P 301 E 2: Synapse Formation

Class 5

Neural Precursor Cells (NPCs)

• are born around ventricular zones

• They diffuse or “climb” along radial glia to reach their destination

  • Brian regions partially the cortex are highly stratified (i.e., different layers handle different things)

• NPCs are exposed to different signals and txn factors, changing their morphology and function

• NPCs have the ability to become ANY neuron or glia (translation applications

Growth Cone

• mass of moving protuberances that seek out target neurons

Part of Growth Cone: Microtubules

• support the axon itself, a hollow mass of alternating tubulin proteins, both skeleton and “railway” for vesicle transport

• provide some rigidity to axon

• also primary means of conducting larger materials like massive protein and vesicles

• motor proteins like kinesin carry substances to their target

Part of Growth Cone: F-actin

• narrow projections that stretch the growth cone and seek out signals

• do the “scouting” that probe the extracellular environment for directional cues

• composed of actin protrusions, which build up in dense networks

Part of Growth Cone: Lamellipodia

• meshwork of actin filaments, carries the axon mass

Synapse Maturation

• synapses broaden, cell adhesion molecules and receptors localize

Synaptic Pruning

• the weaker, unnecessary connections are “pruned” or removed, those mature that keep firing grow stronger

• What are some characteristics of a robust, mature synapse versus a “flimsy” one? What causes these changes

Four Types of Synapses

• Axo-dendritic

• Axo-somatic

• Axo-axonic

• Dendro-dendritic

Axo-Dendritic

• standard conduction of signals, good for modulation where many neurons may need to control a target

Axo-Somatic

• faster, graded potentials don’t have to travel as far

Axo-Axonic

• important for neurocomputation, effects release of NTs

Dendro-Dendritic

• good for binary effects, typically in more sensory systems

What is the importance of circuit pruning?

• The fourth stage of nervous system development, in which neurons that are unsuccessful in finding a place on the appropriate target cell, or that arrive late, die and excess synapses are eliminated

How does a growth cone know where to go?

• attractive and repulsive signals manipulate actin levels in the growth cone

• specific signals are attractive to neurons while others are repulsive

• attractive signals stimulate more actin recruitment, extending the filopodia

• repulsive signals break-up actin filaments, causing them to break down

• has a sort of “turning effect” on the growth cone

Synapse: Adhesion Molecules

• like cadherins, neuroligin, and neurexin make sure the synapse stays in place

Synapse: Actin

• can continue to “shape” the buttons to maximize exposure and physical coherence

Synapse: Receptors

• receptors are recruited, the more stable/strong the synapse the greater the density of the post-synaptic receptors