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by: Maymie Gaylord


Maymie Gaylord
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This 19 page Class Notes was uploaded by Maymie Gaylord on Monday September 21, 2015. The Class Notes belongs to BIOL 4102 at Georgia State University taught by Staff in Fall. Since its upload, it has received 27 views. For similar materials see /class/209921/biol-4102-georgia-state-university in Biology at Georgia State University.




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Date Created: 09/21/15
Biol 41026102 Fundamentals of Neurobiology 111507 Mechanisms of Learning and Memory Chapter 25 What are the changes in the nervous system that underlie learning and memory Types of Learning 0 NonAssociative Habituation Decrement of response to repeated benign stimulus Sensitization Increase in responsiveness after noxious stimulus o Associative Classical Pavlovian Conditioning a learned relationship between two stimuli o A conditioned stimulus CS can predict the occurrence of an unconditioned stimulus US 0 Extinction occurs when the CS stimulus is no longer a good predictor of the US involves a modification ofthe response to the CS Example eyeblink conditionin Instrumental Conditioning Operant Conditioning a learned relationship between a stimulus and the animal39s own behavior involves a modification of the frequency and context Cellular mechanisms of learning in the mollusc Aplysia Habituation is correlated with a decrease in synaptic strength 0 Sensitization involves enhancement of synaptic transmission Due to serotonergic enhancement oftransmitter release Serotonin acts on a receptor that activates adenylyl cyclase Produces cAMP which activates CAMPdependent Protein Kinase PKA PKA has at least 3 actions it phosphorylates K channels and causes them to close 0 causing spike broadening o it phosphorylates Ca2 channels causing them to open more easily in response to voltage allowing more calcium to enter for each action potential 0 it directly enhances exocytosis through an unknown mechanism 0 Classical Conditioning uses the same cellular machinery as sensitization is light siphon stimulus US is tall shock Adenylyl cyclase produces more cAMP when Ca2 is elevated Thus Adenylyl cyclase acts as a coincidence detector of Sensory neuron activity and serotonin p1 Synaptic plasticity in the cerebellum 0 Three layers in cerebellar cortex Granule cell layer ten billion granule cells 0 axons form the parallel fibers and some Golgi cells Purkinje cell layer single layer of large Purkinje cell bodies Purkinje cells are GABAergic and are the only output of the cerebellar cortex Molecular layer axons of granule cells parallel fibers stellate and basket cells dendrites of Purkinje neurons 0 receives input from 200000 granule cells 0 each parallel fiber has a small effect on Purkinje cell 0 Inputs to Cerebellum ossy fibers arise from brain stem nuclei and spinocerebellar tract synapse on granule cells Climbing fibers arise from the inferior olivary nucleus in the medulla make strong excitatory synapses on a small number of Purkinje cells always excite purkinje cell 0 The parallel fibers that are active at the same time as the climbing fiber exhibit Long Term Depression LTD Climbing fiber activation causes strong depolarization and influx of Ca2 Parallel fibers activate metabotropic glutamate receptors which activate PKC The coincidence of Ca2 and PKC leads to a decrease in synaptic strength Studies of Hippocampal Long Term Potentiation LTP 0 Brain slice preparation A3 to CA1 synapse Tetanic stimulation of the Schaffer collaterals potentiates only those syna ses that were stimulated o LTP exhibits features of Hebbian plasticity The Associative properties of LTP are reminiscent of Associative Learning Exhibits cooperativel Due in large part to the properties ofthe NMDA receptor lnflux of Ca2 activates CaMKll Due to insertion of AMPA receptors into synapse and phosphorylation of existing AM PA receptors 0 Long Term Depression LTD counterbalances LTP lf synapses could only strengthen then it would eventually saturate Could be due to relative Ca2 sensitivity of kinases and phosphatases BCM Theory says that plasticity is bidirectional p2 Mechanisms for encoding longterm memory 0 Kinases are active for as long as the second messenger is present Can be modified to be permanently activated Memory will last until kinases are replaced in cell Autophosphorylation can lead to Ca2 independent activation of CaMKll molecular switch hypothesis 0 Protein synthesis can lead to more stable changes 0 Changes in