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Chapter 5

Psych 2NF3 Chapter 5

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Andrew Wade

03/10/2014 03/10/2014 Chapter 5- Communication between neurons Portrait: Otto Loewi Experiment on control of heart rate Electrically stimulating a frogs vagus nerve While heart was immersed in fluid-filled container Channeled fluid from first to second container- not stimulated Fluid transfer slowed beating of second heart Vagus nerve contains a chemical that tells the heart to slow its rate of beating another experiment with accelerator nerve Neurotransmitter discovery Acetylcholine (ACh)- inhibits heart rate Epinephrine or adrenaline (EP) - speed up heart rate Neurons that releaseACh -cholinergic neurons In mammals, norepinephrine, replaces EP as the excitatory neurotrans in noradrenergic neurons ACh= inhibitory by means of a receptor on organs ofANS, mediating rest and digest behaviour BUT excitatory on body muscles connected to SNS The structure of Synapses 3 main parts axon terminal▯ presynaptic membrane (largely of protein molecules) 03/10/2014 within this, other structure slike mitochondria, synaptic vesicles (round granules), tubules some terminals, larger storage granules hold a number of synaptic vesicles membrane encasing tip of adjacent dendritic spine▯ postsynaptic membrane small space separating these 2 structures▯ synaptic cleft Steps in Neurotransmission 4 steps- SUMMARY 1) synthesis, either transmitter is created by DNA or its building blocks are imported and stored in axon terminal 2)release, trans transported to presynaptic membrane and release din response to action potential 3) receptor action- transmitter traverses the synaptic cleft and interacts with receptors on membrane of target cell 4) inactivation- trans either drawn back into axon of presynaptic cell or breaks down in cleft STEP 1- TRANSMITTER SYNTHESISAND STORAGE Some synthesized as proteins in cell body according to DNA instructions Packed into golgi bodied and transported on microtubules to axon terminal RNA messenger may also be transported to synapse OTHER synthesized in axon terminal from building blocks derived from food Transporter proteins absorb these precursor chemicals from blood supply Synaptic vesicles stored in 3 ways Storage granules 03/10/2014 Some attached to microfilaments Others attached to presynaptic membrane STEP 2-RELEASE Action potential triggers the release of a neurotrans from a presynaptic membrane rich in voltage sensitive calcium channels Surrounding extracellular fluid is rich in calcium Incoming calcium ions bind to calmodulin, forming a molecular complex which part in 2 chemical reactions Releases vesicles bound to presynaptic membrane Releases vesicles bound to filaments in axon terminal Vesicles empty contents into cleft through exocytosis STEP 3-ACTIVATION OF RECEPTOR SITES Neuro binds to specialized protein molecules in postsynaptic Called receptors- receive transmitter substance The tyoe of neurotrans and kind of receptor on post synaptic membrane determine whether the neurotrans Depolarizes postsynaptic membrane (excitatory) Hyperpolarizes (inhibitory) Initiates other chemical rxn sequences that can modulate inhibitory or excitatory effect Create new synapses Brings about other changes in cell Neurotrans may interact with autoreceptors on its own presynaptic membrane- may influence cell that just released it 03/10/2014 Autoreceptors receive messages from own axon terminal Amt of neuro trans released depends on Amount of Calcium that enters axon terminal Number of vesicles docked at cell membrane STEP 4- NEUROTRANS DEACTIVATION 4 ways diffuse away from synapse degraded by enzyme sin cleft brought back up in axon terminal- reuptake be taken by glial cells Types of synapses Synaptic variations (page 117) Axodendritic axon terminal synapses on dendritic spine Axomuscular- synapse on muscle cell Axosecretory- secretes into blood Axoaxonic- axon terminal end son another axon Axosomatic- end son cell body Axoextracellular- terminal with no target, secretes into extracellular fluid 03/10/2014 Dendrodendirtic- dendrites send messages to other dendrites Excitatory and inhibitory messages Type 1-Excitatory located on spines of dendrites Round synaptic vesicles Wider cleft Denser material that make up presynaptic and post synaptic membranes Active zone is larger Type 2-Inhibitory- cell body Flattened vesicles Varieties of neurotransmitters Identifying neurotransmitters 1- chemical must be synthesized 2- when released, chemical must produce response in target cell 3-same receptor action must be obtained when chemical is experimentally placed on target 4- must be mechanism for removal after chemicals work is done putative transmitter- suspected of being trans all motor-neuron axons leaving spinal cord are cholinergic and each has an axon collateral within the spinal cord that synapses on a nearby CNS interneuron interneuron in turn, synapses back on the motor neurons’ cell body▯ renshaw loop 03/10/2014 this acts as a feedback circuit that enables the motor neuron to inhibit itself and not become overexcited if it receives many excitatory inputs from other parts of CNS the inhibitory neurotransmitter in renshaw cell, the amino acid glycine, can be blocked by the chemical strychnine, which acts as a poison by causing muscles to contract synchronously, prod convulsions that interfere with breathing and cause death. Classifying neurotrans Sometimes message sent from postsynaptic to presynaptic Classify into 3 groups 1-small molecule trans 2-neuropeptides 3-transmitter gases Small-molecule transmitters Synthesized and packaged for use in axon terminals and act quickly at synapse compared to other classes Can quickly be replaced by presynaptic.mem Main components derived rom foods we eat Levels influenced by diet EXAMPLEs amines Ach Dopamine Norepinephrine Epinephrine 03/10/2014 Serotonin Amino acids Glutamate GABA Glycine Histamine Ach synthesis 1- acetyl CoA carriesAcetate to transmitter synthesis site 2-ChAT transfers acetate to choline to formAch 3- in clef
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