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Definition:
The action potential is a rapid and reversible reversal of the electrical potential difference across the plasma membrane of excitable cells such as neurons, muscle cells and some endocrine cells. In a neuronal action potential, the membrane potential rapidly changes from its resting level of approximately -70 mV to around +50 mV and, subsequently, rapidly returns to the resting level again. The neuronal action potential forms an important basis for information processing, propagation, and transmission. In muscle cells, the action potential precedes, and is necessary to bring about, muscle contraction. Some endocrine cells also exhibit action potentials, where the excitation leads to hormone secretion.

The action potential is also referred to as the electrical impulse or nervous impulse.

Related glossary terms/phrases:
Graded potential

See also:
Neuronal Action Potential



Definition:
Anterior pituitary gland

Related glossary terms/phrases:
Hypophysis
Neurohypophysis



Definition:
An early sexual maturation stage, occurring 1 or 2 years before the onset of puberty, during which the zona reticularis of the adrenal cortex releases increased levels of dehydroepiandrosterone (DHEA) and androstenedione. These adrenal androgens are ultimately responsible for the appearance of pubic and axillary hair, increased oil production by facial skin, acne, and distinct adult body odor.

Related glossary terms/phrases:
Menarche
Pubarche
Thelarche



Definition:
Leading toward a region or structure of interest.

In the nervous system, afferent fibers (i.e., neurons) transmit information from a peripheral receptor to the spinal cord or the brainstem. Afferent neurons are also referred to as sensory neurons.

In the kidneys, the afferent arteriole carries blood to the glomerular capillaries.

Related glossary terms/phrases:
Efferent



Definition:
Absence of menstruation in sexually mature female individuals.



Definition:
Earwax



Definition:
Refers to the concentration gradient of an ion or molecule. The concentration gradient may exist across a biological membrane, where the concentration is higher on one side of the membrane compared to the other side. Concentration gradient may also exist in a solution without an apparent barrier separating the area of higher concentration from the area of lower concentration. In both cases, the free energy that results from the concentration difference drives the movement of the ion/molcule from the area of higher concentration to the area of lower concentration. In free solution, the ion/molecule simply diffuses down its gradient. Movement across a biological membrane is more complicated and is a function of lipid solubility of the ion/molecule as well as the presence of channels or transport proteins that can allow the ion/molecule to cross the membrane (see Lipid Bilayer Permeability and Summary of Membrane Transport Processes).

Related glossary terms/phrases:
Electrical gradient
Electrochemical gradient



Definition:
An electrophysiological technique in which the current passing across the cell membrane is controlled experimentally, and the membrane voltage is measured.

Related glossary terms/phrases:
Voltage clamp

See also:
Neuronal Action Potential - Pharmacological Inhibition of Na+ and K+ Channels



Definition:
The flow of charge. In electrical wires and electronic circuits, current is carried by electrons. In physiological solutions, current is carried by ions in solutions.

Related glossary terms/phrases:
Voltage



Definition:
The first region (i.e., most proximal region) of the small intestine. Chyme leaving the stomach (through the pyloric valve) first enters the duodenum. The contents of the duodenum empty into the jejunum (middle region of the small intestine). In adult humans, the duodenum is about 25-38 cm (10-15 inches) long.

Related glossary terms/phrases:
Jejunum
Ileum



Definition:
Leading away from a region or structure of interest.

In the nervous system, efferent fibers (i.e., neurons) transmit information from the central nervous system to peripheral effector organs (i.e., muscles or glands). Therefore, the cells bodies of efferent neurons reside within the central nervous system, whereas their axonal projections exit the central nervous system to make synaptic contact with effector organs in the periphery. Efferent neurons are also referred to as motor neurons.

In the kidneys, the efferent arteriole carries blood away from the glomerular capillaries.

Related glossary terms/phrases:
Afferent



Definition:
In biological solutions, electrical gradient refers to the electrical potential that acts on an ion to drive the movement of the ion in one or another direction (see Resting Membrane Potential - Establishment of the Membrane Potential).

Related glossary terms/phrases:
Chemical gradient
Electrochemical gradient



Definition:
Refers to the balance of chemical and electrical gradients that act on an ion, particularly as it relates to the movement of an ion across a biological membrane (see Resting Membrane Potential - Establishment of the Membrane Potential and Resting Membrane Potential - Nernst Equilibrium Potential).

Related glossary terms/phrases:
Chemical gradient
Electrical gradient






Definition:
Electrochemical gradient

See:
Electrochemical gradient



Definition:
Electrogenic pumps are primary active transporters that hydrolyze ATP and use the energy released from ATP hydrolysis to transport ions across biological membranes leading to the translocation of net charge across the membrane.

For example, the Na+/K+ ATPase (sodium pump) is an electrogenic pump because during every transport cycle, it transports 3 Na+ ions out of the cell and 2 K+ ions into the cell. This leads to the movement of one net positive charge out of the cell making this process electrogenic.

Related glossary terms/phrases:
Electrogenic



Definition:
An electrogenic transport process is one that leads to the translocation of net charge across the membrane. For example, ion channels such as Na+, K+, Ca2+, and Cl channels are electrogenic.

The Na+/K+ ATPase is electrogenic because for every ATP molecule hydrolyzed, 3 Na+ ions are transported out of the cell and 2 K+ ions are transported into the cell (leading to the translocation of one net positive charge out of the cell).

Many secondary active transporters are also electrogenic. For example, the Na+/glucose cotransporter (found in the small intestine and kidney proximal tubules), transports 2 Na+ ions and 1 glucose molecule into the cell across the plasma membrane (leading to the translocation of two net positive charges into the cell per transport cycle).



Definition:
Endocrine cells are responsible for producing and releasing hormone molecules into the bloodstream. Endocrine cells are typically grouped together in organs referred to as endocrine glands.

Related glossary terms/phrases:
Endocrine gland
Hormone



Definition:
Endocrine glands contain clusters of endocrine cells, whose function is to is to release hormones into the bloodstream.

Endocrine glands are ductless glands in that their secretions do not enter a duct (as for example with salivary glands). Rather, endocrine glands release their secretions (i.e., messenger molecules) into the interstitial fluid in highly vascularized regions, where the molecules then enter the bloodstream. The chemical messenger molecule that is released into the bloodstream is referred to as a hormone.

Related glossary terms/phrases:
Endocrine cell
Hormone



Abbreviation:
Veq. or Eeq.

Definition:
Refers to the membrane potential at which there is no net movement of an ion across the plasma membrane into or out of the cell.

See also:
Resting Membrane Potential - Establishment of the Membrane Potential
Resting Membrane Potential - Nernst Equilibrium Potential
Nernst Potential Calculator



Definition:
Refers to synaptic or receptor potentials that can vary in amplitude and direction. Graded potentials can be depolarizing or hyperpolarizing and do not have a threshold.

Related glossary terms/phrases:
Action potential

See also:
Neuronal Action Potential - Introduction
Neuronal Action Potential - Graded Potentials versus Action Potentials



Definition:
Refers to the hyperpolarization phase of the action potential.

It is also referred to as undershoot.

See figure.

Related glossary terms/phrases:
Hyperpolarization

See also:
Important Features of the Neuronal Action Potential



Definition:
In electrophysiological convention, a negative current value or downward deflection of a current trace is typically referred to as an inward current. A negative current value (i.e., inward current) can reflect either the movement of positive ions (cations) into the cell or negative ions (anions) out of the cell.

See also:
Neuronal Action Potential - Pharmacological Inhibition of Na+ and K+ Channels



Definition:
The one-way valve between the left atrium and left ventricle of the heart. It is also known as the bicuspid valve or mitral valve.

