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Acetylcholine
A chemical neurotransmitter that functions in the transmission at all neuromuscular and glandular junctions of the peripheral nervous system, as well as many synapses within the CNS.
Acetylcholinesterase
An enzyme found in the synaptic cleft that breaks down and inactivates acetylcholine molecules, thereby controlling neurotransmission.
Acetylcholinesterase Inhibitors
Chemicals that inhibit the enzyme acetylcholinesterase at neural synapses.  This prevents the acetylcholinesterase from stopping the action of acetylcholine and allows for continued stimulation of the effector.  The result is spasms and paralysis, which can cause paralysis and death.  Some important acetylcholinesterase inhibitors are organophosphate pesticides, carbamates, and some wargases.
Action potential
A conducted change in the membrane potential of cells, initiated by an alteration of the membrane permeability to sodium ions, and subsequent propagation of an electrical impulse down an axon.  Same as nerve impulse.
Adenosine triphosphate (ATP)
An important high-energy compound located in the cytoplasm of cells, which serves as a source of cellular energy.
Afferent nerve
A nerve that relays sensory information to the CNS.
Amyotrophic Lateral Sclerosis
A disease in which the myelin around nerves is lost causing paralysis and loss of sensory and motor function.  Same as Lou Gehrig's Disease.
Anaplasia
An alteration of cells from normal appearance to poorly-differentiated or undifferentiated morphology.  They have irregular nuclei and cell structure with numerous mitotic figures.  Anaplasia is frequently associated with malignancies and serves as one criterion for grading the aggressiveness of a cancer.
Anoxia
The lack of oxygen supply to tissues.
Anxiety
A feeling of apprehension, uncertainty, and fear without apparent stimulus, and associated with tachycardia, sweating and tremors.
Apoptosis
Individual or single cell death by a process of self-destruction of the cell nucleus.  In apoptosis, dying cells are not contiguous but are scattered throughout a tissue.  Often referred to as "programmed cell death".
Astrocyte
A type of glial cell in the CNS.  They are big cells that maintain the blood-brain barrier and provide rigidity to the brain structure.
Atrophy
A decrease in the size of cells.  If a sufficient number of cells are involved, the tissue or organ may also decrease in size.
Atropine
An anticholinergic drug that blocks acetylcholine receptors.
Autonomic Nervous System
The part of the nervous system involved in the unconscious regulation of visceral functions by transmitting motor information to smooth muscles, cardiac muscle, and various glands.
Axon
The elongation of a neuron that conducts an action potential.  It may extend long distances from one part of the body to another.
Batrachotoxin
A potent neurotoxin of some South American frogs that has been used as arrow poisons.
Benign Tumor
A tumor that does not metastasize or invade surrounding tissue.
Blood-Brain Barrier
The anatomical barrier that isolates the CNS from the general circulation.  The cell responsible is the astrocyte, which forms, layers around capillaries and regulates diffusion of substances from the blood circulation to the neurons.
Botulinum toxin
A potent neurotoxin that blocks the release of acetylcholine at neuromuscular junctions.
b-Bungarotoxin
A potent neurotoxin (venom) of elapid snakes that prevents the release of neurotransmitters, thus causing paralysis and death.
Cancer
A tumor that is malignant, that is, it has the ability to metastasize or invade into surrounding tissues.
Carbohydrates
Organic compounds that serve as sources of energy for the body.  They are converted to glucose, which in turn is used by the cells in cell respiration.
Carcinogenesis
A general term for production of any type of tumor.
Carcinoma
A malignant tumor arising in epithelium.  It is the most common form of cancer and usually spreads via the lymphatic system.
Cardiovascular System
The organ system that transports oxygen and nutrients to tissues and removes waste products.  The main components are the heart, blood, and blood vessels.
Cell Membrane
A phospholipid bilayer which also contains cholesterol and proteins.  It functions to provide support and controls passage of all materials into or out of the cell.
Cell
The smallest living unit in the body.
Cell Proliferation
The process by which cells undergo mitosis and divide into similar cells.
Cellular Swelling
A pathologic condition of a cell that is associated with hypertrophy.  It is due to cellular hypoxia, which damages the sodium-potassium membrane pump.  This in turn changes the intracellular electrolyte balance causing an influx of fluids into the cell and resultant swelling.
Central Nervous System (CNS)
The part of the nervous system that consists of the brain and spinal cord.
Centrioles
Organelles composed of nine microtubule triplets that organize specific fibers of chromosomes and move the chromosomes during cell division.  There are two centrioles, aligned at right angles to each other.
Cerebellum
A posterior portion of the brain that is responsible for voluntary and involuntary motor activities based on memory and sensory input.