Gene transcription can lead to more permanent storage Phosphorylation of transcription factors such as cAMP response element binding protein CREB can lead to upregulation of particular genes Can lead to structural changes at synapse p3 Biol 41026102 Fundamentals of Neurobiology 102507 Movement Higher Control Chapter 14 Input to spinal cord 0 Lateral Pathways Directed movements lateral motor neurons distal muscles Lateral Corticospinal tract axons cross the midline at the pyramidal decussion in the medulla Rubrospinal tract from red nucleus in midbrain o Medial Pathways Posture proximal and axial motor neurons Three tracts that arise from brainstem nuclei Vestibulospinal tracts balance Tectospinal or Colliculospinal tract coordinating head and eye movements Reticulospinal tracts maintenance of posture o pontine reticulospinal tract enhances antigravity reflexes o medullary reticulospinal tracts inhibits antigravity reflexes allowing other movements Motor cortex 0 Primary motor cortex Brodmann s area 4 ntains a motor map of the bod Wilder Penfield used microstimulation in humans to make map Orderly representation explains Jacksonian seizures Over representation of areas of areas of fine motor control The map can change with experience just like somatosensory map Does not have a representation of muscles but coordinates many muscles in a region Neurons in primary motor cortex increase their firing prior to increase in force generation in a particular direction directly activate a group of lower motor neurons Neurons in primary motor cortex are broadly tuned to movements in a particular direction Precise movements are encoded by the population vector A similar map of movement is found in the superior colliculus for encoding saccadic eye movements Premotor and Supplementary Motor Areas 0 Involved in preparation for movement 0 Neurons will fire if there is an intention to move in a particular direction p1 Basal Ganglia 0 Forms a feedback loop with cortex Cortex project to the Striatum Consists of two nuclei 0 Caudate nucleus 0 Putamen Primary neuron is the GABAergic medium spiny neuron Striatum projects to Globus Pallidus also GABAergic Globus Pallidus projects to the Ventral Lateral VLo nucleus of Thalamus VLo has excitatory projection back to cortex 0 The net effect to provide excitation to cortex 0 Substantia nigra provides dopamine input that is excitatory to striatum o Parkinson s Disease Symptoms difficulty in initiating movement movements are slowed lack ability to generate spontaneous movements muscles become rigid and show a characteristic tremor results from loss of DA neurons Leads to loss of excitation in Striatum Treatments LDOPA to supplement DA Also treated by excising Globus Pallidus internal segment Deep Brain stimulation to inactivate GPi o Huntingtons disease chorea overactivity caused by degeneration of striatal neurons defect in Huntingtonin caused by triplet repeat late onset autosomal dominant o Other diseases ofthe basal ganglia include Tourettes and Schizophrenia Cerebellum 0 Also participates in a feedback loop with cortex Cortex to Pons to Cerebellum to cerebellar nuclei to VLc nucleus in Thalamus and back to cortex 0 Compare intended motor output with actual movement Corrects future movements based on sensory input Different motor defects and tremors with different pathologies p2 July 12 2010 Biol 41026102 Fundamentals of Neurobiology Electrical Signaling Membrane Potential Chapter 3 Membrane Potential 0 Basis of cellular signaling Action Potential Synaptic Potential Terminology 0 Charge positive or negative and carried by ions units Coulom s 0 Voltage Potential a measure of how much charge is separated units Volts V Current I the flow of positive charge units Amperes Permeability the relative ease with which ions can flow through channels 0 Conductance G a measure ofthe ease with which charge can flow units Siemens S inverse of Resistance R units Ohms Q Method for recording Neuronal Membrane Potential Voltage Recording with intracellular microelectrodes records the potential difference across the membrane Physical basis for membrane potential 0 Separation of charge by the membrane 0 Differential concentrations of ions across membrane NaK pump maintains distribution 0 Selective Permeability to Na K Cl39 ions esting conductance Based on ion channels Nernst Equilibrium Potential 0 This is very importantlll The Equilibrium potential for an ion is the potential at which there is no net movement of an ion due to a balance between electrical and chemical O O gradients Rim X19 X ZF xi 0 Where EX Equilibrium