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Friday, August 21, 2009

Constantin von Economo's Spindle Cells & The Mind


Today we celebrate the birthday of Constantin von Economo. He was born in 1876. A comprehensive biography of his life entitled "Baron Constantin von Economo His Life and Work"can still be found as a used book. He was known as an avid aviator and a superb physician scientist. Elaborating on Korbinian Brodmann's ground-breaking maps of cerebral cortex, he meticulously charted his own divisions based on the composition of nerve cells seen in histological preparations. In this process, he identified a rare and special type of nerve cell that has intrigued neuroscientists to this day.

The cerebral cortex constitutes our brain's most prominent feature. It consists of a large convoluted sheet of nervous tissue separated at the mid-line into two hemispheres. Its gray matter can be divided into layers of nerve cells of varying shape and density with characteristic in- and output. Pyramidal cells are the predominant type of nerve cell providing output to nerve cells in other layers of the same area, other cortical areas as well as to nerve cells in subcortical structures and to the motor neurons in the spinal cord that innervate the musculature. The roughly triangular cell bodies of the majority of pyramidal cells are located in the deep layers of cortex near the white matter, that Ramon y Cajal defined as layers 5 and 6.

Pyramidal cells feature a distinct distribution of dendrites, that is the processes on which inputs from other nerve cells terminate. The apical dendrite points straight up. It traverses all cortical layers and ramifies in layer 1 under the cortical surface. In addition, a set of dendrites, known as basal dendrites, spreads out radially from the cell body, collecting input from the layer in which the cell body resides. The axon, that is the process conveying the nerve cell's output, emerges from the underside of the cell body. The axons of pyramidal cells are wrapped in sheaths of a fatty substance, known as myelin, for insulation and form the cortical white matter.

Clustered among the ubiquitous pyramidal cells, nerve cells of peculiar, strikingly different shape caught Constantin von Economo's eye. They were unique to only a select few areas in anterior cingulate and insular cortex. The cells possessed conspicuous large cell bodies. Like pyramidal cells, they featured a prominent straight dendrite spanning the cortical thickness and an axon projecting to distant brain structures,  but lacked basal dendrites, resulting in a distinctly bipolar appearance. Von Economo named them "Spindelzellen" or spindle cells in English.

In the past, spindle cells had only been found in humans and other great apes (Nimchinsky and others, 1999). The exclusivity has been taken to suggest that this type of nerve cell may be associated with the complex higher cognitive and affective mental functions that seem to distinguish us from the rest of animaldom, e.g. language, decision making and empathy. Spindle cells are known to be particularly prone to degenerate in people with Alzheimer's disease (Nimchinsky and others, 2004). Recently however, the cells have also been discovered in cetaceans (Marino and others, 2007) and pachyderms (Hakeem and others, 2009).

Little is known about the function of spindle cells. The distribution of their dendrites suggests that they collect input from other nerve cells along narrow radial columns across the cortical thickness, supporting the idea that the cerebral cortex is a functionally organized in discrete modules rather than in broad layers. Their long-distance projections suggest a role in a distributed neural network, processing information across sensory modalities. However, more research clearly needs to be carried out to examine their responsiveness to stimulation and the influences they exert on other nerve cells before any specific role can be assigned with certainty to Constantin von Economo's Spindelzellen.




Monday, August 3, 2009

The Elusive Sense of Magnetism

Pigeons were my constant companions in the early years of academic study. They descended on the university buildings in flocks, settling on the railings of the balconies that ran along the stories outside. They cooed and bantered outside, watching us study hard inside. At times, their habits were annoying. But I always liked to watch them take off in bunches, swaying from one side to the other and then lifting off up and away. The burghers of our city commonly considered pigeons a pest. Pest control was often called in to get rid of them.

