Assessment Statements Obj

E41 State that some neurotransmitters excite postsynaptic transmission and others inhibit postsynaptic transmission 1

E42 Explain how decision-making in the CNS can result from the interaction between the activities excitatory and inhibitory presynapatic neurons at synapses 3

E43 Explain how psychoactive drugs affect the brain and personality by either increasing or decreasing postsynaptic transmission 3

E44List three examples of excitatory and three examples of inhibitory psychoactive drugs

bull Excitatory nicotine cocaine amphetaminesbull Inhibitory benzodiazepines alcohol tetrahydrocannabinol (THC)

1

E45 Explain the effects of cocaine and THC in terms of their action at synapses in the brain 3

E46 Discuss the causes of addiction including genetic predisposition social factors and dopamine secretion 3

Assessment statements from Online IB Biology Subject GuideCommand terms httpi-biologynetibdpbiocommand-terms

Be sure you have a solid understanding of action potentials and synapses as you work through this subtopic

httpwwwyoutubecomwatchv=LT3VKAr4roo

httplearngeneticsutaheducontentaddictionrewardneurontalkhtml

Work through this tutorial

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 bull Action potentials (AP) reach terminal

bud of the pre-synaptic neuron bull Neurotransmitters (NT) chemical

messengers diffuse across the synapse to bind with receptors on the post-synaptic membrane

Neurotransmitters are bull Excitatory which means they excite the

post-synaptic neuron (contributing to depolarisation and propagation of the AP

OR bull Inhibitory hyperpolarising the post-

synaptic neuron and preventing AP

Neurotransmitters can be excitatory or inhibitoryNeurotransmitters (NT) are proteins bull diffuse across the synaptic cleftbull bind with a receptor on the post-synaptic neuron

Whether or not the post-synaptic neuron propagates the action potential depends on bull Which NT diffuses acrossbull Which receptors they bind tobull Which ions flow inout of the post-synaptic neuronbull Whether or not depolarisation reaches threshold

-70mv

0mv

time

threshold

resting

depolarisation

hyperpolarisation

Excitatory NTs cause depolarisationbull eg ACh dopaminebull NT binds Na+ channels open Na+ rushes inbull Membrane potential depolarises AP propagated

Inhibitory NTs cause hyperpolarisationbull eg GABA dopamine (on different pathways)bull NT binds to receptorbull K+ channels open K+ rushes outbull OR Cl- channels open Cl- rushes inbull Membrane potential become more negativebull Action potential is prevented from propagating

httpisgdJellinek

This is a useful example of an inhibitory neurotransmitter

httpwwwyoutubecomwatchv=-pfG6yHAQ5U

Decision-making in the Central Nervous System (CNS)

axon

synapse

axon hillock

actio

n po

tenti

al

Diagram adapted from httphscuweacuksynapses_neurosummationhtm

The axons of many pre-synaptic neurons feed into the dendrites of one post-synaptic

neurons via synapses

The lsquodecisionrsquo whether or not to propagate the action potential along the axon of the

post-synaptic neuron takes place in a region of the cell body

called the axon hillock

This is achieved through summationof the incoming impulses If the total

impulse reaches thresholdthe post-synaptic neuron depolarises and

the 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

Decision-making in the Central Nervous System (CNS)

Diagram adapted from httphscuweacuksynapses_neurosummationhtm

Test out temporal and spatial summation Temporal summationbull Action potentials arrive in rapid

successionbull Depolarisation in the axon hillock is

summativebull If it reaches threshold before

repolarisation the AP is propagated

Spatial summationbull Action potentials arrive simultaneously

from multiple sourcesbull Some neurotransmitters are excitatory

(increasing depolarisation)bull Other NTs are inhibitory

(hyperpolarising) bull Summation in the axon hillock is

summative bull If it reaches threshold the AP is

propagated

httpwwwjellinekeubrainindexhtmlKeepThis=trueampTB_iframe=trueampheight=588ampwidth=672

How do psychoactive drugs affect the brain

Work through the excellent animations and explanations from httpwwwjellinekeu

Before thinking about how drugs affect the synapses be sure you understand how they work and are reset bull Some NTs have a normal excitatory functionbull Other NTs have a normal inhibitory function

In general psychoactive drugs can 1 Increase or decrease the release of NTs (eg

THC ndash cannabis)2 Breakdown re-uptake proteins which are

responsible for returned used components of NTs to the pre-synaptic neuron (ready to use again)

3 Block re-uptake proteins (egcocaine)4 Mimic or block NTs binding to the receptors

on post-synaptic membranes5 Inhibit production of new NTs

Excitatory drugs increase post-synaptic transmission

Examples bull Nicotine amphetamines cocaine

What is the effect of cocaine Normal bull Dopamine acts as excitatory NTbull Dopamine is re-uptaken by pumps on the pre-

synaptic membrane

With Cocainebull Cocaine blocks re-uptake pumpsbull Dopamine remains in synaptic cleftbull More dopamine continues to be releasedbull Summative increase in post-synaptic transmission

Effects on mood bull Dopamine is involved in reward pathways

enhancing feelings of pleasurebull Longer-lasting feelings as dopamine is not re-

uptaken

Effects on behaviourbull feelings of euphoriabull increased energy and alertnessbull highly addictivebull association with depression as body reduces

production of own dopamine over timehttpisgdJellinek

httpisgdJellinek

Inhibitory drugs decrease post-synaptic transmission

Examples bull Alcohol benzodiazapines THC

What is the effect of tetrahydrocannibol (THC) Normal bull Dopamine release is moderated (inhibited) by GABA

With THCbull THC mimics cannabinoids and inhibits GABA release

by binding to cannabinoid receptorsbull GABA cannot inhibit dopamine releasebull More dopamine is released

Effects on mood bull Dopamine is involved in reward pathways

enhancing feelings of pleasurebull Not as extreme release of dopamine as with

cocaine but still higher than normal

Effects on behaviourbull intoxicationbull hungerbull memory impairmentbull potential dependency

httpoutreachmcbharvardeduanimationssynapseswf

httpthebrainmcgillcaintermediairephp

A scale of harm for drugsWhich dots represent bull tobaccobull alcoholbull heroinbull cocainebull THC cannabisbull 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 linkhttpactivepromptherokuappcomSQSNK or httpisgddrugscaleprompt httpactivepromptherokuappcomTYYBH

httpwwwyoutubecomwatchv=bCChf2WHNE4

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