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Transcript
Neuronal Development
• Dr. Donald Allen
Development of the Brain
• Begins on day 28
• Vesicles
• Flexures
Three Primary Vesicles
• Develop during 4th week
• Prosencephalon
• Mesencephalon
• Rhombencephalon
Two Flexures
• Cervical Flexure
• Cephalic flexure
Five Secondary Vesicles
• Develop by 5th week
• Derived from primary vesicles
Five Secondary Vesicles
• Prosencephalon
–
–
• Mesencephalon
• Rhombencephalon
–
–
Adult neuroanatomy terms
• The terms used are strictly used only to
describe parts of the brain during
development
• Some of these terms are used (probably
incorrectly) to describe structures in the
adult brain
– Telencephalon
– Diencephalon
– Mesencephalon
Table 5-1, page 88
• Telecephalon
– Cerebral hemispheres
– Basal ganglia
• Diencephalon
– Thalamus, hypothalamus
• Mesencephalon
– Midbrain
Pontine flexure
• Does not persist in adulthood
• As flexure forms, the walls of neural tube
split apart
Further Development of
Telencephalon
• Page 89, Figure 5-8
• The cerebral hemispheres expand so that
they cover the diencephalon
• The hemispheres become C-shaped
• The insula region does not grow as fast, and
it becomes covered by the frontal and
temporal lobes
• Surface folds to produce gyri and sulci
Processes of Development
•
•
•
•
•
•
•
Cell migration
Growth cone
Neuronal death
Axonal retraction
Muscle fiber types
Myelination
Growing into deficit
Cell migration
• Neurons that divide are located next to the
ventricles
• Where is the gray matter (soma) in the cerebral
cortex?
• Neurons will either migrate by:
– Sending out processes
– Follow radial glia
• Neurons differentiate once they reach their
location
• Location can determine differentiation
Growth cone
• When neurons migrate or send out a process
• Growth cone – process
– Samples environment
• Moves toward attractants
• Moves away from other chemicals
– When growth cone reaches its target
• Vesicles are produced
• Synapse forms
Neuronal death
• The nervous system tends to overproduce
neurons, which then compete for a limited
number of targets
• Neurons which do not make connections
with other neurons or are not active tend to
die
• Motor neurons in spinal cord
Axonal retraction
• During innervation of muscles
• Initially more than one motor neuron
innervates each muscle fiber
• During development, axons retract so that
only one motor neuron innervates each
muscle fiber
Muscle fiber types
• Muscle fibers can be slow-twitch or fast
twitch
• The muscle fiber type is dependent on the
nerves, not the muscle
• If we change the nerve, we can change the
muscle fiber type
Myelination
• Begins 4th fetal month
• Most myelination completed by end of third
year of life
• Myelination occurs at different rates in
different systems
– Lower motor neurons – by 1 month
– Projections from motor cortex to spinal cord –
2 years (Babinski reflex)
Growing into Deficit
• There may be neurological problems that
may not be immediately noticed
• These impairments will become observed as
the impaired systems become functional
• For example, balance or lower extremity
impairments may not be observed until a
child begins to try to stand and walk
Brain Development Postpartum
• At birth, 400 grams
• Increases in brain weight
–
–
–
–
–
Myelination
Number of processes
First 3 years, 3-fold increase in weight
Brain growth then slows down
11 years old – 1400 grams (1100-1700)
Brain weight with age and gender
• Brain weight fairly constant till 50’s, then
slow decline in overall weight
• Men: larger brain by weight
• Women: larger brain by weight of
brain/weight of body
Critical Periods
• Times during development that are crucial for
normal development and outcomes
• Occur when
– Neurons are dividing
– Neurons competing for connections
• Motor neurons :: number of limbs
• Examples in humans
– Vision
– Language
Disorders of Development
• Neural tube disorders
– A neuropore does not close correctly
• Normal development depends on cells making
contact with the proper types of other cells
• Usual structures may not develop, or may develop
incorrectly
Caudal Neuropore
• Failure to close – spina bifida
– Vertebrae do not form normally
– Associated with inadequate folic acid intake by mother
– Several types (page 92)
• Dr. Emmel in Systems Screening I
Rostral Neuropore
• Failure to close
– Failure of forebrain to develop correctly
• Often brainstem is present
– Skull does not form over incomplete brain
– Most fetuses die before birth
• Otherwise, usually die within a week of birth
Arnold-Chiari Syndrome
• Developmental abnormality of hindbrain
• Two types
Arnold-Chiari Type I
• Herniation of cerebellar tonsils through the
foramen magnum
• Medulla and pons often small
• Often have no symptoms
– Often begin in adolescence and early adulthood
• Headache induced by coughing, sneezing, straining
• Possible hydrocephalus (block CSF flow)
• Cranial nerve dysfunction
Arnold-Chiari Type I - Treatment
• If deficits are stable, no treatment
• If deficits are progressing, the bone that is
compressing the hind brain can be removed
Arnold-Chiari Type II
• Signs present in infancy
• Usually associated with meningomyelocele
• Malformation of brainstem and cerebellum
– Extend through foramen magnum
Arnold-Chiari Type II
•
•
•
•
•
Progressive hydrocephaly
Paralysis of sternocleidomastoid muscles
Deafness
Weakness in lateral eye movements
Weakness in facial muscles
Fetal Alcohol or Cocaine
Exposure
• Fetus exposed in utero
• Fetal Alcohol Syndrome
Fetal Alcohol Syndrome
Fetal Cocaine Exposure
• Effect depends on stage of development
• Disrupts neuronal proliferation
• Interferes with neurodevelopmental
processes
• Effects more subtle than those of alcohol
• Also affected by low socioeconomic status,
use of other illegal drugs, smoking, poor
maternal nutrition, and STD
Cerebral Palsy
• Non-progressive and permanent
• Injury to developing brain
– Prenatal, postnatal, or at time of birth
• Defined by motor and postural impairments,
though may also have sensory and cognitive
impairments
• Growing into deficit is common
Types of Cerebral Palsy
•
•
•
•
Spastic
Athetoid
Ataxic
Mixed
Spastic Cerebral Palsy
• Damaged neurons next to ventricles
– Axons of neurons in cerebral cortex
• Toe Walking
• Scissor gait
Athetoid Cerebral Palsy
• Damage in basal ganglia
• Slow, writhing movements of
extremities and/or trunk
Ataxic Cerebral Palsy
•
•
•
•
Damage to cerebellum
Lack of coordination
Weakness
Shaking movements (tremor) during
movements
Mixed Cerebral Palsy
• Combination of above types of cerebral
palsy