Chapter 8 Synap c Transmission and Neural Integra on The ac on poten al an all or none event Depolariza on membrane poten al becoming more posi ve Hyperpolariza on membrane poten al becoming more nega ve So now we have a biological ba ery and a source for crea ng a large signal through the ring of an ac on poten al But how do we ensure that this signal is propagated all the way down the axon to signal the next neuron Passive Current Flow in an Axon Based on the conduc ve proper es of axons you would expect to see a massive loss of the intensity of the signal down the length of the axon Propaga on of an Ac on Poten al Instead you see virtually no loss of the signal down the length of the axon How For most neurons myelina on provides high resistance to ion movement across the plasma membrane so that the signal is not lost along the length of the axon Ac on Poten al Propaga on Nodes of Ranvier are gaps in the myelin where the membrane lacks insula on and has the highest concentra on of sodium channels Because the distance between nodes is short the current remains strong enough to depolarize the membrane at the next node to threshold In this way ac on poten als jumps from node to node called saltatory conduc on and regenerate the signal at each node Saltatory means jump in la n So now we can ensure that this signal is propagated all the way down the axon to signal the next neuron But what happens when the signal gets to the next neuron Cellular Communica on Func onal Anatomy of a Synapse Rapidly degrading synap c violet things Neurotransmi ers Neuropep des are rela vely large composed of 3 36 amino acids The biological ac vity of the pep de neurotransmi ers depends on their amino acid sequence Individual amino acids such as glutamate and GABA are much smaller Come to be called small molecule neurotransmi ers Neurotransmi er systems Mul ple pre synap c neurotransmi er types 1 Glutamate Most common NT At fast excitatory synapses in the Mul ple pre synap c neurotransmi er types 1 Glutamate Most common NT At fast excitatory synapses in the brain and spinal cord 2 Acetylcholine Ach NT at neuromuscular junc on NMJ 3 GABA Most common inhibitory NT in the brain 4 Dopamine DA Plays a cri cal role in reward system Also may play a role in Parkinson s and Schizophrenia cycles and body 5 HT temperature arousal 5 Serotonin 6 Plays role in srystem egula on of mood sleep wake and in uences o n the areward Norepinephrine 7 Gylcine NE monoamine that plays a role in alertness and 8 7 SGylcine ubstance APmino undecapep de Acid plays an responsible inhibitory role for tin ransmission spinal cord of pain 8 Substance P O o Loewi s Experiment Acetylcholine First substance iden ed as a NT O o Loewi called it vagus stu Acetylcholine It is synthesized from Acetyl CoA and choline by choline acetyl transferase CAT Acetylcholine Receptors mediated by fast nico nic Ach receptors nAchR and slower muscarinic receptors mAchR Although these receptors share a common messenger the e ects on the post synap c cell are very di erent Branded Sea Krait There are a number of biological neurotoxins which speci cally bind and block nico nic receptors The snakes venom contains many pep des including bungarotoxin This 74 A A toxin blocks transmission at the neuromuscular junc on by irreversibly and compe vely binding to the acetylcholine receptor at the NMJ causing paralysis respiratory failure and death in the vic m Neurotoxins that act on post synap c receptors Serotonin Synthesis Serotonin is synthesized from tryptophan which is an essen al dietary requirement Serotonin Serotonin and Depression Clinical depression has many symptoms including lack of energy abnormal ea ng habits and di culty sleeping or sleeping too much Much evidence suggests that depression is associated with de ciencies in the biogenic amines Trea ng Depression One class of an depressant is monoamine oxidase MAO inhibitors which inhibits the enzyme which breaks down serotonin The other selec ve serotonin reuptake inhibitors SSRIs inhibit serotonin re uptake transporters Both of these classes ul mately leads to increasing the amount and dura on 5 HT is in the synap c clel and can signal the post synap c neuron 5 HT Serotonin shorthand Post Synap c Events The receptor determines whether the synapse is excitatory or inhibitory EPSP s if they increase the likelihood of a post synap c ac on poten al occurring IPSP s if they decrease the likelihood of a post synap c ac on poten al occurring Excitatory Synapses Inhibitory Synapses Neural Integra on Neurons oHen have many excitatory and inhibitory inputs onto them Both the ming and combina on of these inputs determines the neurons output
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