Potential for ion X Xo is the concentration of ion X outside the neuron Xi is the concentration of ion X inside the neuron R Universal gas constant T Temperature in Kelvin Z Valence F Faraday constant Simplified Nernst Equation At 37 C RTF267mV The conversion of natural logarithm to base 10 logarithm is 23 E Q Io u x 2 g Xi Glial cells are very permeable to K but not permeable to other ions Therefore glial membrane potential is the same as EK o Vmem is determined by the ratio of Ko Ki p1 lf potential is across membrane is different than EX then a current will flow 0 Ix Gx vm Ex This isjust a variation of Ohm s law VR Ix is the current carried by the ion G is conductance to the ion Vm Ex is the driving force on the ion Neurons are permeable to more than just K so it is more complicated to determine their resting potentials based on ion concentrations Very Permeable to K Only slightly permeable to Na Influx of Na balanced by efflux of K Results in a resting potential slightly depolarized from EK Goldman Equation for predicting the membrane potential of a neuron v 62 log PKKo PNaNao PCI Cli PKKi PNaNai PCICIo Where V the membrane potential PX the permeability to ion X 0 At rest this approaches Nernst Equilibrium potential for K herefore membrane potential depolarizes if extracellular K is raised 0 At peak of action potential this approaches Nernst Equilibrium potential for Na Due to high permeability for sodium 0000 Ion Channels General Properties 0 Membrane spanning proteins Comprised of subunits or pseudosubunits hydrophobic amino acids 0 Ion selective due to properties ofthe pore region hydrophilic amino acids Neurons have an Ion Transporter Pump to redistribute Na and K Prevents the ionic gradients from dissipating o Requires ATP 0 Extrudes 3 Na for every 2 K it brings in p2 Biol 41026102 Fundamentals of Neurobiology 91307 Neurotransmitters Chapter 6 Criteria that Define a Neurotransmitter 1 The substance must be present in presynaptic neuron Techniques for Localizing neurotransmitters Immunocytochemistry In Situ Hybridization 2 It must be released in response to presynaptic depolarization Ca2 dependent 3 There must be specific receptors for the substance on the postsynaptic neuron Application ofthe substance should mimic the effect of stimulating the presynaptic neuron Dale s Principle Each neuron uses the same set of neurotransmitters at all of its synaptic terminals Two major Categories of vesiclereleased Neurotransmitters 1 Small Molecule Neurotransmitters Precursors are common metabolic products Synthesized by cytoplasmic enzymes Identifying the enzyme can help determine which neurotransmitter is used by a particular neuron Often done with immunohistochemistry or in situ hybridization Packaged into small clear vesicles in the presynaptic terminal by the Vesicular Transporter Removed from synaptic cleft by specific reuptake mechanism membrane transporters Reused and repackaged 2 Neuropeptides Precursor prepropeptide is synthesized at ribosomes rough endoplasmic reticulum in cell body Propeptide is packaged into dense core vesicles in Golgi Apparatus Transported down microtubules to axon terminal fast axonal transport 400mmday Propeptide is modified to produce peptide neurotransmitter After release neuropeptides are not taken back up Can be degraded by peptidases o Many neurons utilize cotransmitters release two or more neurotransmitters Can be packaged in the same vesicles ex GABA and Glycine Can be packaged in different vesicles with different release kinetics p1 o Examples of Small Molecule Neurotransmitters Am I Acetylcholine ACh First shown to be a neurotransmitter by Otto Loewi s experiments with vagal nerve stimulation Synthetic Enzyme Choline Acetyltransferase ChAT Precursors Choline and Acetyl CoA Broken down in synapse by Acetylcholinesterase AChE o Acetate and Choline are the breakdown products o Choline is taken up and reused o Sarin gas blocks AChE Acts at two main receptor types o nicotinic AChR ligandgated o muscarinic AChR G proteincoupled Found in vertebrate Motor neurons and in some central nuclei such as the basal forebrain complex o Alzheimer s disease results from a loss of cholinergic neurons ino Acids Glutamate o Major excitatory neurotransmitter in the vertebrate CNS Common amino acid Synthetic Enzyme Glutaminase Precursor Glutamine also glucose Removed from synapse by uptake into presynaptic neuron and surrounding glia Receptors gt Two basic types of ionotropic receptors named after selective agonists AMPA I Kainate Permeable to both Na K Erev OmV Rapid kinetics of opening and closing