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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:
Refers to neurons, synapses, or receptors where acetylcholine is used as the neurotransmitter.

For example, cholinergic neurons release acetylcholine as their neurotransmitter.

In cholinergic synapses, acetylcholine is released from the presynaptic neuron, and it acts on acetylcholine receptors in the plasma membrane of the postsynaptic cell.

Cholinergic receptors are those that respond to acetylcholine as the physiological ligand. The two major types are nicotinic and muscarinic cholinergic receptors (may also be referred to as nicotinic and muscarinic acetylcholine receptors).

Cholinergic drugs are compounds that mimic the action of acetylcholine by binding to and activating cholinergic receptors.



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:
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:
Bad, unpleasant, or offensive breath.



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:
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:
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:
Permeability refers to the ease with which molecules cross biological membranes. It may also refer to the ease with which ions or molecules pass through the pore of channel proteins.

Related glossary terms/phrases:
Permeable
Impermeable
Permeant
Impermeant

See also:
Lipid Bilayer Permeability



Abbreviation:
Tg

Definition:
The major glycoprotein found within the colloid of thyroid follicles. The thyroid hormones (T3 and T4) are synthesized on the backbone of thyroglobulin.



Definition:
Thyroid follicular cells are epithelial cells that form the lining of thyroid follicles. Each thyroid follicle is surrounded by a single layer of follicular cells. Follicular cells play the key role in thyroid hormone synthesis and release.

Thyroid follicular cells are also referred to as thyrocytes.



Definition:
Z disk

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



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:
Acetylcholine

See:
Acetylcholine



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



Definition:
An agonist is a molecule that binds to a receptor and activates a physiological response similar to that induced by the naturally occurring physiological ligand of the receptor. Therefore, agonist binding to a receptor mimics the action of the natural ligand.

For example, acetylcholine (ACh) is the naturally occurring physiological ligand that activates nicotinic and muscarinic acetylcholine receptors. Nicotine is an agonist of nicotinic ACh receptors (nAChR), and muscarine is an agonist of muscarinic ACh receptors (mAChR).

Related glossary terms/phrases:
Antagonist



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:
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:
Atropine is a blocker (i.e., inhibitor) of muscarinic acetylcholine receptors. Therefore, atropine blocks the action of postganglionic parasympathetic neurons.



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:
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



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



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:
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.



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



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



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:
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:
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



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:
Excretion refers to the elimination of waste substances from the body. The kidneys and lungs are mainly responsible for excreting metabolites and waste products.

See also:
Secretion



Definition:
Protrusion or bulging of one or both eyeballs out of the orbit. It may be caused by swelling of orbital tissue in response to trauma, or by hyperthyroidism (especially primary hyperthyroidism).

It is also referred to as exophthalmus, exophthalmia, proptosis, or exorbitism.

Other resources:
Exophthalmos (Wikipedia)



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:
Excretion of glucose in urine. This is indicative of an abnormal condition such as hyperglycemia caused by diabetes mellitus.



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



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:
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:
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:
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 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:
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:
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:
Oliguria

See:
Oliguria

Related glossary terms/phrases:
Polyuria

Other resources:
Oliguria (Wikipedia)



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:
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:
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:
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:
Neurotoxins are chemical molecules that have an adverse effect on neuron function and, thus, disrupt the normal function of the nervous system. Neurotoxins could be small molecules or peptides and can be derived from a variety of invertebrate and vertebrate animals, as well as plant species.

The following is a short list of some examples of neurotoxins:

α-Bungarotoxin: A peptide neurotoxin that inhibits the nicotinic acetylcholine receptor.

Chlorotoxin: A peptide neurotoxin that inhibits chloride channels.

α-Conotoxin: A peptide neurotoxin that inhibits the nicotinic acetylcholine receptor.

δ-Conotoxin: A peptide neurotoxin that inhibits voltage-gated sodium channels.

w-Conotoxin: A peptide neurotoxin that inhibits N-type voltage-gated calcium channels.

Picrotoxin: Inhibits GABAA receptor chloride channels.

Tetrodotoxin: Inhibitor of neuronal voltage-gated sodium channels.



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.



Abbreviation:
OAEs

Definition:
Otoacoustic emissions (OAE) are sounds that arise from the organ of Corti within the cochlea of the inner ear in response to sound stimulation. They are thought to result from vibrations of cochlear outer hair cells (OHC) in response to stimulation by sound. Otoacoustic emissions are inaudible to the human ear, but can be recorded by placing a small recording device in the external auditory meatus (ear canal).

Otoacoustic emissions have clinical diagnostic value and are measured as part of hearing exams. In the newborn, otoacoustic emissions are recorded in order to detect blockage in the middle ear cavity (e.g., as a result of fluid accumulation) or external ear canal.



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:
Permeable refers to a property of a membrane or channel pore in allowing substances to pass through. For example, the lipid bilayer portion of biological membranes is highly permeable to fat-soluble molecules, but is not permeable to ions and large polar molecules.

See also impermeable.

Related glossary terms/phrases:
Impermeable
Permeability
Permeant
Impermeant

See also:
Lipid Bilayer Permeability



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



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:
Glial cell found within the posterior pituitary gland. It resembles astrocytes and functions to provide support to unmyelinated axons whose terminals release oxytocin and vasopressin.



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:
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:
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 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:
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:
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



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:
Synaptic pathology. Any pathophysiological condition that leads to abnormal function of synapses within the nervous system. The pathology may be due to pre-synaptic and/or post-synaptic mechanisms, or may involve glial cells surrounding synapses.



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:
Thyroid follicular cell

See:
Thyroid follicular cell



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:
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:
Dry mouth. Dryness in the mouth resulting from abnormally low levels of saliva production.



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