E4: Neurotransmitters & Synapses

Pp 481 - 488

Assessment StatementsE.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 transmissionSome Pre-synaptic

neurons excite postsynaptic transmission

They are called excitatory pre-synaptic neurons

Some Pre-synaptic neurons inhibit postsynaptic transmission

They are called inhibitory pre-synaptic neurons

Excitatory and Inhibitory synapses

Excitatory Pre-synaptic neurons

Inhibitory Pre-synaptic neurons

Depolarises the postsynaptic membrane

Influx of Na+ ions

Possibly reaching threshold

Generation of new action potential

E.g. Acetylcholine

Hyperpolarises the postsynaptic membrane

Influx of Cl- or loss of K+ ions

Makes it more difficult to reach threshold

Lowers the resting potential

E.g. Gama-aminobutryic

acid (GABA)

An action potential in post-synaptic neurone will only occur if the potential difference at the axon hillock (specialized part of the cell body of a neuron that connects to the axon)rises above the threshold value

Decision-making in the CNS can result from the interaction between the activities of excitatory and inhibitory pre-synaptic neurons at synapses.Neurones form

synaptic junctions with the cell body of other neurones

A post synaptic neurone can have many pre-synaptic neurones forming synaptic junctions with it.

Pre synaptic neurones depolarise (excitatory) or hyperpolarise (inhibitory) the post synaptic membrane locally.

The sum of their effects takes place at the axon hillock

SynapsePre-

Pre-

Pre-

Post-

Post-Synaptic Potentials (PSPs)  can be:Excitatory (EPSP)(+);or Inhibitory (IPSP) (-).

Post-synaptic

Pre-synaptic

Pre-synapticP

re-synapticP

re-synaptic

+

+-

-Synapse

The EPSP must build up in the postsynaptic neuron tothe threshold level to allow the formation of an actionpotential.

Synapse+

+

+-

E.g. this Neuron needs a 2 more “+” than “-” before it can generate an action potential.

Neurons can integrate signals from other neurons: 1) through spatial summation (different pre-

synaptic neurones ‘add together’ their local depolarisation's to reach threshold at the axon hillock)

Synapse+

+

+-

Summation of the depolarising effects of three different excitatory pre-synaptic neurones i.e. a+b+c

Having reached the threshold (T), a new action potential is generated in the post synaptic neurone

If the summation of the 3 is below threshold there would be no action potential or response from the central nervous system

+ Synapse++

+++

2) through temporal summation where a number of waves neurotransmitter from the same pre-synaptic neurone depolarises the post synaptic membrane (i.e. waves of depolarisation are added together to reach threshold).

The same pre-synaptic neurone(a)has a high frequency of action potentials arriving at the synapse. This sends a number of waves of neurotransmitter to depolarise the post synaptic membrane.

Waves of depolarisation (from one neurone) are added together to reach threshold

Pre-synaptic neurones depolarises the post synaptic membrane with a number of rapid stimulations.

Decision-making in the CNSsynapses are the sites of decision-making in the

braina post-synaptic neuron's membrane potential is

the summation of input from pre-synaptic neurons EPSPs depolarize post-synaptic neurons whileIPSPs hyper-polarize post-synaptic neurons

if the post-synaptic neuron reaches threshold potential at its axon hillock, it will produce an action potential

pre-synaptic neurons can vary in the frequency, but not intensity of their input, since action potentials are "all-or-none“ rule

Psychoactive drug:A Psychoactive drug is

chemical substancesthat acts upon the CNSaltering the state ofmind by changing(stopping or enhancing)the activity at the postsynaptic  neuron. It result in changesin perception, mood,consciousness,cognition, andbehaviour.

Alcohol

Amphetamine

Marijuana

How psychoactive drugs affect the brain and personality:Psychoactive drugs affect the brain and personality by

either increasing or decreasing post-synaptic transmission.

The post-synaptic membrane contains receptors for the normal neurotransmitters. Normally when the neurotransmitter attaches to these receptors it either:Depolarise the post synaptic neuron resulting in an action

potentialHyperpolarise the post synaptic neuron resulting in no

action potentialA drug can therefore change post synaptic output by:

Blocking or enhancing the depolarising neurotransmitter Blocking or enhancing the hyperpolarise neurotransmitter

Major Neurotransmitters in the BodyNeurotransmitter Role in the BodyAcetylcholine (excitatory)

Dopamine (inhibitory)

Gamma-amino butyric acid

(GABA) (inhibitory)

used by the spinal cord neurons to control muscles and by many neurons in the brain to regulate memory.

produces feelings of pleasure when released by the brain, it has multiple functions depending on where in the brain it acts.

