Chapter 2 – Central Nervous System: Structure and Function
Prenatal development = all steps involved with the formation of various structures and
functions of the body.
• Proper nutrition is critical – low birth weight, smaller head circumference may be
caused by malnutrition. May later lead to obesity and problems in programming
the appetite regulatory system correctly in the fetus
• Abstaining from alcohol and other drugs, decreasing stress level, and avoiding
physical/emotional abuse. Alcohol and drugs can cross the placental barrier
around the fetus
o Fetal alcohol syndrome: can cause serious cognitive difficulties
o Increased stress levels in the mother can raise cortisol amount in the
body. Cortisol: a steroid hormone that elevates blood sugar and
metabolism to help the body adapt to prolonged stress. Too much can
lead to depression of the immune system.
• Severe physical abuse to the mother can lead to brain impairment in the baby
• Shaken baby syndrome: when the baby is shaken so hard that their brain
impacts with the skull. Characteristic injuries subdural hemorrhages, retinal
hemorrhages, fractures of ribs or long bones
• Abuse to mother could lead to her neglecting herself and fail to care for physical
needs. Can affect child and cause neglect of other children.
Development of the Central Nervous System Germinal period: period of time from conception to when implementation in the uterine
wall occurs – about 2 weeks between
• Developing cells are called a zygote
• By end of first week, it’s a blastocyst – consists of tissue that will become the
embryo and the trophoblast (provides nutrition)
• With implantation, the cells are now considered an embryo
Embryonic stage: lasts until the end of the first trimester
Embryo then begins to differentiate into 3 distinct layers:
• Ectoderm: outermost layer. Later develops into the skin, sense organs, and
• Mesoderm: middle layer. Becomes the muscles, blood, and excretory system
• Endoderm: innermost layer. Becomes the digestive system, lungs, other internal
The life support system for the embryo also develops:
• Amnion: sack of fluid where embryo floats for temperature regulation and
• Umbilical cord: connects embryo to placenta
• Placenta: group of tissues in which blood vessels from the embryo and the
mother mix but do not join.
o Particles (water, salt, oxygen) go from mother-to-child while waste and
carbon dioxide go from child-to-mother
Next is development is the beginning of the nervous system
1. Neural plate: made up of ectodermal tissue on the dorsal side of the embryo a. Becomes evident at 3 weeks and appears to be stimulated by signals from
b. At this time, all the cells are totipotent – can be any cell in the body. As the
neural plate develops, the cells lose this ability
2. The neural plate folds to forms the neural groove
3. The edges of the groove fuse to form the neural tube – about 24 days after
a. The inside of the neural tube becomes the spinal canal and the cerebral
b. The swellings at the end of the tube become the forebrain, midbrain, and
At the end of 7 weeks, the embryo is referred to as the fetus and resembles a more
The brain is an almost complete replication of the adult brain at approximately 100 days
from conception, even though the structures are not completely developed.
The neural crest is dorsal to the neural tube. It’s formed from cells that differentiate from
the neural tube as it is being formed.
• This develops into the neurons and glial cells
Prenatal Neuronal Development
Induction: begins when part of the ectoderm becomes the nervous system. This begins
during the development of the neural plate.
• The cells are still totipotent at this stage or stem cells: they are able to develop
into different types of cells and have an almost unlimited capacity for self-renewal
o As the stages progress, these cells become more specific = specialized
Proliferation: used to describe a time of immense cellular division, which occurs when
the neural tube is formed.
• Also termed neurogenesis because is beginning of development of neurons
occurs in the first 5 months of gestation.
Cell migration: begins after the first neurons are developed and continues several
weeks after neurogenesis is complete. As cells migrate to their place, they follow
chemical pathways that lead them to their correct location.
Cell aggregation: occurs once migration is complete. The cells move to other cells that
have migrated to a similar are to form nervous system structures. Maturation: time of axonal, dendritic, and synaptic formation.
• Some axonal growth occurs during migration
• When a primitive neuron reaches a specific location, maturation will give the
neuron all of its distinguishing features
• 1 neuron can grow 1 axon. It takes 2 neurons for one synapse.
o Synaptogenesis: the formation of new synapses. This will occur all
through a person’s life but especially during early periods of development.
Apoptosis: if a neuron does not receive nerve growth factor or does not bond correctly
to receive this factor, the neuron will begin the programmed process of nerve cell death.
