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.