Lecture 7: Evolution of Myelin Flashcards
What is myelin (3)
-Myelin speeds up conduction velocity of action potentials
-gnathosome vertebrates have myelinated axons in their nervous systems (but are absent from agnatha)
-Myelin is not consistently present in invertebrates but has evolved independently several times
Benefits of speeding up conduction velocity (7)
-Speeds up AP’s leading to decreased reaction times (better for avoiding/catching prey)
-More accurate timing
-less cross talk (electrical interference) between adjacent axons
-Better motor coordination
-better learning
-higher intelligence
-allows for organisms to be larger
Myelinated vs giant axons (4)
-Because of their effiency, myelin allows for smaller axons
-Myelinated axons use less energy because they prevent leaks
-giant axons are simpler to build and still get the job done tho
-analogy: Giant axons are an SUV, myelinated axons are a Prius
The proteins of myelin (5)
-The proteins PLP or P0 (“p not”) are required for myelin
-Earliest bilateral animals express the precursor genes for PLP
-there are a few inverts with “myelin-like” coverings based on this ancestral PLP proteins
_spiders, insects, lobster, and mollusks have elaborate connective tissue surrounding their axons that is also “myelin-like”
-p0 is observed only in deuterostomes (chordates, echinoderms, etc.)
Evolution of myelin (2)
-In early Jawed vertebrates (Fish and Sharks), P0 evolved to become the main myelin protein and was found both in the Central nervous system (CNS) and peripheral nervous system (PNS)
-But in tetrapods, there was a significant evolution of the PLP gene, where P0 still formed the PNS, but the CNS was now formed by PLP
Myelin structure in the CNS of tetrapods (3)
-PLP is produced by oligodendrocytes
-PLP holds layers together but not well understood how
-MBP aids in the compaction of layers
myelin structure in PNS of tetrapods (3)
-P0 is produced by Schwan cells
- P0 acts like a zipper to compact the myelin and hold the layers together
-MBP aids in compaction of layers
why did PLP take over form p0? (4)
-only theories but
-PLP takes up less space than p0
-better support of axons, allowing them to “live” longer
-may be an example of “silent dropout” where the more efficient PLP simply replaced p0 due to it becoming irrelevant
How does myelin work? (3)
-in a nonmylenated axon, Ions that form AP’s move forward but also leak out across the axon
-myelin insulates the axon, preventing leakage and therefore speeding up condction velocity as the ions are forced down the axon (think like water in a pipe full of holes vs no holes)
-Ions can still leak out of myelinated axons through gaps in the myelin called “nodes”
vertebrate myelin vs invertebrate myelin (2 and 3)
Vertebrate myelin:
-Always compact
-Always wrapped in a continuous spiral forming multiple layers (cinnamon role style)
invertebrate myelin:
-can be concentric (multiple complete “rings”) or spiral
-may have seams (especially if in ring formation)
-not at all compact