However, this did not happen in our case. Our birds were homing pigeons. Wolfgang Wilschko, the most enthusiastic zoology professor I ever met, his wife and students were studying the birds' ability to use Earth's magnetic field for flight orientation. Other migratory birds have demonstrated this capability. However, how the magnetic sense actually works was little understood. The team transported marked birds to various locations in about a sixty mile radius in a battered, mouse-gray VW microbus on loan from the Deutsche Forschungsgemeinschaft, the German equivalent of the National Science Foundation. They recorded the time the pigeons needed to find back to their loft. Their travel time was compared to the geography they had to navigate, in particular whether magnetic field perturbations produced by intersecting power lines distracted them on their path, delaying their return.

Exposing the pigeons to artificial magnetic fields in the laboratory left them temporarily disoriented. Regardless, the birds commonly found their way back home. For this, they appeared to rely on the inclination, rather than the polarity, of geomagnetic field vectors. That is, the birds could discriminate between equatorial and polar directions of the magnetic field vectors for compass orientation and proved profoundly sensitive to disturbances in the field. Fine-grain local field aberrations may provide additional cues for the birds' navigation (Wilschko and Wilschko, 2005). Sensitivity to the geomagnetic field has been observed in a number of vertebrate animals other than birds (Lohmann and Johnsen, 2000). Begall and others (2008) showed that domestic cattle as well as free roaming deer align their bodies according to the magnetic poles.

Despite the formidable evidence for a magnetic sense, conclusive evidence for magnetoreceptor cells remains elusive to date. An early hypothesis posited that migrating birds could virtually see magnetic field polarity, detecting in the clear sky streaks of light of differing intensity polarized by the geomagnetic field.

Another hypothesis suggested that nerve cells containing minute deposits of biogenic magnetic minerals may detect magnetism. For example, the iron oxides magnetite (Fe3O4) and maghemite (Fe2O3) have been identified in trigeminal nerve fibers innervating the skin on pigeons' bills (Fleissner and others, 2007). In accord, preliminary evidence shows that trigeminal nerve fibers respond to changes in magnetic field strength of geomagnetic magnitude (Semm and Beason, 1990).

In a separate line of investigation, external magnetic fields of strengths comparable to those found in nature proved in principle capable of temporarily altering the concentration of short-lived pairs of radicals intermediate to photo-chemical reactions (Maeda and others, 2006). The interconversion of the pairs' spin states eventually determines the type of reaction product, the yield of which is proportionate to magnetic field strength and direction. Cryptochromes, that is blue light-sensitive flavoproteins, spatially-oriented in the photoreceptor cells of the avian retina are suggested as likely candidates to sustain such reaction (Rodgers and Hore, 2009). The magneto-sensitive process may modulate the birds' perception of light not unlike that originally proposed for magnetic field-related polarized light.

Taken together, much evidence suggests that migratory animals possess a keen sense for magnetic fields, using a "built-in" biological magnetic compass to orient themselves in their environments. Two fundamentally different molecular mechanisms are presently suggested to underlie this ability. One involves magnetic matter embedded in nerve cell fibers of the peripheral sense of touch. The other involves the formation of radical pairs in photoreceptors of the visual system. With either proposal, the precise mechanisms of stimulus reception, stimulus transduction and sensory pathway processing need yet to be uncovered.


Footnote
  • The text of the post is available for download in pdf-format from the scribd store.
Addenda
  • Perhaps this incident was caused by sudden strong local disturbances of the earth's magnetic field that the blackbirds use for navigation (01/05/11):
  • Putman and others (2011) provide behavioral evidence that also loggerhead turtle hatchlings use the Earth's magnetic field to determine latitude as much as longitude for navigation on their migrations. Listen to Joe Palca's segment with the title "For Turtles, Earth's Magnetism Is A Built-In GPS" broadcast today on National Public Radio's Morning Edition (03/02/11).
  • In a blow to the hypothesis that magnetic sensory receptor cells invest the avian beak, Treiber and others (2012) provide histological evidence that the previously implicated magnetite containing cells are iron-rich macrophages. Moreover, Wu and Dickman (2012) showed that nerve cell electrical spiking in the pigeon's vestibular brainstem known for processing information on balance and spatial orientation (equilibrioception) encodes magnetic field direction, intensity, and polarity, suggesting that magnetic receptor cells are located in the inner ear (05/31/2012).
References
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