Every fact in the three brain lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Simplifications and care points: this is general science education, not medical advice. The brain's parts work together; linking one job to one lobe is a simplification. Membrane voltages differ between neurons and between textbooks: the biology textbook gives a resting potential of about −70 mV and the physics textbook about −90 mV, so treat them as typical values. The figure of 86 billion neurons is an average from brain-cell counts, not an exact number. The ruler test measures a whole chain (seeing, deciding, moving) and is affected by attention, practice and guessing; the falling-ruler formula ignores air resistance. Questions about a pupil's own health, headaches or head injuries go to a parent, the school nurse or a doctor.
All sources
[1] Brain Basics: Know Your Brain
National Institute of Neurological Disorders and Stroke (NIH, USA) · Reliability: High
Type: National health research agency
The brain is a three-pound organ lying in its bony shell, washed by protective fluid. The hindbrain (brain stem and cerebellum) controls vital functions such as breathing and heart rate; the cerebellum coordinates movement and learned movements such as playing the piano. The cerebrum is the source of conscious thoughts and actions, holds memories and lets us plan, imagine and think. Its two hemispheres are joined by the corpus callosum; the right hemisphere mainly controls the left side of the body and the left the right. Frontal lobe: planning, imagining, reasoning; the motor cortex controls voluntary movement. Parietal lobe: touch, temperature, reading and arithmetic. Temporal lobe: hearing, forming and retrieving memories. Occipital lobe: processing images from the eyes. Neurons have a cell body, dendrites that receive messages and an axon that sends them; myelin helps signals travel faster and farther.
[2] Anatomy and Physiology, 12.1 Basic Structure and Function of the Nervous System
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
The central nervous system (CNS) is the brain and spinal cord; the peripheral nervous system (PNS) is the rest, including the nerves. Neurons communicate and glial cells support them. Grey matter is dense with cell bodies and dendrites; white matter is mostly axons insulated by myelin. The nervous system has three basic functions: sensation (detecting stimuli), integration (processing information) and response (muscle contraction and gland secretion). The somatic nervous system controls voluntary movement of skeletal muscle; the autonomic nervous system controls involuntary functions of smooth and cardiac muscle and glands.
[3] Biology for AP Courses, 26.2 How Neurons Communicate
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
A resting neuron's inside is about 70 mV more negative than the outside (about −70 mV; the value varies). Na⁺ is concentrated outside and K⁺ inside, kept so by the Na⁺/K⁺ pump. A stimulus must reach the threshold of about −55 mV; then Na⁺ channels open and the membrane rises to about +40 mV (depolarisation); K⁺ leaving the cell repolarises it. The action potential is all-or-nothing. Myelin insulates the axon and speeds conduction; the action potential is regenerated at the nodes of Ranvier (saltatory conduction). At a synapse, neurotransmitters are released into the synaptic cleft and bind to receptors on the next cell.
[4] College Physics 2e, 20.7 Nerve Conduction – Electrocardiograms
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
The potential difference across a nerve cell membrane is 70 to 90 mV (the text uses about −90 mV inside at rest). The membrane is only about 8 nm thick, so the electric field across it is of the order of 11 MV/m. Along an unmyelinated cell membrane the impulse moves slowly, about 1 m/s; myelinated regions transmit signals at very high speed, with the signal regenerated at the nodes of Ranvier.
[5] College Physics 2e, 7.8 Work, Energy, and Power in Humans
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
The basal metabolic rate (BMR), the energy conversion rate of a person at rest, is 85 W in the textbook table; the brain uses 16 W (19%). A full 25% of all energy consumed by the body is used to maintain electrical potentials in living cells.
BrainFacts.org, Society for Neuroscience · Reliability: High
Type: Professional scientific society, public education site
Neuroscientist Suzana Herculano-Houzel's counts give an average of 86 billion neurons in the human brain; she dissolved whole brains into a solution and counted the neuron nuclei.
Eric H. Chudler, University of Washington · Reliability: Medium-high
Type: University neuroscientist's educational site
Ruler test: hold a ruler at the top so it hangs down, the partner holds their hand at the bottom without touching it, drop it some time within 5 seconds, and record where it is caught; repeat 3 to 5 times. Catching at 8 inches means a reaction time of 0.20 s (200 ms). The test is not a simple reflex: the information you see goes to the brain, which sends a command to the arm and hand.
Neuroscience Program, Michigan State University · Reliability: Medium-high
Type: University outreach activity
Ruler-drop activity: test the same person three times, varying when the ruler is dropped; variations include a spoken 'drop' cue, comparing ages and practice. The signal path for the eyes-open test runs from the eyes to the brain (occipital lobe, then motor cortex), down the spinal cord and out to the muscles. Conversion chart: 2.54 cm ≈ 0.07 s, 15.24 cm ≈ 0.18 s, 30.48 cm ≈ 0.25 s.
When air resistance is negligible, all objects fall with the same constant acceleration g = 9.80 m/s² (local values about 9.78 to 9.83 m/s²). For an object dropped from rest, y = y₀ − ½gt².
Used in: Ages 15–16 (experiment); Ages 15–16 (quiz)