Rucete ✏ Campbell Biology In a Nutshell
Unit 7 ANIMAL FORM AND FUNCTION — Concept 48.3 Action Potentials Are the Signals Conducted by Axons
Neurons transmit signals over long distances using action potentials—rapid, all-or-none electrical impulses generated by voltage-gated ion channels. These signals arise from changes in membrane potential and travel directionally along axons due to sequential channel activation and inactivation.
1. Graded vs. Action Potentials
- Graded potentials: variable strength, decay with distance
- Action potentials: uniform spikes that regenerate without loss
- Triggered when depolarization reaches threshold (~–55 mV)
2. Action Potential Phases
- Resting state: Na⁺ and K⁺ channels are closed
- Depolarization: some Na⁺ channels open; Na⁺ enters
- Rising phase: many Na⁺ channels open; membrane potential spikes
- Falling phase: Na⁺ channels inactivate, K⁺ channels open → repolarization
- Undershoot: extra K⁺ outflow makes membrane hyperpolarized
- Return to rest: Na⁺ resets, K⁺ closes
3. Refractory Period
- Na⁺ channels are inactive during falling and undershoot
- No new AP can fire, ensuring one-way travel
- Stronger stimuli = higher action potential frequency
4. Conduction of Action Potentials
- Starts at axon hillock, propagates forward
- Na⁺ influx depolarizes next section to threshold
- Behind wave is refractory → unidirectional flow
- Each segment regenerates the signal with constant size
5. Speed of Conduction
- Wider axons conduct faster due to less resistance
- Myelin sheath enables rapid signal transmission
- Myelin comes from Schwann cells (PNS) or oligodendrocytes (CNS)
- Nodes of Ranvier: gaps rich in Na⁺ channels
- Saltatory conduction: signal "jumps" node to node → faster and more efficient
6. Clinical Relevance and Evolution
- Myotonia: Na⁺ channel defect → muscle spasms
- Epilepsy: excessive AP firing due to Na⁺ channel mutation
- Myelination lets small vertebrate axons match or surpass invertebrate speed
In a Nutshell
Action potentials are rapid, unidirectional electrical signals triggered by voltage-gated ion channels. They arise when membrane depolarization reaches threshold and are regenerated along the axon by Na⁺ and K⁺ channel activity. Myelination and axon diameter influence conduction speed, allowing efficient, high-speed communication in the nervous system.