The Brain, the Nerves, and the Senses

TL;DR. Your brain is about 1.4 kilograms, roughly 2 percent of your body weight, and it consumes about 20 percent of your oxygen and glucose continuously. It contains around 86 billion neurons connected at perhaps 100 trillion synapses, and it cannot store fuel, which is why four minutes without blood flow causes permanent damage. Nerves carry signals to and from it at up to 120 metres per second along insulated fibres. The senses feed it: about 120 million light detectors in each eye, and roughly 15,000 sound-detecting hair cells in each ear that, in humans, never grow back. The recurring theme of this chapter is that the nervous system's parts are the least replaceable in the body, and that its most important property, the ability to rewire, is the reason recovery from damage is possible at all.

Key takeaways

• Four minutes. That is roughly how long brain tissue survives without blood flow, and it is why stroke and cardiac arrest are measured in minutes rather than hours.
• Neurons do not meaningfully regenerate, but connections do rewire. Neuroplasticity is the mechanism behind all rehabilitation, and it is use-dependent, which is why rehabilitation is a treatment rather than a comfort.
• Myelin insulation speeds nerve signals roughly a hundredfold. Losing it is multiple sclerosis (Chapter 40).
• Inner ear hair cells never regenerate in humans. Noise damage is permanent and entirely preventable, and untreated hearing loss is the single largest modifiable midlife risk factor for dementia.
• The lens of the eye never sheds cells. It only adds them, which is why everyone becomes long-sighted after about 45 and why cataract is essentially universal if you live long enough.
• The blood-brain barrier protects the brain and blocks most drugs, which is why brain infections and brain tumours are so much harder to treat than the same problems elsewhere.

The brain

In short: An organ that cannot store fuel, cannot replace its cells, and compensates by being able to rewire the connections between them.

The numbers

The fuel problem is the central vulnerability. Muscle stores glycogen; the liver stores glycogen; the brain stores almost nothing. It requires continuous delivery, which is why an interruption is catastrophic within minutes and why Chapter 22 is organised entirely around speed. During prolonged fasting the brain can shift partly to ketones made by the liver from fat, which reduces but never eliminates its glucose requirement.

The main parts, and what they do

How a neuron works

A neuron receives signals on branching dendrites, sums them at its cell body, and if the total crosses a threshold, fires an all-or-nothing electrical pulse down its axon. At the end, the pulse triggers release of a neurotransmitter across a tiny gap, the synapse, onto the next cell.

Three things follow:

• The signal is electrical within a cell and chemical between cells. Nearly every drug acting on the brain works at the chemical step: SSRIs block serotonin reuptake, opioids activate opioid receptors, benzodiazepines enhance GABA, antipsychotics block dopamine receptors.
• Excitation and inhibition must balance. Glutamate excites, GABA inhibits. Tip the balance toward excitation and neurons fire in synchrony, which is a seizure (Chapter 39).
• Myelin makes it fast. Support cells wrap axons in a fatty insulating sheath with gaps, so the impulse jumps between gaps rather than travelling continuously. This raises conduction speed from about 1 metre per second to up to 120. Stripping it is what multiple sclerosis does.

Plasticity: the one form of repair available

Adult neurons in the cortex are not replaced. What the brain can do instead is rewire: strengthen some synapses, weaken others, grow new dendritic branches, and recruit neighbouring regions to take over lost functions.

This is not a metaphor. It is the mechanism behind:

• Learning and memory, which physically consist of changed synaptic strength.
• Recovery after stroke, where surviving regions progressively take over. It is use-dependent, which is precisely why intensive, repetitive, task-specific rehabilitation works and passive rest does not (Chapter 22).
• Chronic pain, where repeated pain signalling sensitises the system until it fires without a stimulus (Chapter 40).
• Cognitive reserve, the observation that people with more education and more cognitively demanding lives sustain more Alzheimer pathology before showing symptoms (Chapter 37).

Plasticity is greatest in childhood and never disappears. It is the physiological basis for the claim, which is true, that it is never too late to start.

The blood-brain barrier

Brain capillaries are sealed with unusually tight junctions and wrapped by support cells, forming a barrier that admits oxygen, glucose, and small fat-soluble molecules while excluding most large or water-soluble ones, including most drugs and most immune cells.

