pathophisiology
Autism affects the amygdala, cerebellum, and many other parts of the brain.
Unlike many other brain disorders such as Parkinson's, autism does not have a clear unifying mechanism at either the molecular, cellular, or systems level; it is not known whether autism is a few disorders caused by mutations converging on a few common molecular pathways, or is (like intellectual disability) a large set of disorders with diverse mechanisms. Autism appears to result from developmental factors that affect many or all functional brain systems, and to disturb the timing of brain development more than the final product.Neuroanatomical studies and the associations with teratogens strongly suggest that autism's mechanism includes alteration of brain development soon after conception. This anomaly appears to start a cascade of pathological events in the brain that are significantly influenced by environmental factors. Just after birth, the brains of autistic children tend to grow faster than usual, followed by normal orrelatively slower growth in childhood. It is not known whether early overgrowth occurs in all autistic children. It seems to be most prominent in brain areas underlying the development of higher cognitive specialization. Hypotheses for the cellular and molecular bases of pathological early overgrowth include the following:
An excess of neurons that causes local overconnectivity in key brain regions.
Disturbed neuronal migration during early gestation.
Unbalanced excitatory–inhibitory networks.
Abnormal formation of synapses and dendritic spines,for example, by modulation of the neurexin–neuroligin cell-adhesion system,or by poorly regulated synthesis of synaptic protein. Disrupted synaptic development may also contribute to epilepsy, which may explain why the two conditions are associated.
Interactions between the immune system and the nervous system begin early during the embryonic stage of life, and successful neurodevelopment depends on a balanced immune response. It is possible that aberrant immune activity during critical periods of neurodevelopment is part of the mechanism of some forms of ASD.Although some abnormalities in the immune system have been found inspecific subgroups of autistic individuals, it is not known whether these abnormalities are relevant to or secondary to autism's disease processes.[ As autoantibodies are found in conditions other than ASD, and are not always present in ASD, the relationship between immune disturbances and autism remains unclear and controversial.
The relationship of neurochemicals to autism is not well understood; several have been investigated, with the most evidence for the role of serotonin and of genetic differences in its transport. Others have pointed to a role for group I metabotropic glutamate receptors (mGluR) in the pathogenesis of one type of autism, Fragile X. Some data suggest an increase in several growth hormones; other data argue for diminished growth factors. Also, some inborn errors of metabolism are associated with autism but probably account for less than 5% of cases.
The mirror neuron system (MNS) theory of autism hypothesizes that distortion in the development of the MNS interferes with imitation and leads to autism's core features of social impairment and communication difficulties. The MNS operates when an animal performs an action or observes another animal perform the same action. The MNS may contribute to an individual's understanding of other people by enabling the modeling of their behavior via embodied simulation of their actions, intentions, and emotions. Several studies have tested this hypothesis by demonstrating structural abnormalities in MNS regions of individuals with ASD, delay in the activation in the core circuit for imitation in individuals with Asperger syndrome, and a correlation between reduced MNS activity and severity of the syndrome in children with ASD. However, individuals with autism also have abnormal brain activation in many circuits outside the MNS and the MNS theory does not explain the normal performance of autistic children on imitation tasks that involve a goal or object.
Autistic individuals tend to use different areas of the brain (yellow) for a movement task compared to a control group (blue).
ASD-related patterns of low function and aberrant activation in the brain differ depending on whether the brain is doing social or nonsocial tasks. In autism there is evidence for reduced functional connectivity of the default network, a large-scale brain network involved in social and emotional processing, with intact connectivity of the task-positive network, used in sustained attention and goal-directed thinking. In people with autism the two networks are not negatively correlated in time, suggesting an imbalance in toggling between the two networks, possibly reflecting a disturbance of self-referential thought.[94] A 2008 brain-imaging study found a specific pattern of signals in the cingulate cortex which differs in individuals with ASD.
The underconnectivity theory of autism hypothesizes that autism is marked by underfunctioning high-level neural connections and synchronization, along with an excess of low-level processes. Evidence for this theory has been found in functional neuroimaging studies on autistic individuals and by a brainwave study that suggested that adults with ASD have local overconnectivity in the cortex and weak functional connections between the frontal lobe and the rest of the cortex. Other evidence suggests the underconnectivity is mainly within each hemisphere of the cortex and that autism is a disorder of the association cortex.
From studies based on event-related potentials, transient changes to the brain's electrical activity in response to stimuli, there is considerable evidence for differences in autistic individuals with respect to attention, orientiation to auditory and visual stimuli, novelty detection, language and face processing, and information storage; several studies have found a preference for non-social stimuli. For example, magnetoencephalography studies have found evidence in autistic children of delayed responses in the brain's processing of auditory signals.
In the genetic area, relations have been found between autism and schizophrenia based on duplications and deletions of chromosomes; research showed that schizophrenia and autism are significantly more common in combination with 1q21.1 deletion syndrome. Research on autism/schizophrenia relations for chromosome 15 (15q13.3), chromosome 16 (16p13.1) and chromosome 17 (17p12) are inconclusive.
Autistic individuals tend to use different areas of the brain (yellow) for a movement task compared to a control group (blue). 2.Autism affects the amygdala, cerebellum, and many other parts of the brain.
