Download Chapter 3

Chapter 3
The Development and Plasticity of the
Nervous System
The Structural Development of the
Human Nervous System
Brain development begins at the point of
conception when the ovum is fertilized by a
sperm resulting in the formation of a zygote.
 After the zygote divides the resulting developing
human is called:
• Embryo – next 6 weeks
• Fetus – at week 9 and for
the remainder of the
pregnancy

Eight cell zygote
Development of the Nervous System:
The Human Embryo

Layers of cells in the embryo:
• Ectoderm forms the nervous systems as well
as the epidermis and parts of the eyes and
ears
• Mesoderm forms connective tissue, muscle,
blood, blood vessels
• Endoderm forms the linings of the body

Throughout the embryonic and fetal
period different cell types are created; the
process is called differentiation.
Structural Development:
Formation of Nervous System

Embryonic layers thicken to become:
• Neural Plate is the name for the thickened
ectodermal layer.
• Neural folds push up to form a space called
the neural groove.
• Neural tube forms from the neural groove in
23 days. The brain and spinal cord develop
from it.
3 Vesicle Stage
5 Vesicle Stage
Structural Development:
Differentiation of the Brain
Structural Development:
The Developing Spinal Cord
Alar plate - gives rise to sensory neurons
and interneurons of the spinal cord’s dorsal
horn.
 Basal plate - forms ventral portion of the
spinal cord where motor neurons originate
and the interneurons of the ventral root
form
 Sympathetic and Parasympathetic nervous
systems also derive from the basal plate.

Structural Development:
The Developing Ventricular System
Develops in the cavity inside the neural tube,
contains cerebral spinal fluid
 Four ventricles:

• Lateral ventricles
• Third ventricle
• Fourth ventricle
Cellular Development: Formation of
Neurons and Glial Cells
A layer of ectodermal cells form on the inner
surface of neural tube and divide to form:
 Ventricular layer which then divides into daughter
cells
 Daughter cells migrate:
• between the intermediate and marginal layers to form
the cortical plate which develops into the cortex.
• to the subventricular layer, becoming either glial cells
or interneurons.
• daughter cells remaining in the ventricular layer
develop into ependymal cells, which form the lining of
the four ventricles and the central canal of the spinal
cord.
Cellular Development: Formation of
Neurons and Glial Cells
Cellular Development: Formation of
Neurons and Glial Cells

Neurogenesis is the formation of new
neurons.
• Few neurons are formed after birth
• Exceptions are:
 cerebellar cells, olfactory receptor neurons,
hippocampal neurons, and some cortical neurons
• These exceptions allow for neuroplasticity.
Cellular Development: Formation of
Neurons and Glial Cells

Migrating cells
• Guided by radial glial cells
• Glycoproteins allow neurons to bind to other
neurons or radial glial cells (a handhold).
• Failures of the adequate production of
glycoproteins may lead to behavioral deficits.
• Cell migration dysfunction is implicated in
schizophrenia where abnormal distributions of
neurons have been found in the brains of
schizophrenic patients.
Neural Cell Differentiation

Cell-autonomous differentiation is controlled
by genetic programming.
• A Purkinje cell will develop into its distinctive form
even if grown in culture out of its environment.

Induction - other cells
influence the final form.
• The notochord influences new
neurons to become a spinal
motor neurons.
Glial Cell Development
Glial cells develop from the ventricular layer.
 Glial cells develop more after birth.
 A major function of glial cells is the myelination of
neurons:

 Schwann cells in the peripheral nervous system wrap
themselves around nerve axons; a single Schwann cell
makes up a single segment of an axon's myelin sheath
 Oligodendrocytes in the central nervous system wrap
themselves around numerous axons at once.
Formation of Neural Connections

Once a cell has differentiated, it must establish
connections with other neurons.
 Neurons grow toward target cells
 Axon emerges from growth cone
 Filopodia - consist of spine-like extensions that
appear to be searching
The Movement
of Filopodia and
the Growth
Cone
Formation of Neural Connections:
Axonal Growth
Guidepost cells serve as a map; when the
filopodia reach them, the growth cone adheres
to that cell and the guidepost cells redirect
axonal growth to target cells.
 Neurotrophins released by the target cell
• Attract the filopodia of developing neurons
• Repels others to ensure only appropriate
axons move toward the target
 Target cell determines the neurotransmitter
released from the presynaptic neuron

The Importance of Neural Activity




Neural activity is necessary for establishing
appropriate neural connections.
Axonal remodeling is the process of axons
connecting to the correct place; selectively
strengthens the synaptic connection
Neural activity “wires” the connections for
communication within the nervous system
New synaptic connections after birth allow
more refined analysis of stimuli and more varied
behavioral responses
Neural Cell Death
Apoptosis - genetically programmed cell death
 Synaptic pruning
 Theories of cell death

• Neurons compete for connections to target cells and
the unsuccessful ones die.
• Neurons that receive a sufficient amount of chemical
from the target cells survive; neurons that receive less
die.

