Neurons, action potentials and reaction time
Neurons and glia, grey and white matter, resting and action potentials, myelin and synapses, the brain's power use, and calculating reaction time with free fall. With a reaction-time investigation.
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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.[6][2]
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.[2]
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.[3]
The membrane is only about 8 nm thick, so a potential difference of around 90 mV across it gives an electric field of the order of 11 MV/m (E = V / d).[4]
Along an unmyelinated membrane an impulse moves at only about 1 m/s. In myelinated axons the signal is regenerated only at the nodes of Ranvier (saltatory conduction), which is much faster.[4][3]
At a synapse, neurotransmitters are released into the synaptic cleft and bind to receptors on the next cell.[3]
At rest the brain converts about 16 W, 19% of a basal metabolic rate of 85 W, and about a quarter of all the energy the body uses goes into maintaining electrical potentials in cells.[5]
Try it: Reaction time from a falling ruler: sight versus sound
You need
- A 30 cm or 50 cm ruler per pair
- Results table
- Calculator
- Optional: a metre rule to measure the eye–brain–hand path roughly
Safety
Drop the ruler straight down over a short distance and keep faces clear. Use rulers without sharp edges. The 'sound' catcher keeps eyes closed only while seated and still.
What to do
- 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.
- 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.
- 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.
- 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.
- 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.
What happens, and why?
For a ruler released from rest, d = ½gt², so t = √(2d/g); catching at 0.20 m means about 0.20 s. The reaction involves sensing, processing in the brain and a motor command down the spinal cord to the hand muscles. A path of the order of a metre at 1 m/s would take around a second, much longer than a typical catch, which shows that the signal must travel much faster along most of the route; myelinated axons conduct far faster than unmyelinated membrane.[9][7][8][4]
Play: Rest, depolarisation or repolarisation?
Each card describes a moment in a neuron. Is the membrane at rest, depolarising or repolarising?
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Quiz
Other ages: Ages 6–7 · Ages 10–11