The direction of blood flow through the left atrioventricular valve is from the left atrium to the left ventricle.

Related glossary terms/phrases:
Bicuspid valve
Mitral valve



Definition:
The voltage difference across a cell plasma membrane.

The membrane potential is generally inside negative with respect to the outside, where the outside potential is generally set as the reference value. In electrically excitable cells, the value of the membrane potential can be positive (inside with respect to the outside) during electrical activity (i.e., during action potentials).

Related glossary terms/phrases:
Resting membrane potential

See also:
Resting membrane potential



Definition:
Onset of menstruation.

Related glossary terms/phrases:
Adrenarche
Pubarche
Thelarche



Definition:
Newly synthesized or in the process of being synthesized



Definition:
In electrophysiological convention, a positive current value or upward deflection of the current trace is typically referred to as an outward current. A positive current value (i.e., outward current) can reflect either the movement of positive ions (cations) out of the cell or negative ions (anions) into the cell.

Related glossary terms/phrases:
Inward current

See also:
Neuronal Action Potential - Pharmacological Inhibition of Na+ and K+ Channels



Definition:
The voltage difference across a cell plasma membrane in the resting or quiescent state. It is also simply referred to as the resting potential (Vrest). The value of the resting membrane potential varies from cell to cell. Depending on the cell type, it can range from −90 mV to −20 mV.

For example, Vrest is −90 mV in skeletal and cardiac muscle cells as well as in astrocytes. In a typical neuron, Vrest is approximately −70 mV. In many non-excitable cells, Vrest ranges from −60 to −50 mV. In photoreceptors, Vrest is about −20 mV.

See also:
Resting membrane potential



Definition:
The one-way valve between the right atrium and right ventricle of the heart. It is also known as the tricuspid valve.

The direction of blood flow through the right atrioventricular valve is from the right atrium to the right ventricle.

Related glossary terms/phrases:
Tricuspid valve



Definition:
Removal of approximately a pint of blood in a manner similar to that for donating blood. The procedure is generally used as a simple measure to reduce the number of circulating red blood cells (e.g., to treat polycythemia) or to reduce the amount of circulating iron (e.g., to treat hemochromatosis). The procedure may be repeated as needed.

It falls under procedures referred to as phlebotomy (removing blood for therapeutic or diagnostic purposes) or bloodletting (removing blood for therapeutic purposes).



Definition:
The A band is the region of a striated muscle sarcomere that contains myosin thick filaments. In fact, the A band is the entire length of the thick filament of the sarcomere. Its length is approximately 1 μm. The center of the A band is located at the center of the sarcomere (M line).

Related glossary terms/phrases:
H zone
I band
M line
Z disk



Definition:
The absolute refractory period refers to a period during the action potential. This is the time during which another stimulus given to the neuron (no matter how strong) will not lead to a second action potential. The absolute refractory period starts immediately after the initiation of the action potential and lasts until after the peak of the action potential. Following this period, the relative refractory period begins.

Related glossary terms/phrases:
Relative refractory period

See also:
Neuronal Action Potential - Refractory Periods



Abbreviation:
ACh

Definition:
Acetylcholine (ACh) is a chemical neurotransmitter used by the central nervous system (CNS) as well as the peripheral nervous system (PNS). Acetylcholine is a classical neurotransmitter and, in fact, it was the first of the classic neurotransmitters to be discovered. It was discovered in 1914 by Henry Hallett Dale while conducting experiments on the heart.

Acetylcholine is the neurotransmitter used by the somatic division of the nervous system at the neuromuscular junction (where a somatic motor neuron makes synaptic contact with a skeletal muscle cell). Acetylcholine is also used extensively by both branches of the autonomic nervous system; sympathetic and parasympathetic. It is the primary neurotransmitter released in autonomic ganglia by preganglionic autonomic neurons. It is also the primary neurotransmitter released by parasympathetic postganglionic neurons. A few sympathetic postganglionic neurons also release acetylcholine. The diverse actions of acetylcholine are exerted via the activation of nicotinic and muscarinic ACh receptors.

Other resources:
Acetylcholine (Wikipedia)



Definition:
An enzyme found in the synaptic cleft at cholinergic synapses. It degrades acetylcholine to choline and acetate and, thus, terminates the action of acetylcholine at the synapse. Neither choline nor acetate can bind to acetylcholine receptors (nicotinic or muscarinic).



Definition:
A process that requires oxygen (O2); for example, aerobic metabolism.



Definition:
All-or-nothing is usually used when describing the action potential. It refers to the well-known observation that an action potential always occurs in its full size (i.e., full magnitude of voltage change).

Many physiologists use all-or-nothing and all-or-none interchangeably.

See also:
Important Features of the Neuronal Action Potential



Definition:
Amiloride inhibits epithelial Na+ channels (ENaC), and in doing so, it acts as a diuretic by inhibiting sodium reabsorption in the late distal convoluted tubules, connecting tubules, and collecting ducts in the kidneys.

Other resources:
Amiloride (Wikipedia)



Definition:
Lack of oxygen in arterial blood.

Related glossary terms/phrases:
Anoxia
Hypoxia
Hypoxemia



Definition:
Describes a condition in which there is an absence of oxygen supply to tissues.

Related glossary terms/phrases:
Anoxemia
Hypoxia
Hypoxemia



Definition:
An antagonist is a molecule that binds to a receptor, however, it does not activate the physiological response induced by the naturally occurring physiological ligand of the receptor. Moreover, once bound to the receptor, an antagonist prevents the physiological ligand from activating the receptor. Therefore, antagonist binding to a receptor prevents or blocks the action of the natural ligand.

For example, acetylcholine (ACh) is the naturally occurring physiological ligand that activates nicotinic and muscarinic acetylcholine receptors. Tubocurarine and succinylcholine are antagonists of the nicotinic ACh receptor (nAChR), and atropine is an antagonist of the muscarinic ACh receptor (mAChR).

Related glossary terms/phrases:
Agonist



Definition:
The large blood vessel (artery) that receives the output of the left ventricle of the heart. The aorta is the first and largest (in diameter) artery of the systemic circulation.

Following ventricular contraction (systole), hydrostatic pressure within the ventricle exceeds the pressure in the aorta, which forces blood out of the left ventricle, through the aortic semilunar valve, and into the aorta. As the ventricle relaxes during diastole, the pressure within the left ventricle drops to a level below that in the aorta. At this point, the aortic semilunar valve closes, which prevents the backflow of blood from the aorta to the left ventricle.



Definition:
A deficit in expressing or understanding language.

Disorders of language resulting from damage to areas of the brain responsible for language formation and comprehension.



Definition:
The ability of a tissue or organ to regulate its own function without extrinsic neural or hormonal input.

For example, in the kidneys, autoregulation ensures relatively constant blood flow and pressure through the kidney tubules in order to maintain a fairly constant glomerular filtration rate (GFR).



Definition:
The region of the neuron cell body from which the axon originates. The axon hillock is generally the site of action potential initiation. It is also referred to as the initial segment.



Definition:
Proposed model for the inactivation of some voltage-gated ion channels. According to this model, after channel opening, the pore of the open channel is plugged by a globular cytoplasmic portion of the channel protein. The globular portion (ball of amino acids) is tethered to the rest of the protein by a linker part (chain of amino acids).

Related glossary terms/phrases:
Channel inactivation

See also:
Neuronal Action Potential - Important Features of the Neuronal Action Potential



Definition:
Concave on both sides of a structure, usually referring to a disc or a lens.

