It is widely know that the grey matter of the brain is grey because it is dense with cell bodies and capillaries. The white matter is almost entirely composed of lipid-based myelin, but there is also a little room in the grey matter for a few select axons to be at least partially myelinated. A group of well known researchers, mostly from Harvard and MIT, decided to look for possible patterns in the myelin found in cortical grey matter. Their Science published findings suggest that this dynamic balance struck up by each axon, somewhere between zero and full myelination, does not tip to the benefit of action potential speed alone. Instead, it follows a more subtle give and take between different kinds cells.
In looking down the length of an axon, longitudinally that is, each segment of myelin is separated by a node. The thickness of the myelin coat varies significantly from node to node. Presumably then, so does the speed and reliability of the spike propagated in that segment. The researchers suggest however, that it is more the phase and offset of these nodes that matters. The distance to first node in particular is important because it is here that the spike shape is first initiaillized. As Doug Fields points out in a perspective that accompanies the paper, spike shape (usually inconsequential in computational models) has important functional implications including the amount of transmitter released, the refractory period and the spike frequency.
Within the cortical grey, it is now known that the bare initial segment of the axon is irresistible to other cells. Their synaptic overtures are regularly accepted and also reciprocated by the axon’s own transmitter release from bare, noncanonical release sites. The researchers found that the length of the myelin-free axon initial segment had a graded distribution with the more superficially located pyramidal cells in the mouse cortex having longer “open” axon. In layer II/III bare stretches up to o 55 µm were evident. Via Know the brain, and its axons, by the clothes they wear.