Slide 1Cells in the Nervous SystemTwo Types of Information ProcessingHow Neurons WorkHow Electrical Signaling WorksHow Electrical Signaling WorksGeneration of an Action PotentialAnother View of an Action PotentialElectrical Potentials for SignalingSlide 10Signaling - Electrical PotentialsConduction of the Action PotentialRegeneration of Action PotentialWave = Conduction of an Action PotentialRegeneration of Action PotentialRefractory PeriodsConduction VelocitySaltatory Conduction in Myelinated AxonsSlide 19Two Types of SynapsesHow Neurons WorkElectrical SignalingJanuary 25, 2016Cells 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 ProcessingElectrical signaling• excitation• inhibition• graded potentials• action potentialsChemical signaling• neurotransmitters• agonists• antagonistsHow Neurons WorkHow Electrical Signaling WorksSemi- permeablemembrateHow Electrical Signaling WorksAction PotentialGeneration of an Action PotentialAnother View of an Action PotentialElectrical Potentials for Signalinga x o nc e l lb o d ya x o nt e r m i n a l sI n p u t Z o n eL i g a n d - g a t e d c h a n n e l sC o n d u c t i n g Z o n e V o l t a g e - g a t e d c h a n n e l sO u t p u t Z o n eK+N a+a x o n h i l l o c k•Resting Potential•Excitatory Postsynaptic Potential (EPSP)•Inhibitory Postsynaptic Potential (IPSP)•Action Potential•Sodium/Potassium pumpsResting potential (-60 to -70 mV)EPSPAction potentialThresholdNa/K pumps activeHow Neurons Worka x o nc e l lb o d ya x o nt e r m i n a l sI n p u t Z o n eL i g a n d - g a t e d c h a n n e l sC o n d u c t i n g Z o n e V o l t a g e - g a t e d c h a n n e l sO u t p u t Z o n eK+N a+a x o n h i l l o c kSignaling - Electrical Potentialsa x o nc e l lb o d ya x o nt e r m i n a l sI n p u t Z o n eL i g a n d - g a t e d c h a n n e l sC o n d u c t i n g Z o n e V o l t a g e - g a t e d c h a n n e l sO u t p u t Z o n eK+N a+a x o n h i l l o c k•Resting Potential•Excitatory Postsynaptic Potential (EPSP)•Inhibitory Postsynaptic Potential (IPSP)•Action Potential•Sodium/Potassium pumpsResting potential (-60 to -70 mV)EPSPAction potentialThresholdNa/K pumps activeConduction 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 conductionRegeneration of Action Potential•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. +++Na+12Spread of charge+++Na+3+++Na+54Wave = Conduction of an Action PotentialRegeneration of Action Potential+++Na+122Spread of charge+++Na+3+++Na+54• 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), etc. (4,5)4• Backward spread (2,4) of 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 VelocitySpeed 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 carriersSaltatory 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 Worka x o nc e l lb o d ya x o nt e r m i n a l sI n p u t Z o n eL i g a n d - g a t e d c h a n n e l sC o n d u c t i n g Z o n e V o l t a g e - g a t e d c h a n n e l sO u t p u t Z o n eK+N a+a x o n h i l l o c kTwo 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