Of particular importance to physiology is the structure of mature red blood cells (erythrocytes), which is a flattened cell that has assume a biconcave shape. It is thought that the biconcave shape of red blood cells helps with the flow property of blood through blood vessels.



Definition:
The one-way valve between the left atrium and left ventricle of the heart. It is also known as the left atrioventricular valve or mitral valve.

The direction of blood flow through the bicuspid valve is from the left atrium to the left ventricle.

Related glossary terms/phrases:
Left atrioventricular valve
Mitral valve



Abbreviation:
BMI

Definition:
The body mass index is measured by dividing body weight (in kilograms) by body height (in meters) squared (kg / m2). It is highly correlated with body composition, and higher BMI values are correlated with higher body fat content. The BMI value is used to classify one's weight in several categories including underweight, normal, overweight, and obese. The range of BMI values considered to be normal is 18.5 to 24.9. Higher BMI values are associated with an increased risk for diseases such as diabetes mellitus type II, hypertension, and cardiovascular disease.

See also:
Body Mass Index Calculator

Other resources:
Body mass index (Wikipedia)



Definition:
Another name for the parafollicular cell type within the thyroid gland. C cells are situated outside of the thyroid follicles and are interspersed between the follicles. C cells are responsible for producing the hormone calcitonin.

See:
Parafollicular cell



Abbreviation:
Ca2+

Definition:
Calcium (Ca2+) is a divalent cation. It plays an important role in physiological processes such as muscle contraction and synaptic transmission. Calcium is also an intracellular messenger.

The extracellular concentration of Ca2+ is about 2 mM. The intracellular concentration of Ca2+ is about 70 nM.



Abbreviation:
CO

Definition:
Cardiac output is defined as the volume of blood pumped every minute by each (left or right) ventricle of the heart. It is generally reported in L/min or mL/min.

Cardiac output (CO) is the product of heart rate (HR) and stroke volume (SV). Thus,

CO = HR × SV

Heart rate is defined as the number of heart contractions per minute. Stroke volume is the volume of blood pumped per ventricular contraction.

For a typical resting adult human being at rest, CO is approximately 5 L/min.

CO = 70 contractions/min × 70 mL/contraction = 4,900 mL/min



Definition:
Thick connective tissue that separates the atria and the ventricles. It is also referred to as fibrous skeleton of the heart. Despite its name, it is important to note that there is no bone in this tissue.



Definition:
The lowest part of the female uterus facing the vaginal canal. It is cylindrical in shape and 2-3 cm long. A cervical canal runs the entire length of the cervix and connects the lumen of the vaginal canal to the lumen of the uterus. The opening of cervical canal into the uterus is called the internal os, and the opening into the vagina is called the external os.



Definition:
An endogenous, tissue-specific molecule that functions to inhibit mitosis (cell division) in the very tissue releasing the molecule.



Definition:
Refers to a conformational change of a channel protein by which the channel goes from the open state to the inactive state. The inactive state refers to a conformational state in which ions are not allowed to permeate the channel pore. Thus, with respect to ion permeability, the inactive state is similar to the close state of the channel. Ions cannot permeate the channel pore either in the closed or inactive state. However, the channel assumes very distinct and different conformations in the inactive state and closed state.

See also:
Neuronal Action Potential - Important Features of the Neuronal Action Potential



Definition:
An integral membrane protein which contains a pore through which ions, water, or polar molecules permeate. For any given channel, the pore is usually very selective for the particular ion or molecule. For example, sodium (Na+) channels are very selective for Na+ over other cations.

The channel pore may be constitutively open, or it may be gated to the open state by various stimuli such as chemical ligands, voltage, temperature, or mechanical stimulation of the membrane.



Abbreviation:
CI

Definition:
The main anion (negatively charged ion) of the extracellular fluid.

Cloride (Cl) plays an important role in several physiological processes such as the action potential of skeletal muscle cells, CO2 transport in blood (via Cl/bicarbonate exchange across the plasma membrane of red blood cells), and many other processes.

The extracellular concentration of Cl is about 110 mM. The intracellular concentration of Cl is about 10 mM.



Definition:
The semi-solid, partially-digested contents of the stomach that exit via the pyloric valve to enter the duodenum (initial region) of the small intestine.



Definition:
Specifically, thyroid colloid. Refers to the protein-rich fluid within the lumen of thyroid follicles. The major protein component of the thyroid colloid is thyroglobulin.



Definition:
The colon is part of the large intestine within the digestive tract that is located between the cecum and the rectum. The colon is subdivided into the ascending colon, transverse colon, descending colon, and sigmoid colon.



Definition:
Endocrine cell of the anterior pituitary gland responsible for synthesizing and releasing adrenocorticotropic hormone (ACTH).



Definition:
A type of secondary active transport across a biological membrane in which a transport protein couples the movement of an ion (usually Na+ or H+) down its electrochemical gradient to the movement of another ion or molecule against a concentration or electrochemical gradient. The ion moving down its electrochemical gradient is referred to as the driving ion. The ion/molecule being transported against a chemical or electrochemical gradient is referred to as the driven ion/molecule.

In cotransport, the direction of transport is the same for both the driving ion and driven ion/molecule (into the cell or out of the cell).

An example is the Na+/glucose cotransporter (SGLT), which couples the movement of Na+ into the cell down its electrochemical gradient to the movement of glucose into the cell against its concentration gradient.

Cotransport is also commonly referred to as symport.

Transport proteins that are involved in this type of transport are referred to as cotransporters or symporters.

See:
Symport

Related glossary terms/phrases:
Secondary active transport
Exchange

See also:
Lecture notes on Secondary Active Transport



Definition:
Mental retardation, dwarfism, and abnormal bone formation caused by thyroid hormone deficiency during fetal development.



Definition:
A potent blocker of nicotinic cholinergic receptors (nicotinic acetylcholine receptor, nAChR) found at the neuromuscular junction. At small doses, curare can lead to muscle weakness. At high doses, curare can lead to paralysis of skeletal muscles, which would also result in asphyxiation (and ultimately death) due to paralysis of the diaphragm. Curare was commonly the active agent of poison arrow.

Other resources:
See Wikipedia



Definition:
The enzyme found in target tissues of the thyroid hormones that converts thyroxine (also known as tetraiodothyronine or T4) to triiodothyronine (T3).

Specifically, 5'-deiodinase converts T4 to the active form of the thyroid hormone, T3.

5-Deiodinase converts T4 to the inactive form of the thyroid hormone, reverse T3 (rT3).



Definition:
Refers to a change in the value of the membrane potential, where the membrane potential becomes less negative (or more positive) than the resting membrane potential.

Related glossary terms/phrases:
Repolarization
Hyperpolarization

See also:
Resting Membrane Potential - Introduction
Figure showing depolarization, repolarization, and hyperpolarization



Definition:
Accumulation of fluid in the interstitial compartment, which usually leads to a visible swelling of the affected tissue.



Definition:
Movement of a substance out of the cell.

Efflux is reported as a rate. It is the amount of substance that moves through a given area of the plasma membrane per unit time.

Related glossary terms/phrases:
Flux
Influx
Unidirectional flux
Net flux



Abbreviation:
VDF

Definition:
When an ion is not at its electrochemical equilibrium, an electrochemical driving force (VDF) acts on the ion, causing the net movement of the ion across the membrane down its own electrochemical gradient.

The electrochemical driving force is generally expressed in millivolts and is calculated according the following equation:

VDF = VmVeq

where VDF is the electrochemical driving force, Vm is the membrane potential, and Veq is the equilibrium potential.