Cerebrum
The largest portion of the brain that controls thought processes, intelligence, memory, sensations, and complex motor functions.
Chemicals
Atoms or molecules that are the building blocks of all matter.
Cilia
Thread-like projections of the outer layer of the cell membrane, which serve to move substances over the cell, surface.
Cirrhosis
A specific condition of the liver in which the liver cells (hepatocytes) have been replaced by bands of fibrous tissue, which can not carry out the metabolic functions of the liver.
CNS
The part of the nervous system that consists of the brain and spinal cord.
Connective Tissue
One of the four tissues of the body.  It is specialized to provide support and hold the body tissues together (i.e., they connect). It contains more intercellular substances than the other tissues.  Bones, cartilage, and fat are types of connective tissue.  The blood and lymph vessels are immersed in the connective tissue media of the body.
Cytoplasm
A watery solution of minerals, organic molecules, and gases found between the cell membrane and nucleus.
Cytosol
The liquid part of the cytoplasm which distributes materials by diffusion throughout the cell.
Demyelination
The loss of the protective myelin sheath around an axon, usually due to damage to the Schwann and oligodendroglia cells.
Dendrites
Sensory processes of a neuron that are specialized to receive incoming information and send it to the neuron cell body.
Deoxyribonucleic Acid (DNA)
A nucleic acid known as the molecule of life that makes up the chromosomes.  It is composed of a chain of nucleotides containing the sugar deoxyribose and the nitrogen bases, adenine, guanine, cytosine, and thymine wound in a double helix and held together by weak bonds between complementary nitrogen base pairs.
Depression
A clinical psychiatric condition in which a person has a dejected mood, psychomotor retardation, insomnia and weight loss, sometimes associated with guilt feelings and often with delusional preoccupations.
Diencephalon
A portion of the brain that contains the thalamus, hypothalamus, and pituitary gland.  It relays and processes sensory information; control of emotions, autonomic functions, and hormone production.
Digestive System
The organ system that functions to process foods that are ingested, absorb nutrients into body, and provide metabolized nutrients to the body cells.  Consists of the mouth, salivary glands, esophagus, stomach, intestinal tract, liver, and pancreas,
Disease
A malfunction of any component of the body that can result in an abnormal and undesirable physiological or anatomical change.
DNA (Deoxyribonucleic acid)
A nucleic acid known as the molecule of life that makes up the chromosomes.  It is composed of a chain of nucleotides containing the sugar deoxyribose and the nitrogen bases, adenine, guanine, cytosine, and thymine wound in a double helix and held together by weak bonds between complementary nitrogen base pairs.
Dying-back Neuropathy
A neurological condition in which axons begin to die at the very distal end of the axon with necrosis slowly progressing toward the cell body.
Dysplasia
A condition of abnormal cell change or deranged cell growth in which the cells are structurally changed in size, shape, and appearance from the original cell type.
Effector
The body site where a response occurs which counters an initial stimulus and thus attempts to maintain homeostasis.
Efferent Nerve
A nerve that relays motor commands from the CNS to various muscles and glands.
Element
A chemical substance composed of only one atom, e.g., hydrogen, calcium, or singlet oxygen.
Endocrine System
The organ system that regulates body functions by use of chemicals, known as hormones.  Endocrine organs are the pituitary gland, parathyroid gland, thyroid gland, adrenal gland, thymus, pancreas, and gonads.
Endoplasmic Reticulum
A cell organelle, which provides an extensive network of membrane-like channels that, extends throughout the cytoplasm.  It synthesizes secretory products and is responsible for intracellular storage and transport.
Enzymes
A chemical (protein) that catalyses (accelerates) specific biochemical reactions without themselves being permanently changed.
Ependymal Cells
A type of glial cell in the CNS that produces a special fluid, known as the cerebral spinal fluid (CSF).
Epithelial Tissue
One of the four types of tissue in the body that is specialized to protect, absorb and secrete substances, as well as detect sensations.  It covers every exposed body surface, forms a barrier to the outside world, and controls absorption.
Fatty Change
A toxic cellular change that occurs with severe cellular injury.  The cell has become damaged and is unable to adequately metabolize fat, resulting in development of small vacuoles of fat that accumulate and become dispersed within the cytoplasm.  It is usually observed in the liver.
Feed Back Mechanism
A part of the homeostasis in which the body regulates the degree of response to a stimulus.  A negative feed back depresses the stimulus to shut off or reduce the effector response whereas a positive feed back has the effect of increasing the effector response.
Gamma Aminobutyric Acid (GAMA)
A neurotransmitter of the CNS whose effects are usually inhibitory.
