NEUROTRANSMITTERS AND SYNAPSES Option E.4 IB Biology Miss Werba

OPTION E - NEUROBIOLOGY STIMULUS and RESPONSE E.2 PERCEPTION of STIMULI E.3 INNATE and LEARNED BEHAVIOUR E.4 NTs and SYNAPSES E.5 THE HUMAN BRAIN E.6 FURTHER STUDIES IN BEHAVIOUR J WERBA – IB BIOLOGY 2

THINGS TO COVER Inhibitory vs Excitatory neurons Interaction between inhibitory & excitatory neurons Psychoactive drugs Effects of THC & cocaine Addiction J WERBA – IB BIOLOGY 3

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TOPIC 6.5 REVIEW What are the three parts of the synapse? What is the resting membrane potential? What is the threshold level? What is the maximum membrane potential before repolarisation occurs? What are the four phases of the action potential? How does a signal cross the synapse? J WERBA – IB BIOLOGY 5

TOPIC 6.5 REVIEW What are the three parts of the synapse? Presynaptic neuron Synaptic cleft Postsynaptic neuron J WERBA – IB BIOLOGY 6

TOPIC 6.5 REVIEW What is the resting membrane potential? -70mV What is the threshold level? -50mV What is the maximum membrane potential before repolarisation occurs? +30-50mV J WERBA – IB BIOLOGY 7

TOPIC 6.5 REVIEW What are the four phases of the action potential? Resting ( Threshold) Depolarisation Repolarisation Hyperpolarisation ( Resting) J WERBA – IB BIOLOGY 8

TOPIC 6.5 REVIEW How does a signal cross the synapse? Crosses the synapse as a chemical signal, via use of neurotransmitters J WERBA – IB BIOLOGY 9

POST-SYNAPTIC TRANSMISSION Command term = STATE Some pre-synaptic neurons excite post-synaptic transmissions and others inhibit post-synaptic transmissions Pre-synaptic neurons release neurotransmitters (NTs) into the synapse to transfer a stimulus to the post-synaptic neurons Some pre-synaptic neurons generate excitatory post-synaptic potentials (EPSPs)  makes it easier for the post-synaptic neuron to reach threshold Other pre-synaptic neurons generate inhibitory  post-synaptic potentials (IPSPs)  makes it harder for the post-synaptic neuron to reach threshold J WERBA – IB BIOLOGY 10

POST-SYNAPTIC TRANSMISSION Command term = STATE Whether or not the post-synaptic neuron propagates (passes on) the action potential depends on: Which NT diffuses across the synapse Which receptors they bind to Which ions flow in/out of the postsynaptic neuron Whether or not depolarisation reaches threshold J WERBA – IB BIOLOGY 11

POST-SYNAPTIC TRANSMISSION Command term = STATE Excitatory NTs cause depolarisation. This is referred to as an Excitatory Post-Synaptic Potential (EPSP). Inhibitory NTs cause hyperpolarisation. This is referred to as an Inhibitory Post-Synaptic Potential (IPSP). J WERBA – IB BIOLOGY 12

Excitatory NTs cause depolarisation POST-SYNAPTIC TRANSMISSION Command term = STATE Excitatory NTs cause depolarisation eg. acetylcholine (Ach), dopamine NT opens Na+ channels  Na+ influx membrane potential moves closer to threshold action potential propagated J WERBA – IB BIOLOGY 13

Inhibitory NTs cause hyperpolarisation POST-SYNAPTIC TRANSMISSION Command term = STATE Inhibitory NTs cause hyperpolarisation eg. γ–aminobutyric acid (GABA) NT opens K+ channels  K+ efflux OR NT opens Cl- channels  Cl- influx membrane potential moves further from threshold action potential not propagated J WERBA – IB BIOLOGY 14

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SUMMATION Command term = EXPLAIN The signals from the many pre-synaptic neurons accumulate This is called summation The overall charge needs to reach threshold for an action potential to be propagated: J WERBA – IB BIOLOGY 16

