Neuronal Communication: Electrical and Chemical Signaling at Synapses

Electrical Signaling (Within Neuron)

• Resting potential: ~-70 mV (K⁺ leak, Na⁺/K⁺-ATPase).
• Action potential: Voltage-gated Na⁺ influx → depolarization; K⁺ efflux → repolarization. All-or-none.
• Propagation: Along axon; saltatory in myelinated fibers (nodes of Ranvier).

Chemical Signaling (Between Neurons)

• Action potential → Ca²⁺ influx at terminal → vesicle fusion → neurotransmitter release into synaptic cleft.
• Neurotransmitter binds postsynaptic receptor → ionotropic (fast, ion channel) or metabotropic (slow, G-protein) response.
• Excitatory (EPSP: Na⁺/Ca²⁺ influx) or inhibitory (IPSP: Cl^- influx/K⁺ efflux).
• Termination: reuptake, enzymatic degradation, diffusion.

Synapse Types

• Chemical synapses: Majority; unidirectional; plastic (LTP/LTD).
• Electrical synapses (gap junctions): Bidirectional, ultrafast; synchronize networks (e.g., cardiac, escape circuits).

Integration and Output

Spatial and temporal summation of EPSPs/IPSPs at axon hillock determines if threshold reached for next action potential. Neuromodulators (dopamine, serotonin, acetylcholine) tune excitability globally.

Clinical Relevance

• Channelopathies → epilepsy, migraine, ataxia
• Synaptic dysfunction → Alzheimer's, Parkinson's, schizophrenia, autism
• Drug targets: SSRIs, benzodiazepines, anticonvulsants, anesthetics