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Neurons, Synapses, and Brain Development

Brain as an Electro-Chemical / Computational System

  • Brain presented as an electro-chemical machine that obeys the computational theory of mind: every thought, word, movement = electricity + chemicals.
  • Speaking/understanding language, sensing one’s feet on the floor, etc. are framed as concurrent electro-chemical events.
  • Misconception alert: neurons are not neatly organized wires; true histological images (electron microscopy) reveal a chaotic, tangled mesh.

Neuron Anatomy & All-or-None Signaling

  • Basic parts
    • Nucleus (soma/cell body): metabolic hub.
    • Axon: single “leg/wire” conducting outgoing impulses; longest human axon ≈ full length of leg (toe → spinal cord).
    • Dendrites ("branches"): thousands per neuron, receive input. Word root: dendro = tree/branch; firefighting “dendrology” class anecdote.
    • Axon terminals / terminal buttons: chemical release sites.
    • Myelin (Schwann cells): fatty insulation, develops post-natally, raises conduction speed; demyelination → disorders (e.g., MS).
  • Action potential = neural impulse = spike
    • Binary, 0/1; neuron fires or not (“all-or-none”).
    • Typical axonal speed demonstration: ≈ 70 \text{ miles·h}^{-1} measured by squeezing classmates’ feet/shoulders.

Scale & Complexity of the Human Brain

  • Neuron count
    • Traditional “100 \text{ billion}” corrected to 86 \text{ billion} total; 16 \text{ billion} reside in the neocortex.
  • Synaptic explosion
    • Avg. 10^4 connections/neuron ⇒ \approx10^{15} (one quadrillion) synapses.
  • Metaphor: stacking 1\text{moneycard} bills edge-to-edge → 86 \text{ billion} bills stretches ≈ 6{,}800 miles (Fresno → NYC → back → half-way again).
  • Claim: the brain is “the most complex object in the known universe.”

Synapses & Neurotransmitter Mechanics

  • Synapse = cleft/gap between pre- & post-synaptic neurons.
  • Lock-and-key specificity: each neurotransmitter’s molecular shape fits only its own postsynaptic receptor subtype.
  • Sequence
    1. Action potential arrives at terminal.
    2. Vesicles fuse, releasing neurotransmitter into gap.
    3. Binding to postsynaptic receptors → excitatory or inhibitory graded potentials.
    4. Reuptake: surplus transmitter re-absorbed into presynaptic cell & repackaged.
    5. Alternate fate: enzymatic degradation, diffusion, or glial uptake.
  • Number of distinct neurotransmitters ≈ low 20s (depends on whether certain hormones are counted).

Altering Neurotransmission: Drugs, Thoughts, Therapy

  • Key maxim: “Change the neurotransmitter milieu → change behavior.”
  • Chemical interventions
    • L-DOPA supplies dopamine precursor for Parkinson’s (adds transmitter).
    • SSRIs (Selective Serotonin Reuptake Inhibitors) e.g., Prozac, Zoloft.
      • Mechanism: block serotonin reuptake ⇒ more 5\text{-HT} in cleft ⇒ mood-elevating firing in serotonergic circuits.
      • Challenges: wide inter-individual variability; 18+ antidepressants tried empirically; side-effects (e.g., lowered libido).
    • Caffeine, CBT, meditation likewise modulate transmitter levels via pharmacology or cognition.
  • Alzheimer’s aside: failed clearance of metabolic “plaques” chokes neurons to death; sleep is natural nightly cleanup.

Connectome & Consciousness

  • Connectome = full wiring diagram; dynamic, never-static.
  • Two philosophical tiers
    1. Hard problem: how electro-chemical activity → subjective consciousness (possibly beyond physics).
    2. Easy problem: localization of function—what cortical areas do what (memory, vision, language). Current unit focuses here.

Neurodevelopment: From Fetal Boom to Adult Pruning

  • Neurogenesis (new neurons)
    • Peaks during 3rd trimester: \approx2.5\times10^5\;\text{neurons·min}^{-1}.
    • Continues post-natally but slows drastically; many newborn neurons die (neural proliferation vs. death).
  • Synaptogenesis (new connections)
    • Massive from birth to \approx2 yrs; dendritic density skyrockets.
    • Average synapses/neuron:
      • Newborn ≈ 2{,}500
      • Age 2 ≈ 15{,}000 (peak)
  • Pruning
    • Begins ~age 2; eliminates ~½ of synapses for efficiency.
    • Once thought to finish by 12, then 18; now believed to extend into early 20s.
    • Adult mean ≈ 7{,}500-10{,}000 synapses/neuron.
  • Myelination timeline parallels pruning—continues into 20s; frontal lobe maturity peaks early 20s → impulse control anecdote (professor’s tangents when sleep-deprived).

Neuroplasticity: “Wiring by Firing”

  • Principle: neurons that fire together, wire together.
  • Learning typically alters postsynaptic receptor sensitivity minute-to-minute; large-scale wiring changes slower but lifelong.
  • Epigenetic teaser: Worms taught a task → ground up → cannibalistic worms ingest → seem to inherit memory via mRNA; hints at trans-generational info beyond Darwinian genetics (Lamarckian echo).
  • ACES (Adverse Childhood Experiences)
    • Early trauma wires maladaptive circuits; high ACES scores predict mental & physical illness, shorter life.
    • Message: not determinism; neuroplasticity + therapy lets individuals overwrite early wiring.
  • Overarching takeaway the instructor wants remembered: “You have enormous power over your own brain changes.”

Examples, Metaphors & Side Notes

  • Dollar bills are cloth (rags→riches story; Benjamin Franklin).
  • Money in washing machine survives = proof of fabric currency.
  • Classroom foot-squeeze experiment shows slow conduction vs. tiny brain distances.
  • Myelin disorders (MS) briefly referenced.
  • Giraffe necks, Lamarck vs. Darwin: introduces idea of acquired traits vs. natural selection.
  • Cannibalistic worms do not prove psychical inheritance in humans; cow stress hormones in meat ≠ transfer of cow memories.
  • Lecturer’s personal anecdote: PCP asked “Which antidepressant do you want?” ⇒ highlights trial-and-error nature of pharmacotherapy.

Ethical, Philosophical & Practical Implications

  • Complexity makes full connectome mapping and consciousness explanation daunting but motivates ongoing Human Connectome Project.
  • Pharmacological bluntness underscores need for personalized medicine.
  • Brain’s dynamic nature grounds optimism for rehabilitation, learning, and mental-health interventions.
  • Social equity issue: those with highest ACES often have least access to mental-health care; California better than other states, but gap remains.

Key Numbers & Equations

  • Neurons: N \approx 8.6\times10^{10}
  • Synapses: S \approx N \times 10^{4} \approx 10^{15} (one quadrillion).
  • Neurogenesis rate (3rd trimester): 2.5\times10^{5}\;\text{min}^{-1}.
  • Axonal conduction speed: v \approx 70\;\text{mph}\;(\approx31\;\text{m·s}^{-1}).
  • Synapse count per neuron over development:
    \text{Newborn} \;\approx2.5\times10^{3},\;\text{Age 2} \;\approx1.5\times10^{4},\;\text{Adult} \;\approx(7.5-10)\times10^{3}.