The trade-off is stark. It protects the brain from circulating toxins and infections, and it is the main reason brain infections, brain tumours, and neurological diseases are so hard to treat: the drug cannot get in. Levodopa exists in its particular form precisely because dopamine cannot cross it and its precursor can (Chapter 38).

Care and durability

What ages the brain: total brain volume declines slowly from around the age of 30 to 40, with frontal and hippocampal regions affected earliest. Processing speed declines steadily. Crystallised knowledge (vocabulary, accumulated expertise) is largely preserved or improves into the seventies. So "getting slower" and "knowing less" are different, and only the first is typical.

What damages it: high blood pressure (the largest vascular contributor), smoking, diabetes, excess alcohol, repeated head injury, air pollution, untreated hearing loss, social isolation, poor sleep, and untreated depression. That list is the modifiable dementia risk factor list from Chapter 37, and it is strikingly similar to the cardiovascular list, which is not a coincidence.

What protects it: cardiovascular risk control, physical exercise (the most consistent protective association), cognitive and social engagement, treating hearing and vision loss, protecting the head, sleep, and treating depression.

Warning signs: sudden weakness, speech difficulty, or facial droop mean stroke and emergency care now. Progressive memory loss affecting daily function is different from ordinary forgetfulness and warrants assessment. New severe headache, especially sudden and maximal within seconds, is an emergency.

The peripheral nerves

In short: The cabling, which unlike the brain can regrow slowly, and which fails from the longest fibres inward.

Nerves outside the brain and spinal cord come in three functional types: motor (brain to muscle), sensory (body to brain), and autonomic (automatic control of organs, split into sympathetic "fight or flight" and parasympathetic "rest and digest").

Peripheral nerves can regenerate, unlike central ones, at roughly 1 mm per day. A cut nerve in the wrist may take months to a year to restore function, and often does so incompletely.

The longest fibres fail first, which is why diabetic and other neuropathies start in the toes and creep upward in a stocking pattern before appearing in the fingertips (Chapter 40). A neuron whose axon runs from the spinal cord to the foot has to maintain a cellular process a metre long, which is metabolically demanding and the first thing to fail when metabolism is impaired.

The autonomic nervous system deserves specific mention because it runs everything you do not think about: heart rate, blood pressure, digestion, sweating, pupil size, bladder function, sexual function. Damage to it (from diabetes, Parkinson's, or amyloidosis) causes dizziness on standing, gut paralysis, and bladder dysfunction, which are frequently misattributed.

The eye

In short: A camera whose film cannot be replaced and whose lens never stops thickening, which makes two age-related changes universal.

How it works

Light passes through the cornea (which does most of the focusing), through the pupil, through the lens (which fine-tunes focus), and lands on the retina, a sheet of neural tissue that is technically an outgrowth of the brain.

The retina contains two detector types:

Almost all of your detailed vision comes from the fovea, an area about 1.5 mm across. Everything else is periphery, which is why losing the macula (age-related macular degeneration) destroys reading and face recognition while leaving navigation intact, and why glaucoma, which destroys the periphery first, can advance a long way before it is noticed.

The two universal changes

Presbyopia. The lens grows throughout life, adding layers and never shedding cells, so it becomes progressively stiffer. By around 45 it can no longer change shape enough to focus close up. This happens to everyone. Reading glasses are not a failure of eye care; they are an inevitability of lens biology.

Cataract. The same never-replaced lens proteins accumulate damage over decades, and the lens clouds. It is essentially universal with sufficient age, and it is the leading cause of blindness worldwide, almost entirely because of unequal access to a 20-minute operation that replaces the lens and restores vision (Chapter 54).

Care and durability

What damages the eye: ultraviolet light (cataract, and eyelid cancers), smoking (a major risk factor for macular degeneration), diabetes (retinopathy, a leading cause of working-age blindness), raised eye pressure (glaucoma), and, in children, insufficient time outdoors, which is now the best-supported factor in the global rise of short-sightedness.

