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9.2 Electrochemical Impulse 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_uUKD18 No&feature=related • Since then... – ECG (Electrocardiogram) – EEG (Electroencephalograph (EEG) 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. 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. 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 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. 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. Electrical Charge Separation • The separation of electrical charge allows membrane to have potential to do work. • A neuron has charge of -70 mV 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. Now the nerve cell is depolarized... • Sodium gates slam closed and inflow of sodium is halted. 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! Page 420 Read/make notes on this very important figure. Be able to identify resting membrane potential, depolarization , action potential, repolarization. Refractory Period • Nerves cannot be activated again until resting potential restored. • Refractory Period: time required for repolarization. – Lasts 1-10 ms 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. 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) 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. • What does this data tell us? - 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. 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. • Each nerve is composed of many individual nerve cells or neurons. Different neurons may have different threshold levels. 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. 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. Insecticides • Block cholinesterase insect ‘heart’ will remain in contracted state and never relax. 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. 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. 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? NS diseases • Parkinson’s – Characterized by involuntary muscle contractions and tremors due to inadequate dopamine production. • Alzheimer’s – Decreased production of acetylcholine. Seatwork/Homework • Page 426 • #1-13