The most common inhibitory neurotransmitter in the brain.

Examples of excitatory & inhibitory psychoactive drugs:Excitatory psychoactive drugs;

Inhibitory psychoactive drugs;

nicotine,cocaine ,amphetamines.

benzodiazepines, alcohol ,tetrahydrocannabi

nol (THC).

Tetrahydrocannabinol (THC):Marijuana/ganja is the buds and leaves of the Cannabis sativa plant.

Marijuana contains more than 400 chemicals, including tetrahydrocannabinol (THC), the plant's main psychoactive chemical.

Cannabinoid receptors are activated by a neurotransmitter called anandamide.

Anandamide belongs to a group of chemicals called cannabinoids.

THC is also a cannabinoid chemical.

THC mimics the actions of anandamide, meaning that THC binds with cannabinoid receptors and activates neurons, which causes adverse effects on the mind and body.

Effects of Tetrahydrocannabinol (THC)THC affects brain’s short-term memoryTHC affects motor coordination & appetite THC increases heart rate and raises levels of

anxietyTHC contains cancer-causing chemicals similar to

those in tobacco THC affects higher order thinking THC users report feeling mellow and calmTHC mimics the neuro-receptor anandamideTHC binds to the post synaptic membrane receptor

s for anandamide called Cannabinoid ReceptorTHC effect is to hyperpolarise the post synaptic

neuron inhibiting generation of an action potential

Cocaine:Cocaine:- a powdered drug

that is made from the leaves of the coca plant grown primarily in South America.

Cocaine has been in use for centuries, many generations of South American Indians have chewed its leaves to give them strength and energy.

The drug is nowadays taken in by either snorting, injecting or smoking.

Dopamine is released by cells of the nervous system during pleasurable activities such as eating or having sex.

Once released, dopamine travels across a synapse, and binds to a receptor on a post-synaptic neuron.

This sends a signal to that nerve cell, which produces a good feeling.

Under normal conditions, once the dopamine sends that signal it is reabsorbed by the neuron that released it

Cocaine interferes with a chemical messenger in the brain called dopamine, which is involved in the body's pleasure response.

Effects of Cocaine:cocaine is an excitatory psychoactive drugs

that increases synaptic transmissionon post synaptic membrane receptor, it

prevents the removal of acetylcholineon pre-synaptic membrane, it prevents the

recycling of acetylcholineboth effects maintain the stimulation of the

post synaptic membrane cocaine also blocks the recycling of Dopamine

at the Pre-synaptic membrane, making dopamine to remain active in the synapse

dopamine is associated with a feeling of pleasure , thus cocaine creates a mood of euphoria

Drug addiction:Psychoactive drugs often cause

addictive behaviour i.e. an individual persists in use of drugs despite problems related to their use

habitual behaviour associated with addiction includes: craving for the use of the drug; lack of self control in limiting the

taking of the drug;increasing quantities must be taken

to achieve the same mental conditions;

development of withdrawal behaviour if unable to satisfy the craving for the drug.

Causes of addiction:(i) Genetic

predisposition;

Some people are more vulnerable to drug addiction than others,

studies indicates that genetic factors have some influence on addiction,

alcoholism, especially, tends to run in certain families

(ii) Social factors; various social factors correlate positively with addiction, including:cultural traditions; peer pressure; Poverty; social deprivation; traumatic life experiences;

mental health problems; etc

(iii) Dopamine secretion;

many addictive drugs are excitatory at dopaminergic synapses “reward pathway” giving a feel good effect/pleasure

addiction results from dopaminergic synapses responding to regular drug use

tolerance to a drug , leads to increased dosage to produce the desired effect

withdrawal is caused by normal levels of dopamine failing to produce pleasure hence craving for drugs

STOP DRUG ADDICTION!

Any Questions on Drugs?

Thank You!

A Drug-Free World: We Can Do It!

Self Assessment Questions (SAQs)Distinguish between

presynaptic excitatory & inhibitory neurons. [2]

Explain the process of decision-making in the CNS. [3]

Explain how psychoactive drugs affect the brain and personality of an individual. [3]

List 3 examples of excitatory & 3 examples of inhibitory psychoactive drugs. [2]

Explain the effects of THC and cocaine on the brain. [4]

Explain the effects of cocaine and cocaine on the brain. [4]

Discuss the causes of addiction. [6]

Sources: http://www.mun.ca/biology/desmid/brian/

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BiologyIBHL2/5044http://click4biology.info/c4b/E/E4.htmhttp://en.wikipedia.org/wiki/Substance_dependencehttp://www.google.co.in/imageshttp://www.google.co.in/images?

hl=en&gbv=2&q=stohttp://www.daretoact.net/