Neural development recap
Disorders of Development: Down
Syndrome
Genetic condition that causes delays in physical
and intellectual development
 Most common genetic cause of learning
disabilities in children
 Down syndrome results when one of three types
of abnormal cell division involving chromosome
21 occurs

 Trisomy 21
 Mosaic Down Syndrome
 Translocation Down
Syndrome
Neuroplasticity:
Neural Degeneration

Causes





•
Tumors
Seizure Disorders
Cerebrovascular Accidents
Degenerative Disorders
Disorders Caused by Infectious Diseases
Types of degeneration
 Anterograde
 Retrograde
 Transneuronal
•
Chromatolysis - process of breakdown where
degeneration occurs
Neural Degeneration: Tumors

Tumor - Mass of cells whose growth is
uncontrolled and that serves no useful function

Metastasis - Process by which cells break off a
tumor and grow elsewhere in body
• Tumors damage brain tissue two ways




Compression
Infiltration
Glioma - Cancerous brain tumor
Meningioma - Benign brain tumor
Neural Degeneration:
Seizure Disorders

Seizure - a period of sudden, excessive activity of
cerebral neurons
 Briefly alters consciousnesses, movement, or actions
 If neurons that make up the motor system are involved,
convulsions can occur
Convulsion – a violent sequence of uncontrollable
muscular movements caused by a seizure
• Hippocrates was the first to note that seizures
might have a physical cause

Classification of Seizure Disorders
I.
Generalized Seizures
A. Tonic-clonic (grand mal)
B. Absence (petit mal)
C. Atonic
II.
Partial Seizures
A. Simple
1.
2.
3.
4.
5.
Localized motor seizure
Motor seizure with progression of movements
Sensory
Psychic
Autonomic
B. Complex – includes 1-5 as above
III.
Partial seizures evolving to a generalized cortical seizure –
starts as IIA or IIB than becomes a grand mal seizure
Specific Lobe Seizures

Frontal lobe seizures may produce unusual symptoms
that can appear to be related to a psychiatric problem
or a sleep disorder.

Temporal lobe seizures may include having odd feelings
such as euphoria, fear, panic and déjà vu.

Occipital seizures are often mistaken for migraines
because they share symptoms including visual
disturbances, partial blindness, nausea and vomiting, and
headache.

Parietal lobe seizures can involve both sensory and visual
sensations.
Cerebrovascular Accidents:
Stroke
 Hemorrhagic stroke
 Caused by the rupture of a cerebral blood vessel
 Most common cause is high blood pressure
 Ischemic stroke
 Caused by the obstruction of blood flow to the brain
 Thrombus – a blood clot that forms within a blood vessel,
obstructing blood flow
 Embolus – a piece of matter that dislodges from its site of
origin and travels through the system until it reaches a
vessel to small to let it pass thereby obstructing blood
flow
Cerebrovascular Accidents:
Effects of a Stroke

Right Brain
 Paralysis on the left side of the body
 Vision problems
 Quick, inquisitive behavioral style

Left Brain
 Paralysis on the right side of the body
 Speech/language problems
 Slow, cautious behavioral style

Hindbrain
 Can affect both sides of the body
 May leave someone in a ‘locked-in’ state
Cerebrovascular Accidents:
Risk Factors for Stroke











High blood pressure
Cigarette smoking or exposure to secondhand smoke
High cholesterol
Diabetes
Being overweight or obese
Physical inactivity
Obstructive sleep apnea
Cardiovascular disease
Use of some birth control pills or hormone therapies that
include estrogen
Heavy or binge drinking
Use of illicit drugs
Cerebrovascular Accidents:
Traumatic Brain Injury
 Vehicle-related collisions
 Violence
 Sports injuries
 Falls
 Explosive blasts
Traumatic Brain Injury
Head Games
Degenerative Disorders
Transmissible Spongiform Encephalopathy
 Parkinson’s
 Huntington’s
 Alzheimer’s
 Amyotrophic Lateral
Sclerosis (ALS)
 Multiple Sclerosis

Degenerative Disorders:
Multiple Sclerosis
Autoimmune demyelinating disease
 Myelin protein crosses into general circulation
causing an immune system reaction
 Sclerotic plaques interrupt neuronal signals

Disorders Caused by Infectious
Diseases

Viral Encephalitis



Herpes
Polio
Rabies
HIV
 Meningitis
 Bacteria




Syphilis
Lyme Disease
Malaria
Neuroplasticity:
Regeneration of Damaged Neurons

Neural regeneration
• Occurs in embryonic and neonatal nervous system
• In adults usually does not occur in CNS
• Occurs in PNS
Glycoproteins present in mature PNS promote cell
regeneration
 Oligodendrocytes synthesize a glycoprotein that
inhibits axonal growth in CNS
 Collateral Sprouting – neurons compensate for loss
of neural connections in CNS by sending new
axonal endings to vacated receptor sites

Chromatolysis
Neuroplasticity: Transplantation
Animal research - Substantia nigra damage
has been reduced by implanting fetal tissue
from donors into the damaged area.
 Human research - Parkinson’s disease
patients have partial recovery of motor
ability from transplanted fetal tissue.
 Ethics - a major debate over the use fetal
stem cells exists, acceptance might be
higher for adult stem cell use

Neuroplasticity: Stem Cells
Embryonic stem cells are
found in an embryo, fetus or
the umbilical cord blood.
Depending upon when they
are harvested, embryonic
stem cells can give rise to
just about any cell in the
human body.
Adult stem cells - found in
infants, children and adults.
They reside in developed
tissues such as those of the
heart, brain and kidney. They
usually give rise to cells
within their resident organs.