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Spindle neurons are a specific class of neurons that participate in signal transmission in the nervous system, and are characterized by a large spindle shaped soma, gradually tapering into a single apical dendrite (axon) in one direction, with only a single dendrite facing opposite. Whereas other types of neurons tend to have many dendrites, the polar shaped morphology of spindle neurons is unique, having only been found in two very restricted regions in the brains of hominids - the family of species comprising humans and great apes.
Function of spindle neurons
Spindle neurons are relatively enormous cells that refract waves of neural signals as they are transmitted in waves from one region of the brain to other regions. Spindle neurons have been implicated by scientists as having an important role in a myriad of cognitive abilities and disabilities generally unique to humans, ranging from savant perceptiveness and perfect pitch to dyslexia and autism. While rare in comparison to other neurons, spindle neurons are most abundant, and largest, in humans; they have only been found thus far in the anterior cingulate cortex (ACC) and the frontoinsular cortex of hominids.
Spindle cells appear to play a central role in the development of intelligent behavior and adaptive response to changing conditions and cognitive dissonance. They emerge postnatally and eventually become widely connected with diverse parts of the brain, evidencing their essential contributions to the superior capacity of hominids to focus on difficult problems. Evidence of the importantance of their role has been established through single-neuron recording, electrical stimulation, EEG, PET, fMRI, and lesion studies.
The observation that spindle neurons only occur in a highly significant group of animals (from a human point of view) has led to speculation that they are of great importance in human evolution and/or brain function. Their restriction to great apes leads to the conclusion that they developed no earlier than 15-20 million years ago, prior to the divergence of orangutans from the African great apes.
ACC spindle neurons
In 1999, Professor John Allman, a neuroscientist, and colleagues at the California Institute of Technology first published a report on spindle neurons found in the anterior cingulate cortex (ACC) of hominids, but not in any other species. Neuronal volumes of ACC spindle neurons were larger in humans and the gracile chimpanzees than the spindle neurons of the robust gorillas and orangutans.
Allman and his colleagues have delved beyond the level of brain infrastructure to investigate how spindle neurons function at the superstructural level, focusing on their role as 'air traffic controllers' for emotions. Allman's team reports that spindle neurons help channel neural signals from deep within the cortex to relatively distant parts of the brain.
Specifically, Allman's team found signals from the ACC are received in Brodmann's area 10, in the frontal polar cortex, where regulation of cognitive dissonance (disambiguation between alternatives) is thought to occur. According to Allman, this neural relay appears to convey motivation to act, and concerns the recognition of error. Self-control - and avoidance of error - is thus facilitated by the executive gatekeeping function of the ACC, as it regulates the interference patterns of neural signals between these two brain regions. In humans, intense emotion activates the anterior cingulate cortex, as it relays neural signals transmitted from the amygdala (a primary processing center for emotions) to the frontal cortex, perhaps by functioning as a sort of lens to focus the complex texture of neural signal interference patterns. The ACC is also active during demanding tasks requiring judgment and discrimination, and when errors are detected by an individual. During difficult tasks, or when experiencing intense love, anger, or lust, activation of the ACC increases. In brain imaging studies, the ACC has specifically been found to be active when mothers hear infants cry, underscoring its role in affording a heightened degree of social sensitivity - a cognitive function at which women are generally more adept than men.
The ACC is a relatively ancient cortical region, is involved with many autonomic functions, including motor and digestive functions, while also playing a role in the regulation of blood pressure and heart rate. Significant olfactory and gustatory capabilities of the ACC and frontoinsular cortex appear to have been usurped, during recent evolution, to serve enhanced roles related to higher cognition - ranging from planning and self awareness to role playing and deception. The diminished olfactory function of humans, compared to other primates, may be related to the fact that spindle cells located at crucial neural network hubs have only two dendrites rather than many, resulting in reduced neurological integration.
Frontoinsular spindle neurons
At a Society for Neuroscience meeting in 2003, Allman reported on spindle cells his team found in another brain region, the frontoinsular cortex, a region which appears to have undergone significant evolutionary adaptations in mankind - perhaps as recently as 100,000 years ago.
This frontoinsular cortex is closely connected to the insula, a region that is roughly the size of a thumb in each hemisphere of the human brain. The insula and frontoinsular cortex are part of the orbitofrontal cortex, wherein the elaborate circuitry associated with spatial awareness and the sense of touch are found, and where self awareness and the complexities of emotion are thought to be generated and experienced. Moreover, this region of the right hemisphere is crucial to navigation and perception of three dimensional rotations.
Spindle neuron concentrations
The largest number of ACC spindle neurons are found in humans, fewer in the gracile great apes; and fewest in the robust great apes. In both humans and bonobos they are often found in clusters of 3 to 6 neurons. In decreasing order of abundance, they are found in humans, chimpanzees, gorillas and orangutans. While total quantities of ACC spindle neurons were not reported by Allman in his seminal research report (as they were in a later report describing their presence in the frontoinsular cortex, below), his team's initial analysis of the ACC layer 5 in hominids revealed an average of ~9 spindle neurons per section for orangutans (rare, 0.6% of section cells), ~22 for gorillas (frequent, 2.3%), ~37 for chimpanzees (abundant, 3.8%), ~68 for bonobos (abundant/clusters, 4.8%), ~89 for humans (abundant/clusters, 5.6%).
All of the primates examined had more spindle cells in the frontoinsula of the right hemisphere than in the left. In contrast to the higher number of spindle cells found in the ACC of the gracile bonobos and chimpanzees, the number of frontoinsular spindle cells was far higher in the cortex of robust gorillas (no data for orangutans was given). An adult human had 82,855 such cells, a gorilla had 16,710, a bonobo had 2,159, and a chimpanzee had a mere 1,808 - despite the fact that chimpanzees and bonobos are great apes most closely related to humans.
Spindle neurons may develop abnormally in people with autistic disorders, and abnormalities may also be linked to several psychotic disorders characterized by distortions of reality and disturbances of thought and language and withdrawal from social contact, as well as schizophrenia and Alzheimer's disease, but research into these correlations is at a very early stage.
Abnormalities in the physiological activity and anatomy of the anterior cingulate cortex are present in most of the major neuropsychiatric disorders. Allman's team has reported reduced ACC size and metabolic activity in autistic patients, as revealed in structural MRI and PET studies, and activity of the ACC is also reduced in patients, mostly boys, diagnosed with attention deficit disorder, characterized by behavioral and learning disorders.
Studies in humans indicate spindle cells are especially vulnerable to degeneration in Alzheimer's disease, with a loss of approximately 60 percent of ACC spindle neurons.
- Esther A. Nimchinsky et. al., A neuronal morphologic type unique to humans and great apes, Proc. Nat. Acad. Sci. USA, Vol. 96, pp 5268-5273, April 1999.
- John Allman, et. al., Two Phylogenetic Specializations in the Human Brain, The Neuroscientist, Vol. 8, No. 4, pp 335-346, November 4, 2002.
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