Place · Level 3
Nervous System
电信号、髓鞘、神经递质、能量供应和血脑屏障共同决定身体如何感知与行动
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Chapter 1
Electrochemical
Electrochemical
Nerve impulses run on sodium, potassium, calcium, and magnesium ion gradients. The Na⁺/K⁺ pump needs adenosine triphosphate: The cell's universal energy currency — almost everything that costs energy spends it., so the nervous system depends on both electrolytes and energy metabolism — neither pillar can be missing.
Energy budget
The brain is only 2% of body weight but consumes 20% of resting oxygen and 25% of glucose. Almost all of that energy goes to one thing: maintaining ion gradients.The Na⁺/K⁺ ATPase pump fires thousands of times per second on every neuron, pushing out the Na⁺ that leaks in during firing and pulling back the K⁺ that leaks out — each pump cycle consumes 1 adenosine triphosphate: The cell's universal energy currency — almost everything that costs energy spends it.. About 60% of brain ATP is spent on this single enzyme.
So strict hypoglycemia (< 2.5 mmol/L for > 30 minutes) directly threatens consciousness — not as a metaphor for 'too hungry to think', but as the physical reality of ion pumps stalling → membrane potential collapse → loss of neuron function.
Diabetic patients on too much insulin, long-fasting exercisers, and severe alcohol intoxication — these are real clinical scenarios for hypoglycemic coma, not exaggerations.
Chapter 2
Myelin insulation
Myelin insulation
Myelin makes nerve signals faster, more reliable, and more energy-efficient. B12 deficiency damages nerves; high-dose folate alone can mask the anemia while failing to repair the neural damage.
Saltatory conduction
Myelin isn't a uniform plastic insulator — it's wrapped in multiple lipid layers by Schwann cells (peripheral nerves) or oligodendrocytes (central nerves) (~70% lipid, mainly cholesterol + sphingomyelin + galactocerebroside).Between myelin segments are gaps of about 1 μm called nodes of Ranvier, where Na⁺ channels are concentrated.
The action potential doesn't slide smoothly along the axon — it jumps from node to node, called saltatory conduction, which is 50–100× faster than unmyelinated and 10× more energy-efficient.
So two presentations follow from myelin damage:
Slowed conduction → sensory abnormalities (numbness / tingling / walking on cotton wool)Conduction failure → paralysis (the classic mechanism of multiple sclerosis, MS)
Regeneration is slow: peripheral nerve myelin regenerates to a limited extent; central nervous system myelin essentially doesn't regenerate — the core reason spinal cord injuries are irreversible.
Chapter 3
Blood-brain barrier
Blood-brain barrier
Every capillary in the body has gaps, except brain capillaries — whose endothelial cells are welded shut between cells. This barrier is the blood-brain barrier (blood–brain barrier: The 'security gate' on brain vessels that blocks most substances in blood from entering the brain.), and it decides what gets into the brain and what doesn't.
This is the physical moat protecting the brain — the body's most expensive organ.
This is the physical moat protecting the brain — the body's most expensive organ.
Three layers
The blood–brain barrier: The 'security gate' on brain vessels that blocks most substances in blood from entering the brain. is built from three layers:Brain capillary endothelial cells: welded shut by tight junctions like claudin-5, with no fenestrae (windows)Basement membrane: the second sieveAstrocyte end-feet: wrap around the vessel and actively regulate permeability and nutrient transport
What gets in:
Small lipid-soluble molecules (alcohol, caffeine, nicotine, anesthetics) dissolve directly through the membraneGlucose enters via the GLUT1 transporter, matched in number to brain activityAmino acids enter via LAT1 / LAT2 transporters in competition (which is why large doses of BCAA reduce tryptophan entry, altering serotonin synthesis)Ketones enter via the MCT1 transporter
What doesn't get in:
Large proteins (antibodies, albumin) because they're too bigMost antibiotics and chemotherapy drugs because of their high polarity, plus the P-gp efflux pump that kicks them back into bloodDopamine and 5-HT are highly polar, which is why Parkinson's uses L-DOPA instead of supplementing dopamine directly (L-DOPA crosses the BBB and converts to dopamine inside the brain)
BBB damage is a common early presentation of neurological pathology: stroke, encephalitis, early Alzheimer's, and diabetic cerebrovascular disease all show elevated BBB permeability. This is why many neurological diseases relate to systemic inflammation, hyperglycemia, and hypertension — they slowly erode the BBB.
The glymphatic system
A 2013 Science paper from the Nedergaard lab overturned a 100-year-old assumption: the brain isn't lymphatic-system-free — it has a hidden cleanup network that opens primarily during sleep, called the glymphatic system.How it works: cerebrospinal fluid (CSF) flows along peri-arterial spaces into brain tissue; astrocyte AQP4 water channels push CSF through the parenchyma, flushing metabolic waste (β-amyloid, tau protein, lactate) into peri-venous spaces and ultimately into cervical lymphatics.
The core mechanism of sleep: when awake, neurons are active and interstitial space is narrow, so glymphatic flow is low; during sleep, interstitial space widens by ~60% and flow rises ~20×; slow-wave (deep) sleep is the most efficient clearance phase.
Why this discovery matters:
Explains why sleep deprivation produces next-day brain fog and slow thinking — it isn't 'tiredness', it's β-amyloid that didn't get clearedChronic sleep loss + aging degrade glymphatic function, allowing β-amyloid to accumulate — a possible early Alzheimer's mechanismAlso explains why lateral sleeping position increases glymphatic flow versus supine (animal model, Lee 2015)
So sleep isn't 'rest' — it's the brain's physical cleanup time. This is a rare brain-health rationale for '7–9 hours of sleep' that can be explained at the molecular level.
