Neuron Excitation & Synaptic Transmission: How Brain Signals Travel

An "excited" neuron is one that has received sufficient input to generate an action potential — the electrical impulse that propagates along its axon to communicate with other neurons.

The Process: Synaptic Transmission

1. Action potential arrival: Depolarization reaches the presynaptic terminal.
2. Calcium influx: Voltage-gated Ca²⁺ channels open; Ca²⁺ enters the terminal.
3. Vesicle fusion & neurotransmitter release: Ca²⁺ triggers synaptic vesicles to fuse with the membrane, releasing neurotransmitters (glutamate, GABA, dopamine, acetylcholine, etc.) into the synaptic cleft.
4. Receptor binding: Neurotransmitters diffuse across the cleft and bind receptors on the postsynaptic membrane.
5. Postsynaptic response:
   • Excitatory (EPSP): Na⁺/Ca²⁺ influx depolarizes the membrane (e.g., AMPA/NMDA receptors for glutamate).
   • Inhibitory (IPSP): Cl⁻ influx or K⁺ efflux hyperpolarizes (e.g., GABA_A, glycine receptors).
6. Integration: The postsynaptic neuron sums thousands of EPSPs/IPSPs. If net depolarization reaches threshold at the axon initial segment, it fires its own action potential.
7. Signal termination: Reuptake transporters, enzymatic degradation (acetylcholinesterase), and diffusion clear the cleft.

Propagation

The action potential travels down the axon via voltage-gated Na⁺/K⁺ channels. Myelin sheaths (oligodendrocytes in CNS, Schwann cells in PNS) enable saltatory conduction — rapid jumping between nodes of Ranvier.

Network Effect

One neuron synapses onto thousands of others. Excitation spreads through circuits, enabling perception, cognition, movement, and memory. Balance between excitation and inhibition (E/I balance) is critical; dysregulation underlies epilepsy, anxiety, autism, and more.