Antagonist CNQX DA NmethylDaspartate Permeable to Ca2 in addition to Na K Voltagedependent conductance o caused by Mg2 block of the pore Slower kinetics of opening and closing requires glycine as a coagonist Antagonist APV Poreblocker MK801 Properties are important for synaptic plasticity and excitotoxicity gt Metabotropic Glutamate Receptors mGluRs Three classes that differ in their pharmacology GABA yamino butyric acid O O O O O NM I o Major inhibitory neurotransmitter in the vertebrate CNS o Synthetic Enzyme Glutamic acid decarboxylase GAD o Immediate precursor Glutamate o Removed from synapse by highaffinity uptake into presynaptic neuron and surrounding glia o GABA Receptors gt GABAA and GABAC receptors are ionotropic Cl39 channels Erev 50mv Exogenous agonist muscimol Antagonist bicucculine Channel blocker picrotoxin Allosteric modifiers benzodiazepines such as Valium and barbiturates GABAB receptors are metabotropic Activate potassium channels indirectly gt Glycine o Inhibitory neurotransmitter in spinal cord p2 Biogenic Amines Catecholamines o All catecholamines are derived from the amino acid tyrosine o First enzyme is tryosine hydroxylase which produces LDOPA o Dopamine synthesized from LDOPA by DOPA decarboxylase primarily localized to substantia nigra and ventral tegmental area Loss ofthese neurons leads to Parkinson s disease Normally also involved in reward and reinforcement All known receptors are metabotropic o NorepinephrineNoradrenaline Synthesized from Dopamine by DopamineShydroxylase Found in sympathetic ganglia and locus coeruleus o Epinephrine Adrenaline Synthesized from Norepinephrine by phenylathaolamineNmethyltransferase Found in rostral medulla Adrenergic receptors Norepinephrine and Epinephrine act at oc and B adrenergic receptors metabotropic antagonist propanolol Octopamine o Found in invertebrates but not in chordates Serotonin o Synthesis is similar to dopamine but starts with tryptophan being hydoxylated to 5hydroxytryptophan 5HTP o 5HTP is decarboxylated to 5hydroxytrytamine 5HT serotonin o Found in neurons in the raphe nuclei widespread axonal projections o Removed from synapse by Nadependent transporter in presynaptic neuron gt Reuptake inhibitors used as antidepressants Prozac gt Broken down in the cell by Monoamine oxidase MAO and catechol O methyltransferase COMT MAO inhibitors used as antidepressants Histamine o synthesized from Histidine by histidine decarboxylase o found in hypothalamus plays a role in attention and arousal o peripherally released by mast cells in response to allergic reactions and tissue damage Other small molecule transmitters ATP Coreleased as cotransmitter converted by extracellular enzymes into adenosine o Peptide Neurotransmitters More than 100 different peptides There are peptides that form families based similarities of receptors opioids or on similar amino acid sequence Enkephalins and Endorphins Bind to receptors that are activated by opium morphine Sometimes different peptides are produced from the same prepropeptide ie they are encoded by the same gene ACTH y liptropin and Bendorphin Metenkephalin and Leuenkephalin Many peptides have roles as both hormones and neurotransmitters Oxytocin Vasoactive intenstinal peptide VIP Some peptides are involved in pain perception andor analgesia Substance P p 3 I Gasses opioid peptides endorphins enkephalins and dynorphins Nontraditional Neurotransmitters Nitric oxide NO and Carbon monoxide CO NO is synthesized as a byproduct ofthe conversion of arginine to citruline by the enzyme NOS Ca2 dependent enzyme activation NO is a free radical gas that diffuses through membranes and activates intracellular enzymes Acts as an intracellular and an interceuar messenger lnitially identified as vasodiated endothelial relaxing factor Intracellular target is soluble guanylyl cycase produces cGMP from GTP activates Protein Kinase G PKG cGMP is broken down by phosphodiesterases target of Viagra Endocannabinoids Synthesized from membrane lipids long hydrocarbon chains anadamide oeamide Not packaged in vesicles released at point of production Ca2 dependent synthesis Receptors 081 082 can serve as retrograde messen er Example Endocannabinoids such as Anandamide p4 Biol 41026102 Fundamentals of Neurobiology 92707 The Chemical Senses Chapter 8 NOTES Introduction to Sensory Systems 0 There are many Sensory Modalities and some have distinct qualities or submodalities Vision Heanng ch Smell Other senses o All Sensations share similar properties lntens39ty Threshold Sensitivity to differences in stimulus strength depends upon absolute magnitude