Prof. Greg Francis 5/27/081Neurons and neurotransmittersIIE 269: Cognitive PsychologyGreg FrancisLecture 05Why does (nearly) everyone loveProzac?How many neurons? Estimates of 10^11 neurons in the human brain 100,000,000,000; one hundred billion estimates of 100,000 per cubic millimeter (about the resolution of functional MRI) Millions are active at any given timeNo. 21 mmA neuron Dendrite input Soma integrate Axon output Myelin sheath insulateInput / output Electrical signal Inputs change theresting potential of thecell Output identifies whenthe cell potential hasincreased a lotelectrode-70 millivoltsresting levelInput at dendrites Changes the cell membranepotential which causes furtherchanges in the cell’schemistry which causes furtherchanges in the membranepotential Strong enough input crossesa threshold and the cell fires action potentialTime (milliseconds)VoltageA neuron An action potential in a cell sends a signal down itsaxon That signal affects the membranes of other cellsProf. Greg Francis 5/27/082Output Myelin is like insulation for the cell’s axon it insures that the signal generated by the action potential isstrong In multiple sclerosis the body’s immune system attacksmyelin physical problems (paralysis) cognitive problems (memory, reasoning, judgement) cause unknown (300,000 people)Output The output of a neuron is either excitatory or inhibitory on theother neuron it reaches Excitatory: when our neuron sends an output, the receivingneuron is more likely to produce an action potential Inhibitory: when our neuron sends an output, the receivingneuron is less likely to produce an action potential Controlled by type of neurotransmitterNetworks Cognitive behavior is related to groups of neuronsworking together Include excitation and inhibition more laterEpilepsy Disease of central nervous system causes mostly unknown Seizures bursts of electrical activity travelling through networks in thebrain brain activity is out of control epileptic fits Isolated seizures also occur due to high fever, lack ofoxygen, or head injuryEpilepsy EEG recordingsare often used todiagnose epilepsy Many differenttypes of epilepsy,with different EEGpatternsEpilepsy One theory (but not yet proven) is that epilepsypatients’ inhibitory cells are not working properly Excitatory cells activate everything until they exhaustthemselvesProf. Greg Francis 5/27/083Epilepsy Treatment generally involves drugs, diet, avoiding stress,keeping regular schedule In extreme cases surgeryprevents seizures fromspreading throughout thebrainA balanced brain The brain is a dynamic system at multiple levels Neurons balance between “forces” inside and outside of cell membraneallows for action potentials Networks balance between excitation and inhibition Without these balances you do not thinkNeural connections Axon --> dendritesNeurotransmitter effect Bound receptors locally affect the (post-synaptic) cell membraneRRTNeurotransmitter specificity Receptors bind only to certain types ofneurotransmittersRRTTMolecular structure Molecules have a particular three-dimensional shapewater benzeneProf. Greg Francis 5/27/084Molecular structure Different molecules have differentshapesaspirinTNTMolecular structure Neurotransmitters are just molecules At least 50 different neurotransmitters dopamine, norepinephrine, serotonin,acetylcholine, glutamate, gamma-aminobutyric acid (GABA) All with different shapes!Receptor Very largemoleculescalled proteins Similar to a filter accepts someneuro-transmitters rejects othersReceptor When it accepts a neurotransmitter, itstarts a chain reaction of events physical, chemical, electrical locally changes the cell membrane» depolarization (excitation)» hyperpolarization (inhibition)Neurotransmitters Different neurotransmitters are associatedwith different properties actually neurotransmitter and receptor pairs neural cognitive behavioralTourette’s syndrome Inherited (~100,000 in US) Too much dopamine complex tics improved reflexes Treated with Haldol (among others) blocks dopamineProf. Greg Francis 5/27/085Parkinson’s Lack of dopamine Many different causes In extreme cases, patients are “frozen” Give patients large doses of L-DOPA a precursor of dopamine sometimes solves the problem lots of side effects Awakenings, by Oliver SacksDrugs Interact with neurotransmitters in lots ofways, for example Replace: accepted by receptor and withsimilar effect Production: increase or decrease Reuptake: knock out enzymes that removeneurotransmitter from receptor Blocking: partly close receptor proteinProzac Some forms of depression seem to be related tolimits in the use of the neurotransmitter serotonin Prozac is a selective serotonin reuptake inhibitor(SSRI) it keeps serotonin bound to a receptor for longer thanusual, thereby increasing its effect Prozac is one of the most widely prescribeddrugs in the world!Other drugs Amphetamines: release of norepinephrin or dopamine LSD: resembles serotonin Phenothiazine drugs: block dopamine Curare: blocks acetylcholine Cocaine: prolongs effects of dopamine Morphine: resembles small set of neurotransmitterscalled endorphin peptides (modulate pain perception) Tetrahydrocannabinol (active ingredient in marijuana):binds to some neuroreceptors, but it’s not clear what itdoesConclusions Neural action potentials Shape of proteins Specific use of neurotransmitters for certainbehaviors Current work on identification of role ofneurotransmitters Lots of money to be made Lots more complicated than what we’ve seenhereNext time Neural sensitivity Neural codes Receptive fields CogLab on Blind spot due! How do you recognize your