Getting On Your Nerves: Neurones and Drug Mode of Action
The people: Avengers is the most ambitious crossover event in history
Me: *makes a post that covers biology and psychology at the same time*
Today I’m covering my biological psychology module which includes learning about neurones and how drugs work. This isn’t on my biology specification but it might be on yours – so I’m tagging it biology AND psychology. Aren’t I just wild?
Biological psychology assumes that behaviour comes from our biology – that is, our genes, hormones, brain structure etc – and one of the behaviours that it explains is how the body can become addicted. To learn about this, though, you must first learn about the Central Nervous System (CNS).
There are two types of nervous system (clusters of nerves), the other being the Peripheral Nervous System. The CNS consists of the brain and the spinal cord and is pretty much where all communication and decisions occur since messages are sent around the body from the CNS.
Nerves are made up of bundles of specialised cells called neurones. A neuron is pictured below:
Dendrites are short branches at the “top” end of the nerve cell. They receive impulses from the neuron before in a process called neurotransmission. The cell body contains a nucleus, which has the information for the cell. The length of a neuron is called an axon, made up of insulating myelin sheath. Gaps within it are called nodes of Ranvier – electrical impulses jump along these. At the end of the neuron there are axon terminals where neurotransmission takes place over a synapse.
So now you know about the structure of a neuron, the next thing to learn is how messages are passed between them.
Imagine a nerve made up of two neurons. The pre-synaptic neuron is stimulated and action potential is reached. This is kind of like an electrical impulse that travels down the axon towards the terminals.
Between two neurons, there is a gap called the synapse. Neurotransmission is sometimes referred to as synaptic transmission, so it kind of makes sense why the first neuron is called the pre-synaptic neuron.
At one end of the presynaptic neuron, where the terminals are, there are neurotransmitters (chemical messages) in vesicles, which essentially just contain them. When the action potential stimulates the vesicles, they then fuse with the terminal button and release neurotransmitters into the synaptic gap.
On the post-synaptic neuron, there are specific receptor sites on the dendrites that the neurotransmitters are attracted to. The lock and key model is important here; a certain neurotransmitter can only bind to a complementary receptor site. Once the transmitters reach them, they stimulate action potential and the process starts again. Neurotransmitters then detach and either get eaten by enzymes in the synaptic gap or are reabsorbed through the reuptake pump.
There are different types of neurons – sensory neurons carry information from the sense organs (e.g. eyes) to the brain, motor neurons carry information from the CNS down their long axons to muscles and interneurons only communicate within their region.
Since there’s different types of neurons, there’s different types of neurotransmitters too. Excitatory neurotransmitters (also known as neuromodulators or monoamines) pass on electrical impulses and increase the likelihood that a post synaptic neuron will be activated and pass on the message. Examples of this include dopamine, adrenaline and serotonin. On the other hand, there are inhibitory neurotransmitters (endorphins) that block impulses between neurons and therefore decrease the likelihood that the post synaptic neuron will be activated to pass on the impulse.
Here are some examples of each kind of neurotransmitter:
Acetylcholine – excitatory, related to voluntary muscles, inhibition and memory
Dopamine – inhibitory and excitatory, related to voluntary muscles, emotional arousal.
Serotonin – inhibitory and excitatory, related to sleep and temperature regulation.
GABA is an inhibitory neurotransmitter that is closely involved with nearly 40% of synapses in the brain, so most neurones have GABA receptors.
Mode of action describes the action of neurotransmitters at the synapse. Drugs can act through mechanisms like neurotransmitters do. Some drugs can bind to receptors and have the same effect as a neurotransmitter. Some can block the receptor so neurotransmitters cannot fit into them.
In addition to these methods, drugs can work by influencing the making, movement, release or inhibition of neurotransmitters, such as preventing their recycling so they reattach to receptor sites.
Types of drugs that exist include agonists and antagonists. An agonist drug is a drug which mimics a neurotransmitter and increases post-synaptic activity by stimulating post synaptic receptors. An antagonist drug limits the effect of a neurotransmitter therefore reducing post-synaptic activity, perhaps through block post-synaptic receptors or preventing the release of neurotransmitters.
These drugs can further be categorised into psychoactives; depressants and stimulants. Psychoactive stimulants increase post-synaptic activity to cause motor arousal and alertness whereas depressants reduce it, slowing down brain activity and relax muscles.
Cocaine, for example, acts on dopamine. It blocks the dopamine uptake pump in the presynaptic terminal so the dopamine remains active in the synapse, continuing to bind to postsynaptic receptors and cause excitement.
Unfortunately, addiction comes at a cost. Withdrawal is a set of symptoms which occur when there is a sudden drop in blood levels of certain drugs, often the opposite to the initial effects of it. Withdrawal occurs when you are physically dependent on the drug.