Related glossary terms/phrases:
Membrane potential
Equilibrium potential
Electrochemical gradient

See also:
Resting Membrane Potential - Electrochemical Driving Force Acting on Ions
Electrochemical Driving Force Calculator



Definition:
Electrophysiology is the study of the electrical properties of biological macromolecules, cells, tissues, and organs. Electrical signals such as voltage and/or current are generally measured. Examples include measuring changes in the membrane voltage of excitable cells (e.g., neurons, muscle cells, and some endocrine cells) during an action potential. The current carrried by ions as they permeate the pore of ion channels can also be measured - both at the single-channel level (single-channel current), as well as the macroscopic current resulting from the activity of a population of channels. As another example, electrical measurements may involve recording voltage changes at the surface of the skin that result from the activity of skeletal muscles (electromyogram, EMG), cardiac myocytes (electrocardiogram, ECG), or neurons in the brain (electroencephalogram, EEG).



Definition:
A type of secondary active transport across a biological membrane in which a transport protein couples the movement of an ion (usually Na+ or H+) down its electrochemical gradient to the movement of another ion or molecule against a concentration or electrochemical gradient. The ion moving down its electrochemical gradient is referred to as the driving ion. The ion/molecule being transported against a chemical or electrochemical gradient is referred to as the driven ion/molecule.

In exchange, the driving ion and the driven ion/molecule are transported across the biological membrane in opposite directions.

An example is the Na+/Ca2+ exchanger (NCX), which couples the movement of 3 Na+ ions into the cell down its electrochemical gradient to the movement of 1 Ca2+ ion out of the cell against its electrochemical gradient.

Exchange is also commonly referred to as antiport.

Transport proteins that are involved in this type of transport are referred to as exchangers or antiporters.

See:
Antiport

Related glossary terms/phrases:
Secondary active transport
Cotransport

See also:
Lecture notes on Secondary Active Transport



Definition:
Refers to the ability of some cells to be electrically excited resulting in the generation of action potentials. Neurons, muscle cells (skeletal, cardiac, and smooth), and some endocrine cells (e.g., insulin-releasing pancreatic β cells) are excitable cells.

See also:
Resting Membrane Potential - Introduction



Definition:
Facilitated diffusion (or facilitated transport) is a form of passive transport across biological membranes and refers to carrier-mediated transport of molecules/ions down a concentration gradient. Facilitated transport is mediated by facilitative transporters (also referred to as uniporters).

See also:
Facilitated Diffusion



Definition:
The rate of movement of a substance across an interface. The interface could be the plasma membrane (separating the intracellular or extracellular fluid compartments), an epithelial sheet separating two compartments, or where two solutions of different composition meet.

Flux is reported as a rate. It is the amount of substance that moves across a given interface per unit time.

Related glossary terms/phrases:
Influx
Efflux
Unidirectional flux
Net flux



Abbreviation:
GABA

Definition:
GABA is an inhibitory amino acid neurotransmitter in the central and peripheral nervous systems. It is the most abundant inhibitory neurotransmitter in the nervous system. During embryonic development, GABA acts as an excitatory neurotransmitter at some central synapses. GABA is a classical neurotransmitter. Its action is exerted via the activation of GABAA, GABAB, and GABAC receptors. GABAA and GABAC receptors are ligand-gated chloride channels, whereas GABAB receptors are G protein coupled receptors. At GABAergic synapses, the action of GABA is terminated by GABA transporters (GAT), which transport GABA from the extracellular space in synaptic and extrasynaptic regions into neurons and glia.



Definition:
Glutamate (Glu, E) is one of the standard twenty (20) amino acids used by cells to synthesize peptides, polypeptides, and proteins. It has a molecular weight of 147.13 g/mol. Its side chain has a pKa of 4.07 and, therefore, glutamate has a net negative charge at physiological pH.

In the nervous system, glutamate is an excitatory amino acid neurotransmitter. In fact, glutamate is the most abundant excitatory neurotransmitter in the nervous system. Glutamate is a classical neurotransmitter. Its action is exerted via the activation of glutamate receptors (GluR), some of which are ligand-gated ion channels (ionotropic receptors), and some are G protein coupled receptors (GPCRs, metabotropic receptors). At glutamatergic synapses, the action of glutamate is terminated by glutamate transporters (EAAT, excitatory amino acid transporter), which transport glutamate from the extracellular space in synaptic and extrasynaptic regions into neurons and glia.



Definition:
Glycine (Gly, G) is one of the standard twenty (20) amino acids. At a molecular weight of 75.07 g/mol, it is the smallest of the 20 amino acids used by cells to synthesize peptides, polypeptides, and proteins.

In the nervous system, glycine is also an inhibitory amino acid neurotransmitter. Glycinergic synapses are most commonly found in brain stem and spinal cord circuits. Glycine is a classical neurotransmitter. Its action is exerted via the activation of ionotropic glycine receptors (GlyR), which are ligand-gated chloride channels. At glycinergic synapses, the action of glycine is terminated by glycine transporters (GlyT), which transport glycine from the extracellular space in synaptic and extrasynaptic regions into neurons and glia.



Definition:
Goiter is an enlargement of the thyroid gland caused most commonly by iodine deficiency in the diet. Iodine deficiency leads to low levels of thyroid hormone production, and a reduction in thyroid hormone negative feedback on the hypothalamus and the anterior pituitary leads to a compensatory rise in the thyroid stimulating hormone (TSH). Enlargement of the thyroid gland is, therefore, caused by abnormally high circulating levels of TSH, which has a strong trophic effect on the thyroid gland.



Definition:
Endocrine cell of the anterior pituitary gland responsible for synthesizing and releasing follicle stimulating hormone (FSH) and luteinizing hormone (LH).



Definition:
The H zone is in the center of the A band where there is no overlap between the thick and the thin filaments. Therefore, in the H zone, the filaments consist only of the thick filament. The H zone becomes smaller as the muscle contracts and the sarcomere shortens. The center of the H zone is at the M line, which is also at the center of the sarcomere.

Related glossary terms/phrases:
A band
I band
M line
Z disk



Definition:
Bad, unpleasant, or offensive breath.



Definition:
A type of primary hypothyroidism, Hashimoto's disease is an autoimmune disorder, in which anti-thyroid antibodies destroy the ability of the thyroid gland to produce the thyroid hormones. Antibodies against thyroid peroxidase (TPO) and thyroglobulin (Tg) are often seen in this disorder.

Hashimoto's disease is also known as Hashimoto's thyroiditis.

Related glossary terms/phrases:
Hypothyroidism
Primary hypothyroidism



Definition:
Abnormal, unwanted, and excessive hair growth on the face and body. Particularly used to refer to women with such a condition.

Related glossary terms/phrases:
Hirsute



Definition:
The Hodgkin cycle represents a positive feedback loop in neurons, where an initial membrane depolarization from the resting value (∼ −70 mV) to the threshold value (∼ −50 mV) leads to rapid depolarization of the membrane potential to approach the equilibrium potential for Na+ (VNa ≈ +60 mV). The voltage-gated Na+ channels of neurons are responsible for the Hodgkin cycle.

See the figure depicting the Hodgkin cycle.

See also:
Important Features of the Neuronal Action Potential



Definition:
A chemical messenger molecule secreted (i.e., released) by endocrine cells within endocrine glands.

By definition, a hormone molecule is released into the bloodstream and travels throughout the body to find its target cells. Teget cells may be within the intravascular compartment (i.e., within blood vessels), but most hormones have target cells in tissues outside of the blood vessels.

