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Chapter Five
Development and Plasticity of the Brain
Development of the Brain
Growth and Differentiation of the Vertebrate Brain
Early Beginnings
CNS begins to form at two weeks gestation
Development of the neural tube (figure 5.2)
At birth, brain weighs 350g, at one year 1,000g
Growth and Development of Neurons
Proliferation-production of new cells
Migration-move toward final destination
Differentiation-form axons and dendrites
Myelination-addition of insulating sheath
Figure 5.2 Early development of the human central nervous system
The brain and spinal cord begin as folding lips surrounding a fluid-filled canal. The
stages shown occur at approximately age 2 to 3 weeks.
Figure 5.3 Human brain at five stages of development
The brain already shows an adult structure at birth, although it continues to grow
during the first year or so.
Video
Development of the Brain
Determinants of Neuron Survival
Must make correct connections
Must receive support from nerve growth factor
neurotrophins act in several ways
early in development cause cells to survive and
grow
increase the branching of incoming axons
decrease pain and increase regrowth of damaged
axons
apoptosis-programmed cell death that occurs when
connections are not reinforced
Competition Among Axons as a General Principle
We produce redundant synapses
the most successful axons and combinations survive
Development of the Brain
Pathfinding by Axons
Chemical Pathfinding by Axons
Example: Weiss and the grafted salamander leg
Specificity of Axon Connections
Example: Sperry and the rotated eye of newt
Chemical Gradients
cell surface molecule
chemical attractants
Neurotrophins
Figure 5.7 Summary of Sperry’s experiment on nerve connections in newts
After he cut the optic nerve and inverted the eye, the optic nerve axons grew back
to their original targets, not to the targets corresponding to the eye’s current
position.
Development of the Brain
Fine-Tuning by Experience
Effects of Experience on Dendritic Branching
Enriched environments increase dendritic branching
Generation of New Neurons
stem cells in the interior of the brain
scientists have observed new cells in hippocampus and
cerebral cortex
Development of the Brain
Effects of Experience on Human Brain Structures
Example: music training on temporal lobe development
Example: somatosensory cortex in violin players
Combinations of Chemical and Experiential Effects
Development of the Brain
The Vulnerable Developing Brain
Fetal Alcohol Syndrome
decreased alertness, hyperactivity, varying degrees of
mental retardation, motor problems, heart defects, and
facial abnormalities
Fetal Nicotine Exposure
low birthweight, SIDS, decreased intelligence, hyperactivity
Fetal Cocaine Exposure
decrease in IQ and language skills
Recovery of Function After Brain Damage
Causes of Human Brain Damage
closed head injury-sharp blow to the head resulting from a fall,
an automobile accident, a sports accident, an assault, or
other sudden trauma that does not actually puncture the
brain
stroke-loss of normal blood flow to a brain area
ischemia-blood clot or other obstruction closes an artery
hemorrhage-when an artery ruptures
tissue plasminogen activator-breaks up blood clots
Figure 5.20 Mechanisms of neuron death after stroke
Procedures that can preserve neurons include removing the blood clot,
blocking excitatory synapses, stimulating inhibitory synapses, blocking the
flow of calcium and zinc, and cooling the brain.
Recovery from Brain Damage
Adjustments and Potential Recovery After Brain Damage
Learned Adjustments in Behavior-making better use of
unimpaired abilities
Diaschisis-decreased activity of surviving neurons after other
neurons are damaged
The Regrowth of Axons
peripheral axons can regrow
axons will only regrow very short distances
axons regrow better in the young
Recovery from Brain Damage
Adjustments and recovery cont’d
Sprouting-when nearby, uninjured cells form new branches to
the vacant synapses
Denervation Supersensitivity-heightened sensitivity to a
neurotransmitter after the destruction of an incoming axon
Reorganized Sensory Representations and the Phantom Limb
Effects of Age
Older people do not recover as well as younger people,
although, there is a delicate balance here because the very
young brain is also more vulnerable
Figure 5.25 Collateral sprouting
A surviving axon grows a new branch to replace the synapses left vacant
by a damaged axon.
Figure 5.27 Demonstration of denervation supersensitivity Injecting
6-OHDA destroys axons that release dopamine on one side of the brain.
Later amphetamine stimulates only the intact side of the brain because it
cannot cause axons to release dopamine on the damaged side.Apomorphine
stimulates the damaged side more strongly because it directly stimulates
dopamine receptors, which have become supersensitive on that side.
Recovery from Brain Damage
Therapies
Behavioral Interventions
Physical therapy
Occupational therapy
Speech therapy
Drugs
Nimodipine-prevents calcium from entering cells
gangliosides-promote the restoration of damaged brains
through unknown mechanisms
progesterone-women with brain injury recover better than
men and especially if the damage occurs when
progesterone levels are high during their cycle
Recovery from Brain Damage
Therapies
Brain Grafts
Used for Parkinson’s Disease
Currently Experimental