Neurons and reflexes

Think about crossing a busy road. Your eyes watch the traffic, your ears hear a horn, your legs walk, and your brain decides when to go. No single organ can do it alone. The organs must work together, in the right order: that is coordination. In animals, two systems carry the messages: nerves (fast, electrical) and hormones (slower, chemical).

Changes around us are picked up by receptors in the sense organs: gustatory receptors on the tongue detect taste, olfactory receptors in the nose detect smell, and others in the eyes, ears and skin.

The message travels along nerve cells, called neurons, in one direction:

• the dendrites (short branches) pick up the information;
• it travels as an electrical impulse through the cell body;
• then along the long axon to its end.

"How does the message jump to the next neuron?" There is a tiny gap between them, the synapse. Electricity can't jump a gap, just as current stops at a break in a wire. So at the end of the axon, the electrical signal releases chemicals. They cross the gap and start a new electrical impulse in the next neuron. The same happens where a nerve meets a muscle or a gland.

Touch a hot pan by mistake and your hand jumps back before you even think. This quick, automatic response is a reflex. The signal doesn't wait for the brain: the nerves meet in the spinal cord, which sends the order straight back to the muscle.

The path of a reflex is called the reflex arc. Learn it as one chain:

stimulus → receptor → sensory nerve → spinal cord (relay) → motor nerve → muscle (effector) → response

The message also goes on to the brain, which is why you remember to be careful with hot pans next time.