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9.2

9.2. Electrochemical Impulse. Late 18 th Century: Luigi Galvani  leg muscle of dead frog could be made to twitch under electrical stimulation. http://www.youtube.com/watch?v=eI_uUKD18No&feature=related. Since then... ECG (Electrocardiogram) EEG (Electroencephalograph (EEG).

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9.2

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  1. 9.2 Electrochemical Impulse

  2. Late 18th Century: Luigi Galvani  leg muscle of dead frog could be made to twitch under electrical stimulation. http://www.youtube.com/watch?v=eI_uUKD18No&feature=related

  3. Since then... • ECG (Electrocardiogram) • EEG (Electroencephalograph (EEG)

  4. Difference between Electrical and Neuron Transmission • Currents along wire much faster than neuron • Cytoplasmic core offers great resistance • Electrical currents diminish as they move along a wire  neurons do not.

  5. K.S. Cole and H.J. Curtis • Placed a tiny electrode inside the large nerve cell of a squid • Rapid change in electrical potential across membrane • From -10 mV  +40 mV • ACTION POTENTIAL: reversal of potential  does not last more than a few ms before returning to RESTING POTENTIAL.

  6. How do Nerve Cell Membranes become charged? • Neurons have a rich supply of positive and negative ions on inside and outside of cell • Negative ions do not have much to do with action potential  they are large and cannot cross the membrane (stay inside of cell) • Therefore, electrochemical event caused by unequal conc. Of + ions across membrane

  7. Unequal concentration due to... • High concentration of K+ inside of the cell • Tendency to diffuse out. • High concentration of Na+ outside of cell • Tendency diffuse in. THE RESTING MEMBRANE POTENTIAL IS 50 TIMES MORE PERMEABLE TO POTASSIUM THAN TO SODIUM  MORE POTASSIUM IONS DIFFUSE OUT OF NERVE CELL THAN SODIUM DIFFUSE IN. ION-GATES: Control the movement of ions.

  8. So... • There is a more rapid diffusion of potassium ions ______ of the nerve cell than potassium ions ______. • Therefore, net __________ of ions. • During resting potential, net _____________ charge on outside and net ________________ charge inside. • Resting membrane potential: polarized membrane.

  9. Electrical Charge Separation • The separation of electrical charge allows membrane to have potential to do work. • A neuron has charge of -70 mV

  10. Depolarization • Upon excitation, the nerve cell becomes more permeable to sodium than potassium. • Sodium gates are opened • Therefore, sodium ions rush _________ the cell by diffusion and charge attraction. • Potassium gates close • No movement of potassium ions _______ the cell. DEPOLARIZATION! (NERVE CELL TEMPORARILY _____ CHARGED.

  11. Now the nerve cell is depolarized... • Sodium gates slam closed and inflow of sodium is halted.

  12. The Sodium-Potassium Pump • Restores the resting membrane potential by • Transporting sodium ions out • Moving potassium ions in Na+/K+: three Na+ ions out, two K+ ions in. Energy supply: ____________. REPOLARIZATION!

  13. Page 420 Read/make notes on this very important figure. Be able to identify resting membrane potential, depolarization, action potential, repolarization.

  14. Refractory Period • Nerves cannot be activated again until resting potential restored. • Refractory Period: time required for repolarization. • Lasts 1-10 ms

  15. Movement of the Action Potential • Recall: action potential is characterized by the opening of sodium channels in the nerve membrane. • Sodium ions rush INTO the cytoplasm • Causes a charge reversal (depolarization) in that area. • + ions that rush into nerve cell attracted to adjacent negative ions, aligned along the inside of the nerve membrane. • Similar attraction occurs along the outside of the nerve membrane; + charged sodium ions of the resting membrane attracted to the negative charge that has accumulated along the outside of the membrane.

  16. Threshold Levels and the All-or-None Response • Nerve cells respond to changes in pH, changes in pressure, and to specific chemicals. • Major: mild electrical shock (intensity can be regulated)

  17. Classic Experiment • Single neuron leading to a muscle is isolated and a mild electrical shock is applied to the neuron. • Strength of muscle contraction measured. • Intensity slowly increased.

  18. What does this data tell us?

  19. Threshold level: Critical intensity of a stimulus to create an action potential. • All-or-none: Increasing the intensity of the stimuli above the critical threshold value will not produce an increased response. • Neurons fire maximally or not at all.

  20. Detecting the Intensity of a Stimuli • If neurons do not respond correspondingly to intensity of stimuli, how do we know the difference between warm and hot? • The more intense the stimulus, the greater the frequency of impulses • If a warm glass rod is placed in your hand, sensory impulses sent to brain at slow rate • If hot glass rod, frequency is greatly increased  a difference that the brain recognizes.

  21. Each nerve is composed of many individual nerve cells or neurons. Different neurons may have different threshold levels.

  22. Synaptic Transmission • Synapse: small spaces between neurons or neurons and effectors. • Rarely involve just two neurons. • Neurotransmitters: located at end plates of axon. • Impulse reaching axon terminal cause vesicles of neurotransmitters to be released into synapse. • Presynaptic neuron: NTs released from here. • Postsynaptic neuron: effected by presynaptic. • Nerve transmission slows across this synapse. • Reflex arc vs. Solving math problems.

  23. Acetylcholine & Cholinesterase • Example of a neurotransmitter • Acts as an excitatory neurotransmitter by opening the sodium channels of a postsynaptic nerve  causing _______________. • Problem: if acetylcholine is causing sodium channels to open, how can nerve respond to next impulse? • Cholinesterase: an ________________ released from postsynaptic membrane  destroys acetylcholine.

  24. Insecticides • Block cholinesterase  insect ‘heart’ will remain in contracted state and never relax.

  25. Hyperpolarization • Condition in which the inside of the nerve cell membrane becomes more negative than the resting potential. • Many NTs can cause this to happen: open up K+ gates  K+ leaves neuron  negative resting membrane potential.

  26. Different Neurotransmitters • Some neurotransmitters are inhibitory: serotonine, dopamine, gamma-aminobutyric acid (GABA), glutamic acid • All found in central nervous system • Excitatory NTs: norepinephrine found in both excitatory and inhibitory NSs.

  27. Coordination of Inhibitory & Excitatory Responses • Throwing a ball • As triceps excited and contracts, biceps receive inhibitory impulses and relaxes. • Two muscles do not pull against each other • Inhibitory impulses in CNS: more important • Sensory info is received by brain and prioritized. • Less important info is ignored. Why?

  28. NS diseases • Parkinson’s • Characterized by involuntary muscle contractions and tremors  due to inadequate dopamine production. • Alzheimer’s • Decreased production of acetylcholine.

  29. Seatwork/Homework • Page 426 • #1-13

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