Part 3

Eyesight The eyes are the organs that let us see. Light enters through the pupil and is detected by receptor cells at the back of the eye. These send a signal to the brain, which creates an image. 1 Light Light travels in tiny packets called photons. Photons hit receptors in the eye to generate electrical signals. 2 Cornea The cornea is the eye's clear covering. It protects the eye and lets light through. 3 Pupil The black circle in the centre of your eye is a hole through which light enters. It is controlled by the iris. 4 Iris This circular muscle is the coloured part of your eye. It makes the pupil bigger in the dark, to let in more light, and shrinks it in bright light. 5 Lens The clear lens changes shape to focus incoming photons onto the retina. 6 Retina Receptors that detect photons to create electrical signals are found in a sheet, called the retina, at the back of the eye. 7 Optic nerve Signals from the retina are passed to neurons whose axons run to the brain. 8 Thalamus The first stop for visual signals in the brain is the thalamus. From here, they are sent to the visual cortex. 9 Visual cortex The brain forms an internal image of the world when visual signals reach the visual cortex. Making pictures The visual cortex is divided into regions that look after different parts of vision. Some regions focus on colour, for example, others movement, and some combine the two images taken by your left and right eye. Seeing things The sense of vision is very important to humans. The eyes gather light and convert it into neural signals. The brain uses these to create a picture of the world. OPTICAL ILLUSION The brain sometimes gets it wrong. The different colours and shapes in the image to the right trick the brain so that it sees movement. This is called an optical illusion.

Listening Sound is detected by the ears. When sound vibrations travel into the ear they are turned into electrical signals by receptor cells, called hair cells. The brain turns these signals into sounds. 1 Sound waves Sound travels through the air as vibrations. 2 Eardrum This thin membrane vibrates when sound waves reach it. 3 Hammer, anvil, and stirrup In the air-filled middle part of the ear, there are three tiny bones that move when the eardrum vibrates. 4 Oval window The stirrup bone taps on the oval window, another membrane, to send vibrations into the fluid- filled inner ear, or cochlea. 5 Cochlea The inner ear is spiral-shaped and contains hair cells that change vibrations into electrical signals. 6 Nerve A bundle of axons carries signals from the cochlea to the brain. Hearing sounds Sounds, such as the singing of a bird, are vibrating waves of air. You detect them when they enter your ears. There, sounds are converted into signals that are sent to the brain. HAIR CELLS Hair cells are found inside the cochlea. Each cell is topped with tiny hair-like structures that bend when vibrations pass through the fluid inside the cochlea. This bending generates electrical signals that are then sent to the brain. Into the brain When sound messages enter the brain, it compares the timing of the signals from the right and left ears to work out which direction the sound came from. Hearing cortex You become aware of sounds when signals reach the hearing cortex on either side of the brain.

Making us move We need to move in order to do all sorts of things: to search for food, escape danger, to play, or to exercise. The brain makes the body move by controlling its muscles. Many muscles This is what the body looks like beneath the skin! Most muscles are attached to bones. The body moves when these muscles pull on the bones to move the skeleton. Bend When a signal is sent to contract your biceps, your arm bends. Bend or straighten Muscles mainly come in pairs, such as the biceps and triceps in your arms. They are controlled by the nervous system. Signals from the brain tell a muscle to shorten– or contract– to make you move. If there is no signal, the muscle stays relaxed. Straighten When a signal is sent to contract your triceps, your arm straightens. The human body contains roughly 700 muscles. MOVEMENT CORTEX Three areas of the cerebral cortex help control movement. They work with structures inside the brain to make you move. Muscle control To accurately throw a ball, you must move your arm, hand, and finger muscles all at the right time. The brain coordinates the different muscles, and tells them when to move and by how much. The brain constantly receives messages from the senses, and adjusts the muscles to throw the ball accurately. In less than a tenth of a second, signals arrive from the brain to a muscle.

Relaxation If you're resting, your brain uses a part of the ANS called the parasympathetic nervous system to make your body relax. When there's no danger or anything to do, you save and store energy. Ready for anything The brain makes sure the body is ready for what it needs to do– whether that's relaxation or action. It does this using nerves that run to all parts of the body. These make up the autonomic nervous system (ANS). VOLUME CONTROL The more active you are, the more blood your body needs. Your heart always beats, but the ANS can speed it up or slow it down. Action When your body needs to get active, your brain uses a different part of the ANS, called the sympathetic nervous system. Digestion is switched off and the rest of the body gets ready to move. Fight or flight? When a person encounters danger, they need to decide very quickly if they should confront it or run away. The sympathetic nervous system prepares the body to do either. The brain also controls the release of certain chemicals, called hormones, which help prepare the body for action.

Knowing what you need! The brain knows when you are too hot or too cold, hungry or full, and what to do about it. Here, we look at how the brain recognizes that you're hungry and signals that you should eat. 1 Feeling hungry When your stomach is empty, it releases a hormone (chemical messenger) called ghrelin. Ghrelin travels in the blood to the brain, where it sticks to different neurons and makes you feel hungry. 2 Hunt for food When you're hungry, you want food. You might search, or ask, for it. Your brain helps you to notice anything that smells or looks like food. 3 Filling up When you find food, you eat it! As the stomach fills up, it stops producing ghrelin. Neurons attached to the stomach send messages to the brain, which tell it that the stomach is stretched. 4 Feeling full The intestines below the stomach release hormones that signal food has arrived there. These hormones tell the brain when you have eaten enough, so you feel full and stop eating... for a while. You absorb the food you have eaten, use up the energy from the food, and start to feel hungry again. The hypothalamus is the part of the brain that is most important for controlling hunger. NEGATIVE FEEDBACK When you are hungry, or full, the brain and body work together to change your behaviour, so that you stop feeling that way. Your body and brain use this "negative feedback" to keep you feeling just right.

Baby brains In the early stages of life, the brain and spinal cord are formed from a structure called the neural tube. The brain grows fast while the baby is in the womb, and it is about the size of a large orange at birth. 3 WEEKS A neural tube soon develops inside an embryo– the name given to an unborn baby between its second and eighth week. 6 WEEKS As the embryo grows, the front end of the neural tube grows fastest and divides into different parts. 9 WEEKS By nine weeks, sections of the neural tube have started to become the spinal cord, hindbrain, midbrain, and forebrain. The unborn baby is now known as a foetus. 13 WEEKS Over the following weeks, the forebrain grows the most. By 13 weeks, the brain looks similar in shape to an adult brain but much smaller. 25 WEEKS The surface of the brain is still smooth at 25 weeks, but folds are beginning to appear as it gets bigger. The midbrain is now inside the brain. 40 WEEKS Babies are usually born at about 40 weeks. A baby's brain looks like an adult's, but it's just one third of the size. It can't do much– but it can make the baby cry to get attention! At eight weeks old, a human embryo is the size of a raspberry! Making a brain A human brain forms as a baby grows inside its mother's womb. How it develops is controlled by instructions inside the baby's DNA and what the baby senses. SAVING SPACE The cerebral cortex that covers the brain is actually a big flat sheet of neurons, folded up to fit inside the skull. If you scrunch up a sheet of paper, you can see how the folds reduce the area it takes up.