Glial cells
The supporting cells of the neural tissue.  They regulate the environment around the neurons and protect against foreign invaders. They are also known as neuroglia.
Golgi Apparatus
Cell organelles composed of stacks of flattened membranes containing chambers.  They synthesize, store, alter, and package secretory products and lysosomes.
Hepatitis
Inflammation of the liver.
Herpes Simplex Virus
A virus that causes a disease marked by vesicles of the skin, usually on the lips, nares, or genitals.
Hyperplasia
An increase in the number of cells in a tissue.  This generally results in an enlargement of tissue mass and organ size.
Hypertrophy
An increase in size of individual cells.  This frequently results in an increase in the size of a tissue or organ.
Initiation Phase
The initial stage in the carcinogenesis process, which consists of the alteration of the DNA (mutation) of a normal cell.  The initiated cell has thus developed a capacity for unregulated growth.
Inorganic Compounds
Simple molecules that usually consist of one or two different elements.  For example, water (H2O), carbon dioxide (CO2), bimolecular oxygen (O2), and sodium chloride (NaCl).
Integumentary System
The organ system that serves as a barrier to invading environmental organisms and chemicals and serves in temperature control.  Organs include the skin, hair, nails, and exocrine glands.
Interneurons
Interneurons are neurons located only in the CNS and provide connections between sensory and motor neurons. They can carry either sensory or motor impulses.  They are involved in spinal reflexes, analysis of sensory input, and coordination of motor impulses.  They play a major role in memory and the ability to think and learn. They are also known as association neurons.
Karyorrhexis
The rupture of the cell nucleus with the disintegration of the chromatin into granules which are extruded from the cell.
Labile Cells
Body cells that have a limited lifespan and are capable of routine division and replacement.  The squamous epithelium of skin, mouth, vagina and cervix, columnar epithelium of intestinal tract, transitional epithelium of urinary tract, and hematopoietic stem cells of the bone marrow are examples of labile cells.
Latency Period
The time between induction of a lesion and its clinical appearance.  An example is cancer which develops gradually (subclinically) over a period of several years before it becomes manifest to the individual or clinician.  This period is referred to as the "latency period".
Lethal Injury
Damage to a cell or the body so severe that death results.
Leukemia
Cancer of the hematopoietic system, the blood-forming organs.
Lipids
Essential substances of all cells and a major energy reserve for the body.  Lipids may be stored as fatty acids or as triglycerides.
Lou Gehrig's Disease
A disease in which the myelin around nerves is lost causing paralysis and loss of sensory and motor function.  Same as Amyotrophic Lateral Sclerosis.
Lymphatic System
An organ system that returns tissue fluid to blood and defends against foreign organisms.  Organs include the spleen, lymph nodes, thymus, and the lymphatic vessels.
Lysosomes
Organelles that consist of vesicles that contain strong digestive enzymes.  Lysosomes are responsible for the intracellular removal of damaged organelles or pathogens.
Malignant Neoplasm
A tumor or neoplasm that has the ability to metastasize or invade into surrounding tissues. The same as cancer.
Malignant Tumor
Same as malignant neoplasm.
Medulla Oblongata
The segment of the brain that is attached to the spinal cord.  It relays sensory information to the rest of the brain and regulates autonomic function, including heart rate and respiration.
Metaplasia
The conversion from one type of mature cell to a different type of mature cell.  It is a cellular replacement process.  An example is cirrhosis of the liver.
Metastasis
The ability to establish secondary tumor growth at a new location away from the original site.
Metastatic Foci
Secondary tumors in an organ different from the original site of cancer development.
Microglia
A type of glial cell. The microglia are small, mobile, phagocytic cells that function in defense against invading organisms and xenobiotics.
Midbrain
The area of the brain between the cerebrum and brain stem.  It contains the centers that process auditory and visual data and generates involuntary motor responses.
Mitochondria
Oval organelles bound by a double membrane with inner folds enclosing important metabolic enzymes.  They produce nearly all (95%) of the ATP and energy required by the cell.
Motor Neurons
The neurons that relay information from the CNS to other organs, terminating at the effectors.  Motor neurons are the efferent neurons of both the somatic and autonomic nervous systems.  They are also referred to as effector neurons.
Multiple Sclerosis
A disease in which the myelin around nerves is lost causing paralysis and loss of sensory and motor function.
Muscular System
The organ system involved with movement or locomotion and heat production.  The main organs are the skeletal muscles and tendons.
Muscular Tissue
One of the four types of tissue.  It is specialized for an ability to contract.  Muscle cells are elongated and referred to as muscle fibers.  When a stimulus is received at one end of a muscle cell, a wave of excitation is conducted through the entire cell so that all parts contract in harmony.
