neuroplasticity and neurodegeneration
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NEUROPLASTICITY AND NEURODEGENERATION
Adapted from Steven Stahl, MD, PHD
• Neurodevelopment• Neuronal selection• Neuronal migration• Synaptogenesis• Competitive elimination
Synaptic development and synaptic neurotransmission.
Time course of neurodevelopment
Overview of Neurodevelopment
• In adult life neurogenesis occurs in only two areas of the brain:
• in the dentate nucleus of the hyppocampus and in the olfactory bulb.
Neurogenesis also occurs in adult life
Learning, exercise,endogenous growth factors, psychotherapy and antidepressants
Adult Neurogenesis in the hippocampus
• Stress, aging and neurodegeneration can cause loss of the synapse with or without neuronal loss.
• Learning, exercise, growth factors, antidepressants and psychotherapy cause restoration of the synapse and of neurons.
• Transplantation of stem cells is another way to restore the neurons or the synapses.
Synapse loss and restoration
Growth Factors (promote neuronal restoration)
• In necrosis neurons are being destroyed by suffocation or toxins/poisons(neuronal assassination).
• In apoptosis neurons are being destroyed by the activation of a gene inside the cell’s DNA(neuronal suicide).
Necrosis vs. Apoptosis
Neuronal death (necrosis vs. apoptosis)
• During neurodevelopment, neurons are formed in excess (some are normal and some are defective) then they are selected for performing their duties.
• The defective neurons are eliminated.• In developmental disorders the defective
neurons may be selected leading to a neurologic or psychiatric condition.
Neurodevelopment
Neuronal Selection during neurodevelopment
• After neurons are selected, they must migrate to the right parts of the brain.
• In order to migrate, neurons trace either glial cells or the neurons that already migrated.
• Migration is helped by adhesion molecules on the neuronal surfaces and complementary molecules on the glia.
• If migration is successful, the neurons are properly aligned to grow, develop and form synapses.
Neuronal Migration
Good migration vs. defective migration
Proper migration requires recognition and adhesion molecules
• Synaptogenesis is directed by neurotrophins.• Neurotrophins are molecules that cause neurons to
sprout an axonal growth cone.• Once the growth cone is formed the neurons and
glia in the area make recognition molecules that can be ATTRACTIVE or REPULSIVE.
• Repulsive neurotrophins cause the neurons to grow away from them, while attractive neurotrophins cause the neurons to grow towards them.
Synaptogenesis
Attractive and Repulsive neurotrophins
Axonal growth cone “docking”
• Just like the axons, dendritic growth is controlled by growth factors that promote branching of the dendritic tree.
Dendritic growth
Insufficient dendritic arborization leads to defective synaptogenesis.
• Presynaptic axons contain some of the molecular components necessary to form a synaptic connection even before making contact with a postsynaptic site.
• A synapse is formed in stages:
Formation of a Synapse
Stage 1 - Hemisynapse
Stage 2(obtaining supplies)
Stage 3 (extracellular scaffolding)
Stage 4 (intracellular scaffolding)
Stage 5 (adding elements)
Frequent utilization of synapse leads to:• increased flexibility of the postsynaptic site,• Increased neurotransmitter release• postsynaptic receptors increase in number• surface area of the postsynaptic neuron
increases• adjacent postsynaptic sites form
Long term potentiation
Utilization of a synapse increases its flexibility
Released neurotransmitters strengthen the synapse
Adjacent synapses form in the presynaptic and postsynaptic neurons
The dendritic tree is constantly changing throughout life, it can:
• sprout new branches, grow and establish synaptic connections when necessary
• trims, alter or destroy synaptic connections when necessary (pruning)
Dendritic pruning
Normal pruning
Abnormal pruning(in degenerative diseases)
• Can increase the intracellular calcium which• can lead to dendritic death and• even cell death.
Out of control neurotransmission
Abnormal dendritic pruning due to increased neurotransmission
• Between birth and age 6 synapses are formed at an accelerated rate
• During adolescence competitive elimination (pruning) occurs destroying about 50% of the synapses.
Competitive Elimination
Competitive Elimination of synapses in adolescence