However, if you continue to take the drug, you may develop a tolerance ( a decreased sensitivity due to exposure) so more drug is needed to produce the same effect. Tolerance to one drug may also cause tolerance to other drugs with similar mechanisms, and this is called cross tolerance.
Tolerance can essentially be metabolic or functional. Metabolic tolerance is when the amount of drug reaching the sits of action decreases and functional tolerance occurs when there is a reduction in the responsiveness of the receptors. Tolerance to psychoactive drugs is mainly functional. Two naturally occurring processes cause tolerance – down regulation and enzymes.
Down regulation is where the number of receptor sites or their sensitivity is reduced. Since there are less receptors, more drug is needed for the same effect.
Enzymes in the liver break down drugs, so increased exposure to drugs means more of these enzymes are produced, making the body more efficient at removing drug molecules so more drug is needed.
From these facts, you can easily see how addiction occurs. If the symptoms of withdrawal are too much, there is no other option than to continue using. In addition, what starts as a small problem can get quickly larger if the body develops tolerance to the drug.
SUMMARY
· Biological psychology assumes that behaviour comes from our biology – that is, our genes, hormones, brain structure etc.
· Central Nervous System (CNS) consists of the brain and the spinal cord and is pretty much where all communication and decisions occur.
· Nerves are made up of bundles of specialised cells called neurones.
· Dendrites are short branches at the “top” end of the nerve cell which receive impulses from the neuron before in a process called neurotransmission.
· The cell body contains a nucleus, which has the information for the cell.
· The length of a neuron is called an axon, made up of insulating myelin sheath.
· Gaps within it are called nodes of Ranvier – electrical impulses jump along these. At the end of the neuron there are axon terminals where neurotransmission takes place over a synapse.
· In neurotransmission, the pre-synaptic neuron is stimulated and action potential is reached. This is kind of like an electrical impulse that travels down the axon towards the terminals.
· Between two neurons, there is a gap called the synapse.
· At one end of the presynaptic neuron, where the terminals are, there are neurotransmitters (chemical messages) in vesicles. When the action potential stimulates the vesicles, they then fuse with the terminal button and release neurotransmitters into the synaptic gap.
· On the post-synaptic neuron, there are specific receptor sites on the dendrites that the neurotransmitters are attracted to - a certain neurotransmitter can only bind to a complementary receptor site.
· Once the transmitters reach them, they stimulate action potential and the process starts again. Neurotransmitters then detach and either get eaten by enzymes in the synaptic gap or are reabsorbed through the reuptake pump.
· Sensory neurons carry information from the sense organs (e.g. eyes) to the brain, motor neurons carry information from the CNS down their long axons to muscles and interneurons only communicate within their region.
· Excitatory neurotransmitters pass on electrical impulses and increase the likelihood that a post synaptic neuron will be activated and pass on the message e.g. dopamine, adrenaline and serotonin.
· On the other hand, there are inhibitory neurotransmitters that block impulses between neurons and therefore decrease the likelihood that the post synaptic neuron will be activated to pass on the impulse.
· GABA is an inhibitory neurotransmitter that is closely involved with nearly 40% of synapses in the brain, so most neurones have GABA receptors.
· Mode of action describes the action of neurotransmitters at the synapse.
· Some drugs can bind to receptors and have the same effect as a neurotransmitter. Some can block the receptor so neurotransmitters cannot fit into them.
· In addition to these methods, drugs can work by influencing the making, movement, release or inhibition of neurotransmitters, such as preventing their recycling so they reattach to receptor sites.
· An agonist drug is a drug which mimics a neurotransmitter and increases post-synaptic activity by stimulating post synaptic receptors. An antagonist drug limits the effect of a neurotransmitter therefore reducing post-synaptic activity, perhaps through block post-synaptic receptors.
· These drugs can further be categorised into psychoactives; depressants and stimulants. Psychoactive stimulants increase post-synaptic activity to cause motor arousal and alertness whereas depressants reduce it, slowing down brain activity and relax muscles.
· Withdrawal is a set of symptoms which occur when there is a sudden drop in blood levels of certain drugs, often the opposite to the initial effects of it. Withdrawal occurs when you are physically dependent on the drug.
· Drug users develop tolerances (decreased sensitivity due to exposure) so more drug is needed to produce the same effect. Tolerance to one drug may also cause tolerance to other drugs with similar mechanisms, and this is called cross tolerance.
· Tolerance can essentially be metabolic or functional. Metabolic tolerance is when the amount of drug reaching the sits of action decreases and functional tolerance occurs when there is a reduction in the responsiveness of the receptors
· Down regulation is where the number of receptor sites or their sensitivity is reduced. Since there are less receptors, more drug is needed for the same effect.
· Enzymes in the liver break down drugs, so increased exposure to drugs means more of these enzymes are produced, making the body more efficient at removing drug molecules so more drug is needed.
· Addiction occurs as a result of physical dependence, tolerance and the effects of withdrawal.
HAPPY STUDYING!