SUMMATION Command term = EXPLAIN The CNS makes decisions about whether or not to propagate the action potential through the summation of the incoming impulses. There are two main methods of summation: TEMPORAL Multiple APs arrive from one pre-synaptic neuron SPATIAL APs arrive from multiple pre-synaptic neurons J WERBA – IB BIOLOGY 17

SUMMATION Command term = EXPLAIN J WERBA – IB BIOLOGY 18

PSYCHOACTIVE DRUGS Command term = EXPLAIN Psychoactive drugs can affect the brain and personality by influencing NT action. Examples: Increase the release of NTs Decrease the release of NTs – eg. THC (cannabis) Breakdown reuptake proteins so that the NTs are not able to be reused Block reuptake proteins – eg. Cocaine Mimic or block NTs by binding to post-synaptic receptors Inhibit production of NTs J WERBA – IB BIOLOGY 19

PSYCHOACTIVE DRUGS Command term = LIST Psychoactive drugs can therefore be inhibitory or excitatory EXCITATORY PSYCHOACTIVE DRUGS ↑ post-synaptic transmission INHIBITORY PSYCHOACTIVE DRUGS ↓ post-synaptic transmission Cocaine Alcohol Nicotine Opiates (heroin) Caffeine Benzodiazepines (valium) Amphetamines (ecstasy) Tetrahydrocannabinol or THC (cannabis) J WERBA – IB BIOLOGY 20

PSYCHOACTIVE DRUGS Command term = EXPLAIN EXCITATORY PSYCHOACTIVE DRUGS ↑ post-synaptic transmission Cocaine Cocaine blocks dopamine reuptake Dopamine remains in synapse Results in increased post- synaptic transmission Mood: enhances feelings of pleasure longer-lasting feelings Behaviour: Feelings of euphoria Increased energy/alertness Highly addictive Associated with depression as body reduces production of own dopamine over time J WERBA – IB BIOLOGY 21

PSYCHOACTIVE DRUGS Command term = EXPLAIN THC inhibits GABA release GABA normally inhibits dopamine release THC results in increased dopamine release Mood: enhances feelings of pleasure as dopamine is involved in the reward pathway Behaviour: intoxication hunger Memory impairment Potential dependency Associated with depression and other mood disorders INHIBITORY PSYCHOACTIVE DRUGS ↓ post-synaptic transmission Tetrahydrocannabinol or THC (cannabis) J WERBA – IB BIOLOGY 22

PSYCHOACTIVE DRUGS Command term = EXPLAIN J WERBA – IB BIOLOGY 23

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E.4.6 ADDICTION Addiction = a chronic, neurological disorder with genetic, psychosocial and environmental characteristics It is characterised by changes in the brain resulting in a compulsive desire to use a drug. Causes of addiction: Genetic predisposition Social factors Dopamine secretion (reward pathway) Lots of research being done here. Lots still to be done!!! Click on picture of the ‘Mouse Party’ to explore the effects of psychoactive drugs and the science of addiction further J WERBA – IB BIOLOGY 25

E.4.6 ADDICTION Reward centres in the brain play a key role in reinforcing behaviour that may lead to addiction. Dopamine is the most prevalent NT in the reward pathways. Drugs that stimulate dopamine secretion generate feelings of pleasure. This makes the user likely to use the drug again and makes them very addictive. J WERBA – IB BIOLOGY 26

ADDICTION Some people are genetically predisposed to becoming addicts. There is reference to an addiction gene and modified versions of other genes. There is a tendency for children of addicts to become addicts themselves (but may be social/environmental). They are doing research at the moment for gene therapy that could prevent addiction! J WERBA – IB BIOLOGY 27

ADDICTION Psychosocial factors also influence addiction. eg. Peer pressure Age Availability Legality Religious attitudes Community attitudes Stress/abuse Mental health J WERBA – IB BIOLOGY 28

