Lectures - Nervous tissue part 2

most post natal nervous tissue growth is the result of...

"growth" of neurites, especially axons

how are axons formed

neurite of post mitotic neuron adjacent to basal lamina signaled to become axon

how are axons signaled to exit neural tube

morphogens signal axons to leave as layers form in the neural tube

what do axons have to pass through to exit the neural tube

the basal lamina

how do axons grow

axon growth cones

where is the axon growth cone formed

the tip of the axon diffs into autonomous growth cone

what makes up the axon growth cone

cytoplasmic "suitcase" things needed for protein synth

actin molcs and microtubules

filipodia of growth

what is included in the cytoplasmic "suitcase" of the axon growth cone

includes cytoplasmic proteins like ribosomes and mRNA

what does the the cytoplasmic "suitcase" of the axon growth cone enable

enables axon survival, elongation, and navigation proteins

emancipates growth cone from needing to wait for proteins from soma

what does the filipodia of the axon growth cone allow for

constant pull and causes axon to elongate

what control the movement of the filipodia of the axon growth cone

what signals the filipodia encounters

what are some signals that a filipodia of an axon growth cone can encounter

attractant/adhesion molcs

repellent molcs

physical/mechanical barriers

what is an example of an attractant/adhesion molc signal for a filipodia of an axon growth cone can encounter

CAMs (cell adhesion molcs)

what is an example of an physical/mechanical barriers signal for a filipodia of an axon growth cone can encounter

stiff cells (cells with collagen)

define pioneer axons

1st axons to exit the neural tube or grow into a new region

what do pioneer axons have

hyperactive growth cones to move and act fast

how do pioneer axons move/know where to go

follow regional signals and forge pathway to correct target tissues

what do pioneer axons produce

produce region specific CAMs and incorporate into axon mem along the way

what do the region specific CAMs produced by pioneer axons do

attract and guide follower axons

cause follower axons to adhere

where do PNS neurons come from

neural crest cells

how do neural crest cells form

epiblast cells at margins of closing neural tube under go det to form future neural crest cells

describe neural crest cells

pluripotent-like, multipotent cells that migrate throughout body in patterns, can diff into meso or ectoderm

ectodermal cells become what in the PNS

cells of the PNS

mesodermal cells become

mesenchyme

describe the growing axon behavior steps toward synapse

1. moves to intermediate target following attractant signals

2. reaches intermediate target

3. signal that attracted become repulsive after leaving intermediate

4. at final target growth cone branches and each branch diffs into synapse

what does a commissural neuron have to do

has to cross the midline intermediate zone

how does the PNS grow

1. starts with pioneer axons

2. each follower axons produce CAMs to attract, guide and adhere more follower axons

describe the organization of a PNS nerve

single axon → axon bundle → fascicle → nerve

how are PNS axon bundles held together

by layers of connective tissue

what are the layers of PNS axon bundles

endoneurium

perineurium

epineurium

describe the endoneurium

wraps around a single neurons axon, its glial cells, and capillaries

describe the perineurium

wraps around bundles of endoneurium wrapped axons, blood vessels, and adipose tissue to form a fasicicle

describe the epineurium

wraps around bundles of fascicles, blood vessels, and adipose tissue to form a nerve

once synapsed, what happens

fine tuning occurs in multiple ways

what are some ways that an axon will fine tune

single axon will shift connections

respond to activity between axon and target

respond to growth of organism

describe the shifting connections of synapse fine tuning

send out new branches with target are to reinforce connection

allow existing branches to regress if communication is ineffective

describe the response to activity between axon and target of synapse fine tuning

use it or lose

use it a lot, recruit more

describe the response to organismal growth of synapse fine tuning

as organism grows, physical distance to ganglia will change

axon growth shifts to interstitial and is no longer growth cone driven