Usually, people associate more neurons or more brain activity with positive outcomes such as being smarter. This is a theme commonly portrayed in Hollywood especially in the movie Limitless. The sad relativity is that our brains are intricately made to have the perfect number of neurons with too many or too few causing serious detrimental effects. During development, normal brains undergo neuronal pruning which is the process of destroying unnecessary neurons that clog up signaling pathways. This can involve the destruction of an entire brain cell or just the elimination of cell connections called synapses. A synapse is a place where an axon, or end of one neuron, comes into contact with a dendritic spine, or small projection, on another as seen in Fig 1.
Fig. 1. An axon
Neuronal pruning is crucial for proper brain operation and is unfortunately dysfunctional in people with autism spectrum disorders (ASD). Without pruning, ASD patients have an overabundance of the number neurons and their synapses which leads to cortical thickening and interferences in cell to cell communication. This leads to detrimental effects for learning social behaviors and inefficient processing of external stimuli. But what causes this dysfunction? While the symptoms have been thoroughly examined, the direct causes of autism are still not completely understood. However, multiple mechanisms and defective neuronal pathways have been identified and have made monumental progress in the past decade. The most relevant theories for why neuronal pruning/neuron communication is dysfunctional in ASD patients are autophagy and the mTOR pathway and neuroinflammation.
Autophagy and mTOR Pathway:
Autophagy is literally translated as “self-eat” and is the process in which cells degrade excess or damaged intracellular material. Autophagy is responsible for the degradation of excess synapses in neuronal pruning and is inhibited by the mTOR pathway. In ASD patients the mTOR pathway is abnormally overactive leading to the suppression of autophagy and the continued survival of unnecessary synapses. The direct cause of why the mTOR pathway is overactive is not completely known but it can be correlated to neuroinflammation.
Neuroinflammation:
Up to 60% of people with ASD have a systemic immune dysfunction meaning that immune regulation and neuronal communication are closely intertwined. Glial cells and astrocytes are two common cells found in the Central nervous system that also have various immune system functions. It has been noted that these cells have an increased neuro-inflammation activity in ASD brains contributing to the disruption of neuronal communication. Helper T-cells are also abnormally low in quantity making it hard for the brain to understand when to stop an immune response, like inflammation in the brain. The two processes of chronic inflammation and the inhibition of Autophagy are two major causes of ineffective brain communication.
The Importance of Correct Brain Communication:
The brain has an incredible ability to physically change its neuronal connections in order to induce learning, this is called synaptic plasticity. If a certain pathway in the brain is being used frequently, the brain will enhance its synapses and make the neurons more sensitive to stimulation. On the other hand, if a pathway is not being used, it will degrade the connections and make room for the creation of new pathways. This process is what is being disrupted in ASD creating a stress on the brain when located in new environments, locations, or situations.
These dysfunctional pathways are also more abundant in the frontal cortex which means that behavior and decision making would be the most affected. This gives rise to the common symptoms of ASD such as learning disabilities, inability to read others emotions, and compulsive behavior. In recap, more is not always beneficial when it comes to brain functioning. Also, autophagy and neuroinflammation are crucial components of the ASD pathology and are directly related to the overabundance of neural connections. Future research and investigation into these processes hold promise for the future of ASD.
Sources:
https://www.sciencedirect.com/science/article/pii/S0898656814002848
https://www.biooncology.com/pathways/cancer-tumor-targets/pi3k/mtor.html
https://moodle.cord.edu/pluginfile.php/723245/mod_resource/content/0/pathophys%20of%20ASD%202017.pdf
