2 Assessment Statements Obj. E4.1 1 E4.2 3 E4.3 E4.4 State that some neurotransmitters excite postsynaptic transmission and others inhibit postsynaptic transmission.1E4.2Explain how decision-making in the CNS can result from the interaction between the activities excitatory and inhibitory presynapatic neurons at synapses.3E4.3Explain how psychoactive drugs affect the brain and personality by either increasing or decreasing postsynaptic transmission.E4.4List three examples of excitatory and three examples of inhibitory psychoactive drugs.Excitatory: nicotine, cocaine, amphetaminesInhibitory: benzodiazepines, alcohol, tetrahydrocannabinol (THC)E4.5Explain the effects of cocaine and THC in terms of their action at synapses in the brain.E4.6Discuss the causes of addiction, including genetic predisposition, social factors and dopamine secretion.Be sure you have a solid understanding of action potentials and synapses as you work through this subtopic.Command terms:Assessment statements from: Online IB Biology Subject Guide
3 Communication via Synapses Synapses are a fundamental part of neural pathways as they regulate decision-making in terms of exciting or inhibiting the post-synaptic neurons.Review:Action potentials (AP) reach terminal bud of the pre-synaptic neuron.Neurotransmitters (NT), chemical messengers, diffuse across the synapse to bind with receptors on the post-synaptic membrane.Neurotransmitters are:Excitatory, which means they excite the post-synaptic neuron (contributing to depolarisation and propagation of the AP.OR:Inhibitory, hyperpolarising the post-synaptic neuron and preventing AP.Work through this tutorial:
4 Neurotransmitters can be excitatory or inhibitory Neurotransmitters (NT) are proteinsdiffuse across the synaptic cleftbind with a receptor on the post-synaptic neuron.Whether or not the post-synaptic neuron propagates the action potential depends on:Which NT diffuses acrossWhich receptors they bind toWhich ions flow in/out of the post-synaptic neuronWhether or not depolarisation reaches thresholdExcitatory NTs cause depolarisatione.g. ACh, dopamineNT binds, Na+ channels open, Na+ rushes inMembrane potential depolarises, AP propagatedInhibitory NTs cause hyperpolarisatione.g. GABA, dopamine (on different pathways)NT binds to receptorK+ channels open, K+ rushes outOR Cl- channels open, Cl- rushes inMembrane potential become more negativeAction potential is prevented from propagating0mvdepolarisationthresholdresting-70mvhyperpolarisationtime
5 This is a useful example of an inhibitory neurotransmitter. This is a useful example of an inhibitory neurotransmitter.
6 Decision-making in the Central Nervous System (CNS) The axons of many pre-synaptic neurons feed into the dendrites of one post-synaptic neurons via synapses.The ‘decision’ whether or not to propagatethe action potential along the axon of thepost-synaptic neuron takes placein a region of the cell bodycalled the axon hillock.This is achieved through summationof the incoming impulses. If the totalimpulse reaches threshold,the post-synaptic neuron depolarises andthe action potential is propagated.If the sum does not reach threshold,the AP is not propagated.There are two main methods of summation:temporal and spatial.action potentialaxonsynapseaxon hillockDiagram adapted from:
7 Decision-making in the Central Nervous System (CNS) Test out temporal and spatial summation:Temporal summationAction potentials arrive in rapid successionDepolarisation in the axon hillock is summativeIf it reaches threshold before repolarisation, the AP is propagated.Spatial summationAction potentials arrive simultaneously from multiple sourcesSome neurotransmitters are excitatory (increasing depolarisation)Other NTs are inhibitory (hyperpolarising).Summation in the axon hillock is summative.If it reaches threshold the AP is propagated.Diagram adapted from:
8 How do psychoactive drugs affect the brain? Before thinking about how drugs affect the synapses, be sure you understand how they work and are reset.Some NTs have a normal excitatory functionOther NTs have a normal inhibitory functionIn general, psychoactive drugs can:Increase or decrease the release of NTs (e.g. THC – cannabis)Breakdown re-uptake proteins which are responsible for returned used components of NTs to the pre-synaptic neuron (ready to use again)Block re-uptake proteins (e.g.cocaine)Mimic or block NTs, binding to the receptors on post-synaptic membranesInhibit production of new NTsWork through the excellent animations and explanations from
10 Excitatory drugs increase post-synaptic transmission Examples:Nicotine, amphetamines, cocaineWhat is the effect of cocaine?Normal:Dopamine acts as excitatory NTDopamine is re-uptaken by pumps on the pre-synaptic membrane.With Cocaine:Cocaine blocks re-uptake pumpsDopamine remains in synaptic cleftMore dopamine continues to be releasedSummative increase in post-synaptic transmissionEffects on mood:Dopamine is involved in reward pathways, enhancing feelings of pleasureLonger-lasting feelings as dopamine is not re-uptakenEffects on behaviour:feelings of euphoriaincreased energy and alertnesshighly addictiveassociation with depression as body reduces production of own dopamine over time
11 Inhibitory drugs decrease post-synaptic transmission Examples:Alcohol, benzodiazapines, THCWhat is the effect of tetrahydrocannibol (THC)?Normal:Dopamine release is moderated (inhibited) by GABAWith THC:THC mimics cannabinoids and inhibits GABA release by binding to cannabinoid receptorsGABA cannot inhibit dopamine releaseMore dopamine is releasedEffects on mood:Dopamine is involved in reward pathways, enhancing feelings of pleasureNot as extreme release of dopamine as with cocaine, but still higher than normalEffects on behaviour:intoxicationhungermemory impairmentpotential dependency
13 A scale of harm for drugs Which dots represent:tobacco?alcohol?heroin?cocaine?THC/ cannabis?ecstasy?Drag the red dot to point you think represents alcohol.How can drugs cause physical harm?How can drug use lead to addiction (dependency)?What factors contribute to development of addiction?ActivePrompt link:or
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