In days gone by, Neuroanatomy textbooks had drawings of nerve cells showing myelin, an insulating material, wrapped around the cell’s axonal process all the way along its length and spaced at regular intervals. Now, using two massive databases containing electron microscopy data, scientists have been able to reconstruct high-resolution maps of the pattern of myelination on the axons of neurons in the mouse neo-cortex. Their discovery challenges the accepted teachings of traditional neuroanatomy. The research reported on today was conducted at the Harvard Stem Cell Institute (HSCI) and Harvard’s Department of Molecular and Cellular Biology. The research team is headed by Dr. Paola Arlotta of HSCI, and Dr. Jeff Lichtman. Their work is published in the latest issue of Science magazine – one of the most prestigious of the peer-reviewed journals.
Myelin is tremendously important for neural transmission; loss of myelin is seen in several serious pathologies, including schizophrenia and multiple sclerosis. Myelin has been understood for years to be important for speeding the transmission of neural impulses along the axon, at rates that are faster than impulses traveling along non-myelinated axons. For 160 years, neuroscientists believed that the distribution of myelin was identical on every neuron, along the entire length of the axon, throughout the entire central nervous system. The discovery reported on today challenges the accepted teachings of neuroanatomy by demonstrating that patterns of myelination can be different on different neurons.
More specifically, the study showed that some neurons in the neo-cortex (i.e. the more recently-evolved region of the brain), have less myelin than neurons in deeper regions of the brain, and the myelin they do have is “intermittent,” with long axonal processes that lack myelin alternating with myelin-rich axonal tracts. It is thought that the newly discovered pattern of myelination enables neurons to accomplish higher-level information processing. The researchers speculate that because it is the most evolved brain neurons that exhibit the alternative pattern of myelination, it is possible that this new pattern may represent the “future” of neuroanatomy; perhaps the newer pattern of myelination functions to process increasingly complex information. According to Dr Tomassy, the intermittent spacing of myelination may enable synaptic connections along the axon, allowing a higher level of communication and regulation between neurons.
Neurons are exquisitely fine-tuned to transmit electrical impulses, allowing for extremely complex neuronal signaling. Over the years, neuroscientific discoveries have provided an increasingly intricate picture of neurons, and how they process information. Early on, it was taught that synapses – the tiny spaces between neurons across which neurotransmitters flow – only connected the axon of one neuron to the dendrites of the receiving neuron (axodendritic connections). It is now known that axons also synapse on the cell body of the receiving neuron (axosomatic connections); on axons of the receiving neuron (axoaxonal connections); and can even feedback and synapse on themselves. Until the discovery of bipolar neurons, it was supposed that all neurons only had a single axon. More complexity was introduced when it was found that neurons often manufactured more than a single type of neurotransmitter; yet more when it was found that neurons were not confined to expressing a single type of neurotransmitter receptor. Although neurotransmitters are generally supposed to bind to receptors in the membrane of the receiving (post-synaptic) neuron to open ion channels in the post-synaptic neuron, there are also transmitters that, upon binding to post-synaptic receptors, result in a signaling cascade that can alter the pattern of genetic expression in the receiving neuron. Essentially, the message here is if a complexity can be introduced, it is likely to be found. Throughout the history of neuroanatomy, there have been regular discoveries that challenge the accepted teachings. Today’s report adds yet another dimension to an already convoluted story.
By Laura Prendergast