The feeling of sensory overload is one of the most disturbing effects of the autism spectrum disorder (ASD, autism) — sounds seem too loud, light too bright, movement too swift, aromas and flavors too vivid for comfort. Autistic children are thought to withdraw physically or emotionally inward to put some distance between themselves and the amped-up world they perceive. Recent research on a protein known as having to offer insights into how the brain develops structurally and how the protein’s absence could lead to an autistic child’s feelings of too much stimulation from the outside world.
The brain experiences a tremendous growth spurt in the first few years of life. By the time it’s 3 years old, a baby’s brain will have grown to 82% of the size it will be when the child reaches adulthood. By 5, the brain is 90% of its adult size. William Christopher Risher and Çagla Eroglu, of Duke University in Durham, North Carolina, studied the brains of newborn mice in search of the catalyst that ignites such rapid brain expansion.
The Duke research team used a state-of-the-art technology called three-dimensional electron microscopy to watch how the brain sculpts itself as the newborn brain develops. The brains of newborn mice were used in the study.
Sensory impulses — pain, sounds, sights, aromas, the urge to wiggle a toe or yawn, etc. — are conducted from one neuron (nerve cell) to the next by sending neurotransmitter chemicals or electrical impulses from one neuron’s synapses to the next. Neurons are not firmly connected one to the other as other cells are; synapses are the spaces from which a sensory impulse leaves one neuron and jumps to the next.
Historical understanding holds that each synapse serves as a receiving site for just one sensory impulse at a time but the Duke study of newborn neurons indicate synapses receive impulses from multiple sources at once. As the brain matures, synaptic activity becomes more controlled so that eventually each synapse does receive just one impulse at a time. As the synapses become quieter or specialized this way, the brain develops a larger and more complex neural network to accommodate and process the vast amount of new information a baby faces as it discovers its new world.
The Duke research team discovered the protein, hevin, plays a vital role in converting a developing synapse from receiving multiple impulses at once into a mature synapse that processes just one impulse at a time. Hevin is produced by astrocytes (non-neuronal cells in the brain). When the gene responsible for activation of astrocytes is absent or malfunctioning, hevin is not produced and the synaptic connections remain open to multiple impulses all the time.
Previous studies have linked hevin to depression, suicide, and autism but, until the Duke study, there was no understanding of how the protein affects the brain. In the case of autism, the sensory overload so many patients describe could be the result of multiple impulses being processed by one synapse all the time.
Source:
- Risher, W. Christopher., et al. "Astrocytes refine cortical connectivity at dendritic spines." eLife. eLife Sciences Publications Ltd., 17 Dec. 2014. Web. 15 Jan. 2015.
- "90% of a Child's Brain Develops in the First 5 Years of Life." First 5 California. First 5 California, n.d. Web. 15 Jan. 2015.