Related glossary terms/phrases:
Endocrine cell
Endocrine gland



Definition:
A condition characterized by an inability to tolerate, or over-sensitivity to, sounds of moderate loudness encountered in daily life.

It is also spelled hyperacousis.

Other resources:
Hyperacusis (Wikipedia)



Definition:
A total plasma calcium level that is greater than the normal range of 2.2 - 2.6 mM (9 - 10.5 mg/dL). The free calcium concentration in the plasma is approximately 1.5 mM (6 mg/dL), and the remaining amount is bound to plasma proteins.

Related glossary terms/phrases:
Hypocalcemia



Definition:
Condition in which the plasma glucose concentration is abnormally high. In general, a fasting plasma glucose concentration higher than 110 mg/dL, or a postprandial plasma glucose concentration higher than 180 mg/dL is considered abnormally high.

Normal, fasting glucose concentrations range from 70 to 110 mg/dL.

Related glossary terms/phrases:
Hyperglycemic
Hypoglycemia
Hypoglycemic



Definition:
Related to, or pertaining to hyperglycemia.

An agent that causes an increase in plasma glucose concentration (i.e., induces hyperglycemia).

Related glossary terms/phrases:
Hyperglycemia
Hypoglycemia
Hypoglycemic



Definition:
An increase in the number of cells in a tissue. It is generally brought about through mitotic cell division of cells within a tissue.

It also refers to an enlargement of an organ or body part due to an increase in the number of cells within the organ or body part.

Related glossary terms/phrases:
Hypertrophy



Definition:
Refers to a change in the value of the membrane potential, where the membrane potential becomes more negative than the resting membrane potential.

Related glossary terms/phrases:
Depolarization
Repolarization

See also:
Resting Membrane Potential - Introduction
Figure showing depolarization, repolarization, and hyperpolarization



Definition:
Hyperthyroidism refers to a pathophysiological condition in which the thyroid gland produces and releases abnormally high levels of the thyroid hormones (T3 and T4).

The symptoms of hyperthyroidism may include high metabolic rate, weight loss, nervousness, excess heat production, tachycardia, and tremor.

Two main forms of hyperthyroidism exist: (1) Primary hyperthyroidism, and (2) Secondary hyperthyroidism.

In primary hyperthyroidism, the thyroid gland produces high levels of the thyroid hormones, either as a result of a secretory tumor of the thyroid gland, or under the control of thyroid stimulating immunoglobulins (such as in Graves' disease).

Secondary hypothyroidism is caused by high levels of thyroid stimulating hormone (TSH) produced by the anterior pituitary gland. TSH then stimulates the thyroid gland to produce excessive amounts of the thyroid hormones.

Related glossary terms/phrases:
Hypothyroidism
Euthyroidism



Definition:
An increase in the size of a cell such as muscle. It is generally brought about through the addition of cellular components.

It also refers to an enlargement of an organ or body part due to an increase in the size of the cells within the organ or body part.

Related glossary terms/phrases:
Hyperplasia



Definition:
A total plasma calcium level that is less than the normal range of 2.2 - 2.6 mM (9 - 10.5 mg/dL). The free calcium concentration in the plasma is approximately 1.5 mM (6 mg/dL), and the remaining amount is bound to plasma proteins.

Related glossary terms/phrases:
Hypercalcemia



Definition:
Condition in which the plasma glucose concentration is abnormally low. In general, a plasma glucose concentration lower than 70 mg/dL is considered abnormally low.

Normal, fasting glucose concentrations range from 70 to 110 mg/dL.

Related glossary terms/phrases:
Hypoglycemic
Hyperglycemia
Hyperglycemic



Definition:
Related to, or pertaining to hypoglycemia.

An agent that causes a reduction in plasma glucose concentration (i.e., induces hypoglycemia).

Related glossary terms/phrases:
Hypoglycemia
Hyperglycemia
Hyperglycemic



Definition:
Hypothyroidism refers to a pathophysiological condition in which the thyroid gland does not produce sufficient amounts of the thyroid hormones (underactive thyroid), leading to abnormally low levels of the thyroid hormones (T3 and T4).

The symptoms of hypothyroidism may include reduced metabolic rate, chronic fatigue, weight gain, myxedema, and depression.

Three main forms of hypothyroidism exist: (1) Primary hypothyroidism, (2) Secondary hypothyroidism, and (3) Tertiary hypothyroidism.

In primary hypothyroidism, the thyroid gland itself is incapable of producing normal levels of the thyroid hormones.

Secondary hypothyroidism is caused by low levels of thyroid stimulating hormone (TSH). TSH, produced by the anterior pituitary gland, is required to stimulate the thyroid gland to produce the thyroid hormones.

Tertiary hypothyroidism is caused by low levels of thyrotropin releasing hormone (TRH). TRH, produced by the hypothalamus, is required to stimulate the anterior pituitary gland to produce TSH which, in turn, is required to stimulate the thyroid gland to produce the thyroid hormones.

Related glossary terms/phrases:
Primary hypothyroidism
Secondary hypothyroidism
Tertiary hypothyroidism

Hyperthyroidism
Euthyroidism



Definition:
Decreased oxygen content in arterial blood.

Related glossary terms/phrases:
Anoxia
Anoxemia
Hypoxia



Definition:
Describes a condition in which there is a decrease in oxygen supply to tissues. It may be generalized (whole body) or local (e.g., brain).

Related glossary terms/phrases:
Anoxia
Anoxemia
Hypoxemia



Definition:
The I band is the region of a striated muscle sarcomere that contains thin filaments. This region is closest to the Z disk, and is the lightest region of the sarcomere when viewed in under the light or electron microscope. The I band is occupied by the thin filaments only. Each Z disk runs through the middle of the I band. Therefore, half of each I band belongs to one sarcomere, and the other half belongs to the neighboring sarcomere. The I band shortens as the muscle contracts and the sarcomere shortens.

Related glossary terms/phrases:
A band
H zone
M line
Z disk



Definition:
The last region (i.e., most distal region) of the small intestine. Ileum receives the contents of the jejunum and, in turn, the contents of ileum leave the small intestine by emptying into the cecum. In adult humans, the ileum is about 2-4 meters (about 6.5-13 feet) long.

Related glossary terms/phrases:
Duodenum
Jejunum



Definition:
Not permeable. Not allowing the passage of substances. Impermeable refers to a property of a membrane or channel pore in preventing or restricting the passage of substances. For example, the lipid bilayer portion of biological membranes is highly impermeable to ions and large polar molecules.

See also permeable.

Related glossary terms/phrases:
Permeable
Permeability
Permeant
Impermeant

See also:
Lipid Bilayer Permeability



Definition:
Not permeant. Incapable of passing through or penetrating. Impermeant refers to the inability of a substance (e.g., ion or molecule) to cross (i.e., permeate or penetrate) a biological membrane or channel pore. For example, it can be said that ions are membrane impermeant.

See also permeant.

Related glossary terms/phrases:
Permeant
Permeability
Permeable
Impermeable

See also:
Lipid Bilayer Permeability



Definition:
Movement of a substance into the cell.

Influx is reported as a rate. It is the amount of substance that moves through a given area of the plasma membrane per unit time.

Related glossary terms/phrases:
Flux
Efflux
Unidirectional flux
Net flux



Definition:
Refers to the ability of the thyroid gland to accumulate iodide (I) against a steep electrochemical gradient. While the iodide concentration in plasma and interstitial fluid is approximately 300 nL, iodide concentration in the cytoplasm of thyroid follicular cells, as well as the lumen of thyroid follicles can be many folds higher. The protein that enables iodide transport into the thyroid gland against an electrochemical gradient is the Na+/iodide symporter (NIS), which is located in the basolateral membrane of thyroid follicular cells. Within the lumen of thyroid follicles, iodide is incorporated into the tyrosine residues of thyroglobulin during thyroid hormone biosynthesis, hence, allowing very high iodide concentrations in the colloid.



