells: Cells found in the body under normal conditions that can destroy target virus-infected and tumor cells by an as yet unknown recognition mechanism. Neoplastically transformed cells: Tissue culture cells changed in vitro from growing in an orderly pattern and exhibiting contact inhibition to growing in a pattern more like that of cancer cells. Neural imaging: Term used to refer to the application of dyes or radioactive compounds that allow the visualization of neuron morphology (e.g., computer tomography, magnetic resonance imaging). Neurolab: Shuttle mission (1998) dedicated to studies in the neurosciences. Neuroplasticity: Long-term changes in structure or function exhibited by neurons after changes in their activity. Neutrophil: Type of white blood cell. Nonstochastic effect: See deterministic effects. Nuclear matrix: Fibrous material within the cell nucleus. Nystagmus: Rhythmic oscillation of the eyeballs. Oculomotor: Related to the control of eye movement. Os calcis: Hell bone. Osteoblast: Bone-forming cell. Osteocalcin: Protein selectively expressed by bone-forming cells and present in bone matrix. Osteoclast: Multinucleated, bone-resorbing cell related to macrophages.

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--> Osteogenic: Forming bone. Otoconia: Calcium carbonate crystals embedded in a gelatinous membrane of the inner ear, whose motion depends on acceleration. Otolith: Component of the inner ear sensitive to linear acceleration. Oxidative burst: Sign of neutrophil function indicating oxygen-based destruction of a foreign invader. Paracrine: Referring to a mode of action in which hormones act on neighboring cells. Parasympathetic: Pertaining to the division of the autonomic nervous system that controls most of the basic metabolic functions essential for life. Parathyroid hormone: Hormone produced by the parathyroid gland in response to low plasma calcium. Patch clamp: Method for measuring voltage and current across cell membranes by using glass microelectrodes. Periosteum: Exterior layer of bone composed of several cell layers and fibrous tissue. Plantar flexion: Movement at the ankle joint that results in the ball of the foot moving toward the floor. Plasmodesmata: Complex cytoplasmic tubes that connect the cytoplasm of two plant cells and allow small molecules to pass freely between the cells. Polar auxin transport: Cell-to-cell movement of auxin in plants. Prolactin: Hormone responsible for control of lactation in females. Proprioception: Conscious awareness of the positions of various parts of the body in space provided by joint and muscle sensory inputs; also, sensory stimulation arising from receptors within muscles. Prostaglandin: Regulatory molecule derived from lipids that act locally. Protonema: Filament of cells that grows from a fern spore. Proto-oncogene: Gene that codes for proteins involved in the signaling pathways stimulated by growth factor; when mutated, this gene causes cancer and is called an oncogene. Protoplasts: Plant cells from which the cell wall has been removed. Quality factor: LET-dependent factor by which absorbed doses are multiplied to obtain the equivalent dose for radiation protection purposes. Radiosensitivity: Relative susceptibility of cells, tissues, organs, and organisms to the injurious action of radiation. Recall antigen: Standard testing antigen to which most individuals have been exposed and will respond by exhibiting delayed hypersensitivity. Relative biological effectiveness: Biological potency of one type of radiation compared with another that produces the same biological end point.

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--> Reporter genes: Genes that, when fused to the gene of interest, yield a hybrid protein that can be readily detected or visualized in intact cells and tissues. Reticulocyte: Young red blood cell. Retrograde amnesia: Loss of memory for events during a circumscribed period prior to a traumatic event, brain injury, or damage. Rhizoid: Root-like organ of some algae, mosses, and ferns that consists of a single cell or filament of cells. Root cap: A protective cap of cells at the distal end of a root that covers its meristem and senses the direction of gravity. Saccadic: Pertaining to rapid movement of the eyes from one position to another. Saccule: Smaller chamber of the membranous labyrinth of the inner ear. Sarcolemma: Connective tissue sheath surrounding a muscle fiber that is composed of the muscle plasma membrane, the basement membrane, and the adjoining connective tissue or endomysium. Sarcomere: Fundamental unit of contraction arranged in repeating series to form myofibrils within a striated muscle cell. Saturation mutagenesis: Treatment of a sufficient population of some organism with a mutagen, so that descendants have a high probability of exhibiting mutations in all genes. Sensory deprivation: Term referring to the deficits that a neuron undergoes after losing input from part of its afferent fibers due to nerve lesion, exposure to drugs, or decreased exposure to adequate sensory stimulus. Sievert: SI unit of radiation equivalent dose, equal to dose in grays times a quality factor, times other modifying factors (1 sievert = 100 rem). Signal transduction: Process by which a cell responds to a stimulus. Skylab: Prototype U.S. space station that flew three missions of 28, 56, and 84 days' duration in the early 1970s. Sled: Rail-mounted device for delivering controlled linear acceleration to a subject positioned on it. Slow-twitch fibers: Skeletal muscle fibers that contract slowly and are maximally activated for each muscle fiber action potential. Solar maximum: Period of maximum probability of emission of solar event radiation (e.g., protons, alpha radiation, electromagnetic energy). Solar minimum: Period of minimum probability of emission of solar event radiation. Solar particle event: Flux of energetic ions and/or electrons of solar origin. Soleus: Slowly contracting muscle in the leg that produces plantar flexion of the foot to raise the body up against gravity.

