Скачать или смотреть USMLE-1Neuroscience: Action Potential & Synapse Quiz

Neuroscience: Action Potential & Synapse
The Action Potential is the neuron's all-or-none electrical signal for rapid, long-distance communication.
Key Phases:
Resting State: Stable -70 mV potential maintained by Na⁺/K⁺ ATPase.
Depolarization: Stimulus opens voltage-gated Na⁺ channels; if threshold (~ -55 mV) is reached, rapid Na⁺ influx causes spike to +30 mV.
Repolarization: Na⁺ channels inactivate; K⁺ channels open → K⁺ efflux.
Refractory Periods: Absolute (no new AP possible) and Relative (stronger stimulus needed).
Propagation: In myelinated axons, signal jumps between Nodes of Ranvier (saltatory conduction), increasing speed and efficiency.

The Synapse is the junction where a neuron communicates with its target cell (neuron, muscle, gland). Chemical synapses are most common.

Synaptic Transmission Steps:

AP reaches presynaptic terminal.

Voltage-gated Ca²⁺ channels open → Ca²⁺ influx.

Ca²⁺ triggers exocytosis of neurotransmitter (NT) vesicles.

NT diffuses across synaptic cleft and binds to postsynaptic receptors.

Receptor activation opens ion channels, creating a graded potential:

Excitatory (EPSP): Depolarizes (e.g., glutamate opens Na⁺ channels).

Inhibitory (IPSP): Hyperpolarizes/stabilizes (e.g., GABA opens Cl⁻ channels).

NT action is terminated by enzymatic breakdown (e.g., ACh by acetylcholinesterase) or reuptake (e.g., serotonin, dopamine).

Integration & Output: Multiple EPSPs and IPSPs summate (spatial and temporal summation) at the axon hillock. If net depolarization reaches threshold, a new action potential fires in the postsynaptic neuron.

Component Function Key Feature
Voltage-gated Na⁺ Channels Rapid depolarization Inactivate after opening.
Voltage-gated K⁺ Channels Repolarization Open more slowly.
Voltage-gated Ca²⁺ Channels Synaptic release Convert electrical to chemical signal.
Ligand-gated Ion Channels Generate EPSPs/IPSPs Direct, fast synaptic action.
Metabotropic Receptors Modulate synaptic strength Slow, longer-lasting effects via G-proteins.
Clinical Links:

Myasthenia Gravis: Autoimmune attack on ACh receptors impairs neuromuscular transmission.

SSRIs: Block serotonin reuptake, enhancing mood-regulating pathways.

Local Anesthetics: Block voltage-gated Na⁺ channels, preventing AP initiation.

Epilepsy: Often involves excessive neuronal excitability and synchronous firing.

In essence: The action potential is the electrical signal that travels along a neuron. At the synapse, this signal is converted into a chemical message (neurotransmitter) to influence the next cell, completing the chain of neural communication.