1. SBI 4U Electrochemical Impulse

2. Some Interesting Facts about the Neuron • Longevity – can live and function for a lifetime • Do not divide – fetal neurons lose their ability to undergo mitosis; neural stem cells are an exception • High metabolic rate – require abundant oxygen and glucose

3. How do nerve cells pass along a message? • Nerve impulses remain as strong at the end of a nerve as they were at the beginning • Use cellular energy to generate current (which is the message) • Deals with a change in electrical potential energy across a membrane

4. Action vs Resting Potential • Action potential  voltage difference across a membrane when a nerve is excited • Resting potential  voltage difference during resting stage

5. How do nerve cell membranes become charged? • Molecular Level of the Nerve Cell • Neurons have a large amount of both + and – ions both inside and outside cell (unlike most cells) • Negative ions don’t contribute much charge (too big) • Key Focus: • Electrochemical message is caused by an unequal concentration of + ions across the nerve cell membranes

6. How do nerve cell membranes become charged? • Note • Sodium likes to diffuse into cell • Postassium likes to diffuse out of cell • Both diffuse at the same time • However, diffusion is unequal; cell more permeable to potassium, so more potassium diffuse out • Since more K+ out, exterior of membrane more positive than interior • Conversely the excess negative ions accumulate along the inside of the membrane • Creates a polarized membrane

8. How a charge is generated • Ion gates allow ions into and out of cell • Steps • Nerve excited • Na+ gates open allowing more sodium ions inside cell, while K+ gates close • Rapid inflow of + ions causes charge reversal (a.k.a. Depolarization) • Inside of cell now positive so Na+ gates close • Sodium-potassium pump restores resting membrane by transporting 3 Na+ out for every 2 K+ in (a.k.a. Repolarization)

9. Process of Depolarization and Repolarization Note how the action potential is moving away from the site of origin

10. Refractory Period • Before a nerve can produce another action potential, it must repolarize • This recovery time is called the refractory period • Often lasts from 1 to 10 ms

11. Movement of Action Potential • Once a nerve is stimulated, the message needs to be carried along the length of the axon • Therefore, depolarization has to move from the zone it initiated in to adjacent regions • How does this happen?

12. Movement of Action Potential • Once an action depolarization has occurred, there are now more positive ions on the inside of the membrane • These positive ions are attracted to the negative ions in the adjoining regions that have not been stimulated • As these positive ions move toward the negative ions (the resting membrane), the nerve impulse is carried with them • This resting membrane then undergoes depolarization

13. Movement of Action Potential

14. Movement of Action Potential • Once depolarization happens, it stimulates the sodium channels to open which results in the movement in the action potential • This wave of depolarization therefore moves along the length of the nerve membrane • Depolarization of the membrane causes the sodium channels to close, the potassium ions to reopen • Note that every wave of depolarization is followed by a repolarization and a refractory period

15. Movement of Action Potential

16. Nodes of Ranvier • How does one action potential move down the axon? • Nodes of Ranvier are located between myelinated sections of the axon • Figure 8.15, p.358 • Nodes contain many Na+ channels • Nodes are the specific site of triggering an action potential • Remember: sodium ions ENTER via the channels • This DEPOLARIZES the membrane • THRESHOLD occurs • Prior membrane can not be stimulated. Why not? • Next membrane in front can be. Once Threshold occurs, action potential is triggered. Signal continues down axon. • This process can also be referred to as : Saltatory Conductions • No jumping in non-myelinated axons. So which is faster?

17. Movement of an Action Potential, cont’d

18. Threshold Levels • Threshold Level • A minimum level of stimulus is required to produce a response • Varies depending on the neuron

19. All-or-None Response • Once you reach the threshold level, adding more stimulus will not elicit a greater response • A nerve muscle fibre responds completely or not at all to a stimulus • Note: the intensity of the response will change depeding on: • If the brain recognizes a change in the frequency of responses • if neurons that have a higher threshold are excited Higher threshold of B, so two neurons are excited

20. Synaptic Transmission • Messages need to be transmitted between neurons • Occurs via synapses • Small spaces between neurons, or between neurons and effectors (e.g. Muscles) • As impulse moves along axon it reaches the endplate and releaes vesicles that contain neurotransmitters (via exocytosis) • Neutrotransmitters are released from the presynaptic neuron, travel across the synaptic cleft, and create a depolarization of the post synaptic neuron

21. Synaptic Transmission

22. Example of Neurotransmitter • Acetylcholine • Acts as excitatory neurotransmitter on many post synaptic neurons • Opens sodium channels, helping with depolarization • Potential problem  keeps sodium channels open, so keeps cell in constant state of depolarization; not able to respond to next impulse since no refractory period • Solution: membrane enzyme cholinesterase destroys acetylcholine • Real world application: insecticides blocks cholinesterase so insect heart remains contracted