Nerves consist of bundles of different types of neurones. Fig. 5.1 shows the mean transmission speed of impulses for three types of sensory neurone, C, Aδ and Aβ. [Figure 5.1] • Impulses from pain receptors in the skin are sent along sensory neurones C and Aδ. • Sensory neurones C and Aδ synapse in the spinal cord with relay neurones known as projection neurones. • Projection neurones send impulses to the part of the brain that perceives pain. • Impulses from touch receptors in the skin pass along sensory neurones Aβ, which can also synapse with the projection neurones in the spinal cord. • Sensory neurones Aδ and Aβ are myelinated. • Sensory neurones C are unmyelinated. Back pain in humans can be reduced by using a transcutaneous electrical nerve stimulation (TENS) machine. A TENS machine uses electrical impulses to stimulate nerve endings near the site of the pain. It is thought that TENS triggers the release of natural painkillers called endorphins at synapses. Fig. 5.2 shows synapses in a pain pathway. [Figure 5.2]