Definition:
The middle region of the small intestine between the duodenum and the ileum. The jejunum receives the contents of the duodenum and, in turn, its contents empty into the ileum. In adult humans, the jejunum is about 2.5 meters (about 8 feet) long.

Related glossary terms/phrases:
Duodenum
Ileum



Definition:
The tapping sounds heard through a stethoscope placed over a partially compressed peripheral artery.

For the purpose of non-invasive blood pressure measurements, the stethoscope is usually placed on the skin overlying the brachial artery just distal to an inflated pressure cuff wrapped around the upper arm. As the pressure cuff is gradually deflated, the Korotkoff sounds result from the pulsations of the blood through the partially constricted brachial artery. With each ventricular systole, blood is forced to flow through the partially constricted brachial artery, and the turbulence in blood flow leads to the generation of the Korotkoff sounds. When the pressure cuff is fully deflated, flow through the artery resumes its normal laminar flow and, at that time, the Korotkoff sounds disappear.



Definition:
Endocrine cell of the anterior pituitary gland responsible for synthesizing and releasing prolactin (PRL).



Definition:
Lidocaine is a local anesthetic and an antiarrhythmic drug. It is a commonly used local anesthetic for minor surgery and in dental procedures. Lidocaine is also used topically to relieve itching, burning, and pain from skin inflammations.

Lidocaine's mechanism of action is to block fast voltage-gated Na+ channels of neurons and cardiac myocytes.

Other names used for lidocaine are xylocaine and lignocaine.

See also:
Pharmacological Inhibition of Na+ and K+ Channels

Other resources:
Lidocaine (Wikipedia)



Definition:
In striated muscle sarcomere, the M line is the attachment site for the thick filaments. The M line is in the center of the A band and, thus, it is in the center of the sarcomere.

Related glossary terms/phrases:
A band
H zone
I band
Z disk



Definition:
The one-way valve between the left atrium and left ventricle of the heart. It is also known as the left atrioventricular valve or bicuspid valve.

The direction of blood flow through the mitral valve is from the left atrium to the left ventricle.

Related glossary terms/phrases:
Left atrioventricular valve
Bicuspid valve



Definition:
Chemical warfare agent belonging to sulfur (or sulphur) mustards. Causes severe chemical burns and water blisters (i.e., it is a vesicant). It is cytotoxic, mutagenic, and carcinogenic.

Other resources:
Sulfur mustard (Wikipedia)



Definition:
A condition associated with hypothyroidism (especially primary hypothyroidism) in adults. It is characterized by thick, course skin, skin swelling (edema), and decreased metabolic rate and mental activity.



Definition:
An equation used to calculate the equilibrium potential (Veq.) of an ion. The equilibrium potential for an ion is also referred to as the Nernst potential for that ion. It is the membrane potential at which no net movement of the ion in question occurs across the membrane.

General form of the Nernst equation

where Veq. is the equilibrium potential, R is the universal gas constant, T is the temperature in Kelvin, z is the valence of the ionic species, F is the Faraday's constant, and [X]o and [X]i are the extracellular and intracellular, respectively, concentrations of the ion in question.

See also:
Resting Membrane Potential - Nernst Equilibrium Potential
Derivation of the Nernst Equation



Definition:
Refers to the action potential of neurons. It is also referred to as the nervous impulse.

Related glossary terms/phrases:
Action potential

See also:
Neuronal Action Potential



Definition:
Refers to the action potential of neurons. It is also referred to as the nerve impulse.

Related glossary terms/phrases:
Action potential

See also:
Neuronal Action Potential



Definition:
Net flux represents the amount of substance moved in or out of the cell. It is the mathematical difference between influx and efflux.

Net flux = Influx − Efflux

Similar to influx and efflux, net flux is reported as a rate. It is the net amount of substance that moves through a given area of the plasma membrane per unit time.

Related glossary terms/phrases:
Flux
Influx
Efflux
Unidirectional flux



Definition:
Neurohormones are chemical messenger molecules that are released by neurons, but enter the bloodstream where they travel to distant target sites within the body. Therefore, neurohormones share characteristics with both neurotransmitters and hormones. Similar to neurotransmitters, neurohormones are released by neurons. Similar to hormones, neurohormones travel in the bloodstream.

Two well-known examples of neurohormones are oxytocin and the antidiuretic hormone (also referred to as vasopressin).

Related glossary terms/phrases:
Hormone
Neurotransmitter



Definition:
Chemical messenger molecules released by neurons into the synaptic cleft.

Neurotransmitter molecules may be small molecules such as glutamate, γ-aminobutyric acid (GABA), acetylcholine, dopamine, norepinephrine, serotonin (5-hydroxytryptamine), and glycine. These are referred to as classic neurotransmitters. A neuron generally releases only one type of small classic neurotransmitter.

Neurotransmitters may also be peptide molecules, such as substance P, opioids, and somatostatin. These are referred to as neuropeptides.

Gas molecules, such as nitric oxide (NO), may also act as neurotransmitters.



Definition:
Refers to cells that do not generate action potentials. With the exception of neurons, muscle cells, and some endocrine cells, all cells in the body are non-excitable.

See also:
Resting Membrane Potential - Introduction



Definition:
Refers to an abnormally low volume of urine production. Generally, a urine production rate of less than 400 mL/day is referred to as oliguria.

Related glossary terms/phrases:
Polyuria

Other resources:
Oliguria (Wikipedia)



Definition:
Auditory sensory organ within the inner ear cochlea, where the inner and outer hair cells are located.

Related glossary terms/phrases:
Cochlea



Definition:
Ouabain binds to and inhibits the transport activity of the Na+/K+/ATPase (i.e., sodium pump).

Ouabain is plant derived and belongs to the class of drugs referred to as cardiac glycosides. Similar to other cardiac glycosides, ouabain increase heart muscle contractility. However, ouabain is used only experimentally and not in humans (as for example digoxin is for the treatment of congestive heart failure).

There is some evidence that ouabain may be produced endogenously in humans.

Related glossary terms/phrases:
Vanadate

Other resources:
Ouabain (Wikipedia)



Definition:
Refers to that part of the action potential where the membrane potential is positive (inside with respect to the outside).

See figure.

See also:
Neuronal Action Potential - Important Features of the Neuronal Action Potential



Definition:
Body weight that exceeds the normal or standard weight range. The standard range is generally determined based on height and frame size.

Related glossary terms/phrases:
Body Mass Index

See also:
Body Mass Index Calculator



Definition:
The component of the electrocardiogram (ECG) that corresponds to the depolarization of atrial myocytes. Atrial depolarization precedes atrial contraction.

Related glossary terms/phrases:
QRS complex
T wave



Definition:
Cells within the thyroid gland. Parafollicular cells are situated outside of the thyroid follicles and are interspersed between the follicles. Parafollicular cells are responsible for producing the hormone calcitonin. Parafollicular cells are also referred to as C cells.



Definition:
Permeant refers to the ability of a substance (e.g., ion or molecule) to cross (i.e., permeate or penetrate) a biological membrane or channel pore. For example, it can be said that fat-soluble molecules are membrane permeant.

See also impermeant.

Related glossary terms/phrases:
Impermeant
Permeability
Permeable
Impermeable

See also:
Lipid Bilayer Permeability



Abbreviation:
PCG

Definition:
A recording of the sounds produced by the heart during the cardiac cycle. The main sounds recorded are those associated with turbulent blood flow caused by the closure of heart valves.



