🧠 Neural Control & Synaptic Transmission

The nervous system coordinates rapid biophysical communication through electrical impulse propagation along axonal membranes and chemical neurotransmitter release across synaptic clefts.

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1. ⚡ Resting Membrane Potential & Action Potential Kinetics

  Membrane Potential (mV)
      ▲
  +30 ┼                      /───\  <-- Depolarization Peak (+30 mV: Voltage-gated Na+ channels open)
      │                     /     \
    0 ┼────────────────────/───────\────────────────
      │                   /         \  <-- Repolarization (Voltage-gated K+ channels open, Na+ close)
  -55 ┼──────────────────/           \  <-- Threshold Potential (-55 mV)
  -70 ┼─────────────────/             \──────/───  <-- Resting Potential (-70 mV maintained by Na+/K+ Pump)
      │                                \____/  <-- Hyperpolarization (-80 mV)
      └──────────────────────────────────────────► Time (ms)

Phase	Membrane Potential	Dominant Ion Flux & Channel Status
Resting State	$-70\text{mV}$ (Polarized)	Axolemma is more permeable to $K^{+}$ than $N{a}^{+}$. Maintained by $N{a}^{+}/K^{+}$ ATPase Pump ($3N{a}^{+}$ pumped out for every $2K^{+}$ pumped in using 1 ATP).
Depolarization	$-70\text{mV}⟶+30\text{mV}$	Threshold stimulus opens voltage-gated $N{a}^{+}$ channels, triggering rapid influx of $N{a}^{+}$ into the axoplasm down electrochemical gradient.
Repolarization	$+30\text{mV}⟶-70\text{mV}$	$N{a}^{+}$ channels inactivate; voltage-gated $K^{+}$ channels open, causing rapid $K^{+}$ efflux restore negative resting interior.
Hyperpolarization	$-80\text{mV}$ (Undershoot)	Delayed closure of $K^{+}$ channels; corrected back to $-70\text{mV}$ by the electrogenic $N{a}^{+}/K^{+}$ pump.

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2. 🌉 Synaptic Transmission: Chemical vs Electrical Synapses

  [PRE-SYNAPTIC AXON TERMINAL]
  Action Potential arrives ──► Voltage-Gated Ca2+ channels open ──► Ca2+ influx
                                                                        │
                                                                        ▼
  Synaptic Vesicles fuse with Pre-synaptic Membrane ──► Exocytosis of Neurotransmitter (e.g., Acetylcholine)
                                                                        │
                                                                        ▼ [Diffusion across 20nm Cleft]
  Binds to Ligand-Gated Ion Receptors on Post-Synaptic Membrane ──► Generates EPSP / Action Potential

Feature	Chemical Synapse	Electrical Synapse
Synaptic Cleft Width	Fluid-filled cleft ($20-40\text{nm}$)	Minimal ($0.2-2.0\text{nm}$) with physical Gap Junctions (Connexons)
Conduction Speed	Slower (Synaptic delay $\text{a}\approx 0.5\text{ms}$)	Extremely rapid (Almost instantaneous)
Directionality	Strictly Unidirectional	Can be Bidirectional
Neurotransmitter Required?	Yes (Acetylcholine, GABA, Dopamine)	No (Direct ionic current flow)

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3. 🧠 Human Brain Anatomy & Functional Localizations

• Forebrain (Prosencephalon):
  • Cerebrum: Sensory, motor, and association areas (memory, communication); connected by Corpus Callosum (tract of myelinated commissural nerve fibers).
  • Thalamus: Major relay center for sensory and motor signals.
  • Hypothalamus: Master homeostatic center (thermoregulation, hunger, thirst, sleep-wake cycles, neurohormones).
• Midbrain (Mesencephalon): Corpora quadrigemina (4 optic/auditory colliculi lobes) and Cerebral Aqueduct of Sylvius.
• Hindbrain (Rhombencephalon):
  • Cerebellum: Arbor vitae; coordinates complex voluntary muscular movements, balance, and posture.
  • Pons: Contains Pneumotaxic respiratory center.
  • Medulla Oblongata: Contains vital involuntary centers (cardiac, vasomotor, respiratory rhythm, vomiting, swallowing).