BIOLOGY

Efferent Neurons

Neurons that carry information from the central nervous system to other parts of the body. Also called motor neurons.

Depolarization

A process that occurs when the voltage-gated Na⁺ channels open, allowing Na⁺ to rush into the cell and depolarize it.

Dendrite

An extension of the neuron that transmits impulses toward the cell body.

Afferent Neurons

Neurons that carry information to the central nervous system from the periphery. Also called sensory neurons.

Synpase

The space between the axon terminal of one neuron and the dendrite of another neuron (or membrane of an effector organ) where neurotransmitters are released

Somatic Nervous System

Division of the peripheral nervous system that is responsible for voluntary movement.

Saltatory Conduction

A means by which action potentials jump from node to node along an axon.

Resting Potential

The charge difference across the cell membrane of a neuron or a muscle cell while at rest. Most often maintained by the sodium-potassium pump.

Nodes of Ranvier

Gaps between segments of myelin sheath where action potentials can take place, allowing for saltatory conduction.

Neurotransmitters

Chemical messengers released from synaptic terminals of a neuron that can bind to and stimulate a postsynaptic cell.

Axon Hillock

The portion of the neuron that connects the cell body (soma) to the axon. The impulses the neuron receives from all the dendrites are summed up at the axon hillock to determine whether or not an action potential will be initiated.

Action Potential

A sharp change in the membrane potential of neurons or muscle cells caused by a change in the selective permeability to Na⁺ and K⁺ using voltage-gated ion channels. Action potentials are all-or-nothing events.

Epiphysis

Dilated ends of long bones in the appendicular skeleton.

Diaphysis

Cylindrical shaft of a long bone. Filled with bone marrow for the production of blood cells.

White Fibers

Fast-twitch muscle fibers. They are primarily anaerobic and fatigue more easily than red fibers.

Transverse Tubules (T-Tubules)

A system of tubules that provides channels for ion flow throughout skeletal and cardiac muscle fibers to facilitate the propagation of an action potential.

Spongy Bones

Lighter and less dense than compact bone, it consists of an interconnecting lattice of bony spicules (trabeculae). The cavities between the spicules contain bone marrow.

Smooth Muscle

Nonstriated muscle, responsible for involuntary action. Controlled by the autonomic nervous system.

Skeletal Muscle

Type of muscle responsible for voluntary movement, consisting of multinucleated, striated (striped) muscle fibers.

Sarcoplasmic Reticulum

A modified form of endoplasmic reticulum; stores calcium that is used to trigger contraction when muscle is stimulated.

Sarcomere

The structural unit of striated muscle. It is composed of thin (mostly actin) and thick (mostly myosin) filaments.

Red Fibers

Slow-twitch muscle fibers. They are primarily aerobic and contain many mitochondria and high levels of myoglobin.

Osteons

The structural unit of compact bone that consists of a central canal (either a Haversian or Volkmann's canal) surrounded by a number of concentric rings of bony matrix called lamellae.

Osteoclasts

Cells in the bone matrix that are involved in bone degradation.

Osteoblasts

Cells in the bone tissue that secrete the organic constituents of the bone matrix. Osteoblasts develop into osteocytes.

Compact Bone

Much more dense than spongy bone, compact bone consists of Haversian systems (osteons).

Cartilage

A firm, elastic, translucent connective tissue consisting of collagenous fibers embedded in chondrin. Produced by cells called chondrocytes. Cartilage is the principal component of embryonic skeletons and can harden and calcify into bone (ossify).

Cardiac Muscle

Type of muscle found within the heart; may contain one or two nuclei. Involuntary, like smooth muscle, but appears striated, like skeletal muscle. Able to depolarize independent of the nervous system.

Axial Skeleton

The skull, vertebral column, ribcage, and hyoid bone

Appendicular Skeleton

The bones of the pelvis, the pectoral girdles, and the limbs.

Innate Immunity

Nonspecific immunity provided by structures and cells. Structures, such as the skin, and cells, such as macrophages, are able to recognize invaders and kill them. Some cells of the innate immune system, such as macrophages and dendritic cells, are able to signal the presence of an invader to the adaptive immune system.  Contrast with adaptive immunity

Immunoglobulin

Synonymous with antibody; produced in response to a specific foreign substance that recognizes and binds to that antigen and triggers an immune response.

Cell-Mediated (Cytotoxic) Immunity

Branch of the immune system in which intracellular pathogens are eliminated by killing their host cells. T-cells are the primary mediators of cytotoxic immunity.

Antigen

A substance that is bound by an antibody, causing an immune reaction.

Secondary Response

Subsequent infections by pathogens that trigger a more immediate response from the memory cells produced during the primary immune response.

Primary Response

The initial response to a specific antigen. During a primary response, T- and B-cells are activated and specific antibodies and memory cells for the antigen are produced.

Lymphatic System

A system of vessels and lymph nodes that collect interstitial fluids and return them to the circulatory system, thereby maintaining fluid balance. The lymphatic system is also involved in lipid absorption and lymphocyte activation.

