ADHD is a neurobiological developmental disorder that impedes one’s executive functions, but what causes it? ADHD has long been a subject of intense scientific scrutiny, from its origins to whether it’s even a real thing (it is, btw). However, with an explosion of research on the topic in the last 20 years thanks to the completion of the human genome project in 2003, we’re getting closer to solving to the riddle of what triggers the disorder that affects millions of children (and adults) worldwide.

Executive (Mal)functions

Let’s make this quick and dirty. There are seven main neural networks in the brain, one of them being the central executive network. It has four circuits that carry out our executive functions. These are:

• The frontal-striatal-thalamic circuit (the “what” network) – our working memory that holds information in our mind to guide our behavior over time (the sticky note of the brain). “There’s a test coming up Friday. I should study for it to get a good grade.”
  
  
• The frontal-cerebellar circuit (the “when” network) – the timing circuit, helping us determine the duration of time (seconds, minutes, hours) as well as the timing of our actions and behaviors (when to hit a ball or when it’s our turn to speak).
  
  
• The frontal-limbic circuit (the “why” network) – handles our emotional regulation as well as the motives behind our decisions/actions. It’s the thinking before we act/react.
  
  
• The frontal-cingulate-parietal network – the self-awareness circuit, allowing us to acknowledge and recognize our actions, mistakes, how it affects others, and ways to improve and adapt.

In ADHD, these circuits are disrupted, resulting in a dysfunction in:

• Working memory – forgetting things
• Time management – “time blindness”, no accurate or consistent concept of time
• Emotional regulation – difficulties calming down or even modulating voice and facial expressions
• Planning – difficulties planning and organizing actions towards a goal
• Forethought – not thinking things through
• Learning from consequences – wanting to fix the problem, but not knowing how
• Organization – difficult to work in hierarchical/linear thinking, like a traditional outline
• Reconstitution of information – difficulties recalling information, especially in multi-step directions

The ADHD Difference

People with ADHD have been described as acting immature for their age, but there’s a reason for it. Neuro-imaging data noted some key differences between the ADHD brain and non-ADHD brain, specifically the size and thickness of brain structures associated with executive functions. Overall, researchers found that these regions are 3-10% smaller with less activity and neural connections than typical. These include the prefrontal cortex, cerebellum, and basal ganglia. However, researchers also discovered an early maturation of the motor cortex. This suggests that the reason behind many of the symptoms seen in ADHD is due to a rapidly maturing motor system with an executive system that cannot manage it.

Another difference is in neurotransmitter levels. Neurotransmitters are chemical messengers that transmit signals between neurons in the brain and throughout the nervous system. Studies have shown that there’s a difficulty in the reuptake of certain neurotransmitters which lead to the characteristic symptoms of ADHD. The neurochemicals mainly affected are:

• Dopamine (the feel-good hormone), necessary for motivation, reward-seeking behavior, and the ability to focus on tasks; considered the primary neurotransmitter affected in ADHD
• Norepinephrine (aka noradrenaline), needed for arousal, attention, and cognitive function
• Serotonin (the happy hormone), essential for mood regulation and sleep patterns
• GABA, the brain’s primary inhibitory transmitter responsible for calming neural activity down

Smaller brain structures, reduced connectivity, and compromised neurochemicals. These all contribute to the clinical presentations of ADHD.

Genes Made Me Do It

The role of genetics in ADHD has become increasingly apparent in recent years, with mounting evidence suggesting a strong genetic component to the disorder. Genes serve as the blueprint for our structure and function. However, the genes that build and operate the executive functions and its circuitry are different in the ADHD brain.

How is it different? They have too many copies of the same gene, known as a copy number variance. This alters the structure of a protein and how it will function. For example, if there are too many copies of DRD4 (a dopamine receptor gene), it results in a diminished sensitivity to dopamine. Because the dopamine signal is reduced, those with ADHD may engage in high risk/high reward behaviors to boost these neurotransmitter levels.

It’s worth mentioning that there is no “ADHD gene”, but rather an involvement of multiple genes that contribute to the diagnosis of the disorder. There are at least 20-45 genes that are attributed to ADHD. The greater the number of each gene expression, the greater the chances of an ADHD diagnosis.

Family studies have consistently shown that ADHD tends to run in the same family tree, with close relatives of individuals with ADHD being at a higher risk of developing the disorder themselves. In fact, ADHD heritability is high, exceeding that of intelligence and equal to that of height. However, recent discoveries are proposing that some cases of ADHD are not inherited, but rather a product of natural gene mutations within the individual.

While genetics accounts for the majority of ADHD etiology, external factors can also increase the likelihood of ADHD as they can impact typical brain development. These include premature birth/low birth weight, maternal stress during pregnancy, exposure to toxins such as lead or pesticides, or nutritional deficiencies during critical developmental periods.

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When you piece all these findings together, it paints a clear clinical picture of ADHD. The neurobiological differences in brain structure and function result in different behaviors from the norm. Therefore, the way the ADHD brain pays attention and takes in information is biologically different.

It’s important to note that just because your child may be genetically predisposed to ADHD, it doesn’t necessarily mean that those genes will express themselves to the point of a diagnosis. And even if they did, ADHD is among the most treatable disorders to date. Next week, we’ll get into effective methods of treatment in managing ADHD symptoms and what parents can do to optimize routines for the ADHD brain.

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