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Glossary of Physiology Terms – I
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I band
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



Ileum
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



Impermeable
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



Impermeant
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



Inactivation
Definition:
Channel inactivation

See:
Channel inactivation



Influx
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



Intrinsic factor
Definition:
A glycoprotein released by parietal cells (also know as oxyntic cells) located in the fundus region of the stomach. Intrinsic factor is required for vitamin B12 absorption in the small intestine.



Inward current
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



Iodide trap
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.



Ion-coupled transport
Definition:
Secondary active transport

See:
Secondary active transport









Posted: Tuesday, March 5, 2013
Last updated: Friday, January 16, 2015