Humoral Immunity

The synthesis of specific antibodies by activated B-cells in response to an antigen. These antibodies bind to the antigen and either clump together to become insoluble, neutralize the antigen, or attract other cells that engulf and digest the pathogen.

Thyroid-Stimulating Hormone (TSH)

Synthesized and released by the anterior pituitary, stimulates the thyroid gland to absorb iodine and to synthesize and secrete thyroid hormones. is regulated by thyroid releasing hormone (TRH), which is released by the hypothalamus.

Growth Hormone (GH)

Synthesized and released by the anterior pituitary, stimulates bone and muscle growth as well as glucose conservation. is inhibited by somatostatin and stimulated by growth hormone releasing hormone (secreted by the hypothalamus).

Exocrine Glands

Glands that synthesize and secrete substances through ducts. The mammary glands and sweat glands are examples of it

Endocrine Glands

Glands that synthesize and secrete hormones into the circulatory system. Examples include the hypothalamus, pituitary gland, pineal gland, pancreas, testes, ovaries, adrenal glands, thyroid gland, and parathyroid glands.

Antidiuretic Hormone (ADH)

A peptide hormone, also known as vasopressin, that acts on the collecting duct to increase water reabsorption. is produced by the hypothalamus and stored in the posterior pituitary.

Adrenocorticotropic Hormone (ACTH)

Synthesized and released by the anterior pituitary, stimulates the adrenal cortex to synthesize and secrete glucocorticoids. is regulated by corticotropin releasing factor (CRF), which is released by the hypothalamus.

Tropic Hormones

Hormones that travel to a target tissue and cause the release of another hormone. A hormone downstream will cause the physiological effect.

Thyroid Hormones

Synthesized and released by the thyroid gland, thyroid hormones triiodothyronine (T₃) and thyroxine (T₄) stimulate cellular respiration as well as protein and fatty acid synthesis and degradation.

Steroid Hormones

Nonpolar hormones that cross the cell membrane and act by binding intracellular receptors.

Somatostatin

Produced and secreted by the /δ/-cells of the pancreas, somatostatin inhibits the release of glucagon and insulin.

Signaling Cascade

Series of events, starting with the binding of a peptide hormone to a surface receptor. This sequence of events ultimately results in a change in cellular behavior.

Second Messenger

A small molecule that transduces a hormonal signal from the exterior of the cell to the interior. Usually released when a peptide hormone binds to its receptor; cAMP is a common example.

Prolactin

A hormone synthesized and released by the anterior pituitary that stimulates milk production and secretion in female mammary glands.

Posterior Pituitary

Stores and releases hormones (oxytocin and ADH) synthesized by the hypothalamus. The release of these hormones is triggered by an action potential that originates in the hypothalamus.

Peptide Hormones

Polar hormones incapable of permeating the cell membrane that bind to surface receptors and act through secondary messengers.

Parathyroid Hormone (PTH)

Synthesized and released by the parathyroid gland, increases blood Ca²⁺ concentration by increasing Ca²⁺ reabsorption in the kidneys

and by stimulating calcium release from bone.

Insulin

Produced and secreted by the *β*-cells of the pancreas,

decreases blood glucose concentrations by facilitating the uptake of glucose by muscle

and adipose cells and the conversion of glucose to glycogen in muscle and liver cells.

Glucocorticoids

Synthesized and released by the adrenal cortex,

raise blood glucose levels while decreasing protein synthesis.

Glucagon

Produced and secreted by the *α*-cells of the pancreas,

increases blood glucose concentration by promoting gluconeogenesis

and the conversion of glycogen to glucose in the liver.

Epiphyseal Plate

Portion of the bone where growth occurs; located in the epiphysis.

Endorphins

Synthesized and released by the anterior pituitary,

inhibit the perception of pain.

Direct Hormones

Hormones that travel to a target tissue to cause an action without another hormone acting as an intermediary.

Calcitonin

Hormone synthesized and released by the thyroid gland that decreases plasma Ca²⁺ concentration.

Anterior Pituitary

Synthesizes and releases many vital hormones, including follicle-stimulating hormone, luteinizing hormone, adrenocorticotropic hormone, thyroidstimulating hormone, prolactin, endorphins, and growth hormone ("FLAT PEG"). is under the hormonal control of the hypothalamus

Amino Acid-Derivative Hormones

Hormones that are synthesized by modifying amino acids. Most act via secondary messengers, while some act in a fashion similar to steroid hormones.

Aldosterone

A steroid hormone produced in the adrenal cortex that is responsible for reabsorption of sodium and water and excretion of potassium and hydrogen ions.

Adrenal Medulla

Synthesizes and releases epinephrine and norepinephrine, which stimulate an increase in the metabolic rate and blood glucose levels.

Adrenal Cortex

Synthesizes and releases corticosteroids.