Definition:
Physiology is the study of how living systems function. The scope of physiological studies ranges from the subcellular level (molecules and organelles) all the way to the level of the whole organism and how organisms adapt to vastly different environmental conditions such as hot, cold, dry, humid, or high altitude.

See also:
What is Physiology?



Definition:
Plasma is the fluid portion of whole blood, which makes up about 40% to 60% of the total volume of whole blood. Plasma has a light yellow color and is generally obtained by separating the fluid portion from the blood formed elements through sedimentation or centrifugation. Plasma contains mostly water and, in addition, minerals, nutrients, proteins, hormones, and gases (oxygen and carbon dioxide). Unlike serum, in which fibrinogen and other clotting factors have been removed by coagulation, fibrinogen and other clotting factors remain present in plasma.

Plasma is one the main fluid compartments of the human body, making up nearly 10% of the total volume of body fluids. Plasma makes up the intravascular fluid compartment; itself a subcompartment of the extracellular fluid compartment.



Definition:
Refers to an abnormally large volume of urine production. Generally, a urine production rate of higher than 2.5 L/day is referred to as polyuria.

Related glossary terms/phrases:
Oliguria
Hypouresis

Other resources:
Polyuria (Wikipedia)



Abbreviation:
K+

Definition:
The main cation (positively charged ion) of the intracellular fluid.

Potassium (K+) plays an important role in the action potential of neurons and muscle cells.

The extracellular concentration of K+ is about 4 mM. The intracellular concentration of K+ is about 150 mM.



Definition:
Primary hypothyroidism is characterized by abnormally low levels of thyroid hormone (T3 and T4) production, where the defect is at the level of the thyroid gland itself.

In primary hypothyroidism, the circulating levels of the thyroid hormones are low, however, the levels of the thyroid stimulating hormone (TSH) are high due a lack of thyroid hormone negative feedback on the anterior pituitary.

Common causes of primary hypothyroidism include iodine deficiency (which leads to goiter), and Hashimoto's disease, in which autoimmune antibodies destroy the ability of the thyroid gland to produce the thyroid hormones.

Related glossary terms/phrases:
Hypothyroidism
Secondary hypothyroidism
Tertiary hypothyroidism



Definition:
Peptide hormone produced by the anterior pituitary gland. Prolactin is best known for its action in stimulating the mammary glands to produce milk (lactation).

Prolactin is known to be involved in many other physiological processes including enlargement of the mammary glands in preparation for milk production, sexual gratification, metabolism, regulation of the immune system, and others.

Other resources:
Prolactin (Wikipedia)



Definition:
Development of axillary and pubic hair during puberty.

Related glossary terms/phrases:
Adrenarche
Menarche
Thelarche



Definition:
A period of rapid physical growth and change, brought about by hormonal changes, that culminates in sexual maturity. In most human girls and boys, it starts between the ages of 10 and 14 and it is complete by the age of 16 in most girls, and by the age of 17 in most boys.

Related glossary terms/phrases:
Adrenarche
Menarche
Pubarche
Thelarche



Definition:
Pulse pressure is defined as the arithmetic difference between the systolic pressure (the highest blood pressure) and the diastolic pressure (the lowest blood pressure) recorded at any point along the vascular bed. Therefore:

Pulse pressure = Systolic pressure - Diastolic pressure

Pulse pressure values can be reported at any point along the vasculature starting from the left ventricle and aorta all the way to the vena cava and the right atrium. As with systolic and diastolic pressure values, pulse pressure is typically reported in mm Hg (millimeters of mercury). A typical arterial pulse pressure is 40 mm Hg (120 - 80 = 40 mm Hg). This value decreases as one proceeds along the vascular bed from arteries to arterioles, capillaries, venules, and veins. The most significant drop occurs along the arterioles.

See also:
Mean Arterial Pressure Calculator



Definition:
The component of the electrocardiogram (ECG) that corresponds to the depolarization of ventricular myocytes. Ventricular depolarization precedes ventricular contraction.

Related glossary terms/phrases:
P wave
T wave



Definition:
The relative refractory period refers to a period during the action potential. This is the time during which a stronger than normal stimulus is needed in order to elicit an action potential. The relative refractory period immediately follows the absolute refractory period.

Related glossary terms/phrases:
Absolute refractory period

See also:
Neuronal Action Potential - Refractory Periods



Definition:
Refers to the return of the membrane potential toward the normal resting value after a membrane depolarization.

Related glossary terms/phrases:
Depolarization
Hyperpolarization

See also:
Resting Membrane Potential - Introduction
Figure showing depolarization, repolarization, and hyperpolarization



Definition:
Secondary active transport is a type of active transport across a biological membrane in which a transport protein couples the movement of an ion (typically Na+ or H+) down its electrochemical gradient to the movement of another ion or molecule against a concentration or electrochemical gradient. The ion moving down its electrochemical gradient is referred to as the driving ion. The ion/molecule being transported against a chemical or electrochemical gradient is referred to as the driven ion/molecule.

This transport process is referred to as active transport because the driven ion/molecule is transported against a concentration or electrochemical gradient. It is referred to as secondary active transport because no ATP hydrolysis is involved in this process (as opposed to primary active transport). The energy required to drive transport resides in the transmembrane electrochemical gradient of the driving ion.

Secondary active transport is also referred to as ion-coupled transport. Those utilizing Na+ as the driving ion are called Na+-coupled transporters. Those utilizing H+ as the driving ion are called H+-coupled transporters.

Two types of secondary active transport exist: cotransport (also known as symport) and exchange (also known as antiport). Na+/glucose cotransporter and H+/dipeptide cotransporter are examples of cotransporters. Na+/Ca2+ exchanger and Na+/H+ exchanger are examples of exchangers.

Related glossary terms/phrases:
Cotransport
Symport
Exchange
Antiport

See also:
Lecture notes on Secondary Active Transport



Definition:
Secondary hypothyroidism is characterized by abnormally low levels of thyroid hormone (T3 and T4) production, where the defect is at the level of the anterior pituitary gland.

In secondary hypothyroidism, the anterior pituitary gland is unable to produce sufficient levels of the thyroid stimulating hormone (TSH) which, in turn, leads to insufficient stimulation of the thyroid gland to produce the thyroid hormones (T3 and T4).

Thus, in secondary hypothyroidism, the circulating levels of both TSH and the thyroid hormones (T3, and T4) are abnormally low

Related glossary terms/phrases:
Hypothyroidism
Primary hypothyroidism
Tertiary hypothyroidism



Definition:
Secretion refers to cellular release of substances (ions and small and large molecules) to the external environment of the cell. Secretion may be accomplished by exocytosis (fusion of transport vesicles with the plasma membrane and release of vesicle contents to the external environment), by transport of molecules across the plasma membrane (via the activity of transport proteins such as pumps, transporters, and channels), or by simple diffusion of fat-soluble molecules through the plasma membrane out of the cell.

For example, endocrine cells secrete hormone molecules that then enter the bloodstream. Neurons release (i.e., secrete) neurotransmitter molecules into the synaptic cleft. Some neurons secrete neurohormones; which similar to hormones, travel in the bloodstream to reach distant target cells. Epithelial cells secrete molecules in luminal spaces, such as digestive enzymes secreted into the digestive tract by various cell types.

See also:
Excretion



Abbreviation:
Na+

Definition:
The main cation (positively charged ion) of the extracellular fluid.

