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Neurotransmitters and synapses. Option E.4. Assessment Statements. E.4.1 State that some presynaptic neurons excite postsynaptic transmission and others inhibit postsynaptic transmission.

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assessment statements
Assessment Statements
  • E.4.1 State that some presynaptic neurons excite postsynaptic transmission and others inhibit postsynaptic transmission.
  • E.4.2 Explain how decision-making in the CNS can result from the interaction between the activities of excitatory and inhibitory presynaptic neurons at synapses.
  • E.4.3 Explain how psychoactive drugs affect the brain and personality by either increasing or decreasing postsynaptic transmission.
  • E.4.4 List three examples of excitatory and three examples of inhibitory psychoactive drugs.
  • E.4.5 Explain the effects of THC and cocaine in terms of their action at synapses in the brain.
  • E.4.6 Discuss the causes of addiction, including genetic predisposition, social factors and dopamine secretion.
synaptic transmission
Synaptic transmission

Neurons communicate with each other chemically across a space called a synapse

One side of the synapse is the presynaptic membrane of the sending neuron and on the other side of the synapse is the postsynaptic membrane of the receiving neuron

Molecule which moves across the space (synaptic cleft) between the two membranes is called a neurotransmitter

Specific neurotransmitter is received by a specific receptor

excitation and inhibition
Excitation and Inhibition
  • Some neurotransmitters are excitatory and stimulate the next neuron to forward the message
  • Do this by increasing the permeability of the postsynaptic membrane to positive ions, making it easier for positive ions to move in
  • Some neurotransmitters are inhibitory and cause the positive ions to move out of the postsynaptic cell
  • Positive ions move back in to the synaptic cleft chemically depressing the postsynaptic cell and makes it much harder to excite
decision making in the cns
Decision making in the CNS

Impulse which moves down the presynaptic neuron is called the action potential

As action potential reaches axon bulb, calcium ions rush into the end of the neuron

Vesicles containing neurotransmitters fuse with the presynaptic membrane

Neurotransmitter released into the synaptic cleft

Neurotransmitter binds to specific receptors on the postsynaptic membrane

Receptors let ions enter or leave when the neurotransmitter binds to them

excitatory neurotransmitters
Excitatory neurotransmitters
  • One example: acetylcholine
  • Generates action potential
  • Increases permeability of the postsynaptic membrane to positive ions
  • Causes positive sodium ions to diffuse into the postsynaptic neuron
  • Localized depolarization occurs
  • Inside of neuron develops a net positive charge compared to the outside
  • Depolarization continues as sodium ions diffuse to the next area of the neuron
  • Impulse is carried along the nerve
  • If threshold is not met, the neuron does not carry the impulse to the next neuron
inhibitory neurotransmitters
Inhibitory neurotransmitters
  • Example: GABA
  • Inhibit action potentials
  • Causes hyperpolarization of the neuron
  • Inside of the neuron becomes more negative making it difficult for an action potential to be generated
  • GABA binds to specific receptor
  • Causes negatively charged chloride ions to move across the postsynaptic membrane into the postsynaptic cell OR it can cause positively charged K+ ions to move out of the postsynaptic neuron
  • Movement of Cl- into the neuron or K+ out causes hyperpolarization
putting it together
Putting it together
  • A neuron is always on the receiving end of many excitatory and inhibitory stimuli
  • Neuron sums up the signals
  • If the sum of the signals is inhibitory then the axon does not fire
  • If the sum of the signals is excitatory, the axon fires
  • Summation of the messages is the way that decisions are made by the CNS
cholinergic synapses
Cholinergic synapses
  • Acetylcholine is released by all motor neurons and activates skeletal muscle
  • If it remained in the synapse, the postsynaptic membrane would go on firing indefinitely
  • Acetylcholinesterase breaks it down
  • Acetylcholine is involved in the parasympathetic nervous system
  • Causes relaxation rather than flight

Nicotine stimulates transmission in cholinergic synapses which is why it has a calming effect on the body and personality

  • People addicted to nicotine become very agitated if they cannot have a cigarette
adrenergic synapses
Adrenergic synapses
  • Noradrenaline depolarizes the postsynaptic neuron
  • Noradrenaline is involved in the sympathetic sysem
  • It causes a “fight or flight” reaction
  • Cocaine and amphetamines both cause increased alertness, energy and euphoria
effect of drugs on the brain
Effect of drugs on the brain

Can alter your mood or your emotional state

Excitatory drugs (nicotine, coaaine, amphetamine) increase nerve transmission

Inhibitory drugs (benzodiazepines, alcohol, and THC) decrease the likelihood of nerve transmission

Drugs can change synaptic transmission in the following ways:

Block a receptor for a neurotransmitter (structure similar to the transmitter)

