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Glossary of Physiology Terms – M
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M line
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



Membrane potential
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



Menarche
Definition:
Onset of menstruation.

Related glossary terms/phrases:
Adrenarche
Pubarche
Thelarche



Mitral valve
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



Motor unit
Definition:
A motor unit is composed of a motor neuron and all of the muscle fibers (i.e., muscle cells) it innervates.

Alternatively, a motor unit is a group of muscle fibers innervated by a single motor neuron.



Mustard gas
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)



Myolemma
Definition:
Plasma membrane of a muscle cell. It is also referred to as sarcolemma.

Related glossary terms/phrases:
Sarcolemma



Myoplasm
Definition:
Cytoplasm of a muscle cell. It is also referred to as sarcoplasm.

Related glossary terms/phrases:
Sarcoplasm



Myxedema
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.









Posted: Wednesday, March 6, 2013
Last updated: Thursday, February 11, 2016