What protects it: sunglasses with UV protection, not smoking, glucose and blood pressure control, regular eye examinations (which detect glaucoma before any symptom), and, for children, roughly two hours a day outdoors, which trials show meaningfully reduces the onset of myopia.

Warning signs that are emergencies: sudden loss of vision, sudden onset of many new floaters or flashes (possible retinal detachment), a curtain across the visual field, sudden painful red eye with visual loss, and double vision of sudden onset.

The ear

In short: Fifteen thousand hair cells per ear that never grow back, which makes noise damage permanent and hearing protection a lifelong investment.

Sound reaches the eardrum, is amplified about twentyfold by three tiny bones (the smallest in the body), and enters the fluid-filled cochlea, a spiral in which different frequencies displace different positions along a membrane. Roughly 15,000 hair cells sit along it and convert mechanical displacement into nerve signals.

Those hair cells do not regenerate in mammals. Birds and fish regrow theirs, which is why the research field exists, and humans do not. Every one destroyed by noise, disease, or certain drugs is permanently gone.

Noise damage is cumulative and dose-dependent. Sustained exposure above roughly 85 decibels causes progressive loss, and the exposure limit halves for every 3 dB increase. Personal audio at high volume, power tools, live music, and industrial noise all contribute, and the damage is painless and invisible until enough cells are gone.

High frequencies go first, which is why the earliest symptom is not "everything is quieter" but "I can hear you but I cannot make out the words," particularly in a noisy room. Consonants carry most of the information in speech and they are high-frequency.

Two consequences worth stating plainly:

• Untreated hearing loss is the largest modifiable midlife risk factor for dementia (Chapter 37), and a randomised trial found hearing aids slowed cognitive decline in older adults at higher risk.
• The average person waits about a decade between noticing hearing difficulty and doing something about it, during which social withdrawal and its consequences accumulate.

The vestibular system shares the inner ear: three fluid-filled loops at right angles detecting rotation, plus two organs detecting linear acceleration and gravity. Its failure causes vertigo, and the commonest cause, benign paroxysmal positional vertigo, is produced by displaced crystals and cured in minutes by a positioning manoeuvre, which is one of the highest-value interventions in medicine relative to its cost.

Smell and taste

In short: The one sensory system that does regenerate, and an early warning indicator for two major diseases.

Smell receptors are neurons directly exposed to the outside air, and uniquely among human neurons they are replaced throughout life, roughly every one to two months.

Taste detects only five qualities (sweet, salty, sour, bitter, umami). Nearly everything people call taste is actually smell, arriving at the nose from the back of the mouth, which is why food is tasteless with a blocked nose.

Loss of smell is clinically informative out of proportion to its inconvenience:

• It appears years before motor symptoms in Parkinson's disease (Chapter 38) and early in Alzheimer's.
• It was a distinctive early feature of COVID-19.
• It causes real harm: undetected gas leaks and spoiled food, plus loss of appetite and a measurable effect on mood and quality of life that is routinely underestimated.

Sources and notes

Neuron count of about 86 billion: Azevedo, Herculano-Houzel et al., Journal of Comparative Neurology, 2009. Brain energy consumption, blood flow, and ischaemic tolerance are standard physiology (Kandel et al., Principles of Neural Science). Nerve conduction velocities and peripheral regeneration rate of approximately 1 mm/day are standard neurology. Photoreceptor counts: Curcio et al., Journal of Comparative Neurology, 1990. Cochlear hair cell numbers and the absence of mammalian regeneration are standard otology. Noise exposure limits: NIOSH and WHO occupational standards. Hearing aids and cognitive decline: the ACHIEVE trial, Lin et al., The Lancet, 2023. Time outdoors and myopia incidence: He et al., JAMA, 2015, and subsequent cluster-randomised trials. Olfactory neuron turnover and its role as a prodromal marker in Parkinson's: Doty, Nature Reviews Neurology, 2012.

Open questions. Whether adult humans produce meaningful numbers of new hippocampal neurons is genuinely contested, with careful studies reaching opposite conclusions. Restoring hair cell regeneration in mammals remains an unsolved research goal.

Next: the frame that holds you up, the covering that keeps the world out, and the glands that set the pace. 👉