MID 3-DENDRITES

dendrites have their own…

growth cones

• appear as mini growth cones

dendrite growth cone behavior can be…

variable

• sometimes response to signals in reverse to axon growth cone

• when synapsing with axon required, responds identically to axon growth cone

complexity of dendrite branching dependent on…

amount of incoming innervation

• multi vs simple dendrite

• innervation acitivy levels also affect complexity

dendrite complexity is

VERY dynamic

• in mice, some dendrites can appear and disappear within minutes when exposed to stimuli

myelin in CNS

present as oligodendrocytes

myelin in PNS

present as schwann cells

myelin appears as..

white matter of CNS

• gray matter=not myelinated

myelin enables…

fast electrical conductance along axons

• electrical insulator

myelin electrical insulation…

accelerates conductance by localizing Na+ channels at short gaps between adjacent myeline sheaths = node of ranvier

myelin dynamic over…

throughout life

• CNS plasticity different from PNS plasticity

CNS: oligogenesis

oligodendrocyte precursor cells (OPCs) can

• undergo division to produce more

AND/OR

• differentiate into a post mitotic pre myelinating oligodendrocyte (PO)

CNS myelin plasticity

oligogenesis occurs thoruhgout life

• human adults, OPCs still active

• numbers peak during middle age

PNS: 2 schwann cell (SC) classes

1. myelinating SC

2. remak SC

myelinating SC

• produce and wrap axons in myelin sheaths

• nourish axons

remak SC

• non myelinating

• nourish axons

PNS: schwann cell development step 1

• nueral crest cells → schwann cell precursors (SCP)

PNS: schwann cell development step 2

• SCPs migrate bounded by associated axon

  • once associated, SCPs become dependent on that axon for survival

PNS: schwann cell development step 3

SCPs stop migrating = immature schwann cells (ISC)

• axon still growing

PNS: schwann cell development step 4

immature schwann cells (ISCs):

• figure out which axons to myelinate = radial sorting

• based on diameter of axon

  • big diameter = myeline

  • small diameter= no myelin

PNS: schwann cell development step 5

become self supporting and mature into myelinating schwann cells

PNS myeline plasticity: response to injury

• peripheral nerves can undergo regeneration

• PNS regeneration possible through SChwann cell plasticity

PNS myelin plasticity: response to injury caveats

1. effective only on minor injuries

• regeneration rate too slow to completely heal traumatic nerve damage

2. SCs competence diminishes with age

SC mediated PNS regeneration step 1

both myelinating and remak cells SCs undergo reprogramming

• myelinating de-myelinate

• both become repair SCs

SC mediated PNS regeneration step 2

newly programmed repair SCs remove damaged part of axon

• behave similarly to macrophages=wallerian degeneration

SC mediated PNS regeneration step 3

clear out damaged tissue

• produce trophic factors to nourish regrowing axon axon

SC mediated PNS regeneration step 4

repair SCs form scaffold to guide regrowing axon back to target

SC mediated PNS regeneration step 5

repair SCs proliferate; reprogram to turn on myelinating genes

SC mediated PNS regeneration step 6

newly reprogrammed myelinating SCs re myelinate axon