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Sept 19th.docx

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BIOL 4370

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Sept 19 th • AP in somatosensory system are slow compared to mechanical wires - axon is leaky: some of K can go out (via leaky channels) instead of being pushed/diffused (low resistance Rm) - axon is sticky: when K+ tries to go down axon, they may be attracted and “stick” to negative ions through the membrane (high membrane capacitance Cm) - axon is thin: diameter of axon is small and increases resistance slowing K to move (high axoplasmic resistance Ra) • Invertebrates evolved wide axons to speed up Aps (low axoplasmic resitance) - disadvantage: not going to reach threshold as fast - disadvantage: limits number of axons invertebrates can have • Vertebrates evolved myelin (lipid bi-layer) to reduce leakiness and stickiness - where this isn’t myelin is called “nodes of ranvier” - acts as an insulation, covers up holes (leaky channels) and prevents positive and negative ions from attracting to each other by increasing distance - in nodes of ranvier there is high concentration of ion channels - salutatory conduction: potassium diffuses really quickly (as a result of myelin), and when it gets to next node there is new AP causing short (1ms) pause. “regenerate signal at each node” • Glial cells myelinate axons in CNS. - oligodendrocytes myelinate in CNS (wrap themselves around axon multiple times) - schwann cells myelinate in PNS (wrap themselves around axon multiple times) • We have different diameters in axons - c-fibers (c-axons) are very thing and conduct very slowly. They are for pain • Multiple sclerosis damage axons in NS - autoimmune disorder - liberation procedure proven to not work • Guillain-Barre Syndrome damages axons in PNS - autoimmune disease, rare, - can go away •
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