1 Psychology 304: Brain and Behaviour Lecture 20

2 The Development of the Nervous System 1. What are the phases of prenatal neurodevelopment? (continued) 2. What developmental changes are observed in the nervous system after birth?

3 By the end of today’s class, you should be able to: 1. describe the role of growth cones, filopodia, and lamellipodia in axon growth. 2. describe the role of astrocytes in synaptogenesis. 3. review the mechanisms involved in neuron death. 4. discuss the process of synaptic rearrangement. 5. discuss neurodevelopment in infancy through to adolescence.

4 4. Axon growth and synaptogenesis  Once cells have differentiated into region-specific neurons, axons and dendrites begin to grow from them.  Axonal and dendritic growth are directed at appropriate targets. What are the phases of prenatal neurodevelopment? (continued)

5  Filopodia and lamellipodia extend and retract from growth cones, in search of appropriate targets.  Chemoattractants and chemorepellents guide axonal growth.

6 Axonal Growth in Response to Chemoattractants

7 Growth Cone with Filopodia

8 Filopodia and Lammelipodia

 Synapse formation occurs once axons and dendrites have reached their target locations.  Astrocytes are involved in the formation of synapses.  Synapses that do not function properly after formation are typically eliminated.

10 5. Neuron death  Large-scale neuron death (i.e., apoptosis) in the brain occurs during neurodevelopment.  Neurons compete for connections to target organs. Those neurons that form connections are exposed to life-preserving chemicals called neurotrophins.  The most well-studied neurotrophin is nerve growth factor.

11  Neuron death is an active process. The absence of neurotrophins triggers a genetic program that causes the neuron to “commit suicide.”

12 Apoptosis

13  Following neuron death, some of the synapses formed in earlier stages of development are eliminated (through retraction of axons) and new synapses are formed. 6. Synapse rearrangement

14 Synaptic Rearrangement

15 What developmental changes are observed in the nervous system after birth? With two exceptions, all of the neurons that will compose the adult human brain develop by the 7 th month of pregnancy. Nevertheless, the brain grows substantially after birth. Postnatal brain growth results from synaptogenesis, myelination of axons, and increased branching of dendrites.

16 The rates of postnatal synaptogenesis and myelination vary across brain regions. Once maximum postnatal synaptic density is achieved, synaptic loss often occurs. The prefrontal cortex does not “mature” until late adolescence. The relatively late maturation of this brain region is thought to contribute to the impulsivity of early adolescence.

17 The Development of the Nervous System 1. What are the phases of prenatal neurodevelopment? (continued) 2. What developmental changes are observed in the nervous system after birth?