(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. Suggestions for the advantages include:
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):
- 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 (Zalc and Colman 2000)
- Increases temporal precision
- Enhances precision in event timing by reducing the absolute travel time and thus absolute temporal variability in communication between two points.
- Permits greater precision in spatial localization of stimuli (Wilson & Hartline 2011)(this may be especially useful for localizing sounds through binaural comparison, and might be expected to improve success in evading predatory attacks as well as be an advantage for auditory predators such as owls).
- Enables better synchronization of spatially-distributed targets
- Synchronizes different regions of a muscle sheet or synergistic byt spatially disperssed muscles
- Provides shorter delays in feedback loops
- Increases intrinsic stability of feedback loops
- Increases the frequency response of neural systems
- Provides faster communication between brain and distant body parts
- Enables larger organisms (Zalc and Colman 2000)
- Enables better positioning of sensory organs (Zalc, Goujet and Colman, 2008, 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 an energy-hungry 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 to other uses. Were we an amyelinate species, we would have greatly diminished problems with being overweight :-)!
- It may promote the ability of organisms to withstand hypoxic (low oxygen) conditions (Lenz 2012)
- It may enhance an organism's ability to withstand periods of starvation (Lenz 2012)
- 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 engaged in massively parallel computation.
- To attain the same inter-hemispheric travel time for nerve impulses using unmyelinated axons would require scaling up brain dimensions over 100-fold.
- Reducing currents surrounding myelinated fibers, which reduces the "cross-talk" between adjacent fibers, permitting closer associations without requiring special arrangement to decrease such potentially disruptive interactions.
- Reducing the capacitance between inside and outside of a nerve cell making it easier for the cell's electrical signaling to vary rapidly, so reactions to changes in sensory or synaptic inputs can occur more rapidly -- not just conduction is speeded.
- 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
Lenz, P.H. (2012)
The biogeography and ecology of myelin in marine copepods.
J. Plankton Res. 34: 575-589.
Wilson, C. and D.K. Hartline (2011)
The novel organization and development of copepod myelin. I. Ontogeny
J. Comp. Neurol. 519: 3259–3280
Zalc, B. (2006)
The acquisition of myelin: a success story.
In Chadwick, D.J. and Goode, J., eds Purinergic Signalling in Neuron-Glia
Interactions, No. 276 Wiley, Chichester. pp 15-25.
Zalc, B. and D.R. Colman (2000)
Origins of vertebrate success
Science 288(5464) 271-272
Zalc, B., D. Goujet and D.R. Colman (2008)
The origin of the myelination program in vertebrates.
Curr. Biol. 18(12): R511-R512
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.