Origin of copepod myelin

(last revision 2011-02-18)

How does copepod myelin differ from that of vertebrates?

What are the functional and structural features of copepod myelin?

Several distinct features distinguish copepod myelin from that of vertebrates:
  1. Concentric layers of compacted membrane
  2. Lack of "seams" or junction points at cell margins in the myelin layers (in contrast to shrimp myelin)
  3. Breaks in the sheath at intervals that are not necessarily circumferential as in the vertebrate "nodes of Ranvier."
  4. Derivation from the nerve cell itself, not glial cells as in vertebrates, shrimp and annelids.
However, copepod myelin functions in exactly the same way as does that of vertebrates. Its functionally significant features include:
  1. Multiple layers of membrane surounding an axon. These reduce the electrical capacitance and increase of resistance between the interior and the exterior of a nerve fiber.
  2. "Node" -- breaks in the sheath -- just not the vetebrate form
  3. The occurrence of the necessary molecular components for nerve impulse generation and recovery at the exposed axon membrane.
  4. Mechanisms that prevent current leak and short circuiting of the insulating properties of the sheath.

How does copepod myelin work?

Essentially the same way as vertebrate myelin. Current from a nerve impulse enters the fiber at a node, and flows with relative ease along the axon interior to the next node, assisted by the insulating qualities of the sheath. See the description on the previous page.

What is the source of copepod myelin?

Vertebrate myelin derives from the elaboration of sheets of membrane that are wrapped spirally around an axon by a special glial cell termed a "Schwann cell" in the peripheral nervous system (the nerves to the muscles and organs of the body) and an "oligodendrocyte" in the central nevoyus system (the brain and spinal cord). That of coepods derives from the nerve cell itself. Exactly how this works is not yet known. The figure below shows several possible alternative scenarios:
  1. : Myelin internal to the axon might derive from and be extended by intracellular membrane sources such as coalescence of vesicles (e.g. from endoplasmic reticulum, as in vertebrate Schwann cells), forming hollow tongues that elongate down the axon and also spread internally around the interior perimeter, ultimately encircling it. Multiple layers of tongues would form the membrane "stacks" observed in electron microscopic cross sections. The margins of the stacks would fuse to form complete myelin composed of concentric layers. Of several possible variants, on the left is depicted a sequence with the growing margins staggered, as might occur with the addition of layers in small increments. Alternatively, membrane might be added at a fixed location with migration of material within the myelin layers (vesicle profiles on the right).
  2. : Myelin could be derived from the invagination of the surface of the axon ("axolemma"), with access to extracellular medium remaining. Continuity between axolemma and the myelin tongues has not been observed.
  3. : Myelin tongues might derive from axonal invagination as in 2, but ultimately pinch off either directly (as shown) or indirectly as "pinocytotic" vesicles, isolating the internal cisternae from the external medium and limiting penetration by extracellular markers.
  4. : Instead of coming directly from the outer surface of an axon, the myelin tongues in a stack might derive from adjacent tongues. Continuity between adjacent layers has not been observed, but pinch-off as in 3 might occur.
  5. : While unluikely, myelin might still derive from glial membrane, although this is stretching the logical point. In such a scenario, glial cytoplasm would have to be eliminated to orm a myelin layer, and the layer would have to be precisely aligned with axonal structures to present the appearance of a seamless intra-axonal hollow tongue. No evidence for such glial involvement has been observed.

    Why do copepods need myelin?

    To escape their predators by giving them rapid raeaction times made possible by fast conduction of nerve impulses along nerve fibers. For more details, click here.

    See also Myelin Evolution page

    References (additional references on the preceding page )
    1. Davis, A.D.
    2. Wilson, C.H. and D.K. Hartline (2011) "The novel organization and development of copepod myelin. I. Ontogeny" J. Comp. Neurol. (submitted)
    3. Wilson, C.H. and D.K. Hartline (2011) "The novel organization and development of copepod myelin. I. Ontogeny" J. Comp. Neurol. (submitted)