1. Option A: Neurobiology & Behavior
A1: Neural Development
Use the info in the presentation to complete A1 Notes Guide
Information taken from:
HL Biology 2nd Edition Damon, McGonegal, Tosto, and Ward
Biology for the IB Diploma Preparation Guide Walpole

2. Essential Idea: Modification of neurons starts in the earliest stages of embryogenesis and continues to the final years of life The study of neurogenesis in the embryonic brain has shown that neurons are producing and responding to chemical messages. Immature nerve cells migrate to their final locations and the brain matures as millions of connections are formed. These connections are reinforced by experience and remain as learning and memory.

3. How does the neural tube develop in an embryo?
Embryonic chordates
Nerve cord is made along the dorsal side early in development
In humans this happens in the 1st month in utero
Embryogenesis: the study of embryonic development
Allows scientists to learn key principles of neural development using the frog as a model of chordate development
3 distinct tissue layers develop after fertilization
Ectoderm – outer most layer becomes the brain & spinal cord
Endoderm – inner layer forms the lining of the gut & other organs
Mesoderm – middle layer develops into the other systems of the body

4. Embryonic Tissues of Xenopus (a frog species)
You should be able to draw and label this diagram. You should be able to annotate the diagram by stating the fate of each part as it develops into the adult frog.

5. How are neurons produced from the neural tube?
Cells of the neural plate form the neural tube
Some of these cells will differentiate and become neurons, some become glial cells (cells that do not carry messages, but provide supporting functions to the neurons)
Neurons continue to develop in the brain and spinal cord
Majority of neuronal cell division stops before birth
Majority of new neurons are formed between the 5th week and 5th month of development in a human
Neuroblasts: immature neurons that are precursor cells
Neurogenesis: the process of differentiation from neuroblasts to neurons

6. Neural Tube Development
Ectoderm cells on the dorsal surface form the neural plate region
This region folds inward to form a groove
The neural groove becomes a neural tube, growing longer as the embryo develops
The neural tubes becomes separate from the rest of the embryo
The notochord develops from the mesoderm

7. Neural Tube Development

8. What if the neural tube doesn’t close?
Incomplete closure of the embryonic neural tube can cause spinal bifida.

9. How do immature neurons reach their final destination?
Neural migration
Glial cells provide a scaffolding network along which immature neurons migrate
Cytoplasm and organelles move to one side of the cell & the whole cell follows in that direction
During development neurons move from one side of the brain to the other
Mature neurons do not migrate
Axons extend from the cell body in response to chemical stimuli (CAMS)
This determines the length and direction of the axon growth
Some axon are very short, some are very long
Only one axon per cell, but they can be branched

10. Migration along glial cells

11. Neuronal Migration and CAMs Cell Adhesion Molecules

12. How do synapses develop and what happens to those not used?
As axons and dendrites grow from developing neurons that start making connections with other neurons
Motor neurons also create synapses with muscle cells
*Synapses that are not used, disappear. Synaptic transmission causes a synapse to become stronger and be retained.
When an infant is 2 or 3 years old, they have synapses per neuron, twice as many as in an adult brain.

13. What is the consequence of plasticity?
Neural Plasticity is the ability of the nervous system to change as a person has new experiences.
Unused neurons are pruned (removed) and allows the nervous system to rewire itself.
New experiences cause new connections to form and old ones to be lost.
Plasticity is important for neural repair.
Functional plasticity – ability of the brain to move functions from a damaged area to an undamaged area
Structural plasticity – ability of the brain to actually change its physical structure as a result of learning

14. Stroke
Following a stroke, both functional and structural plasticity can be observed.
Using primate models, we have learned improvement can be made with intervention. After a stroke resulting in weak hand movements, monkeys that did exercises to move the hand recovered more rapidly than those that did not exercise. The part of the brain that improved shoulder movement took over the movement of the hand. The brain reorganized itself in the monkeys that received the exercise therapy.

15. Check your understanding
Describe spina bifida.
Outline the differentiation and migration of immature neurons
Explain neural pruning
Compare structural and functional plasticity