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Assessment Statements Obj.

E4.1 State that some neurotransmitters excite postsynaptic transmission and others inhibit postsynaptic transmission. 1

E4.2 Explain how decision-making in the CNS can result from the interaction between the activities excitatory and inhibitory presynapatic neurons at synapses. 3

E4.3 Explain how psychoactive drugs affect the brain and personality by either increasing or decreasing postsynaptic transmission. 3

E4.4List three examples of excitatory and three examples of inhibitory psychoactive drugs.

• Excitatory: nicotine, cocaine, amphetamines• Inhibitory: benzodiazepines, alcohol, tetrahydrocannabinol (THC)

1

E4.5 Explain the effects of cocaine and THC in terms of their action at synapses in the brain. 3

E4.6 Discuss the causes of addiction, including genetic predisposition, social factors and dopamine secretion. 3

Assessment statements from: Online IB Biology Subject GuideCommand terms: http://i-biology.net/ibdpbio/command-terms/

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

http://www.youtube.com/watch?v=LT3VKAr4roo

http://learn.genetics.utah.edu/content/addiction/reward/neurontalk.html

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: • 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.

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

Whether or not the post-synaptic neuron propagates the action potential depends on: • Which NT diffuses across• Which receptors they bind to• Which ions flow in/out of the post-synaptic neuron• Whether or not depolarisation reaches threshold

-70mv

0mv

time

threshold

resting

depolarisation

hyperpolarisation

Excitatory NTs cause depolarisation• e.g. ACh, dopamine• NT binds, Na+ channels open, Na+ rushes in• Membrane potential depolarises, AP propagated

Inhibitory NTs cause hyperpolarisation• e.g. GABA, dopamine (on different pathways)• NT binds to receptor• K+ channels open, K+ rushes out• OR Cl- channels open, Cl- rushes in• Membrane potential become more negative• Action potential is prevented from propagating

http://is.gd/Jellinek

This is a useful example of an inhibitory neurotransmitter.

http://www.youtube.com/watch?v=-pfG6yHAQ5U

Decision-making in the Central Nervous System (CNS)

axon

synapse

axon hillock

actio

n po

tenti

al

Diagram adapted from: http://hsc.uwe.ac.uk/synapses_neuro/summation.htm

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

neurons via synapses.

The ‘decision’ 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 threshold,the 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: http://hsc.uwe.ac.uk/synapses_neuro/summation.htm

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

succession• Depolarisation in the axon hillock is

summative• If it reaches threshold before

repolarisation, the AP is propagated.

Spatial summation• Action potentials arrive simultaneously

from multiple sources• Some 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.

http://www.jellinek.eu/brain/index.html?KeepThis=true&TB_iframe=true&height=588&width=672

How do psychoactive drugs affect the brain?

Work through the excellent animations and explanations from http://www.jellinek.eu/

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

In general, psychoactive drugs can: 1. Increase or decrease the release of NTs (e.g.

THC – 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 (e.g.cocaine)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: • Nicotine, amphetamines, cocaine

What is the effect of cocaine? Normal: • Dopamine acts as excitatory NT• Dopamine is re-uptaken by pumps on the pre-

synaptic membrane.

With Cocaine:• Cocaine blocks re-uptake pumps• Dopamine remains in synaptic cleft• More dopamine continues to be released• Summative increase in post-synaptic transmission

Effects on mood: • Dopamine is involved in reward pathways,

enhancing feelings of pleasure• Longer-lasting feelings as dopamine is not re-

uptaken

Effects on behaviour:• feelings of euphoria• increased energy and alertness• highly addictive• association with depression as body reduces

production of own dopamine over timehttp://is.gd/Jellinek

http://is.gd/Jellinek

Inhibitory drugs decrease post-synaptic transmission

Examples: • Alcohol, benzodiazapines, THC

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

With THC:• THC mimics cannabinoids and inhibits GABA release

by binding to cannabinoid receptors• GABA cannot inhibit dopamine release• More dopamine is released

Effects on mood: • Dopamine is involved in reward pathways,

enhancing feelings of pleasure• Not as extreme release of dopamine as with

cocaine, but still higher than normal

Effects on behaviour:• intoxication• hunger• memory impairment• potential dependency

http://outreach.mcb.harvard.edu/animations/synapse.swf

http://thebrain.mcgill.ca/intermediaire.php

A scale of harm for drugsWhich 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:http://activeprompt.herokuapp.com/SQSNK or http://is.gd/drugscaleprompt http://activeprompt.herokuapp.com/TYYBH

http://www.youtube.com/watch?v=bCChf2WHNE4

http://learn.genetics.utah.edu/content/addiction/reward/

http://learn.genetics.utah.edu/content/addiction/factors/environment.html

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