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

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.

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

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.

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

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

Neural Tube Development

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

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

Migration along glial cells

Neuronal Migration and CAMs Cell Adhesion Molecules

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 15000 synapses per neuron, twice as many as in an adult brain.

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

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.

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