of stimulus Weber39s law Adaptation Localization on body or in space ensory systems share a common organization Sensory receptor cells Specialized cells for translating the energy of the stimulus into a neural code Sometimes these cells are not neurons Primary Sensory Neurons 15 order neurons are the first neurons to ceive sensory information Sensory transduction The process by which the stimulus is translated into a change in neuronal activit Molecular mechanism that leads from sensory stimulus to a change in ionic currents The ionic currents lead to a Receptor Potential The receptor potential is a graded response that is proportional to the magnitude of the stimulus lfthe receptor potential is large enough it leads to action potential firing Neural encoding Stimuli must be represented by a consistent change in neuronal activity Strength of Stimulus 0 Frequency code The firing rate of a neuron provides information about the strength of the stimulus 0 Population code Comparing the activity of different neurons can provide more information than the single neuron Duration of Stimulus Modality of Stimulus o Labeled Line Code the identity ofthe axons tells the nervous system what type of stimulus arrived Topographicalorganization All sensory pathways except taste and smell are organized to produce a central map of the receptive field surface Olfaction has a map of olfactory space Receptor tuning A sensory receptor responds best to one quality ofthe stimulus However it will respond to other qualities as well 3gt I 0 p1 Gustation o Stimulus Tastants are nonvolatile hydrophilic molecules generally related to palatability 0 Organization Tongue has regions of higher sensitivity to different tastes but all tastes can be sensed at all positions on the tongue The tongue is covered with raised papillae fungiform vallate and foliate On the papillae are tastebuds taste buds are also found in the soft palate pharynx larynx and upper Within each tastebud are taste receptor cells Taste receptor cells have a lifetime of 2 weeks and are regenerated from basal cells When exposed to an appropriate tastant there is a change in the resting potential of the receptor cell a receptor potential Taste receptor cells synapse with primary sensory neurons Individual taste afferents respond to a variety of stimuli Therefore population coding determines taste sensation o Transduction there are a number of different transduction mechanisms Salt direct passage of Na through amiloridesensitive ion channels in the apical membrane Sour Ac39 protons can also pass through amiloridesensitive ion channels and other channe s protons can block some potassium channels Bitter a variety of very different compounds taste bitter Alkaloids quinine and caffeine amino acids divalent salts Different receptors have been identified for different compounds T2Rs Act via the G protein gustducin Sweet Sugars and some amino acids T1R2T1R3 heterodimer Umami Monosodium Glutamate MSG T1R1T1R3 0 Central connections Axons of primary sensory neurons run in three cranial nerves Chorda tympani branch of cranial nerve VII the facial nerve anterior 23s of tongue The glossopharyngeal nerve cranial nerve IX posterior 13 of tongue The vagus nerve cranial nerve X regions around throat Project to portions of the nucleus of the solitary tract in the medulla gustatory nucleus Neurons from the gustatory nucleus project to the ventral posterior medial nucleus of the thalamus Thalamic axons project to several cortical regions including the gustatory cortex in the temporal lobe Brodmann s area 36 43 p2 Olfaction o Stimulus Airborne odorants volatile substances Classification of odorants into categories has been difficult What is the basic unit of smell Pheromones are chemical signals that are used for communication by members of the same species 0 Olfactory receptor cells are located in the olfactory epithelium have axons that project to the olfactory bulb have chemosensitive cilia that extend into a mucus layer are replaced every 6 8 by new receptor neurons that arise from the basal cells in the epithelium o Transduction Olfactory receptor molecules are G protein coupled receptors Linda Buck and Richard Axel Nobel prize Binding ofthe odorant either directly to the receptor or via an olfactory binding protein activates the olfactory G protein Golf The on subunit of Golf activates adenylyl cyclase which produces cAMP cAMP activates a cyclicnucleotidegated channel Na and Ca2 enter through the channel Ca2 activates a Cl39 channel that causes further depolarization Note that Cl39 leaves the cell because