Glucocorticoids are stimulated by adrenocorticotropic hormone (ACTH),

whereas mineralocorticoids are stimulated by angiotensin II.

Cortical sex hormones include androgens such as testosterone.

Mechanical Digestion

Breakdown of food particles into smaller particles through such activities as biting, chewing, and churning.

Intestinal Glands

Contains brush-border enzymes such as maltase, sucrase, and lactase to digest disaccharides. Other enzymes of these glands include aminopeptidase, dipeptidase, and enteropeptidase.

Cholecystokinin (CCK)

A hormone that is secreted by the duodenum in response to the presence of chyme.

stimulates the release of bile and pancreatic enzymes into the small intestine, and promotes satiety

Bile

An alkaline fluid synthesized in the liver, stored in the gall bladder, and released into the duodenum.

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aids in the emulsification, digestion, and absorption of fats.

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Villi

Fingerlike projections that extend out of the small intestine in order to increase surface area for maximal absorption

Small Intestine

Section of the digestive tract that can be subdivided into three sections: duodenum, jejunum, and ileum.

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Most digestion takes place in the duodenum and most absorption takes place in the jejunum and the ileum.

Pyloric Sphincter

A valve between the stomach and the small intestine that regulates the flow of chyme into the duodenum.

Pyloric Glands

Glands located in the walls of the stomach that secrete the hormone gastrin to increase gastric acid production.

Peristalsis

Involuntary muscular contractions that push food down the digestive tract.

Pepsin

Secreted as pepsinogen by the chief cells of the stomach, this enzyme cleaves peptide bonds, starting the digestion of proteins into individual amino acids

Pancreas

Its exocrine functions include secreting pancreatic amylase, trypsinogen, chymotrypsinogen, procarboxypeptidases A and B, and pancreatic lipase into the small intestine.

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Its endocrine functions include secretion of insulin and glucagon.

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Liver

Essential organ of the human body responsible for the production of bile,

detoxification of ingested substances,

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production of urea,

and the processing and modification of nutrients for storage.

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also produces albumin (a protein that maintains blood oncotic pressure) and clotting factors.

Large Intestine

Section of the GI tract that consists of the cecum, the colon, and the rectum. The main function of the large intestine is to absorb salts, water, and some vitamins

Intrinsic Factor

A protein secreted by parietal cells of the stomach that is necessary for vitamin B₁₂ absorption.

Hepatic Portal Vein

Carries nutrients (monosaccharides, amino acids, and small fatty acids) absorbed in the small intestine to the liver, where they are modified to enter circulation.

Gastric Glands

Located in the stomach; secrete HCl and various enzymes (such as pepsin) when stimulated by gastrin.

Enteropeptidase

Digestive enzyme secreted by cells in the duodenum.

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This enzyme converts trypsinogen to trypsin. Trypsin is then able to activate other pancreatic enzymes to allow digestion to continue within the duodenum.

Enteric Nervous System

A collection of millions of neurons within the gastrointestinal system that governs the function of the GI tract.

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This system is able to function independently of the brain and spinal cord.

Chyme

Combination of partially digested food and acid that forms in the stomach.

Chemical Digestion

Enzymatic breakdown of large molecules into smaller molecules.

Ribonucleic Acid (RNA)

A nucleic acid found in both the nucleus and cytoplasm and most closely linked with transcription and translation, as well as some gene regulation.

Peroxisome

Organelle that contains hydrogen peroxide and participates in the breakdown of very long chain fatty acids.

Obligate Intracellular Organisms

Organisms that require a host cell to express their genes and reproduce.

Facultative Anaerobe

An organism that makes ATP by aerobic respiration if oxygen is present, but that can switch to fermentation for sufficient ATP when oxygen is not available.

Centrosome

The portion of the cell containing the centrioles.

Reverse Transcriptase

An enzyme in retroviruses that uses RNA strands as templates for synthesizing cDNA molecules.

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Mitochondria

The site of aerobic respiration that provides the cell with a majority of its energy in the form of ATP.

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is a semiautonomous organelle enclosed by two membranes with an intermembrane space between the two membranes

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and a mitochondrial matrix enclosed by the inner membrane.

Lytic Cycle

Phase in viral replication in which the host cell is lysed and releases new virions. 

Lysosome

A membrane-bound vesicle that contains hydrolytic enzymes used for intracellular digestion

Lysogenic Cycle

Phase of viral replication in which the DNA of the bacteriophage becomes integrated in the host's genome and replicates as the bacteria replicates. 

Golgi Apparatus

An organelle that plays a role in the packaging and secretion of proteins and other molecules produced intracellularly

Episomes

Plasmids that have the ability to integrate into the host genome.

Conjugation

The temporary joining of two organisms via a tube called a pilus, through which genetic material is exchanged; a form of sexual reproduction used by bacteria.

Cell Theory

A foundational belief in modern biology that all living things are composed of cells, the cell is the basic functional unit of life, that all cells arise from preexisting cells, and that DNA is the genetic material.