Anatomy and Physiology / Unit 6: The Nervous System and Senses
6.2 Resting Potential and the Action Potential
Notes
Neurons are excitable: they produce electrical signals by moving ions across the membrane.
- At rest the membrane potential is about -70 mV, inside negative. The sodium-potassium pump keeps Na+ high outside and K+ high inside, and K+ leak channels make the inside negative.
- A stimulus that reaches threshold (about -55 mV) opens voltage-gated Na+ channels. Depolarization: Na+ rushes in and the inside becomes positive. Then Na+ channels close and K+ channels open: repolarization as K+ leaves. A brief hyperpolarization may follow.
- The action potential is all-or-none: either it fires at full strength or not at all. Stronger stimuli produce a higher frequency of impulses, not a bigger impulse.
- During the refractory period a neuron cannot fire again, so impulses travel one way.
- Myelinated axons use saltatory conduction, jumping node to node, so they conduct faster.
Practice
What happens during depolarization of a neuron?
Sodium channels open and Na+ enters, making the inside less negative then positive.
A stimulus too weak to reach threshold is applied to a neuron. What is the result?
Action potentials are all-or-none and need threshold.
Why do myelinated axons conduct impulses faster than unmyelinated axons of the same diameter?
The action potential regenerates only at the nodes, skipping the insulated segments.
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