Myelin
Protein layers that surround neurons and serves like insulation.  Myelinated neurons usually transmit impulses at high speed, such as needed in motor neurons. Loss of myelination allows interruption of the action potential (like leakage) and causes a dysfunction of these cells.  This can cause paralysis and loss of sensory and motor function.
Necrosis
The death of a cell caused by a progressive failure of essential metabolic and structural cell components, usually in the cytoplasm.  Necrosis generally involves a group of contiguous cells or occurs at the tissue level.
Neoplasia
A new growth of tissue with abnormal and unregulated cellular proliferation.  There are two types of neoplasia, benign and malignant.  Same as a tumor.
Neoplastic Conversion
The second major step in the carcinogenesis process in which specific agents (referred to as promoters) enhance the further development of the initiated cells.
Nervous System
The organ system that coordinates activities of other organ systems and responds to sensations.  It is composed of the central nervous system and peripheral nervous system.
Nervous Tissue
One of the four body tissues that is specialized so as to be capable to conduct electrical impulses and convey information from one area of the body to another.  Most of the nervous tissue (98%) is located in the central nervous system, the brain, and spinal cord.
Neural Synapse
The junction between the axon of one neuron and the dendrite of another neuron or an axon and a connection with a muscle cell (neuromuscular junction).
Neuroglia
Cells of the nervous system that provide physical support for the nervous tissue, control tissue fluids around the neurons, and help defend the neurons from invading organisms and xenobiotics.  Same as glial cells.
Neurons
The functional nerve cells directly responsible for transmission of information to and from the CNS to other areas of the body.
Neurotoxin
A chemical that damages an area of function of the nervous system.
Neurotransmitters
These are chemicals that move information across a synapse by diffusing across the synaptic junction, binding to receptors on the postsynaptic membrane, and stimulating generation of an action potential.
Nicotine
A neurotoxin that binds to certain cholinergic receptors thus preventing normal neural function and stimulation.
Norepinepharine
A chemical neurotransmitter of adrenergic nerves of both the central and peripheral nervous systems. It is also produced by the adrenal medulla in response to stimulation.  It is the same as noradrenaline.
Nucleic acids
These are large organic compounds inside virtually all body cells (RBCs is an exception) that store and process information at the molecular level.
Nucleolus
This is a dense region of the nucleus, which contains the RNA and DNA.  It is the site for rRNA synthesis and assembly of the ribosome components.
Nucleus
A membrane-bound part of a cell that contains nucleotides, enzymes, and nucleoproteins.  The nucleus controls metabolism, protein synthesis, and the storage and processing of genetic information.
Oligodendrocyte
A type of glial cell in the CNS that wraps itself around an axon to form myelin, which serves like insulation.
Oncogene
Altered or misdirected proto-oncogene which then has the ability to transform the normal cell into a neoplastic cell.  Most oncogenes differ from their proto-oncogenes by a single point mutation.
Organ System
A group of organs that contribute to specific functions within the body.
Organelle
An intracellular structure that performs a specific function or group of functions.
Organic Compound
A substance that contains covalently-bonded carbon and hydrogen and often other elements.
Organophosphate Chemical
Organic chemicals that contain a phosphate group.  Many are highly toxic, as they are capable of inhibition of the enzyme acetylcholinesterase at neural synapses.  Many pesticides and some warfare agents are organophosphate chemicals.
Organs
A group of tissues precisely arranged so that so they can work together to perform specific functions.
p53 Gene
A normal suppressor gene that controls cell division and stimulation of repair enzymes to rebuild and restore damaged regions of the DNA. Damage or inactivation of the p53 gene is considered a contributing cause of most cancers.
Peripheral Nervous System (PNS)
All nervous tissue outside the central nervous system.
Peripheral Neuropathy
Abnormal and detrimental changes to nervous tissue outside the brain or spinal cord.
Permanent Cells
Body cells that never divide and do not have the ability for replication even when stressed or when some cells die.  Examples are neurons and muscle cells.
Peroxisomes
Very small, membrane-bound organelles which contain a large variety of enzymes that perform a diverse set of metabolic functions.
Physiological Adaptation
The ability of the body to adapt to changes or stresses so that the change is beneficial.  Increase in muscle mass with exercise is an example of physiological adaptation.
PNS
The peripheral nervous system.  All nervous tissue outside the CNS.
Point Mutation
A change in the DNA sequence in a gene.
Pons
A section of the brain that functions as a relay center and assists in somatic and visceral motor control.
Poorly-differentiated
The change in a cell so that it has lost much of the normal appearance.
