The process of formation of spermatozoa, including both spermatocytogenesis and spermiogenesis.

A ringlike muscle that normally maintains constriction of a body passage or orifice and that relaxes as required by normal physiological functioning. Also called anatomical sphincter.

The fusiform figure occurring during metaphase of cell division, composed of microtubules radiating from the centrioles and connecting to the chromosomes at their centromeres.

A small, usually single-celled asexual or sexual reproductive body that is highly resistant to desiccation and heat and is capable of growing into a new organism, produced especially by certain bacteria, fungi, algae, and nonflowering plants.

An unspecialized cell that gives rise to a specific specialized cell, such as a blood cell.

Any of a group of lipids with a specific 7-carbon-atom ring system as a nucleus, such as progesterone, adrenocortical and gonadal hormones, bile acids, sterols, toad poisons, and some carcinogenic hydrocarbons.

Any agent, act, or influence which produces functional or trophic reaction in a receptor or an irritable tissue.

The part of the autonomic nervous system originating in the thoracic and lumbar regions of the spinal cord that in general inhibits or opposes the physiological effects of the parasympathetic nervous system, as in tending to reduce digestive secretions or speed up the heart. Produces the \"flight-or-fight\" response.

The site of functional apposition between neurons, where an impulse is transmitted from one to another, usually by a chemical neurotransmitter released by the axon terminal of the presynaptic neuron. The neurotransmitter diffuses across the gap to bind with receptors on the postsynaptic cell membrane and cause electrical changes in that neuron (depolarization/excitation or hyperpolarization/inhibition).

The side-by-side association of homologous paternal and maternal chromosomes during early meiotic prophase.

The fusion of two gametes in fertilization.

The contraction, or period of contraction, of the heart, especially of the ventricles.