Myelin

This mural depicts a neuron (brain/spinal cord cell). The parts of a neuron (left to right) are dendrites (receive transmissions), soma or cell body (normal cell function, includes nucleus), axon (moves neural impulse), myelin sheath (speeds up transmission of neural impulse), and axon terminals (send the messages across the synaptic gap to another cell's dendrites). Neurons do not start out with a myelin sheath, it 'gets on' the nerves as they age, which occurs in humans from birth to adolescence. It is partially responsible for the increase in cognitive function over this time. Saying something/one gets on your nerves means they are annoying, so it is a joke that myelin literally gets on your nerve cells.