• The space that is left by the dead cells is filled with the axons of the living
neurons = rearrangement of synaptic connections.
At the 12 week, the brain starts to assume more of a concrete model of the adult brain.
• Most noticeable characteristics: ventricles; telencephalon; diencephalon;
A baby may have as many as 100 billion neurons at birth.
Structure of the Neuron
Neurons are the main cells in the body that specialize in communicating with one
another or with muscles, glands, and other tissues.
Soma: area where neurons assemble proteins, generate energy, and maintain
• Nucleus: contains DNA (deoxyribonucleic acid)
o Each strand of DNA is made up of chromosomes: long strands of
nucleotides that are paired together
o When nucleotides form sequences, they make up the components of
o Each gene carries instructions that can be added to other genes in the
DNA segment to synthesize a specific protein necessary to form
• Cytoplasm: internal fluid that holds organelles in place within the cell
• Mitochondria: site of energy production for cells o Adenosine triphosphate (ATP): the energy source for neurons and other
cells; consists of adenosine bound to ribose and three phosphate group.
Fats, sugars, proteins from food react with oxygen to produce ATP
o Cycle in which ATP is formed is called the Krebs cycle
• Endoplasmic reticulum (ER): network of tubules within a cell that transports
synthesized lipids and membrane proteins to other locales
o Smooth ER: does not contain ribosomes; important for the synthesis and
production of lipids that are used in carbohydrate metabolism. Also
involved in the detoxification from drugs and poisons
o Rough ER: contains ribosomes (takes in materials and synthesizes
• Golgi complex: system of membranes that package molecules into vesicles or
can modify the molecules further
• Lysosome: contains digestive enzymes and provides the neuron help in recycling
and reusing materials
• Microtubules: tubules that quickly break down materials within the neuron and
maintaining cellular structural support
• Messenger RNA: information coded in the DNA is transcribed on this
o Transcription: synthesis of RNA from a DNA template.
o Accomplished through RNA polymerase (enzyme) pretty much unzipping
the DNA helix and reading one half of the DNA strand while concurrently
building the same strand to match the strand that is being transcribed
o The RNA strand breaks away from the DNA and is carried elsewhere
o RNA is similar to DNA except that it contains uracil instead of thymine. Its
backbone is phosphate and ribose instead of ribose and deoxyribose
o Messenger RNA carries the genetic code from the neuron to the rough ER
where it is transcribed into proteins
• Lipid bilayer: membrane that covers the neuron and is made of two layers of fat;
it allows selective permeability to certain substances
o Made up of hydrophilic exterior and hydrophobic interior. This gives it
polarity and the selective permeability
o Channel proteins: certain molecules can pass through here, providing
nutrients and ions into the cell o Signal proteins: transfer a signal to the inside of the neuron when certain
molecules bind to them on the outside of the membrane
Dendrite: structures that receive information and send it to the body of the neuron
• Neuron many have many dendrites that branch out to other neurons
• Dendritic branches are divided into segments, called orders, named based on
their location in relation to the soma
o First order is closest to the soma and then so on
Axons: structure that sends information from the cell body to the synapse
• Signals from the axon usually travel in one direction, starting from the soma
• A neuron usually only has one axon. The axon can have many branches.
• Axons can vary in length from millimeters to a meter
Axon hillock: structure on the body of the axon that determines whether an impulse is
strong enough to cause an action potential
Action potential: massive momentary reversal of the membrane potential from -70 to
+50 mV; synonymous with firing of the neuron
• Of the sum of depolarization and hyperpolarization is sufficient to depolarize the
membrane to the threshold of excitation, the action potential is generated
• All-or-nothing principle: either the signal is strong enough to produce an action
potential or it dissipates
• Resting state: without any stimulation, the electrical charge within the neuron is
-70 mV and the neuron is polarized. The concentration of Na and Cl are greater
outside the neuron while the concentration of the K is more concentrated outside
o Sodium-potassium pump: functions to maintain the cell potential; pumps
out sodium ions and potassium ions in by active transport
o Diffusion: tendency of molecules to move from areas of high concentration
to areas of low concentration
o Concentration gradient: attraction of a region of high levels of molecules to
an area of low concentration
• Absolute refractory period: brief period of time (1-2 milliseconds) when it is
impossible to have another action potential take place • Relative refractory period: period when the neuron can respond to a series of
impulses that have a greater depolarization charge. This is when the neuron has
started to repolarize but has not returned to resting potential