How Neurons Work Electrical Signaling January 25 2016 Cells in the Nervous System Neurons 100 to 150 billion Glia Supporting cells Form a barrier between the blood and the brain blood brain barrier Two Types of Information Processing Electrical signaling excitation inhibition graded potentials action potentials Chemical signaling neurotransmitters agonists antagonists How Neurons Work How Electrical Signaling Works Semipermeable membrate How Electrical Signaling Works Action Potential Generation of an Action Potential Another View of an Action Potential Electrical Potentials for Signaling N a axon c e ll body axon te r m in a ls a x o n h illo c k K In p u t Z on e L ig a n d g a te d c h a n n e ls C o n d u c tin g Z o n e O u tp u t Z o n e V o lta g e g a te d c h a n n e ls Action potential EPSP Threshold Resting potential 60 to 70 mV Na K pumps active Resting Potential Excitatory Postsynaptic Potential EPSP Inhibitory Postsynaptic Potential IPSP Action Potential Sodium Potassium pumps How Neurons Work N a axon c e ll body axon te r m in a ls a x o n h illo c k K In p u t Z on e L ig a n d g a te d c h a n n e ls C o n d u c tin g Z o n e O u tp u t Z o n e V o lta g e g a te d c h a n n e ls Signaling Electrical Potentials N a axon c e ll body axon te r m in a ls a x o n h illo c k K In p u t Z on e L ig a n d g a te d c h a n n e ls C o n d u c tin g Z o n e O u tp u t Z o n e V o lta g e g a te d c h a n n e ls Action potential EPSP Threshold Resting potential 60 to 70 mV Na K pumps active Resting Potential Excitatory Postsynaptic Potential EPSP Inhibitory Postsynaptic Potential IPSP Action Potential Sodium Potassium pumps Conduction of the Action Potential As the action potential is conducted along the axon the potential does not change size or shape This is because the potential is regenerated at each point along the axon No regeneration potential decreases graded conduction Regeneration of Action Potential Na 1 Spread of charge Na 3 2 4 Na 5 After reaching threshold Na enters through the voltage gated channels The entry of Na causes the membrane potential to reach 30 mV The positive charge opposes Na entry equilibrium Positive charge spreads along the membrane depolarizes adjacent parts and the process repeats Wave Conduction of an Action Potential Regeneration of Action Potential Na 1 Spread of charge 2 Na 3 2 4 4 Na 5 Membrane depolarization opens voltage gated Na channels and Na enters the neuron 1 The spread of charge 2 depolarizes adjacent points along the membrane opens voltage gated channels Na enters 3 4 5 spread 2 4 of etc Backward charge does not open voltage gated Na channels because these channels not only close they inactivate Refractory Periods Absolute refractory period Lasts about 1 msec 1 1000 sec Sodium channels closed and inactivated so the neuron will not generate another action potential Limits neuron to a maximum of 1000 action potentials per second Relative refractory period Lasts 3 to 4 msec Hard to generate action potential but possible Conduction Velocity Speed of conduction in uninsulated axon varies from 1 meter sec to 35 meters sec depends on axonal thickness Thick axon fast conduction more charge carriers ions Thin axon slow conduction fewer charge carriers Saltatory Conduction in Myelinated Axons Many axons are insulated by myelin which is made by Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system Current flows to the next Node of Ranvier so the action potential jumps from node to node saltatory conduction Action potential can fail at two nodes and still be regenerated Speed of saltatory conduction is up to 120 meters sec 4 times faster than unmyelinated axons How Neurons Work N a axon c e ll body axon te r m in a ls a x o n h illo c k K In p u t Z on e L ig a n d g a te d c h a n n e ls C o n d u c tin g Z o n e O u tp u t Z o n e V o lta g e g a te d c h a n n e ls Two Types of Synapses Chemical synapse Most common Terminal filled with vesicles that release neurotransmitter Synaptic delay 1 ms Can be modulated Electrical synapse Tight junction Fast No cleft Electrical potential travels directly to next neuron