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The human brain contains on average about 86 billion neurons, counted by dissolving whole brains and counting neuron nuclei. Neurons communicate; glial cells support them.

Grey matter is dense with cell bodies and dendrites; white matter is mostly axons, insulated by lipid-rich myelin. In the CNS the somatic system controls voluntary movement of skeletal muscle; the autonomic system controls smooth and cardiac muscle and glands.

At rest, the inside of a neuron is about 70 mV more negative than the outside, with Na⁺ concentrated outside and K⁺ inside, kept that way by the Na⁺/K⁺ pump. If a stimulus reaches the threshold of about −55 mV, Na⁺ channels open and the membrane depolarises to about +40 mV; K⁺ leaving the cell then repolarises it. The action potential is all-or-nothing.

Experiment: Reaction time from a falling ruler: sight versus sound

1. The holder hangs the ruler with 0 cm between the catcher's open thumb and finger. Drop it at an unpredictable moment within 5 s; record the catch distance d.
2. Condition A (sight): 10 tries. Condition B (sound): the catcher closes their eyes and the holder says 'drop' as they release; 10 tries. Alternate the order between pairs.
3. For each try calculate t = √(2d / g), with d in metres and g = 9.80 m/s². Find the mean, the range and the uncertainty (half the range) for each condition.
4. Decide whether the difference between A and B is larger than the uncertainties. Discard and note any try where the catcher clearly anticipated the drop.
5. Estimate the length of the signal path eye → brain → spinal cord → hand. If signals moved at 1 m/s all the way, how long would that path take? Compare with your mean reaction time.

Try	Sight: d (m)	Sight: t (s)	Sound: d (m)	Sound: t (s)
1	Predict which condition gives the shorter mean reaction time.
2	Predict which condition gives the shorter mean reaction time.
3	Predict which condition gives the shorter mean reaction time.
4	Predict which condition gives the shorter mean reaction time.
5	Predict which condition gives the shorter mean reaction time.
6	Predict which condition gives the shorter mean reaction time.
7	Predict which condition gives the shorter mean reaction time.
8	Predict which condition gives the shorter mean reaction time.
9	Predict which condition gives the shorter mean reaction time.
10	Predict which condition gives the shorter mean reaction time.
Mean ± uncertainty	Predict which condition gives the shorter mean reaction time.

Questions

1. Describe the changes in membrane potential and ion movement during an action potential, with values.

2. Explain how myelin makes conduction faster and what white matter is made of.

3. Give your mean reaction times (sight and sound) with uncertainties. Is the difference significant? Explain.

4. Using your estimate of the signal path, show why nerve signals must travel much faster than 1 m/s along most of the route.

Sources for every fact on this sheet: echoesof.science/school/brain/sources/

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