PSYA01H3 Chapter Notes - Chapter 3: Amniote, Neural Tube, Cerebral Cortex

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12 Aug 2016
Evolutionary development of the CNS
The central nervous system evolved from the very simple one found in simple animals
Even the simplest animals have sensory neurons and motor neurons for responding to the
oFor example, single-celled protozoa have molecules in their cell membrane that are
sensitive to food in the water. These molecules trigger the movement of tiny threads
called cilia, which help propel the protozoa toward the food source.
The first neurons appeared in simple invertebrates, such as jellyfish
oThe sensory neurons in the jellyfish’s tentacles can feel the touch of a potentially
dangerous predator, which prompts the jellyfish to swim to safety. This simple neural
system is sufficient to keep it alive.
The first central nervous system worthy of the name, though, appeared in flat-worms.
oThe flatworm has a collection of neurons in the head—a simple kind of brain— that
includes sensory neurons for vision and taste and motor neurons that control feeding
oEmerging from the brain are a pair of tracts that form a spinal cord
oThey are connected by commissures, neural fibers that cross between the left and right
side of the nervous system to allow communication between neurons at symmetrical
positions on either side of the body
oThe tracts are also connected by smaller collections of neurons called ganglia, which
integrate information and coordinate motor behavior in the body region near each
During the course of evolution, a major split in the organization of the nervous system occurred
between invertebrate animals (those without a spinal column) and vertebrate animals (those
with a spinal column)
In all vertebrates, the central nervous system is organized into a hierarchy: The lower levels of
the brain and spinal cord execute simpler functions, while the higher levels of the nervous
system perform more complex functions
oIn humans, reflexes are accomplished in the spinal cord.
oAt the next level, the midbrain executes the more complex task of orienting toward an
important stimulus in the environment
oFinally, a more complex task, such as imagining what your life will be like 20 years from
now, is performed in the forebrain
The forebrain undergoes further evolutionary advances in vertebrates.
In lower vertebrate species such as amphibians (frogs and newts), the forebrain
consists only of small clusters of neurons at the end of the neural tube
In higher vertebrates, including reptiles, birds, and mammals, the forebrain is
much larger, and it evolves in two different patterns.
Reptiles and birds have almost no cerebral cortex.
Mammals have a highly developed cerebral cortex, which develops
multiple areas that serve a broad range of higher mental functions.
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