Spindle cells are in large whales
Spindle cells, a type of cell previously thought to be found only in great apes, have been found in large whales. Spindle cells are also called Von Economo neurons.
What part of the brain?
In humans, the cells are in layer Vb of anterior cingulate gyrus (ACC) (Nimchinsky et al., 1995) and in the frontoinsular cortex (FI) (Allman et al 2005; Watson et al., 2006). In humpback whales, the cells are found in the homologous areas, and additionally in the polar portion of the frontal cortex. Also, in humpback whales, a few scattered spindle cells were found in various other parts of neocortex where they are not found in humans.
Which species?
The cells are thought to have evolved by convergent evolution in the great apes and the whales.
The cells have been found in humpback whales, sperm whales, and orcas. The cells are not present in smaller, yet very intelligent bottleneck dolphins. The cells had previously been found in great apes but not in any other primate or mammalian species (Nimchinsky et al., 1999) (humpback whales are mammals but had not been tested).
“This suggests that as shown previously in hominids (Nimchinsky et al., 1999), the presence of these cells is not necessarily related to high encephalization quotient, but rather to absolute brain size. Spindle cells are prevalent in species with the largest brains in primates and in cetaceans, but not in the cetaceans with the highest encephalization levels, as they were never seen in smaller-bodied, yet relatively larger-brained delphinids such as the bottlenose dolphin, the Pacific white-sided dolphin, or the tucuxi.”
However, “Their occurrence should, however, not be deemed a by-product of developmental consequences of increasing brain size (Finlay and Darlington, 1995; Finlay et al., 2001) …. in both cetaceans and hominids, spindle cells are a feature of the largest brains within the orders but not across, as many delphinids do not have spindle cells, yet their brains are substantially larger than the human brain.”
The spindle cells is “remarkable for its discrete distribution in a few, functionally related, cortical regions in a very restricted yet significant number of highly social species, all characterized by a relatively recent evolution, a slow maturation, a low reproduction rate and few offsprings (i.e., K-selective species), a very large brain and a large body size within their groups.”
What do they look like?
(only (b); (a) is a pyramidal cell)
“These neurons are characterized by a very elongate, tapering, large-size perikaryon mostly symmetric about its vertical and horizontal axes, and extensive apical and basal dendrites. Some dendrites may be truncated shortly after the perikaryon, ending in a brush-like pattern. They are usually lightly stained on Nissl preparations compared to the surrounding pyramidal neurons. These studies also demonstrated that the spindle cells represent a class of projection neurons that send an axon to the subcortical white matter and likely contribute to the connectivity of the prefrontal cortex and select subcortical centers”
Function?
“The function of spindle neurons is not understood.” They seem to be projection neurons that send an axon subcortically and possibly callosally (Nimchinsky et al., 1999; Allman et al 2005; Watson et al., 2006). “Current hypotheses based on available data in human state that spindle cells may provide an output from the anterior cingulate cortex and frontoinsular cortex to prefrontal and temporal association cortices involved in theory of mind. …. these neurons may be involved in the control of complex integrations involving emotions, vocalization control, facial expression, or autonomic function as well as regulation of visceral, olfactory, and gustatory functions, as well as visceral functions and alimentary behaviors.”.
NewScientist indicates that Hof’s theory is that these are “express trains” for long-distance connectivity: ““The velocity of the signal is faster, and they miss out junctions on the way,” says Hof. “They are like the ‘express trains’ of the nervous system” that bypass unnecessary connections, enabling us to instantly process and act on emotional cues during complex social interactions.”
One hypothesis is that they help with “rapid intuitive choice in complex social situations” (Allman et al 2005).
Links
- Science Daily article
- Interview with Hof
- AlphaPsy gives a concise summary of the sorts of speculation that spindle cells engender:
“When they were discovered in our line in 1999, they fueled a wealth of speculation about their putative role in breaking the modularity of mind and achieving intermodal integration, that sort of things. The more sobering idea that they may be nature’s way to cope with brain size and complexity while avoiding disrupting the brain’s connectivity is rarely raised (but I am simply a waiter, mind you, our cooks, the Neurophilosopher and the Neurocritic, know better). Their discovery in whales is exciting since these animals seem to show a high degree of tool use, cultural transmission and social complexity (but Monsieur, if I may make a remark, this is the bare minimum for any self-respecting stylish animal species nowadays).”
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