One really, really long nerve!

1. One really, really long nerve! • Dissection on a giraffe: investigating the recurrent laryngeal nerve http://www.youtube.com/watch?v=cO1a1Ek-HD0

2. Electrochemical Impulses

3. Electrochemical Impulses • Nerve impulses are electrochemical messages created by the movement of ions through the neuron cell membrane • Negative and positive ions are both found in large numbers inside & outside neurons.

4. Negative ions are large & tend to stay inside the cell. • Movement of positive ions involves diffusion, active transport and a Na+/K+ pump.

5. K+ found inside neurons tends to diffuse out of the cell. • Na+ found outside the cell tends to diffuse in.

6. Electrochemical Gradient • Since the cell membrane is more permeable to K+ than Na+ , K+ moves out of the cell faster than Na+ moves in. • This results in an electrochemical gradient across the cell membrane, and an overall external positive charge which is referred to as resting potential.

7. The Electrochemical Gradient that is created is referred to as the neuron’s resting potential

8. Action Potential • When a message is received by the dendrites of a neuron, the following changes, referred to as an action potential occur: • The neuron cell membrane becomes more permeable to Na+ than to K+ (Na+ gates open) • Na+ rushes into the neuron by diffusion.

9. Action Potential • A reversal of charge occurs, called depolarization (this is the ‘firing’ of the neuron) • Once the inside of the neuron becomes positive, the Na+ gates close. • A Na+ - K+ pump in the cell membrane moves sodium out and potassium in, restoring the resting potential (called re-polarization)

11. Movement of the Action Potential • Depolarization occurs as a wave along the membrane of the axon • The wave of depolarization is followed by a wave of re-polarization • The action potential moves along the axon by jumping from one node of Ranvierto another

13. Some Important Terminology Refractory period: time it takes for the nerve cell membrane to re-polarize and be able to conduct another action potential Threshold level: minimum level of stimulus required for a neuron to respond (varies between neurons)

15. Some Important Terminology All-or-none response: neurons fire maximally or not at all (means that increasing the stimulus above threshold level does not increase the response; see p. 422) Speed of transmission: determined by intensity of the stimulus, amount of myelin and diameter of the axon