• M line
• Membrane potential
• Menarche
• Mitral valve
• Motor unit
• Mustard gas
• Myolemma
• Myoplasm
• Myxedema

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.

A band
H zone
I band
Z disk

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).

Resting membrane potential

Resting membrane potential

Onset of menstruation.

Adrenarche
Pubarche
Thelarche

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.

Left atrioventricular valve
Bicuspid valve

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.

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.

Sulfur mustard (Wikipedia)

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

Sarcolemma

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

Sarcoplasm

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.