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Glossary of Physiology Terms – S
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Salpingectomy
Definition:
Surgical removal of the fallopian tube.



Sarcolemma
Definition:
Plasma membrane of a muscle cell. It is also referred to as myolemma.

Related glossary terms/phrases:
Myolemma



Sarcoplasm
Definition:
Cytoplasm of a muscle cell. It is also referred to as myoplasm.

Related glossary terms/phrases:
Myoplasm



Secondary active transport
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



Secondary hypothyroidism
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



Secretion
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



Sodium
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.



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



Spike phase
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



Spike
Definition:
Action potential

Related glossary terms/phrases:
Action potential



Square-wave pulse
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)



Sub-threshold
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



Supra-threshold
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



Symport
Definition:
Cotransport

See:
Cotransport



Synaptopathy
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.









Posted: Friday, May 13, 2011
Last updated: Sunday, January 25, 2015