EC is less negative than Vm o Coding In mice there are about 1000 different olfactory receptor molecules about 300 in humans An odorant would bind to more than one receptor allowing a tremendous number of different combinations to be activated by different odorants Each olfactory receptor neuron expresses only one type of receptor molecule is sensitive to many different odorants 0 Organization The olfactory bulb has glomeruli regions of synaptic input from olfactory receptor cells All olfactory receptor cells that express a particular receptor molecule project to the same glomerulus Each Mitral Cell the main output ofthe olfactory bulb receives input in a single glomerulus Optical imaging shows that an odorant activates a distinct and consistent pattern of glomeruli spatial coding Also have temporal coding of information Oscillations and synchronous activity contain information that is used by downstream elements The output neurons ofthe olfactory bulb project to the olfactory tubercle and olfactory cortex in the temporal lobe Do not project to thalamus before arriving in cortex A variety of different forebrain areas receive olfactory input p3 Biol 41026102 Fundamentals of Neurobiology 11805 Vision ll Central Pathways Chapter 10 NOTES The brain extracts information about the world 0 Visual illusions show that this information is not always accurate The Central projections of the Retina e optic nerves leave the retina They join at the optic chiasm After the chiasm the axons are referred to as the optic tracts The retina projects to four areas ofthe brain Hypothalamus Suprachiasrnatic nucleus Circadian rhythm The Pretecta Area Used for pupillary responses The Superior Colicuus sed for generating saccadic eye movements and orienting to stimuli Contains retinotopic map The Lateral Geniculate Nucleus LGN of Thalamus Involved in visual perception Organization of Visual Space 0 Binocular field central area seen by both eyes Monocularfields lateral areas seen by one eye only Approximately half ofthe Retinal Axons cross at the optic chiasm 0 Only axons from the nasal retina cross 0 Axons from the temporal retina remain ipsilateral 0 Thus left visual field is represented in right optic tract 0 Lesions in the visual pathway produce specific visual defects The LGN maintains an orderly map ofthe retina o Retinal Ganglion axons segregate into 6 eyespecific layers Input from each eye remains segregated in different layers Each layer has a complete retinotopic ma Layers l and 2 are the Magnocellular layers Layers 46 are the Parvocellular layer Koniocellular layers located in between main layers All retinal input to cortex passes through the LGN But only 20 ofthe synapses are from RGCs the rest are feedback connections 0 Neurons in LGN have receptive fields with properties similar to those of the retinal ganglion cells that they receive input from Axons of LGN neurons form the optic radiation to Primary Visual Cortex o This cortical area is also known as V1 area 17 Striate Cortex Structure of Striate Cortex 0 Six cell layers 0 pyramidal cells are projection neurons spiny stellate cells are local interneurons Different layers have different functions Cells in the same column get the same thalamic input Magnocellular input stays segregated from parvocellular input Inputs from each eye project to different columns ocular dominance columns binocular neurons combine input from both eyes Some neurons in Primary Visual cortex respond to visual disparity Basis for Random Dot Stereograms O O O O 0000 p1 Receptive field properties of Neurons in primary visual cortex 0 O O 0 Simple cells respond to oriented edges Some neurons are directionselective Complex cells respond to bars of a particular length and orientation anywhere in receptive field Receptive field properties are thought to arise from a summation of the receptive field properties of input neurons Cortex has a columnar organization 0 O Neurons of similar orientation selectivity are aligned in vertical columns Blobs are areas of nonorientation selective but color selective cells Each area of visual space is multiply represented by different orientation columns Orientation columns form a repeated pinwheel structure determined with optical imaging Extrastriate cortex 0 O isual processing continues in other areas of cortex Two pathways Dorsal Where pathway object location Area MT Medialtemporal cortex involved in motion perception Our brains can distinguish the difference between movement of our eyes and movement of a target Movement is perceived the same regardless of whether the image is