Myelin Advantages

(and a few disadvantages)

Myelin affects the nervous system, and hence the physiological and behavioral capabilities of an organism, in many ways. The primary impact is two-fold: on conduction speed and on the metabolic costs of nerve impulses. Beyond these, however, the list grows as more studies are made on organisms possessing it:

Myelin speeds the conduction of nerve impulses by a factor of 10 compared to unmyelinated fibers of the same diameter.
Increased conduction speed increases the nervous system's information processing speed
Decreases reaction times to stimuli:
- Promotes the ability to escape from sudden predatory attack
- Promotes the ability to recognize and rapidly react to available prey
Increases temporal precision
- Permits more finely calibrated temporal responses
- Permits greater precision in spatial localization of stimuli
Enables better synchronization of spatially-distributed targets
- Synchronizes different regions of a muscle sheet
Provides shorter delays in feedback loops
Increase intrinsic stability of feedback loops
Increases the frequency response of neural systems

Provides faster communication between brain and distant body parts

Enables larger organisms
Enables better positioning of sensory organs (Zalc, Colman and __ found that fossils of putatively myelinate placoderms show evidence of longer optic pathways than do those of their amyelinate predecessors the ostracoderms, likely permitting better positioning of the eyes).

Provides a several hundred-fold improvement in metabolic efficiency for recouping the energy cost of nerve impulse traffic.

For a nervous system such as ours, which already accounts for an age-dependent 20-50% of the body's resting metabolic energy budget, this is not an inconsequential advantage, allowing the energy saved to be put oto other uses. Were we an amyelinate species, we would have greatly diminished problems with being overweight :-)!
It may promote te ability of organisms to withstand hypoxic (low oxygen) conditions

Provides economy of space. Its speed-up of impulses permits a trade-off with size that allows a much more compact nervous system for a given axonal conduction speed

Promoting nervous systems such as ours with large numbers of neurons and correspondingly (so we suppose) greater computing power.
To attain the same inter-hemispheric travel time for nerve impulses using unmyelinated axons would require scaling up brain dimensions over 100-fold.

Reduced currents surrounding myelinated fibers reduces the "cross-talk" between adjacent fibers, permitting closer associations without requiring special arrangement to decrease such potentially disruptive interactions. These advantages conferred by myelin provide clear sources of selective pressure for its evolutionary invention. Myelin has a few disadvantages as well, that may deter its evolution or indeed promote the loss of myelin in the evolution of some organisms (see Myelin Evolution pages):

It costs a significant amount in metabolic energy to producte the many layers of lipid-rich membrane that comprise myelin.
- This can be a particularly bothersome problem in environments such as the "oligotrophic" open ocean, which is distant from continent-based sources of nutrients.
Key biosynthetic resources required fo myelin may be limited for some organisms in some ecosystems
- Lipids required for myelin membranes may be less readily available from a diet where food quality is poor, and metabolically expensive to manufacture
- Cholesterol, which is not synthesized by protostomes (the most common invertebrates) and hence is an essential "vitamin" in their diets
This material has been assembled and presented as a public service by Dan Hartline, Bekesy Laboratory of Neurobiology, Pacific Biosciences Research Center, University of Hawaii at Manoa (danh at hawaii.edu). Opinions expressed here are those of the author and do not represent the positon or policies of the University or any funding agency.