Ouch! Touching a hot pan
Your hand lets go before you even feel the burn. Follow the signal through the nerves.
Meet your nerves
Nerves are the wires of your body. They run from your fingertips, up your arm, into a thick cable inside your backbone: the spinal cord. From there the cable goes up into your brain. Messages race along these wires so fast you never notice them. This lesson follows one message: your right hand touching a hot pan.
Peripheral nerves are bundles of nerve fibres (axons) wrapped in connective tissue. The nerves of the arm come off the brachial plexus, a web of roots, trunks and cords in the neck and armpit that sorts fibres from spinal segments C5 to T1 into the median, ulnar, radial, musculocutaneous and axillary nerves. The spinal cord is the body’s main cable: grey matter in the middle (posterior horns receive sensory input, anterior horns hold the motor neurons) and white matter around it, carrying tracts up to the brain and down from it. The other nerves of the body are hidden here so the arm’s wiring stands out.
The signal races in
Your fingertip touches the pan. Tiny heat sensors in the skin fire, and a signal (yellow) shoots up the nerve in your arm. It goes through the web of nerves near your shoulder and into the back of the spinal cord. The whole trip takes less time than a blink.
Heat-sensitive nerve endings (nociceptors) in the skin fire when the skin gets dangerously hot. Their signal travels along sensory fibres in the digital branches and then the median nerve, through the brachial plexus, past the cell body in the spinal (dorsal root) ganglion and in through the posterior root to the posterior (dorsal) horn of the spinal cord. Sharp, well-located pain travels in thin myelinated A-delta fibres at 3 to 30 m/s, so the metre from fingertip to cord takes somewhere between a thirtieth and a third of a second. The nerve also carries much faster fibres (the A-alpha fibres from muscle spindles and tendons reach 80 to 120 m/s), but those report position and stretch, not pain.
The spine answers by itself
The spinal cord does not wait for the brain. Inside the cord the signal hops from the back to the front, and a new order (green) rushes straight back out to the arm muscle: the biceps. The muscle tightens and yanks your hand away. Your brain has not heard about any of this yet. This is a reflex.
This is the withdrawal (flexor) reflex, and it is polysynaptic: in the posterior horn the sensory fibre synapses on interneurons, which excite the motor neurons in the anterior (ventral) horn of the same segments. Their axons leave through the anterior root, pass through the brachial plexus and run in the musculocutaneous nerve to the biceps brachii and brachialis, which flex the elbow. Other interneurons quieten the opposing triceps (reciprocal inhibition), and the same wiring can brace the other side of the body (the crossed extensor reflex, most obvious when you step on something sharp and the other leg takes your weight). The loop runs entirely within the spinal cord, and the hand is clear of the pan within about half a second.
Now you feel it
At the same time, the spinal cord sends a second message (red) up to the brain. It has more stops on the way: it climbs the cord, crosses to the other side, is passed on at a relay station deep in the brain called the thalamus, and finally reaches the strip of brain that feels touch, which has to work out what happened and where. That is the moment you say "ouch!". By then your hand is already safe.
In the posterior horn a second neuron picks up the pain signal, crosses the midline through the anterior white commissure one or two segments higher, and climbs the lateral spinothalamic tract (pain and temperature; the anterior tract carries crude touch). It ends in the thalamus, mainly the ventral posterolateral nucleus, where a third neuron relays it to the primary somatosensory cortex in the postcentral gyrus. Because of the crossing, the right hand is felt by the left side of the brain. The dull, burning after-pain travels separately in unmyelinated C fibres at only 0.5 to 2 m/s, which is why it arrives a moment after the sharp sting.
All of it, in order
Watch the whole thing: the signal races up the arm (yellow). At the spinal cord it splits in two: an order goes straight back out to the biceps (green), which pulls, while the pain message (red) starts its long slow climb to the brain. Pull away first, feel it second. That order keeps you safe, and it is built into your spinal cord.
The timings here are stretched so you can see them. In the real body the sharp-pain signal on A-delta fibres reaches the cord within a few tenths of a second, the reflex loop through the cord adds only a few milliseconds, and the arm is moving before you are aware of anything; the hand is clear within about half a second. The signal to the brain leaves the cord at the same time; what takes longer is not the distance but the processing: two more synapses on the way, then the cortex has to turn the signal into a felt, located pain. The dull burning pain that follows travels on unmyelinated C fibres at only 0.5 to 2 m/s, arriving a second or so later. Myelin is what makes the difference: a myelinated fibre conducts up to 120 m/s, an unmyelinated one 0.5 to 2 m/s. The brain can still overrule a reflex, which is how you manage to hold on to a hot plate long enough to put it down.