Part 2

Making connections The cells that carry messages around your brain– making you think, feel, sense, and act– are called neurons. A human brain contains roughly 86 billion neurons and 84 billion other cells! Neurons The brain and nerves in your body contain neurons. Neurons connect to each other to create a network that information travels around. Each neuron collects incoming messages, then sends messages to other neurons or body parts. Dendrites Around their cell body, neurons have branches– called dendrites– that collect incoming messages. Axon Each neuron has a long, wiry axon, along which it sends outgoing messages. Myelin sheath Most axons are wrapped in a fatty substance called myelin, which makes messages travel faster. The longest axon in your body runs from your big toe to the base of your brain! Synapses Where the axon of one neuron meets the dendrite of another neuron is called a synapse. Connecting the brain The right and left sides, or hemispheres, of the brain are actually quite separate structures. Large bundles of axons connect one side to the other. The largest bundle is called the corpus callosum. Supporting cells Neurons might be the most important brain cells, but the brain couldn't function without other types of cell, too. Find out about some of these below. Oligodendrocytes These are fatty cells that wrap around axons in the brain to make the myelin sheaths. Microglia Microglia fight any germs that get into the brain and remove broken bits of cells. Astrocytes Astrocytes create the brain's structure, supply nutrients, and repair damage. Pericytes These cells control blood flow and decide what leaves the blood to enter the brain.

Spikes Information travels through neurons by electrical impulses, called spikes. The number and pattern of spikes carries the information– about incoming sensory signals, memories and feelings, or outgoing instructions to control the body. 1 Spike starts When enough messages are received by a neuron's dendrites, a spike begins where the cell body meets the axon. 2 Spike travels The electrical spike then travels down the axon, away from the cell body toward the synapses. 3 On to the next neuron When the spike reaches a synapse, it causes the release of chemicals that pass the message to the next neuron. Sending impulses Neurons carry information around the brain and the rest of the nervous system using two things: tiny electrical and chemical messages. Even your thoughts are carried by electrical impulses! Spikes travel down axons without myelin a bit like a wave moving across the sea. WHITE AND GREY MATTER Some neurons are wrapped in a fatty myelin sheath. This helps speed up electrical spikes. Neurons with a myelin sheath look white, and neurons without one look grey. INCREASING THE SIGNAL Imagine a neuron that tracks how hungry you are– if you're full, it doesn't make any spikes, but as your stomach empties, the more spikes it creates. More spikes make you feel hungrier. Weak signal The fewer messages a neuron receives telling it to spike, the fewer spikes it sends. Strong signal When lots of messages arrive at a neuron's dendrites, it fires lots of spikes.

Crossing the divide When a spike reaches the end of an axon, it causes the release of chemical messengers called neurotransmitters at a tiny structure known as a synapse. The chemicals pass on the message to the next neuron. Synapses Synapses link neurons together. They contain the end of one neuron's axon, a tiny gap, and part of the dendrite of the next neuron. Chemical messages cross the gap in about one thousandth of a second! Most brain neurons form synapses with thousands of other neurons. Neurotransmitter packet Ready to go Neurotransmitters are stored in little round packets in an axon's end. When a neuron is quiet, they remain still but ready to move. Neurotransmitters cross the gap Neurotransmitter release An electrical spike causes some packets to move to the edge of the neuron and release the neurotransmitters inside into the gap. Neurotransmitters stick to receptors Passing on the message The neurotransmitters spread to the next neuron and stick to receptors there, which change that neuron's electrical activity and can cause it to spike. Stop or go? There are two types of synapse: inhibitory (STOP) and excitatory (GO). STOP synapses make it less likely that the next neuron will spike. GO synapses do the opposite. Neurons add up all the STOP and GO signals they receive to decide whether to spike. STOP synapses tell other neurons not to spike. They're important for making sure the brain doesn't get overexcited. GO synapses tell other neurons it's time for them to spike. Most neurons need to receive multiple GO signals before they spike.

Reflexes Reflexes are automatic responses to a particular event– for example, sneezing when something irritates the nose. All animals have reflexes, and the brain isn't always involved in them. Spikes can travel directly from sensory organs to the spinal cord, then straight back to the muscles to cause a response. Sneezing clears the nose automatically– there's no need for the brain to think about it. Basic brains Human brains are very complicated, so many neuroscientists study animals with much simpler brains. This allows them to figure out the basic ways in which brains work, which helps us to understand all brains. Reflexes vs. thinking The brain and nervous system use information to help an animal survive. The simplest brains mainly use reflexes. Thinking is slower, as new sensory information is mixed with an animal's knowledge and thoughts about what it needs, before an action is chosen. The more complex a brain is, the more thinking it does. C. elegans uses mainly reflexes. If you touch either end of it, it automatically slithers away in the opposite direction. Not all animals have brains. PLANTS VS. ANIMALS Animals need brains because they move around. Plants live their lives in one place, so they don't need brains– although some, such as the Venus flytrap, have developed ways of moving quickly without neurons!

Amazing animal brains There are millions of different species of animal, and they all have different brains. The structure of an animal's brain is closely linked to how that creature lives its life. Many brains Each of an octopus's eight arms contains its own "mini-brain", which is called a ganglion. Octopuses use their arms for catching prey, exploring, and tasting. Arms Each arm can move on its own, without the main brain telling it what to do. The octopus is the most intelligent animal without a backbone. BRAIN DIVERSITY These three creatures have brains that are very different from ours. Shrinking shrews In winter, food is scarce, so the common shrew shrinks its brain and skull to save energy. In spring, the brain grows back! Walking brains Tiny spiders have fairly big brains to help them build webs. In some species, the brain takes up 80 per cent of the body and even spills into the legs! No-brainers Sea squirts have disappearing brains! Baby sea squirts can swim, but adult sea squirts stick to rocks, so they don't need a brain. As they grow they absorb it.

Evolution Humans belong to the ape family. By studying fossils of other species of human that no longer exist, we can see how modern humans evolved. Fossil skulls tell us how their brains changed in size and shape. Australopithecus africanus This ape lived in Africa and had certain features resembling a human. It could walk on two legs, had a more rounded skull, and probably used simple tools. Homo habilis The cranium of Homo habilis was expanded and its face and teeth were smaller than most apes. It used more complex stone tools. Homo erectus Homo erectus walked upright like modern humans and its brain was bigger than that of Homo habilis. It used stone axes and it may have made fires. Homo neanderthalensis Neanderthals were modern humans' closest relatives. They were shorter and more muscular, but their brains were the same size. They wore primitive clothes. Homo sapiens Our species evolved around 300,000 years ago. Our brains have not changed much since then, but human lifestyles are different because of what we've learned. Becoming human To understand where humans came from, scientists look at fossils and closely related animals. One of the most important parts of human evolution was the brain getting bigger. Family tree Evolution doesn't happen in a straight line. Many types of human species branched off from our ape ancestors, but only one branch led to modern humans. Close cousins Chimpanzees are our closest living relatives– 99% of their DNA is the same as ours! Their brain is three times smaller than a human's, but they make and use simple tools.