![]() Among other features, these neurons are born early in development and have less distinct axon and dendritic regions compared to neurons at the cortical surface ( Suárez-Solá et al., 2009). ![]() Second, humans have an unexpectedly high number of AcDs (8.86–13.23%) in the white matter compartment below the cortex, which largely contains bundles of axons intermixed with a population of scattered neurons. found that inhibitory interneurons do not follow the same trend as pyramidal cells: primates have similar numbers of AcD interneurons as other mammalian species. The careful quantification and systematic survey of pyramidal neurons from different species and brain regions also yielded three other observations. ![]() propose that humans and non-human primates have fewer AcDs because they already have other cellular specializations that can boost the strength of the electrical signal sent through pyramidal neurons. This is supported by prior studies showing that AcDs generate stronger and more frequent electrical spikes, and also require a lower threshold to trigger an action potential ( Thome et al., 2014 Kole and Brette, 2018). So, what might be the reason for humans and non-human primates having fewer AcDs? It is thought that AcDs enhance the electrical behavior of pyramidal neurons by allowing signals to bypass the soma and flow directly from the dendrite to the axon. Image credit: Adapted from Figure 2 in Wahle et al., 2022. Scale bars represent 25 micrometers for all panels. Fewer than ten examples of apical AcDs were identified in the rat, ferret and macaque tissue samples studied, and none were identified in the human tissue samples. It is very rare for the AcD to emerge from the apical dendrite: an example of this is indicated by the asterisk in panel C2. In most pyramidal neurons, the AcD emerges from the base of the soma. The site where the axons emerge is indicated by a large white arrow, with smaller black arrows highlighting the direction the axon takes through the neocortex. Tissue samples were stained using one or two of the following techniques: biocytin (black and white images), immunofluorescence (multi-color images), or the Golgi method (human auditory cortex only). Neurons with axon carrying dendrites (AcDs) in the rat visual cortex ( A1, A2), the cat visual cortex ( B1, B2), the ferret visual cortex ( C1, C2), the macaque premotor cortex ( D1, D2), and the human auditory cortex ( E1, E2). Moreover, AcDs were rarely found in the upper layers of the neocortex: these layers are thicker in non-human primates and humans, and are associated with complex behaviors and higher cortical functions. This revealed that the proportion of pyramidal neurons with an AcD was around 10–20% in non-primate mammals (rat, cat and ferret), but much lower (typically a few percent) in macaque monkeys and humans. used a range of histological techniques to compare the morphology and structure of pyramidal neurons in postmortem tissue samples extracted from six cortical areas at different stages of the animals’ development ( Figure 2). Now, in eLife, Petra Wahle from Ruhr University Bochum and co-workers in Germany, Austria and Spain report that the proportion of axon carrying dendrites (AcDs) varies between mammalian species and different areas of the brain ( Wahle et al., 2022). Researchers are particularly interested in features that only occur in humans and primates, as these may be associated with cognitive behaviors as well as neurological and psychiatric conditions. ![]() These kinds of morphological differences are important because they influence how individual neurons and neuronal groups compute information. ![]() These ‘axon carrying dendrites’ are unusual because the signals dendrites receive are usually processed in the soma before they are sent out via the axon to other neurons ( Förster, 2014). Image credit: Adapted from Figure 1 in Wahle et al., 2022 Scale bars represent 100 micrometers for panel A, and 25 micrometers for the other panels. In some neurons, however, the axon emerges from a dendrite ( C), or initially co-joins with a dendrite as it exits from the soma ( D). Most pyramidal neurons contain an axon (indicated by the white arrow) that exits from the base of the soma ( B). Branching off the base of the soma are several basal dendrites, and a single apical dendrite protrudes from its apex. ( A) The neocortex of an infant macaque monkey that has been stained with dyes that label the dendrites and axons (initial segment only) of pyramidal neurons (shown in green). ![]()
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