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Transmission of Nerve Impulses Section 9.2 in your Bio 12 textbook Nerve Impulse An “electrochemical” signal Nerves use cellular energy (ATP) to generate a current & move the signal down the length of the neuron Cellular chemistry… There are + and – ions in both the external & internal environments around the nerve cell At rest: outside of cell has a net + charge inside of cell has a net – charge more + ions outside of the cell than inside Difference in voltage = resting potential At rest: cell has a potential of -70 mV As a nerve impulse passes along the length of the axon, the voltage changes Excited: outside of cell has a net – charge inside of cell has a net + charge more + ions inside the cell than outside Difference in voltage = action potential When excited: cell has a potential of +40 mV What causes the difference in charge? - ions stay put too large to cross the cell membrane + ions pass through the membrane unequal concentrations produce the difference in charge K+ tend to diffuse: inside outside Na+ tend to diffuse: outside inside Diffusion of both types of ions happens simultaneously But: at rest, cell membrane is more permeable to K than to Na therefore: there is an unequal concentration of + ions this is why the outside is + compared to the inside Polarization Membrane is “polarized” when at rest Polarized = charged Question: Can you hypothesize the effect on the charge of the resting membrane if there were a greater concentration difference of + ions? Depolarization When the cell is excited… An electrical impulse runs down the length of the axon “gates” for Na open & more Na+ diffuse rapidly into the cell Cell is more permeable to Na than K now Cell is now “depolarized” at that area causes a reversal in charge Once the inside becomes + relative to the outside, the Na gates close Repolarization Resting potential is restored Achieved via sodium-potassium pumps Move 3 Na+ out For every 2 K+ in Pumps are powered with ATP How often can an action potential be initiated? Depends on the length of the refractory period The time that it takes (1-10 ms) for the resting potential to be restored Movement of Action Potential Copy Figure 6, page 421 in your textbook Impulse moves along the cell’s axon, away from the cell body, toward the adjacent cell Impulse moves from the area of depolarization to the next, resting, region Threshold level Minimum amount of stimulus required to produce a nerve impulse All-or-nothing! Neurons only “fire” at one intensity Once the threshold level is reached, the impulse is transmitted Larger stimulus does not cause a larger or faster impulse Neurons fire or they don’t, there is nothing in-between This is the “all-or-none” response How do we feel the difference in stimuli? E.g. a warm stove vs. a hot stove ????? The frequency of the nerve impulse OR The number of nerves stimulated McGraw-Hill Animation http://highered.mcgrawhill.com/sites/0072495855/student_vi ew0/chapter14/animation__the_nerv e_impulse.html So… Once the action potential reaches the end of the axon, how does it get to the next neuron? Axon terminals are (almost) in contact with the dendrites of other neurons Synapse: space between neurons Chemical signals Transmission of the signal is chemical rather than electrical Chemicals: neurotransmitters Speed of impulse transmission decreases during this chemical phase because diffusion is a relatively slow process More synapses: slower the speed of the nerve impulse Reflex arc has few synapses (very quick responses) Acetylcholine Common neurotransmitter Excitatory Causes the postsynaptic neuron to become more permeable to Na Depolarization Cholinesterase Enzyme (enzymes always end in –ase) Released from postsynaptic neuron Destroys acetylcholine Na gates close, neuron can recover McGraw-Hill Animation http://highered.mcgrawhill.com/sites/0072495855/student_vi ew0/chapter14/animation__transmiss ion_across_a_synapse.html More about neurotransmitters… If a neurotransmitter is inhibitory rather than excitatory, membrane becomes more permeable to K K+ leave the cell Cell is “hyperpolarized” Postsynaptic neurons become inactive Summation >1 neuron can provide an impulse at a time Branching, axon terminals of >1 neuron may be in contact with the next neuron The effect produced by neurotransmitters from >1 neuron is “summation” What are some other neurotransmitters? Serotonin Dopamine Gamma-aminobutyric acid (GABA) Glutamic acid Norpinephrine/noradrenaline Homework/Practice questions Section 9.2, Questions, page 426 #1-11