Spiders on speed get weaving SPIDERS ON DRUGS Spiders on speed get weaving 29 April 1995 NewScientist.com news service SPIDERS on marijuana are so laid back, they weave just so much of their webs and then ... well, it just doesn't seem to matter any more. On the soporific drug chloral hydrate, they drop off before they even get started. A spider's skill at spinning its web is so obviously affected by the ups and downs of different drugs that scientists at NASA's Marshall Space Flight Center in Alabama think spiders could replace other animals in testing the toxicity of chemicals. Different drugs have varying effects on the average arachnid addict. On benzedrine, a well-known upper, the house spider spins its web with great gusto, but apparently without much planning, leaving large holes. On caffeine it seems unable to do more than string a few threads together at random. The more toxic the chemical, the more deformed the web. NASA researchers think that with help from a computer program they can quantify this effect to produce an accurate test for toxicity. J WERBA – IB BIOLOGY 29

NORMAL SPIDER WEB J WERBA – IB BIOLOGY 30

SPIDER EXPOSED TO MARIJUANA J WERBA – IB BIOLOGY 31

SPIDER EXPOSED TO CAFFEINE J WERBA – IB BIOLOGY 32

SPIDER EXPOSED TO SPEED J WERBA – IB BIOLOGY 33

SPIDER EXPOSED TO SLEEPING PILLS J WERBA – IB BIOLOGY 34

Sample questions Q1 State one example of an excitatory and one example of an inhibitory psychoactive drug. [2] Explain the effects of cocaine on the brain. [3] Discuss causes of addiction. [3] J WERBA – IB BIOLOGY 35

Sample questions Q2 OUTLINE Pavlov’s experiments into conditioning in dogs. [2] Q3 OUTLINE the role of inheritance and learning in the development of birdsong in young birds. [2] J WERBA – IB BIOLOGY 36

Sample questions Q2 Identify the percentage of homozygotes among drug users only. [1] Calculate the actual number of homozygotes in group 2. [2] Evaluate the evidence to support the hypothesis that the presence of the homozygous mutation is a risk factor in drug and alcohol use. [3] Suggest a reason for the high incidence of homozygotes among drug and alcohol users. [1] J WERBA – IB BIOLOGY 37

Sample questions A1 excitatory: cocaine / nicotine / amphetamines; inhibitory: benzodiazepines / alcohol / THC; [2] excitatory (psychoactive) drug; cocaine attaches to dopamine pumps/transporters (on presynaptic membrane); blocks uptake/recycling / causes dopamine to persist in the synaptic cleft; amplifies synaptic transmission / causes constant stimulation of postsynaptic neuron; causes euphoria/feelings of happiness/pleasurable effects; [3] J WERBA – IB BIOLOGY 38

Sample questions A1 addictive drugs trigger secretion of dopamine which causes feelings of pleasure/well-being/reward / users become dependent on feelings; genetic predisposition is most common with addiction to alcohol; social factors affect the incidence of addiction; it is not certain that a person who is genetically predisposed will develop addiction when exposed to the drug / OWTTE; although many drugs are (potentially) addictive, not every user becomes an addict; named social factors which must be explained: e.g. cultural traditions / peer pressure; social deprivation / traumatic life experiences / mental problems; J WERBA – IB BIOLOGY 39

Sample questions A2 7.8% (allow answers in the range of 7.7% to 7.9%) [1] (15/100) * 80 = 12 [2] J WERBA – IB BIOLOGY 40

Sample questions A2 group 1 and group 4 have small differences; supports that the homozygous condition is not a risk factor in alcohol use; group 1 and group 2 have large difference; group 2 have more chance of being homozygotes for the mutation; group 1 small difference with group 3; homozygous condition appears to be a risk factor in group 2and in group 3; numbers are very different in each population / small percentages; [3] Homozygotes cannot inactivate cannaboids/THC, thus increased needs for drug or alcohol intake / chances of addiction [1] J WERBA – IB BIOLOGY 41