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Chapter 5

PSY 220 Chapter Notes - Chapter 5: Skeletal Muscle, Temporal Lobe, Neural Development

Course Code
PSY 220
Hurwitz Barry

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PSY220- Psychobiology
Main Ideas:
Neurons begin by migrating to their proper locations and developing axons, which
extend to their targets by following chemical pathways.
The nervous system forms far more neurons than it needs and then eliminates
those that do not establish suitable connections or receive sufficient input. It also
forms excess synapses and discards the less active ones.
Experiences alter brain anatomy. Plasticity is greatest early in life but continues
throughout life.
Many mechanisms contribute to recovery from brain damage, including
restoration of undamaged neurons to full activity, regrowth of axons, readjustment
of surviving synapses, and behavioral adjustments.
5.1- Development of the Brain
The human central nervous system begins to form when the embryo is about 2 weeks old.
1. Dorsal surface thickens
2. Long, thin lips rise, curl, and merge, forming a neural tube that surrounds a fluid-
filled cavity
3. As the neural tube sinks under the surface of the skin, the forward end enlarges
and differentiates into the hindbrain, midbrain and forebrain. The rest becomes
the spinal cord
4. The fluid-filled cavity within the neural tube becomes the central canal of the
spinal cord & the four ventricles of the brain, containing CSF
At birth, the average human brain weighs about 350g; by the end of the first year, it
weighs 1,000g, close to the adult weight of 1,200 to 1,400g.
Growth & Development of Neurons:
1. Proliferation- the production of new cells
o Early in development, the cells lining the ventricles of the brain divide;
some remain where they are (stem cells), continuing to divide, while
others become primitive neurons & glia that begin migrating to other
o Neuron proliferation is similar among vertebrates, differing mainly in the
number of cell divisions (*human brains differ from chimpanzee brains
mainly because human neurons continue proliferating longer)
2. Migration- the moving of cells to their eventual location after they have
differentiated as neurons or glia
o Some neurons migrate faster than others, and a few don’t reach their final
destinations until adulthood

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o Some neurons move radially from the inside of the brain to the outside,
some move tangentially along the surface of the brain, and some move
tangentially and then radially
o Chemicals known as immunoglobulins and chemokines guide neuron
migration; a deficit in these chemicals leads to impaired migration,
decreased brain size, decreased axon growth, and mental retardation
&Differentiation- neuron forms its axons & dendrites
o The axon grows first
o After the migrating neuron reaches its destination, its dendrites begin to
3. Myelination- process by which glia produce the insulating fatty sheaths that
accelerate transmission in many vertebrate axons
o Myelin forms first in the spinal cord and then in the hindbrain, midbrain,
and forebrain
o Unlike the rapid proliferation and migration of neurons, myelination
continues gradually for decades
4. Synaptogenesis- the formation of synapses
o Synaptogenesis begins before birth and continues throughout life, as
neurons form new synapses and discard old ones
o Synaptogenesis generally slows in older people, as does the formation of
new dendritic branches
5. Pruning & cell death- once an area of the brain has enough connections, any
others growing towards the same point will die by switching on a “cell-suicide”
program called apoptosis; improves cognitive function
The olfactory bulb (bulbus olfactorius) is a neural structure of the vertebrate forebrain
involved in olfaction, or the sense of smell. Flow of olfactory information from receptors
to glomeruli layer.
Within the olfactory bulb are discrete spheres of nerve tissue called glomeruli. They are
formed from the branching ends of axons of receptor cells and from the outer (dendritic)
branches of interneurons, known in vertebrates as mitral cells, which pass information to
other parts of the brain.
In general, animals learn most easily when they are young; as they grow older, their
neurons become less changeable.
Newly formed neurons of the hippocampus go through a stage when they are highly
changeable, like those of youth. During this period, they integrate into new circuits that
represent new memories.

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More of the newly formed neurons survive during times of new learning; a supply of new
neurons keeps the hippocampus “young” for learning new tasks.
Every cell keeps its DNA molecules until it dies.
Skeletal muscles are replaced slowly, making the average skeletal muscle cell 15 years
old, while all of your skin cells are less than a year old. Cells of the heart are, on average,
almost as old as the person, indicating that the body replaces no more than 1% of heart
cells per year.
The mammalian cerebral cortex generally forms few or no new neurons after birth.
However, new neurons may form after brain damage.
In adults, new neurons form in the olfactory receptors, the hippocampus, and in the song-
producing areas of certain bird species.
Q: What evidence indicates that new neurons seldom or never form in the cerebral
A: The ^14C concentration in the DNA of cerebral cortex neurons corresponds to the
level during the year the person was born, indicating that all or nearly all of those neurons
are as old as the person is.
Paul Weiss (1924) Experiment- grafted an extra leg to a salamander; after the axons
reached the muscles, the extra leg moved in synchrony with the normal leg next to it.
Weiss suggested that the nerves attached to muscles at random and then sent a variety of
messages, each one tuned to a different muscle.
Later evidence provided by Roger Sperry suggested that the salamander’s extra leg
moved in synchrony with its neighbor because each axon found exactly the correct
1. Sperry cut the optic nerves of some newts
2. The damaged optic nerve grew back and connected with the tectum
(amphibians’ main visual area), thereby reestablishing normal vision.
3. Sperry cut the optic nerve and rotated the eye by 180 degrees
4. The axons grew back to their original targets and the newt saw the world
upside down and backwards
*Conclusion: the axons regenerated to their original places because they follow a
chemical trail.
A growing axon follows a path of cell-surface molecules, attracted by some chemicals
and repelled by others, in a process that steers the axon in the correct direction.
Eventually, axons sort themselves over the surface of their target area by following a
gradient of chemicals.
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