Chapter 2: Brain DevelopmentTJHSST Neuroscience Society

Prepared by Usnish Majumdar

Development

• First neurons must be created (differentiated cells)

• Neurons must then migrate• Axon-dendrite migration and connection

occurs

3 Layers of Embryo

• Ectoderm (“ecto” latin = outside)– Skin –Neural Tissue

• Mesoderm (“middle” latin = middle)• Endoderm (“endo” latin = within)

http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/Bio%20102/Bio%20102%20lectures/Animal%20Diversity/Protostomes/Lophotrochozoans/Image7.gif

Neural Tube

• Formed 3-4 weeks after conception

• Two parallel ridges form from ectoderm tissue

• Hollow• Forms spinal cord

and Brain

http://scienceblogs.com/retrospectacle/2007/06/neurological_defect_spotlight.php

Cell Differentiation

• Most embryonic cells are pluripotent stem cells• A variety of chemicals signal cells to turn into

specialized cells• Ectodermal cells are inhibited by molecules, signalling

a development into neural cells and not skin cells• After neural cell determination:– Sonic hedgehog – protein secreted from mesoderm – Higher concentration = glial cell– Lower concentration = motor neuron– Even lower concentration = interneuron

More Cell Differentiation (Yes, this is important)

• Signals help determine the specific neurotransmitters that can be used by a neuron

• If neurons are cultured by themselves = norepinephrine

• Cultured with cardiac tissue = acetylcholine• Based on different genes turning off and on

Neuronal Migration in The Brain•Neurons produced in center of tube, migrate outward•Ventricular zone -> Marginal zone•Migration prominent in cortex•Mechanisms• Neurons migrate along fibers to destination• inhibitory interneurons migrate tangentially

•External forces that can disrupt proper neuronal migration:• Alcohol, Cocaine, Radiation, etc.

Connections

• Axons travel long distances until they find dendrites

• In the case of a motor neuron, axon may travel from spinal cord to foot

Axon Growth• Controlled by Growth Cone• Contains receptors for certain signaling

proteins• Signaling proteins (actually classes of proteins)

– Netrin– Semaphorins– Ephrins

• Possible effects– Move forward– Stop – Recoil– Change Direction

• Axon meets Dendrite and forms synapse– Additional molecules help with target recognition

• Last process is the wrapping of axon in myelin

Cervical

Thoracic

Lumbar

Sacral

Paring Back• Neural network is ‘pruned’ to create a more

efficient and accurate system (too many connections at first)

• Apoptosis: programmed cell-death– AP Biology: regulated by several proteins

• Neuron doesn’t receive enough trophic factor to deter these proteins, it dies

• Each trophic factor supports a specific group of neurons– Ex: nerve growth factor = sensory neuron survival

Critical Periods

• Important period when nervous system must obtain certain critical experiences.

• “use it or lose it”• Enriched environments bolster brain

development

Development Continues

• Even in your late teens, your brain is still maturing.

• Frontal lobe is last to become connected with myelinated axons– Judgement, insight, and impulse control– …college students …

Quiz: 5 minutes, 15 Questions