Touch something hot and one message leaves your skin, but the spinal cord handles it twice. An interneuron hands it straight to a motor neuron, and the arm is already lifting; the same message is also relayed up to the brain, which decides and sends its own command back down the cord. Fire the stimulus, or advance the time since the stimulus yourself, and watch where the amber message is at the moment the purple reflex reaches the muscle.
Fire the stimulus. Watch which colour reaches the motor neuron first, and where the other one is at that moment.
Example latencies: reflex ≈ 50–80 ms, conscious ≈ 150–250 ms. This model uses 70 ms and 200 ms.
Both responses begin identically: a receptor in the skin fires and a sensory neuron, part of the somatic nervous system in the peripheral nervous system, carries the message into the spinal cord. Inside the cord the message is handled twice. An interneuron passes it straight to a motor neuron, which contracts the arm muscle: the spinal reflex, complete at about 70 ms in this model. At the same time the message is relayed up the cord to the brain, which registers the heat and sends its own command back down the cord to a motor neuron, finishing at about 200 ms. The spinal cord therefore has two roles here: it initiates the reflex and it relays messages to and from the brain.