stable on the retina or moves across it Ventral What pathway color and object recognition Input from parvocellular pathway V4 Inferotemporal cortex Face recognition cells p2 Biol 41026102 Fundamentals of Neurobiology Sept 11 2007 Intracellular Signal Transduction Chapter 6 Styles of Neuronal communication 0 Synaptic Pointtopoint o paracrine local hormone volume transmission endocrine hormonal Types of Signaling molecules Cellimpermeant neurotransmitters o Cellpermeant steroid and thyroid hormones gases Cellassociated cell adhesion molecules Four Types of Receptors 1 Ionotropic Receptors Ligandgated ion channels 2 Metabotropic or Gprotein Coupled Receptors GPCR 3 Enzymelinked Receptors Receptor Tryrosine Kinases growth factor receptors 4 Intracellular Receptors Soluble guanylyl cyclase steroid nuclear receptors Two types of Receptor are most important at synapses o Ionotropic Receptors Ligandgated ion channels Comprised of 5 subunits Each subunit has 3 or 4 transmembrane segments Many ionotropic receptors have similar amino acid sequences suggesting an evolutionary connection 0 Metabotropic Receptors Gproteincoupled receptors GPCRs ven transmembrane spanning domains G protein binding site on C terminus Binding site for neurotransmitter is in a pocket formed by the trans membrane regions Terminology o Agonist a substance that activates a receptor Exogenous Agonist not normally present Endogenous Agonist the natural agonist found in the body the neurotransmitter o Antagonist a substance that prevents the agonist from binding 0 Desensitization the process by which prolonged exposure to an agonist can lead to a decreased activation of the receptor Examples of Receptors o Acetylcholine Receptors Nicotinic Acetylcholine Receptors nAChR Ionotropic Exogenous agonist nicotine Antagonists oc bungarotoxin curare Five subunits 0 two on subunits amp 3 others 3 y 5 s o The on subunits have the ACh binding sites Neuronal and Muscle nAChRs differ o sensitivity to antagonists 0 subunit composition 0 Ca2 permeability Muscarinic Acetylcholine Receptors mAChR Metabotropic Exogenous Agonist muscarine Antagonists atropine scopolamine p1 GTPbinding Proteins G proteins 0 Heterotrimeric G Proteins mediate most ofthe actions of G proteincoupled receptors on 3 y subunits When the receptor is activated on subunit exchanges GDP for GTP oc subunit with GTP dissociates from M In some cases By can directly activate a protein such as an ion channel muscarinic receptor the shortcut pathway oc subunit can activate effector proteins Can be positively or negatively coupled to effector Gs and Gi oc subunit has GTPase activity and stimulated by GTPaseActivating Proteins GAP hydrolyzes GTP to GDP allowing up and yto reassociate Primary Effector Proteins ead to production of Second Messen e s g r Diffusible molecules that convey signal inside the cell cAMP P3H DAG and Ca2 are examples 0 Adenylyl Cyclase produces cAMP from ATP cAMP is broken down by Phosphodiesterases PDE cAMP can activate Protein Kinase A PKA cAMP binds to regulatory subunits allowing catalytic subunits to dissociate PKA can phosphorylate proteins cAMP can also directly open cyclicnucleotidegated ion channels Leads to amplification of signal 0 Phospholipase C produces Inositol trisphoshate IP3 and Diacylglycerol DAG from Phosphatidylinositol bisphosphate a membrane lipid P3 acts at receptors to release Ca2 from intracellular stores DAG and Ca2 activate Protein Kinase C PKC PKG translocates to the membrane when activated 2 0 Ca is a common intracellular messenger Elevated in Cytoplasm due to Membrane voltagegated channels Membrane ligandgated channels P3 receptors on the endoplasmic reticulum Mitochondrial release binds to Calmodulin leads to activation of Ca2Calmodulindependent protein kinase type II CaMKII can directly gate ion channels a2 activated K channels Ryanodine receptors Ca2 stimuated Ca2 release from intracellular stores Ca2 is rapidly removed from the cytoplasm Calcium pump NaJ39ICa2 exchanger Ca2 binding proteins Kinases and Phosphatases Protein phosphorylation is a reversible process Kinases and Phosphatases act in dynamic equilibrium 0 PKA PKG and CaMKH are SerThr kinases p2 Functions of Intracellular signaling omosynaptic Plasticity One neuron alters its own properties as a function of its own activity Sometimes homeostatic in nature 0 Heterosynaptic Plasticity Neuromodulation One neuron alters the properties of another neuron Alter ion channel activation change excitability Alter receptor sensitivity Alter synaptic release p3


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