The reward system in your brain is responsible for reinforcing actions and behaviours. This important system causes you to desire a reward or stimulus (craving water on a hot day), causes you to learn certain behaviours that help you receive that reward (taking a sip of water from your water bottle), and allows you to experience the pleasure of receiving that reward (the satisfaction of quenching your thirst). The reward system is essential for our very survival as it ensures that we are motivated to eat, drink, sleep, build relationships, and procreate. In fact for most animals, their ability to be successful in life (to survive and reproduce) depends on whether or not their reward system can function properly and guide them to the optimal behaviours.
In order to fully understand our reward system in relation to drugs, we need to learn more about the structures in your brain that comprise the reward system. The reward system begins with the ventral tegmental area (labelled VTA in the above image) which responds to stimuli from your surroundings that tell you that a reward is present; this could be food you are eating, the time you lie down in your bed to go to sleep after a long day, a new notification on your phone, or even a drug like heroin or alcohol. The VTA will release a molecule called dopamine towards your nucleus accumbens. When your nucleus accumbens receives dopamine it will give you the feeling of pleasure by activating your limbic system (system that allows you to feel emotions) and will send signals to your brain to get you to remember and learn the behaviours that gave you this reward. When your VTA detects a reward, it also sends dopamine to your prefrontal cortex; the part of your brain responsible for organising your thoughts, as well as planning and making decisions that will allow you to achieve your goals. When your prefrontal cortex receives this dopamine signal, it will deliberate on whether or not this is a good reward to seek out and may allow you to control your impulses (of note: teenagers and adults under the age of 25 have a harder time controlling their impulses because the prefrontal cortex is not developed until later in life).
Now that we have covered how the reward system works, let us review a real life example of this:
Lilly is studying for her math exam and is getting quite bored as she has reviewed lots of her material and is finding it quite easy. She promised herself that she would review at least three more concepts so she can stay on track with her study schedule (planning and decision making in order to achieve a goal – prefrontal cortex in action). A notification pops up on her phone letting her know that a new episode of her favourite show is now released. Upon seeing the notification her VTA detects that a reward (watching the show – something she enjoys) is near. Her VTA releases dopamine that signals to her nucleus accumbens and to her prefrontal cortex. When her nucleus accumbens receives this signal, it activates her limbic system which makes her feel happy and it gives her the impulse to go lie down on her bed with her phone and watch the new episode. When her prefrontal cortex receives this signal, it makes her deliberate “I want to watch this new episode, but I should finish my studying because it will help me do well on my math exam.” The following situation could unfold in scenario A) or scenario B) depending on whether or not her prefrontal cortex is able to control her impulse.
A) In this scenario, Lilly is able to control her impulse with her prefrontal cortex and get back to studying. Lilly’s prefrontal cortex overcomes the urge and she decides that she will watch the show immediately after she is done reviewing her math homework – it will be a reward for finishing her studying! She controls her impulses – something that is not always easy to do! Think about the times that you have procrastinated or not been able to control your impulses.
B) In this scenario, Lilly gives into her impulse and begins watching the latest episode of the show. Her VTA responds favourably to watching the show (i.e. she enjoys it) and releases a lot of dopamine which stimulates her nucleus accumbens. Her nucleus accumbens sends signals to the rest of her brain instructing it that in order to receive pleasure, she should pick up her phone and start watching her favourite show. Her behaviour is reinforced. The next time she notices there is a new episode of the show, her brain will remember the joy that watching the last episode brought her and her VTA will release dopamine resulting in an impulse to watch her show.
The example above is more or less a harmless one, but when it comes to certain drugs (especially opioids, but many others too), they work by directly overstimulating your reward system such that you no longer have the ability to control your impulses. For example, when an individual takes an opioid like heroin, the drug travels in that person’s body to their brain and in the brain the drug causes your brain (specifically your VTA) to release a huge amount of dopamine. The dopamine released makes you feel good but also reinforces in your brain that you should use heroin in order to get this reward again. The amount of dopamine released in your brain in response to heroin greatly exceeds what you would typically experience from your day-to-day life and as such the reinforcement in your brain to crave heroin is so strong that your prefrontal cortex is not able to control your impulses at all. The reinforcement for drugs like heroin is so strong in some cases, an individual’s craving for heroin can outweigh their desire for basic needs such as food and water.
This hijacking of the reward system by drugs that cause dopamine release is one of the main reasons that individuals use drugs frequently and have a difficult time stopping their substance use. The drug is so powerful that people feel like they no longer have a choice and their brain feels as though they need the drug to survive.
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