Chapter 7 Nervous System Ionic Basis for Ac on Poten al Genera on and Propaga on Exam avg 88 Quiz next class Genera8ng membrane poten8al So now we have a biological ba ery in the form of a res8ng membrane poten8al of 70 mV But how do we use that poten8al energy to create a large signal we can send to the next neuron Cellular Communica8on Now that we have these ba eries how do we use them to propagate signals We can use the poten8al energy of one of these ba eries by changing the permeability of the membrane for an ion Which ba ery do we want to use Which one has the greatest di erence between Eion and Vm Vm Eion This is called a driving force ENa 60 mV EK 90 mV ECl 50 mV ECa 200 mV Neurons use Na ac on poten als spikes Neurons again take advantage of the concentra8on gradients The ac8on poten8al an all or none event Depolariza8on membrane poten8al becoming more posi8ve Hyperpolariza8on membrane poten8al becoming more nega8ve Proper8es of Na channels determine the structure of the AP Sodium channels Fast inac8va8ng Refractory Period During and immediately aber an ac8on poten8al the membrane is less excitable than it is at rest This is because most of the sodium channels are closed and inac8vated and cannot be opened by a second s8mulus This results in the unidirec8onal propaga8on along an axon Anima8on h p www blackwellpublishing com ma hews channel html Proper8es of the Ac8on Poten8al All or none Has a threshold because Sodium channels Stereotypical shape Uni direc8onal because of refractory period Has a refractory period because all channels are closed Sort of like a toilet Epilepsy Epilepsy is a brain disorder characterized by periodic and unpredictable seizures caused by the rhythmic ring of large group of neurons Most of the evidence suggests that abnormal ac8vity in a region that is hyper excitable provides the trigger for a seizure that then spreads to other brain areas Up to 1 of the popula8on is a icted making epilepsy one of the most common neurological problems The pa8ent is shown reading quietly in the period preceding the seizure in A and the period when she reported feeling fear in B and nally the period when there was an altera8on of consciousness and an audible scream in C Ion channels involved in epilepsy Can be caused by cor8cal damage or trauma stroke tumors etc but most forms arise from one or more gene8c muta8ons 1 Na channels 2 GABA receptors 3 Potassium channels 4 Ca2 channels 5 Nico8nic acetylcholine receptors Treatment of Epilepsy No e ec8ve preven8on or cure exists for epilepsy which over 8me can lead to cell death for many neurons in the brain and loss of brain func on Pharmacological therapies that successfully inhibit seizures focus on two things Enhance the func8on of inhibitory synapses that use GABA Limit ac on poten al ring by ac ng on sodium channels Stabilizes the inac8vated state of sodium channels and poten8ates GAGA receptors Stabilizes the inac vated state of sodium channels meaning that fewer of these channels are available to subsequently open So now we have a biological ba ery and a source for crea8ng a large signal through the ring of an ac8on poten8al But how do we ensure that this signal is propagated all the way down the axon to signal the next neuron Passive Current Flow in an Axon Based on the conduc ve proper es of axons you would expect to see a massive loss of the intensity of the signal down the length of the axon Propaga8on of an Ac8on Poten8al Instead you see virtually no loss of the signal down the length of the axon How For most neurons myelina on provides high resistance to ion movement across the plasma membrane so that the signal is not lost along the length of the axon Ac8on Poten8al Propaga8on Nodes of Ranvier are gaps in the myelin where the membrane lacks insula8on and has the highest concentra8on of sodium channels Because the distance between nodes is short the current remains strong enough to depolarize the membrane at the next node to threshold In this way ac on poten als jumps from node to node called saltatory conduc on and regenerate the signal at each node
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