Unit 2: The Developing Brain

What are the three stages of neuron development?

Induction, proliferation, migration

Neural Induction / proliferation

The mesoderm (layer formed in embryonic development) releases signaling molecules that turn on and off certain genes, triggering some ectoderm cells to become nerve tissue. Further signaling interactions refine the nerve tissue into basic neurons or glia (support cells).

Describe how the sonic hedgehog signaling molecule determines the differentiation of nerve cells.

The proximity of cells to the signaling molecules determines their fate. Cells closest to the secretion of sonic hedgehog become glia cells, slightly farther will become motor neurons (control muscles), and the farthest will become interneurons (relay messages to other neurons).

Migration

The movement of neurons to the proper position in the brain. The ectoderm thickens and builds up in the middle to form the forebrain, midbrain, and hindbrain. Neurons move from the neural tube’s (built up matter) ventricular zone (inner surface) to the marginal zone (outer surface).

Describe glia as a guiding mechanism during migration.

Ushers neurons to their final destination through radial migration. Takes place in an “inside out manner,” where older cells form the deepest layer of the cortex and the younger cells form the outer layer of the cortex.

How do neurons become interconnected?

Through the growth of axons and dendrites.

Growth cones

Enlargements on the axon’s tip, actively explore the environment as they locate their precise destination. Special molecules can help guide growth cones (ex. netrin, semaphorin), the binding of particular signals to the growth cone receptors can dictate direction and movement.

How does the synapse differentiate once contact occurs?

Small portion of axon that contacts the dendrite becomes specialized for the release of neurotransmitters, small portion of dendrite becomes specialized to receive and respond to the signal. Special molecules pass between sending and receiving cells to ensure connection. Other molecules coordinate the maturation of the synapse in order to adapt to changes in our body and behavior. Signals also determines the type of. neurotransmitters that a neuron will use to communicate with other cells.

Myelination

The wrapping of axons by extensions of glia, increases the speed at which signals are sent form one neuron to another. Occurs throughout the lifespan.

Nodes of Ranvier

Gaps in the myelin sheath that the electrical signal jumps over, moving faster over the insulated portions.

Paring back of the neural network

The elimination of neurons through apoptosis to create a more efficient system. Apoptosis is activated when a neuron loses its battle with other neurons to receive trophic factors (life sustaining chemical signals).

Critical periods

Periods characterized by high learning rates and enduring consequences for neuronal connectivity. Paring down of connections occurs during these periods. Connections diminish in number but the remaining ones are stronger and more precise. Turn into sensory, motor, or cognitive maps.

Plasticity

The ability of the brain to modify itself and adapt to the environment. Can be categorized into experience-expectant or experience-dependent.

Experience-expectant plasticity

integration of environmental stimuli into patterns of development. Occurs as a result of common experiences shared by most members of a species.

Experience-dependent plasticity

Stimuli unique to the individual that shapes their development.