KS3 / KS4 Biology: The human ear

Deep inside your head is a remarkably beautiful structure. A maze of tunnels and caverns submerged in fluid. These are the semicircular canals and the cochlea, which are part of your inner ear. They are crucial for both your balance and hearing.

When sound hits your ear, it sets off a wonderful chain of events. It enters as pressure waves, which push and pull your eardrum, making it vibrate. On the other side of the eardrum, slowing time allows us to see how these vibrations set a series of bones jiggling. They end with the smallest bone in your entire body, called the stirrup. It is smaller than a grain of rice. These bones transmit the vibrations to a sensitive area called the oval window. They also protect your ear. If a sound is too loud, they pull the stirrup away from the most sensitive parts. Temporarily at least, you go a bit deaf, but the rest of your ear is protected.

Beyond the stirrup is a liquid-filled cavern called your cochlea. The sound waves enter the water, tickling clumps of tiny hair-like sensors on the floor, which begin to dance to the sound of the world outside. Thousands of clusters of hair cells each pick out a different part of the sound. Each sends a tiny piece of information to your brain, where it's interpreted so you can make sense of the sounds around you. The ear has evolved to be most sensitive to the sounds of another voice, allowing your brain to tune in to the words of another human.

But your ear doesn't just allow you to hear. It also plays an essential role in allowing you to walk. For you to take just one step, your brain has to coordinate the precise movement of over 100 different muscles, bones, and tendons. And there is a place where people learn to walk sooner than anywhere else on Earth.

This is Koarmba. She is mother to a baby girl called Kossini. They live in Rhumsiki, a tiny village in the remote northern highlands of Cameroon. Here, most mothers believe in actively teaching their babies to stand and walk, to get them off their backs as soon as possible. And it works. These people have trained their brains to find their feet much earlier than you. Ever since Kossini was a month old, her mother has repeated the ancient ritual of katete, which means "to make jump." Every day, she takes hold under the arms and bounces her. This daily encouragement helps the gradual development into mature walking.

But before you can stand up for any length of time, your brain must learn to understand the orientation of your body, and that's why your inner ear is so important. The semicircular canals that form three twisting tunnels inside your ear are all oriented in a different direction. In each lies a saddle-topped fleshy mountain known as your crista. The mountain's slopes are covered in a thick forest of tiny hair cells. For the moment, they lie still, waiting. But this inner sea never remains calm for long. Because every now and then, there's a tsunami. A shockwave races through the tunnel and pummels the mountain. On its flanks, the hair cells are thrown about in the turbulent waters. The pressure builds until electricity flows, creating a powerful electrical current.

Every time your head moves, the hair cells are thrown about inside one or more of the canals. And in a fraction of a second, electrical impulses are fired straight to your brain. The feeling that emerges is your sense of balance. To walk, your brain has to learn to sense when you are over-balancing to one side and then instruct your leg to bring your weight back to the centre. All of this happens within a fraction of a second. To begin with, it's a real struggle. But Kossini's half-brother is already a master of bipedalism. And he is just ten months old. From now on, and for the rest of his life, walking will be automatic.

The amazing construction of your ear allows you to both sense and explore the planet you inhabit.