The Nervous System

1. The Nervous System Chapter 17: Inquiry Into Life 10th Edition

2. The Nerve Impulse How do nerve impulses actually travel along a neuron? What are the steps involved?

3. The Nerve Impulse • Definition: • An electrochemical chain reaction that travels in one direction along a neuron. • Also called an action potential.

4. The Nerve Impulse • Resting Potential: • The axon is not conducting an impulse along it. • The neuron is said to be at rest. • If we use and oscilloscope (a type of voltmeter that records voltage differences) and attach one end to the outside of the axon (extracellular fluid) and the other end into the inside of the axon, here’s what we would see on the screen during a resting potential:

5. The Nerve Impulse • Resting Potential: diagram (a) • Shows that the inside of the axon is negatively (-) charged in relation to the outside of the axon which is positively (+) charged. • This is the polarity – the difference in charge between two different regions.

6. The Nerve Impulse • Resting Potential: diagram (b) • The sodium/potassium pump always moves 3 Na+ ions to the outside and 2 K+ ions to the inside • The axon has more Na+ ions on the outside and more K+ ions on the inside. • The outside of the axon becomes (+) charged when compared to the inside. Closed gates separate charges on the inside and the outside.

7. The Nerve Impulse • The Action Potential: • This refers to the firing of a nerve impulse along the axon. • The action potential has two phases on the oscilloscope:

8. The Nerve Impulse • The Action Potential: diagram (c) • A rapid change in polarity across the membrane occurs following a stimulus.

9. The Nerve Impulse • The Action Potential: Step 1 • The sodium gates open, Na+ ions rush into the axon. • Potential changes from -65 mV to +40 mV. • This is called depolarization – the inside of the axon becomes (+) and the outside becomes (-).

10. The Nerve Impulse • The Action Potential: Step 2 • Potassium gates open and K+ ions flow to the outside of the axon (sodium gates close). • Potential goes down from +40mV to -65mV. • This is called repolarization because the inside of the axon goes back to (-) charged and outside becomes (+) charged.

11. The Nerve Impulse • The Action Potential • As the action potential moves along the axon, each new portion undergoes depolarization then repolarization.

12. The Nerve Impulse • Important Notes: • 1. Refractory Period: after the action potential has moved on, the previous portion of the axon keeps the sodium gates closed for a while to prevent the impulse from moving backward.

13. The Nerve Impulse • Important Notes: • 2. Sodium/potassium pump will eventually return Na+ to the outside and K+ to the inside (polarization). • 3. Na + and K + gates are concentrated at the nodes of Ranvier – ion exchange (Na + /K +) happens only at the nodes.

14. The Nerve Impulse • Important Notes: • 4. The action potential in myelinated axons is much faster than in non-myelinated axons (12-120 m/s) • Impulse jumps from node to node – called saltatory conduction. • In non-myelinated axons, the impulse travels point to point which is much slower.

15. The Nerve Impulse • Important Notes: • 5. All or none response : if a stimulus causes depolarization to a certain level, and action potential occurs. This level is the threshold. If a threshold is not reached, then there is no action potential (all or nothing). • Stronger stimuli do not cause stronger action potentials.