Neuroscience: How the Brain Works

Neurons and Synaptic Transmission

Neurons: ~86 billion in the human brain, each with ~7,000 synaptic connections. Structure: soma (cell body), dendrites (input), axon (output), myelin sheath (insulation). Resting potential: -70 mV (K⁺ leak channels, Na⁺/K⁺-ATPase). Action potential: all-or-none, threshold ~-55 mV, Na⁺ influx → depolarization → K⁺ efflux → repolarization. Propagation: saltatory conduction along myelinated axons at 1-120 m/s. Synaptic transmission: Ca²⁺-triggered vesicle fusion → neurotransmitter release into synaptic cleft → receptor binding on postsynaptic membrane. Major neurotransmitters: glutamate (excitatory), GABA (inhibitory), dopamine, serotonin, acetylcholine, norepinephrine.

Brain Organization

Cerebral cortex: 2-4 mm thick, 6 layers, ~16 billion neurons. Four lobes: frontal (planning, motor, speech — Broca's area), parietal (somatosensory, spatial), temporal (auditory, memory — Wernicke's area), occipital (visual). Prefrontal cortex: executive functions, working memory, decision-making. Subcortical structures: basal ganglia (motor control, reward), thalamus (sensory relay), hypothalamus (homeostasis, hormones). Limbic system: amygdala (emotions, fear), hippocampus (memory formation, spatial navigation). Cerebellum: motor coordination, motor learning, balance. Brainstem: autonomic functions (breathing, heart rate), sleep-wake cycles.

Neuroplasticity

Neuroplasticity: the brain's ability to reorganize its structure and function. Hebbian learning: "neurons that fire together wire together." Long-term potentiation (LTP): persistent strengthening of synapses — cellular basis of learning. LTP mechanism: NMDA receptor activation → Ca²⁺ influx → AMPA receptor insertion → enhanced transmission. Long-term depression (LTD): weakening of synaptic connections. Critical periods: windows of heightened plasticity (e.g., vision: first 5 years). Adult neurogenesis: new neurons in hippocampus (dentate gyrus) — involved in learning and mood. Rehabilitation: stroke recovery through intensive practice — cortical remapping. Environmental enrichment: increases dendritic branching, synaptogenesis, BDNF expression.

Memory Systems

Working memory: temporary storage and manipulation (prefrontal cortex, 7±2 items). Declarative memory: facts (semantic) and events (episodic) — hippocampus → neocortex consolidation. Procedural memory: skills and habits (basal ganglia, cerebellum). Memory consolidation: hippocampal replay during sleep (sharp-wave ripples), systems consolidation over weeks-years. Engrams: physical traces of memory — Tonegawa lab optogenetically activated specific memory cells. Molecular mechanisms: CREB transcription factor, protein synthesis, synaptic tagging and capture. Forgetting: interference (proactive, retroactive), decay, retrieval failure, motivated forgetting. Pathological: Alzheimer's disease (amyloid-β plaques, tau tangles), hippocampal atrophy.

Brain-Computer Interfaces and Future

Non-invasive BCI: EEG-based (P300 speller, SSVEP, motor imagery). Invasive BCI: Utah array, Neuralink N1 (1024 electrodes). BrainGate: quadriplegic patients control computer cursors and robotic arms. Speech decoding: UCSF lab decodes attempted speech at 78 words/min from ECoG. Neuromodulation: transcranial magnetic stimulation (TMS), transcranial direct current stimulation (tDCS). Deep brain stimulation (DBS): Parkinson's, epilepsy, treatment-resistant depression. Brain organoids: 3D brain tissue grown from stem cells — model development and disease. Connectomics: complete wiring diagram — C. elegans (302 neurons, complete), Drosophila (139,255 neurons, 2024). Human Brain Project and BRAIN Initiative: mapping the human connectome.

Try it live

Everything above runs in your browser — open Michaelis-Menten Kinetics and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.