Autism physiology explores the biological functions and processes of Autism Spectrum Disorder (ASD). This field moves beyond behavioral observations to investigate the underlying mechanics of the body and brain, providing a more complete picture of the factors contributing to the diverse experiences of autistic individuals.
Brain Structure and Connectivity
The development of the autistic brain follows a distinct trajectory, beginning with a period of accelerated growth in early life. This rapid expansion is then followed by a phase of slowed or arrested growth later in childhood. This atypical growth pattern occurs when brain circuits are forming, potentially disrupting the establishment of mature neural networks.
Structural variations are observed in specific brain regions. The amygdala, an area for processing emotions, shows inconsistent findings but may be enlarged in early development before its size normalizes over time. These differences may relate to how emotions and social cues are processed. The cerebellum, known for motor control, also contributes to cognition and social interaction, and autistic individuals often show a reduction in tissue in certain areas.
Beyond the size of specific regions, the way different parts of the brain communicate is a significant area of study. A leading model suggests the autistic brain has a unique pattern of connectivity. This pattern is described as having long-range under-connectivity, with weaker connections between distant brain regions, and local over-connectivity, where neurons within a specific area have an unusually high number of connections.
This connectivity pattern can be compared to a city’s road system with an overabundance of local streets but not enough major highways connecting different neighborhoods. This setup would make travel across the city difficult. Similarly, this brain connectivity pattern may affect the integration of information from different brain areas, influencing higher-order functions like social communication.
Neurotransmitter and Chemical Signaling
The brain’s function relies on chemical messengers called neurotransmitters, which transmit signals between nerve cells. In autism, research has pointed to differences in the balance of these chemical signals. One theory focuses on the relationship between excitatory and inhibitory neurotransmission, involving glutamate (the primary excitatory signal) and gamma-aminobutyric acid (GABA) (the main inhibitory signal).
The “excitatory/inhibitory (E/I) imbalance theory” suggests that in some autistic individuals, the brain’s signaling environment may be more excitatory, due to increased glutamate activity or reduced GABAergic signaling. This state of heightened neural excitability could help explain differences in sensory processing, such as hypersensitivity to sound, light, or touch.
Studies using magnetic resonance spectroscopy (MRS) have investigated the levels of these neurotransmitters, though findings can be inconsistent. Some research has found reduced levels of GABA or a decreased GABA-to-glutamate ratio in specific areas like the frontal and occipital lobes. This chemical imbalance is theorized to disrupt the brain’s ability to filter information efficiently and maintain stable neural circuits.
Other chemical messengers, known as neuromodulators, also play a part. Serotonin is one such modulator involved in regulating mood, sleep, and social behavior. Research has found that a significant portion of autistic individuals have elevated levels of serotonin in their blood. In the brain, however, serotonin levels or the function of its transporter protein may be altered, which could influence repetitive behaviors and social functioning.
Genetic Foundations
Autism is highly heritable, but there is no single “autism gene.” Instead, its development is polygenic, influenced by the combined effects of many genes and environmental factors. Common genetic variants, widespread in the general population, account for a substantial portion of this heritability.
These common variants each contribute a small amount to the likelihood of developing autism. When an individual inherits a specific combination of these variants, their cumulative effect can surpass a certain threshold, leading to the neurological differences associated with the condition. This polygenic model helps explain why autism often runs in families.
In addition to inherited genes, another type of genetic influence comes from de novo mutations. These are spontaneous genetic changes that appear for the first time in an individual and are not present in the parents’ DNA. While de novo mutations are rare, they can have a significant impact and are more frequently found in autistic individuals.
The presence of de novo mutations helps to explain cases where autism appears in a family with no prior history of the condition. These spontaneous changes can occur in hundreds of different genes involved in brain development and synaptic function. The interplay between inherited common variants and rare de novo mutations creates a complex genetic architecture.
Systemic Physiological Variations
The physiological characteristics of autism extend beyond the brain, affecting other interconnected body systems. A notable area of research is the gut-brain axis, the bidirectional communication between the central nervous system and the gastrointestinal tract. Gastrointestinal issues are frequently reported in autistic individuals, often accompanied by differences in the gut microbiota.
Studies have identified distinct compositions of gut bacteria in autistic individuals. Since the gut microbiome influences the production of neurotransmitters, including a significant amount of the body’s serotonin, an imbalance in these microbes could affect brain chemistry and function. This connection suggests that gut health and brain processes are closely linked.
Immune system dysregulation is another systemic factor observed in autism research. This can manifest as neuroinflammation, an inflammatory response within the brain or spinal cord. Studies have found elevated levels of immune molecules called cytokines in the brains and blood of some autistic individuals, which can contribute to a chronic inflammatory state.
One theory related to early development is Maternal Immune Activation (MIA), which proposes that a significant immune response in a pregnant person could affect fetal neurodevelopment. Furthermore, some research points to differences in metabolism related to mitochondria, the energy-producing structures within cells. Evidence of mitochondrial dysfunction has been reported in a subset of autistic individuals, which could impact the high energy demands of the developing brain.