Block release of neurotransmitter from the presynaptic membrane

Enhance neurotransmission by mimicking a neurotransmitter

Block removal of neurotransmitter from the synapse and prolong the effect of the neurotransmitter

excitatory drugs and how they act
Excitatory drugs and how they act

Nicotine mimics acetylcholine (Ach)

Acts on the cholinergic synapses of the body and the brain to cause a calming effect

After Ach is received by the receptors, it is broken down by acetylcholinesterase but it cannot break down nicotine

This excites the postsynaptic neuron and it begins to fire, releasing a molecule called dopamine

Dopamine gives you a feeling of pleasure

Cocaine stimulates transmission at adrenergic synapses and causes alertness and euphoria

Causes dopamine release

Cocaine blocks removal of dopamine from the synapse so that it builds up

Leads to overstimulation of the postsynaptic neuron

Leads to euphoria

Both of these drugs causes addiction


Amphetamine stimulates transmission at adrenergic synapses and gives increased energy and alertness

Amphetamine acts by passing directly into the nerve cells which carry dopamine and noradrenaline

It moves directly into the vesicles of the presynaptic neuron and causes their release into the synaptic cleft

Normally these would be broken down, but amphetamines interfere with breakdown

High concentrations of dopamine cause euphoria and high concentrations of noradrenaline may be responsible for the alertness and high energy effect

inhibitory drugs and how they act
Inhibitory drugs and how they act

Benzodiaizepine reduces anxiety

Can be used against epileptic seizures

Modulates the activity of GABA which is the main inhibitory neurotransmitter

When GABA binds to the postsynaptic membrane, it causes Cl-to enter the neuron

Causes hyperpolarization and resists firing

B increases the binding of GABA to the receptor and therefore greater hyperpolarization

Alcohol acts similarly to B in that it increases the binding of GABA to the postsynaptic membrane

Decreases activity of glutamate, an excitatory neurotransmitter

Alcohol helps to increase the release of dopamine by a process which is not well understood

Appears to stop the activity of the enzyme which breaks down dopamine in the synaptic cleft

Tetrahydrocannabinol (THC) is the main pschoactive chemical in marijuana

THC mimics the neurotransmitter, anandamide

THC binds to the same receptor and cause the postsynaptic neuron to be hyperpolarized

Anandamide may play a role in memory functions, such as eliminating information from our memory that is not needed

Marijuana disrupts short-term memory in humans

thc effects
THC effects

Feelings of relaxation, lightheadedness, haziness

Decrease in learning, coordination, problem-solving, and short-term memory

Stays in synapse longer than anandamide

High concentrations of receptors found in hippocampus (short-term memory); cerebellum (coordination)

cocaine effects
Cocaine effects

Euphoria, talkativeness, increase in mental alertness

Temporary decrease in the need for food and sleep

Large amts. Cause erratic and violent behavior

Sustains level of dopamine in the synapse

causes of addiction
Causes of addiction

Body develops a tolerance and needs more of the drug to produce the same result

Chemical dependency is caused by drug “rewiring the brain” and becoming an essential biochemical in the body

Role of abused drugs is to stimulate the reward pathway located in the brain

Withdrawal symptoms

Anxiety, depression, craving

Seizures, severe shaking

Continued addiction

Lung damage

Risk of contracting HIV, hepatitis B and C

Kidney disease

genetic predisposition
Genetic predisposition

Studies in male twins find that when one twin suffers an addiction to alcohol or drugs, the rate of addiction in the second twin is 50% greater among identical twins than among fraternal twins

Other experiments indicate that a genetically determined deficiency of dopamine receptors predisposes certain people to addiction

Persons who become addicted to drugs that increase dopamine levels do so to compensate for that deficiency

social factors of addiction
Social factors of addiction
  • Family addiction, family parenting skills, mental health problems of family or child
  • Often related to peer group; users teach new users what effects to expect and what altered state is desirable
  • Alcohol at social gatherings fosters the paradigm that it must be available to have a party
  • Alcohol very rare in Saudi Arabia since it is prohibited
dopamine secretion
Dopamine secretion
  • Neurotransmitter which activates the reward pathway and gives us a sense of pleasure or satisfaction
  • During cocaine use, dopamine builds up in the synapse
  • Over-stimulation decreases the number of receptors and the remaining receptors become less sensitive to dopamine (desensitization or tolerance)

With tolerance, exposure to the drug causes less response that it previously caused

  • More and more of the drug is needed to have even the normal sense of well-being
  • Type of neuroadaptive change
knockout mice
Knockout mice
  • Genetically manipulated mice addicted to cocaine
  • Studies show that glutamate may oversee the learning and memories which lead to cocaine-seeking
  • Mouse party