The Brain and Nervous System In Depth
Our brain weighs about 1.4 kilograms, roughly 2 percent of our body weight, yet it uses about 20 percent of all the energy the body burns. Inside it are about 86 billion neurons, joined by trillions of connection points. This chapter goes deep into how a single neuron sends a signal, which chemicals cross the gap, and what each part of the brain does.
The structure of a neuron
A neuron is shaped like no other cell in the body. It stretches far out so it can send signals over long distances. The longest neuron in us runs from the tip of the toe up to the lower spinal cord, almost a meter long in one single cell. Tap the dots on the picture to see each part.
A nerve signal is a wave of electricity running along the cell membrane
At rest, the inside of a neuron is about 70 millivolts more negative than the outside. This is called the resting potential. When the cell is stimulated strongly enough, sodium channels open and positively charged sodium rushes in, turning the inside positive. Then potassium channels open to pull the charge back. All of this takes about one thousandth of a second.
Try it: is it strong enough to fire?
The sodium potassium pump keeps pushing the ions back where they started once the signal has passed. That job takes a lot of energy, and it is a big reason why the brain uses one fifth of the body's energy.
At the junction, the signal changes from electrical to chemical
The axon terminal does not really touch the next cell. A very small gap separates them, and that gap is called the synapse. When the electrical signal arrives, tiny sacs release chemicals across the gap to bind to receptors on the next cell. Tap to see what each chemical does.
Each job lives in a different place in the brain
The brain does not work as one single lump. Each region takes care of a different job, and they stay connected to one another all the time. Tap each lobe of the brain to see what it does.
Sometimes the spinal cord decides on its own
If your hand touches something hot, you pull it back before you even feel the pain, because the signal runs into the spinal cord and turns straight back out to the muscle without waiting for the brain. Press the button to see the path.
Compare the timing
The simplest reflex arc uses only two neurons joined together in the spinal cord, a sensory fiber and a motor fiber. The classic example is tapping the tendon below the knee and watching the leg kick, which doctors use to check whether that stretch of the nerve pathway is working normally.
If this pathway is damaged, the reflex disappears or becomes unusually brisk. So tapping the knee is not a ritual. It really is a test of the body's electrical circuit.
How much can our working memory hold?
Working memory is the brain's temporary holding space, like scratch paper inside your head. Most adults can hold about 7 random digits, plus or minus two. Children aged 11 to 12 usually manage about 6 to 7. Try measuring your own.
Try it: digit span
A short string of digits will appear for you to remember, then disappear. Type the whole string back correctly. Each time you get it right, it grows by one more digit.
A trick that psychologists call chunking means that instead of remembering 0 8 1 2 3 4 5 one digit at a time, you remember the chunks 081 2345. The brain can take in far more that way. This is why phone numbers are written in groups.
The hippocampus, the memory factory
New memories are encoded in the hippocampus, which sits deep inside the temporal lobe. Then, while we sleep, those memories are moved out and stored in the cortex. Someone whose hippocampus is damaged on both sides cannot form any new memories at all, even though their old memories are still complete.
Spaced review beats one long cram
Research on learning agrees: several short reviews spread out over time work far better than one long read the night before a test. And testing yourself helps you remember better than just reading it over again. This is why every chapter in this series has a quiz.
Test yourself
Fancy a game on this lesson?
Every question from this lesson is in the Body 101 arena. Race the clock, duel someone at home, play as a team, beat the bosses, or practise with no timer at all.