Sodium (Na+) plays an important role in several physiological processes such as the action potential of neurons and muscle cells, secondary active, sodium-coupled transport of ions, nutrients, neurotransmitters across the plasma membrane of cells, and many other processes.

The extracellular concentration of Na+ is about 145 mM. The intracellular concentration of Na+ is about 15 mM.



Definition:
Endocrine cell of the anterior pituitary gland responsible for synthesizing and releasing growth hormone (GH).



Definition:
Refers to the rapid depolarization of the membrane early in the action potential. In neuronal, skeletal muscle, and cardiac muscle action potentials, the Hodgkin cycle is responsible for the spike phase of the action potential.

See figure.

See also:
Important Features of the Neuronal Action Potential



Definition:
Action potential

Related glossary terms/phrases:
Action potential



Definition:
A rectangular signal waveform used in physiological studies to perturb (i.e., challenge) the system under study. The response of the system to the pulse is then studied carefully to learn about how the system responds to challenges.

Examples include pulses of voltage or current in electrophysiological experiments. Other examples include pulses of light, pressure, temperature, ligand, etc.

A square-wave pulse is defined by the amplitude and duration of the pulse, as well as by the frequency at which the pulse is applied to the system under study.

See also:
Neuronal Action Potential - Introduction

Other resources:
Square wave (Wikipedia)



Definition:
Sub-threshold (or subthreshold) refers to a stimulus that is too small in magnitude to produce an action potential in excitable cells.

In general, a sub-threshold stimulus leads to the depolarization of the membrane, but the magnitude of the depolarization is not large enough to reach the threshold voltage. Therefore, sub-threshold stimuli do not elicit action potentials.

Related glossary terms/phrases:
Threshold
Supra-threshold

See also:
Neuronal Action Potential - Introduction



Definition:
Supra-threshold (or suprathreshold) refers to a stimulus that is large enough in magnitude to produce an action potential in excitable cells.

In general, a supra-threshold stimulus leads to the depolarization of the membrane, and the magnitude of the depolarization is larger than that necessary to simply reach the threshold voltage. Therefore, supra-threshold stimuli elicit action potentials.

Related glossary terms/phrases:
Threshold
Sub-threshold

See also:
Neuronal Action Potential - Introduction



Definition:
The component of the electrocardiogram (ECG) that corresponds to the repolarization of the ventricles. Therefore, the T wave also corresponds to relaxation of the ventricles.

Related glossary terms/phrases:
P wave
QRS complex



Definition:
Tastants are taste-provoking chemical molecules that are dissolved in ingested liquids or saliva.

Tastants stimulate the sense of taste. It can also be said that tastants elicit gustatory excitation.

A tastant is the appropriate ligand for receptor proteins located on the plasma membrane of taste receptor cells.



Definition:
In humans, taste buds are the sensory receptor areas primarily on the tongue, but also in the throat, where taste receptor cells are located. In humans, there are approximately 10,000 taste buds in the oral cavity (tongue) and throat. A typical human taste bud contains about 50 taste receptor cells. Stimulation of taste receptor cells by tastants elicits the sense of taste (also referred to as gustation).



Definition:
Tertiary hypothyroidism is characterized by abnormally low levels of thyroid hormone (T3 and T4) production, where the defect is at the level of the hypothalamus.

In tertiary hypothyroidism, the hypothalamus is unable to produce sufficient levels of thyrotropin release hormone (TRH). Low TRH levels lead to low production of the thyroid stimulating hormone (TSH) by the anterior pituitary gland which, in turn, leads to insufficient stimulation of the thyroid gland to produce the thyroid hormones (T3 and T4).

Thus, in tertiary hypothyroidism, the circulating levels of TRH, TSH, T3, and T4 are all abnormally low.

Related glossary terms/phrases:
Hypothyroidism
Primary hypothyroidism
Secondary hypothyroidism



Abbreviation:
TEA

Definition:
An inhibitor of voltage-gated potassium (K+) channels.

TEA is a quaternary ammonium cation (positively charged ion). It is also commonly used as a cation replacement for sodium (Na+) in physiological buffers used in ion replacement experiments.

See also:
Pharmacological Inhibition of Na+ and K+ Channels



Abbreviation:
TTX

Definition:
Inhibitor of fast voltage-gated sodium (Na+) channels of neurons and muscle cells. It is an extremely potent and toxic neurotoxin.

See also:
Pharmacological Inhibition of Na+ and K+ Channels

Other resources:
Tetrodotoxin (Wikipedia)



Definition:
Development of breasts as an early event during female puberty.

Related glossary terms/phrases:
Adrenarche
Menarche
Pubarche



Definition:
The membrane voltage that must be reached in an excitable cell (e.g., neuron or muscle cell) during a depolarization in order to generate an action potential. At the threshold voltage, voltage-gated channels become activated. Threshold is approximately −50 to −40 mV in most excitable cells.

Related glossary terms/phrases:
Sub-threshold
Supra-threshold

See also:
Neuronal Action Potential - Introduction



Definition:
Endocrine cell of the anterior pituitary gland responsible for synthesizing and releasing thyroid stimulating hormone (TSH).



Definition:
The condition of hearing a sound in the absence of an external sound stimulus. The sound may be ringing, buzzing, whistling, or hissing. The sound may be soft, loud, low-pitched, or high-pitched. It may be heard in one ear or both ears (simultaneously or independent of one another).

Other resources:
Tinnitus (Wikipedia)



Definition:
The one-way valve between the right atrium and right ventricle of the heart. It is also known as the right atrioventricular valve.

The direction of blood flow through the tricuspid valve is from the right atrium to the right ventricle.

Related glossary terms/phrases:
Tricuspid valve



Definition:
Refers to the hyperpolarization phase of the action potential.

It is also referred to as hyperpolarizing afterpotential.

See figure.

Related glossary terms/phrases:
Hyperpolarization

See also:
Important Features of the Neuronal Action Potential



Definition:
The rate of movement of a substance across an interface in only one, and not the opposite, direction (i.e., flux in only one direction). For example, when referring to the plasma membrane of cells, we can think of unidirectional flux of a substance ino the cell (referred to as influx), as well as unidirectional flux of the substance out of the cell (referred to as efflux). The difference between two unidirectional fluxes is referred to as net flux, which is the net amount that moves into or out of the cell.

Related glossary terms/phrases:
Flux
Influx
Efflux
Net flux



Definition:
An electrophysiological technique in which the voltage of a cell membrane is controlled experimentally, and the current passing across the membrane is measured.

Related glossary terms/phrases:
Current clamp

See also:
Neuronal Action Potential - Pharmacological Inhibition of Na+ and K+ Channels



Definition:
The difference in electrical potential between two points.



Definition:
Areas of the central nervous system (brain and spinal cord) which consist mostly of myelinated axons (rather than cell bodies). The characteristic white color is due to the presence of myelin, which is mostly composed of lipids.



Definition:
The part of the embryonic gonad that develops into the male reproductive ducts (epididymis, vas deferens, and seminal vesicles). In the female, this structure disappears because of the absence of anti-Müllerian hormone secreted by the Sertoli cells.



Definition:
The Z disk (or Z line) defines the boundaries of a muscle sarcomere. Two adjacent Z disks along the myofibril mark the boundaries of a single sarcomere. The Z disks are the attachment sites for the thin filaments. Therefore, from each Z disk, thin filaments extend to two neighboring sarcomeres. When a muscle fiber contracts, the Z disks of a sarcomere move closer together (i.e., the sarcomere also shortens).

Related glossary terms/phrases:
A band
H zone
I band
M line









Posted: Sunday, March 31, 2013
Last updated: Friday, August 28, 2015