PSYC 210 Behavioral Neuroscience More on How Neurons Work Wednesday Sept 3 2014 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 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 More on 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 Events at a Chemical Synapse Neurotransmitter manufactured in cell body by ribosomes along with rough endoplasmic reticulum ER Moved by smooth ER to Golgi apparatus and packaged into synaptic vesicles Events at a Chemical Synapse Microtubules transport synaptic vesicles and other material including enzymes that can synthesize neurotransmitters down the axon to the synaptic terminal Events at a Chemical Synapse 1 Action potential invades synaptic terminal 2 Open voltage gated calcium channels Ca2 enters the terminal 3 Ca2 causes synaptic vesicles to bind to presynaptic membrane 4 Vesicles burst open and release contents 5 Neurotransmitter diffuses across cleft and binds to receptors 6 Neurotransmitter becomes unbound Reuptake into vesicles Removal from cleft Two Type of Receptors Ionotropic Receptors Neurotransmitter diffuses across cleft and binds to receptors If the receptor is an ionotropic receptor it is connected to an ion channel which opens and allows ions to enter or leave the neuron This produces a fast response EPSP or IPSP Metabotropic Receptors Neurotransmitter diffuses across cleft and binds to metabotropic receptors e g a dopamine receptor If the receptor is a metabotropic receptor it is connected to a G protein which can activate either an ion channel or additional chemical messengers within neurons second messengers This process is slower Synaptic Transmission Neurotransmitter acts on receptor at ligand gated channels to produce an EPSP or IPSP Agonists mimic the action of a neurotransmitter Antagonists block the action of a neurotransmitter e g curare at neuromuscular junction Receptor can be coupled to ion channel ionotropic fast acting Receptor may be coupled to a protein G protein metabotropic slower because part of the protein breaks off and changes a function within the cell e g release Ca2 from intracellular store More Definitions Agonists chemical compounds that mimic the action of neurotransmitters Note the action may not be at receptor but may act on the reuptake mechanisms Antagonists chemical compounds that block the action of neurotransmitters Competitive blocks binding site Non competitive does not prevent binding but neurotransmitter has no effect Ligand a chemical that binds at a receptor and may or may not be a neurotransmitter Affinity how well a compound binds to a receptor Notes No drug has just one action in the nervous system A drug may have a primary effect BUT There are always side effects different pathways use the same neurotransmitter the drug binds to other receptors at lower affinities The events at chemical synapses are the major targets for medications and drugs of abuse Two Major Dopamine Systems Reward Movement Functions of Dopamine ADHD Cocaine amphetamine and methylphenidate increase the concentration of dopamine in the synapse