A Leydig cell is a steroid-producing biological cell in the interstitial tissue between seminiferous tubules of the testis. Adult Leydig cells produce testosterone, with their activity regulated in part by luteinizing hormone. They are anatomically and functionally distinct from the Sertoli cells supporting developing germ cells inside the tubules.

A 2014 adult-mouse experiment showed that this distinction does not make the populations independent. Selective removal of Sertoli cells was followed by a major decline in Leydig cell number. Yet average circulating testosterone was not significantly reduced in the same simple proportion, and the high values associated with pulses were largely absent. Cell number, average hormone level and the pattern of hormone release were therefore different observations.^[ref]

For an illustrative accounting model, total production is cell number multiplied by mean production per cell. Halving cell number need not halve total production if remaining cells change their activity. A circulating concentration additionally depends on removal from the blood. This explains why one hormone measurement cannot serve as a direct count of Leydig cells.

A neuromuscular junction is the specialized synapse through which a somatic motor neuron excites a skeletal muscle fibre. Arrival of an action potential at the nerve terminal opens voltage-gated calcium channels; calcium entry triggers release of acetylcholine. Binding of acetylcholine to receptors on the muscle's motor end plate produces a local depolarization that normally triggers a muscle action potential.^[ref]

The electrical signal then initiates release of stored calcium within the muscle fibre, enabling contraction. Synaptic release and contraction are separate steps: the acetylcholine does not itself pull the contractile filaments. Breakdown of acetylcholine by acetylcholinesterase helps terminate the synaptic signal.^[ref]