Progression Stage
The third recognized step in the carcinogenic process that is associated with the development of the initiated cell into a biologically malignant cell population.
Proliferation
The reproduction or multiplication of similar forms, especially cells.
Promotion Phase
The second step in the carcinogenesis process in which specific agents (referred to as promoters) enhance the further development of the initiated cells.
Proteins
The most diverse and abundant of organic compounds in the body.  There are about 100,000 different kinds of proteins that perform a large variety of important functions, such as the protein pores in cell membranes, keratin in skin and hair, collagen in ligaments and tendons, myosin in muscles, and hemoglobin in RBCs. The building blocks for proteins are the 20 amino acids.
Proto-oncogenes
Normal or good cellular genes that instruct the production of the regulatory proteins and growth factors within the cell or its membrane. Activation of a proto-oncogene can cause alteration in the normal growth and differentiation of cells, which leads to neoplasia.
Pulmonary Fibrosis
Changes in the lining of the pulmonary alveoli in which the normal epithelial cells are replaced by fibrous tissue.  Gases poorly diffuse across the fibrous tissue and thus gas exchange is drastically reduced in the lungs.
Pyknosis
A degenerative change in a cell in which it thickens with a shrinking of the nucleus and the chromatin condenses to a solid, structureless mass or masses.
Receptor
The site within the body that detects or receives the stimulus, senses the change from normal, and sends signals to the control center.
Reproductive System
The organ system that produces germ cells (eggs and sperm) and provides the environment for growth of the fetus (women).  The main reproductive organs are the ovaries, uterus, mammary glands, testes, prostate gland, and the external genitalia.
Respiratory System
The organ system responsible for oxygen and carbon dioxide exchange.  The main organs are the lungs, trachea, larynx, nasal cavities, and pharynx.
Reversible Cell Damage
A type of cellular damage in which the response of the cell to toxic injury may be transient and once the stress has been removed or the compensatory cellular changes made, it returns to full capability.
Ribonucleic Acid (RNA)
A nucleic acid consisting of a chain of nucleotides that contain the sugar ribose and the nitrogen bases adenine, guanine, cytosine, and uracil.
Ribosomes
Very small cell organelles that consist of RNA and proteins, and function in protein synthesis.
Sarcoma
A malignant tumor arising in connective or muscle tissue.  They are usually spread by the blood stream and frequently metastasize to the lung.
Saxitoxin
A potent neurotoxin present in some shellfish poisoning that produces its effect by blocking sodium channels.
Schwann Cells
A very important glial cell present in the peripheral nervous system.  They wrap themselves around all axons outside the CNS and form myelin, which serves like insulation.
Sensory Neurons
Neurons that carry information from sensory receptors (usually processes of the neuron) to the CNS.  They are also known as afferent neurons.
Skeletal System
The organ system that supports and moves the body, protects internal organs, provides for mineral storage, and provides for blood formation.  The main organs are the bones, cartilage, ligaments, and bone marrow.
Somatic Nervous System
The portion of the nervous system that carries motor information to skeletal muscles.  Also known as the Voluntary System.
Somatic System
The part of the nervous system under voluntary control.
Stable Cells
Body cells that have a long lifespan with normally a low rate of division but the ability to rapidly divide upon demand.  Examples are liver cells, alveolar cells of the lung, and kidney tubule cells.
Stimulus
A change in the environment, such as an irritant, loss of blood, or presence of a foreign chemical.
Strychnine
An extremely poisonous natural substance that inhibits the neurotransmitter glycine at postsynaptic sites, resulting in an increased level of neuronal excitability in the CNS.
Synapse
The junction between the axon of one neuron and the dendrite of another neuron or an axon and a connection with a muscle cell (neuromuscular junction).
Target Organ
The site of action of a toxin.  Most toxins are specific in their toxicity and damage only particular tissues or organs.
Tetrodotoxin
A potent neurotoxin produced in some species of frogs, puffer fish and other invertebrates.
Tissue
A group of cells with similar structure and function. There are four types of tissues: epithelial tissue, connective tissue, muscle tissue, and nerve tissue.
Tumor
A general term for an uncontrolled growth of cells that becomes progressively worse with time.  Tumors may be benign or malignant.  Same as neoplasm.
Tumor Suppressor Gene
Genes present in normal cells that serve to prevent a cell with damaged DNA from proliferating and evolving into an uncontrolled growth.  Sometimes referred to as anti-oncogenes.  The p53 gene is a tumor suppressor gene.
Urinary System
The organ system responsible for the elimination of wastes; regulation of pH and the volume of blood.  The main organs are the kidneys, urinary bladder, and urethra.