Transcutaneous electrical nerve stimulation (TENS) was developed in the 1960s and utilizes the gate-control theory of pain relief. It ‘closes the gate’ to pain impulses, in the case of labour, at the synapse in the spinal cord of afferent uterine nerve impulses. It does this by the generation of a barrage of below-pain-threshold impulses by a small current generator which mediates its effects via leads attached to two sets of conduction pads attached to the woman's back. The unit thus consists of a pulse generator and amplifier, together with a hand held ‘boost’ button that generates continuous impulses when engaged. The strength and frequency of impulse discharge is controlled by the woman and therefore it enhances her control. Additionally, it has the ability to stimulate natural endorphin release.