Chapter 4
Neurotransmitters
Neurotransmitters
B6 participates in synthesizing GABA, serotonin, dopamine and other neurotransmitters. Choline is the acetylcholine precursor.
But being a precursor isn't the same as 'more supplement = smarter / happier' — brain regulation is far more complex than any single pathway.
But being a precursor isn't the same as 'more supplement = smarter / happier' — brain regulation is far more complex than any single pathway.
Six neurotransmitters
1. Glutamate: the main excitatory transmitter; nearly all fast learning, memory, and sensory signaling rely on it. Made from ordinary amino acid metabolism with no nutritional bottleneck.2. GABA: the main inhibitory transmitter (sedation, anti-anxiety, sleep). The GAD enzyme converts glutamate → GABA and requires B6 (PLP) as a cofactor, so severe B6 deficiency can cause seizures (neonatal form).
3. Dopamine: reward, motivation, motor control. Phenylalanine → tyrosine → DOPA → dopamine; the pathway depends on iron (TH enzyme cofactor), B6, and vitamin C. Iron-deficiency fatigue and partial depression mechanisms originate here.
4. 5-HT (serotonin): mood, appetite, sleep rhythm. Tryptophan → 5-HTP → 5-HT; the rate-limiting enzyme TPH needs iron and vitamin C. Brain 5-HT accounts for only 5% of body total; 95% is in the gut — the anatomical root of the gut-brain axis.
5. Acetylcholine (ACh): attention, memory, motor neurons. Choline + acetyl-CoA → ACh; choline is often insufficient in normal diets (egg yolk, liver, soybeans are main sources).
6. Norepinephrine (NE): alertness, stress. Starts from dopamine; the DBH enzyme needs copper, and copper-metabolism disorders (Menkes / Wilson) have neurological manifestations.
Note: evidence is weak for supplements like '5-HTP or tryptophan boosts mood', and they carry serotonin-syndrome risk when combined with antidepressants (SSRIs). Neurotransmitter synthesis is a feedback-regulated closed loop, not 'more material = more product'.
Chapter 5
Energy hungry
Energy hungry
The brain and nerves consume energy continuously. B1, B2, B3, and B5 participate in the pathways that convert food energy into usable adenosine triphosphate: The cell's universal energy currency — almost everything that costs energy spends it..
What passes for 'brain nutrition' starts with stable energy supply, adequate sleep, and avoiding chronic energy deficits.
What passes for 'brain nutrition' starts with stable energy supply, adequate sleep, and avoiding chronic energy deficits.
Ketones · backup fuel
The brain prefers glucose, but it isn't the only choice — during prolonged fasting / strict low-carb / type 1 diabetes decompensation, the liver produces ketones (β-hydroxybutyrate BHB + acetoacetate + acetone), which cross the blood-brain barrier (blood–brain barrier: The 'security gate' on brain vessels that blocks most substances in blood from entering the brain.) to fuel the brain directly.The energetics of ketone supply:
Full keto-metabolism can supply 60–70% of brain energy (after 3–4 weeks of adaptation; the remaining 30% still requires glucose for specific neurons)BHB yields ~10% more adenosine triphosphate: The cell's universal energy currency — almost everything that costs energy spends it. per molecule than glucose and produces fewer ROS — the metabolic basis for the ketogenic diet's effectiveness in some refractory epilepsiesKetones → neuroprotection: upregulates BDNF and suppresses the NLRP3 inflammasome; reduces pathology in animal AD/PD models
But distinguish clearly:
Ketogenic diet (KD) — strict low-carb (< 50 g/day) over several weeks before entering real ketosis (BHB > 0.5 mmol/L)'Exogenous ketones' — drinking BHB salts / esters directly raises blood ketones short-term without entering ketogenic metabolic state, with weak evidence for anti-epilepsy / anti-ADMCT oil — medium-chain triglycerides; partially converted to ketones by the liver, but limited
Practical: healthy people don't need to chase ketosis. The ketogenic diet is a clinical tool (refractory epilepsy, specific metabolic diseases), not a mass-market brain-health formula.
Chapter 6
Movement trains nerves
Movement trains nerves
Strength and skill aren't just muscle getting stronger — they also include neural recruitment, coordination, reaction speed, and updating motor maps.
Exercise is the nervous system's training ground, not just calorie burn.
Exercise is the nervous system's training ground, not just calorie burn.
BDNF · brain fertilizer
Brain-derived neurotrophic factor (BDNF) is one of the most-studied 'nutritive factors' in the central nervous system — it promotes new neuron growth, synapse formation, hippocampal LTP (long-term potentiation, the basis of memory), and adult neurogenesis.BDNF and exercise:
Acute aerobic exercise: serum BDNF rises ~30% short-term, short half-lifeRegular exercise (8–12 weeks): resting serum BDNF stays elevated ~20%, with hippocampal volume +2% (Erickson 2011, PNAS, elderly RCT)HIIT > steady-state aerobic on serum BDNF, but strength training + aerobic combination is the most reliable
BDNF and depression:
Depressed patients show reduced hippocampal BDNF expression; SSRIs work partly by raising BDNFRegular exercise's antidepressant effect in RCTs is equivalent to SSRIs (mild-to-moderate depression, Cooney Cochrane 2013)
Relationship with nutrition:
Omega-3 EPA/DHA raises BDNF expression (animal + partial human data)Polyphenols (flavonoids, blueberries / cocoa): weak evidenceChronic high sugar / high saturated fat suppresses BDNF (clear in animal models, inferred in humans)Sleep < 6 h suppresses BDNF
So 'exercise + fish + sleep + learning new skills' is the most solid brain-health combination by current evidence, far more powerful than any single brain-supplement product.