The Nervous System

Your nervous system is your body's electrical control network. At its heart is your brain: a 1.4 kg lump of jelly-like tissue containing approximately 86 billion neurons (nerve cells), all networked together. Branching out from the brain is the spinal cord, then a vast network of nerves reaching every corner of your body. The nervous system senses what is happening, decides what to do about it, and sends out instructions, all in milliseconds, using tiny electrical signals.

• Brain neuronsapprox. 86 billionPlus approx. 85 billion support cells
• Brain weightapprox. 1.4 kgAbout 2% of your body
• Brain's share of energyapprox. 20%A massive amount for its size
• Nerve signal speedUp to 120 m/sapprox. 430 km/h
• Spinal cord lengthapprox. 45 cmConnects the brain to the rest of the body
• Reflex timeapprox. 50 msFaster than thinking

The main parts

The nervous system is split into two main parts.

• Central nervous system (CNS): the brain and spinal cord. The control centre.
• Peripheral nervous system (PNS): all the nerves that branch out from the CNS to the rest of the body.

The peripheral system itself is split into two parts: the somatic nervous system (controls voluntary movements like walking) and the autonomic nervous system (controls automatic things like heart rate and breathing).

The brain: your control centre

Your brain sits inside your skull and weighs around 1.4 kg, roughly 2% of your body weight. Despite its small size, it uses around 20% of your body's total energy: the most energy-hungry organ pound for pound. The brain is divided into several main areas:

• Cerebrum (the big folded outer part): handles thinking, memory, decision-making, language and conscious awareness. Split into left and right hemispheres.
• Cerebellum (smaller, at the back): coordinates muscle movements and balance.
• Brain stem (at the base, connecting to the spinal cord): controls automatic functions like breathing, heart rate and reflexes.

Neurons: the body's wires

The basic unit of the nervous system is the neuron, or nerve cell. Each neuron has:

• A cell body containing the nucleus.
• Many dendrites: short branching extensions that receive signals from other neurons.
• One long axon: the wire that carries signals out to other neurons or to muscles.

Neurons communicate by sending electrical pulses down their axons. When the pulse reaches the end of one axon, it triggers the release of chemicals called neurotransmitters that pass the signal across a tiny gap (a synapse) to the next neuron. Your brain alone has approximately 86 billion neurons and over 100 trillion synapses between them. The complexity is staggering.

How nerve signals travel

An electrical signal in a nerve travels at speeds of up to 120 metres per second (over 400 km/h), which means a message from your toe takes about 20 milliseconds to reach your brain. The fastest signals travel along nerves that are covered in an insulating sheath called myelin. Where myelin is missing (as in the disease multiple sclerosis), signals slow down and movements become uncoordinated.

The spinal cord

The spinal cord is a thick bundle of nerves running down the inside of your spine. It is your brain's main motorway: signals from your brain to the rest of your body travel down it, and signals from your body back to your brain travel up it. The spinal cord is so important that it is protected inside the bony spine, with extra layers of tough membrane and cushioning fluid.

The spinal cord also handles reflexes on its own. When you touch something hot, your hand jerks away in about 50 milliseconds, before the signal even reaches your brain. The spinal cord makes the decision locally and sends the instruction back to your muscles, then tells your brain about it afterwards. This is much faster than waiting for the brain to think about it.

Fact Your brain has roughly 86 billion neurons. To get a sense of that number, the entire population of Earth is around 8 billion people. So your brain has more than 10 times as many neurons as there are humans on the planet. Each neuron connects with thousands of others, giving over 100 trillion connections in total: more connections than there are stars in our galaxy.

Voluntary vs involuntary control

Some of what your nervous system does is under your conscious control (you choose to do it). Other things happen automatically without you having to think about them.

• Voluntary actions: walking, typing, talking, looking around, picking things up. Controlled by the somatic nervous system.
• Involuntary actions: heart beating, breathing, digesting food, controlling body temperature, pupil width. Controlled by the autonomic nervous system.

The autonomic nervous system itself has two opposing modes. The sympathetic mode is the "fight or flight" response: it speeds up your heart, sends blood to your muscles, and prepares you for action when threatened. The parasympathetic mode is the "rest and digest" response: it slows your heart, sends blood to your gut, and helps you relax and recover.

Did you know? Most of your brain's cells are not actually neurons. About half of your brain consists of glial cells (also called neuroglia), which support and protect the neurons. They provide nutrients, clean up waste, insulate the axons with myelin, and may also play roles in memory and brain repair. For a long time scientists treated glial cells as boring "glue" cells; recent research has shown they are much more important than that.

Deeper dive: how memories actually form

One of the great mysteries of the nervous system is how the brain stores memories. There is no single "memory cell" that holds a particular memory. Instead, memories are stored as patterns of connections across many millions of neurons.

When you experience something new (say, meeting a friend for the first time), the experience activates a particular pattern of neurons across many brain areas: ones that processed the visual image of the face, ones that heard the voice, ones that felt the emotion of the meeting, ones that recorded where and when it happened. The brain strengthens the synapses between all of these neurons, so that next time any one of them fires (say, you see the same face again), the whole pattern is reactivated, and the memory comes back.

This is called long-term potentiation and it works through a chemical change at the synapses. The more often a connection between two neurons is used, the stronger the synapse gets, and the easier it is to fire the connection in the future. The phrase often used is "neurons that fire together, wire together".

Different kinds of memory use different brain areas. Short-term memory (the address you just heard, the number you are about to dial) is held in a region called the prefrontal cortex. Long-term factual memory (the capital of France, your grandmother's birthday) is consolidated by a curved structure called the hippocampus and stored throughout the cortex. Muscle memory (riding a bike, playing the piano) is stored separately in the cerebellum and basal ganglia.

Damage to the hippocampus (from disease or injury) typically wipes out the ability to form new memories while leaving old ones largely intact. This is famously what happened to the patient H.M., whose hippocampus was surgically removed in 1953 and who became a key subject of memory research for the next 55 years.

For other systems controlled by nerves, see the muscular system and the circulatory system.