Muscle cell membrane capable of propagating action potentials.

The functional contractile unit of striated muscle.

The endoplasmic reticulum of a muscle cell; it envelops the myofibrils.

A force, resulting from natural selection parameters, that causes changes within the gene pool of a population.

Fluid discharged at ejaculation in the male, consisting of secretions of glands associated with the urogenital tract and containing spermatozoa.

Either of a pair of pouchlike glands situated on each side of the male urinary bladder that secrete seminal fluid and nourish and promote the movement of spermatozoa through the urethra.

See afferent neuron.

A gene located only on a sex chromosome; such genes exhibit different inheritance patterns in males and females.

A small mass of specialized cardiac muscle fibers located in the posterior wall of the right atrium of the heart that acts as a pacemaker of the cardiac conduction system by generating at regular intervals the electric impulses of the heartbeat. Also called sinoauricular node, sinus node.

Autosomal cells; all cells in the body except germ cells and gametes.

A taxonomic category subordinate to a genus (or subgenus) and superior to a subspecies or variety.

A male gamete or reproductive/sex cell; a spermatozoon.