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Ejaculation moves semen through and out of the urethra. In male reproductive physiology, emission brings sperm and glandular fluids into the urethra, while expulsion uses coordinated muscular contractions to discharge them. Contributions from the autonomic nervous system and somatic nervous system couple secretion and transport with the activity of skeletal muscles.

The coordination involves the spinal cord, rather than being an isolated contraction of one reproductive organ. A rat experiment identified a population of spinal neurons whose loss disrupted ejaculation while leaving other components of mating behavior relatively preserved.^[ref] The result supports a spinal cord coordinating mechanism in that animal model; it does not by itself establish that every aspect of human ejaculation or sexual experience has the same organization of the nervous system.

Lique M. Coolen investigates neural control of reproductive behavior. With William A. Truitt, she tested the role of a lumbar spinal cord neuron population in male rats. Selective destruction using SSP-saporin disrupted ejaculation while leaving other components of sexual behavior intact.^[ref]

The dissociation supported a specific contribution to coordinating ejaculation, rather than merely a general inability to move or mate. It did not establish that the targeted neurons act alone, or that the same manipulation has been demonstrated in humans.

The penis is an external organ containing the terminal urethra and erectile tissue. In human anatomy it participates in urinary outflow and reproductive delivery. Its vascular erectile mechanism is distinct from the neural and muscular coordination of ejaculation.

Semen is the fluid discharged by the male reproductive tract during ejaculation. In humans it normally contains sperm suspended in secretions from reproductive glands, including the prostate and seminal vesicles. Semen is therefore a mixture, whereas a sperm is an individual gamete.^[ref]

Fluid volume and the number or movement of sperm describe different properties. A sample can contain fluid but few or no sperm, so visible semen alone does not establish successful gametogenesis or fertilization.

William A. Truitt coauthored an experimental identification of spinal neurons important for ejaculation in male rats. With Lique M. Coolen, he used a toxin targeted to a receptor-bearing lumbar neuron population. Removing these cells disrupted ejaculation while sparing other observed sexual behaviors.^[ref]

The design goes beyond observing neurons active during a behavior: altering the population and measuring a selective behavioral loss tests its causal importance. The result supports a role in a spinal coordinating circuit, while leaving the circuit's other necessary components to be identified.