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--> Somatic effects of radiation: Effects arising from damage produced in various tissues of an irradiated individual's body. Somatosensory: Pertaining to sensory stimulation of tactile receptors of the body surface. Somites: Paired blocks of mesodermally derived cells organized segmentally along the developing spinal cord to generate bone (vertebrae) and skeletal muscle cells. Sopite syndrome: Form of motion sickness associated with prolonged exposure to unusual motion or gravity conditions whose primary features are drowsiness, fatigue, lack of initiative, apathy, and irritability. Spacelab: Module specially constructed for the Shuttle payload bay for use in scientific experiments; a self-contained laboratory that can be loaded onto shuttle missions as needed. Space map: Organization of groups of neurons into specific arrangements in the central nervous system that reflect the structure and/or function of the spatial environment representing the sensory or motor systems. Spemann's organizer: Region of developing amphibian embryo in which cells play a critical role in signaling positional information to nearby cells. Statolith: Dense body within a plant cell, whose sedimentation provides information to the cell about the direction of the gravity vector. Stochastic effects: Effects resulting from random events; their probability of occurrence in an exposed population of cells or individuals is a direct function of dose; these effects are commonly regarded as having no threshold. Stromal cells: Resident cells in the bone marrow that do not develop into blood cells but may support this process; they can differentiate into bone or fat cells. Subtractive hybridization: Method by which genes expressed in a tissue-specific manner can be enriched for cloning. Sympathetic: Pertaining to the division of the autonomic nervous system that is active in emergency conditions of extreme cold, violent effort, and emotions. Systolic: Peak blood pressure that occurs at the end of heart contraction (e.g., for a blood pressure of 120/80, the systolic pressure is 120). Telomere: Special DNA structure at the ends of chromosomes. Th1 cytokine profile: Profile of cytokines produced by helper T cells that promote the development of a cell-mediated immune response. Th2 cytokine profile: Profile of cytokines produced by helper T cells that promote the development of an antibody-mediated immune response. Third-quarter phenomenon: Marked decline in individual and group performance believed to occur between the midpoint and the beginning of the last quarter of a specified period of isolation and confinement. Thrombin: Protease that stimulates clotting and other cellular processes via a specific receptor.

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--> Tibialis anterior: Predominantly fast muscle that dorsiflexes the ball of the foot off the floor during walking. Tonus: Muscle tension that is present independent of voluntary innervation. Topographic: Orderly representation reflecting regional anatomy and/or function. Trans-acting factor: Protein that binds to DNA and regulates gene expression. Transformed cell lines: See cell transformation. Transgenic mice: Mice into which genetic material from another organism has been experimentally inserted. Transport code: Computer program that calculates the particle distributions and energy behind a specific shield, derived from the basic nuclear cross sections for interactions and fragmentation in shielding. Trochanter: Anatomical region in the hip bone. Tropic response: Change in direction of growth by a plant organ in response to an environmental factor such as the direction of gravity. Tumor growth factor-: Regulatory peptide that stimulates bone formation when injected into bone. Tumor necrosis factor: Peptide cytokine involved in inflammation; produced by macrophages. Tumor necrosis factor-: Cytokine that destroys tumor cells and regulates immune responses. Tumor suppressor: Gene or gene product that acts to regulate cell replication. T-wave: Brain wave having a frequency of only 5 to 7 cycles per second. Ubiquitination: Covalent linkage of a small protein, ubiquitin, to the lysine amino groups of proteins, which targets them for proteolysis. Vestibular: Pertaining to the sensory system concerned with maintenance of posture and balance by perceiving gravity (linear acceleration) and rotary movements of the head. Vestibular hair cells: Cells in the vestibular system of the inner ear. Vestibular ocular reflex: Passive eye movements elicited by activation of receptors in the vestibular apparatus. Visual field: Entire expanse of space visible at a given instant without moving the eyes. Water use efficiency: Ratio of CO2 assimilated into carbohydrates during photosynthesis to H2O lost from leaves by transpiration; a high ratio is desirable for maximum efficiency in plants. Xeroderma pigmentosum: Inherited disease in which individuals are highly susceptible to sun-induced cancer. Z-axis: Longitudinal axis of the body. Zeitgeber: Periodic feature in the social and physical environment that entrains the circadian rhythms of an organism.