Transcript Neural Development

IB Biology
Brain Development
Neural Development
• Neural tube formation– How do organs form from 1 fertilized egg?
• After fertilization cells differentiate into 3 tissue layers:
– Ectoderm- outermost layer
» Brain and nervous system
– Endoderm-inner layer
» Lining of gut and other organs
– Mesoderm-middle layer
» Skeleton, muscle system
» https://www.youtube.com/watch?v=lGLexQR9xGs
» http://www.dnatube.com/video/12257/The-humanembryonic-brain-development
Neural Development, cont.
• First organ to form from neural tube brain
• Presence of tissue that is developing triggers
the development of another tissue
• Notochord (mesoderm) ectoderm  neural
plate folds in and closes neural tube
elongates into brain and spinal cord
• Closure of neural tube happens in stagesbrain forms before caudal (tail) area closes
Neural Devel., cont.
• Failure to completely close- spina bifida
Activity
• In your notebook, create a flow chart
illustrating the development of the neural
tube. Begin with the cell layers.
– Pages 497-498
https://www.youtube.com/watch?v=6I
i_v3t9hpU
Neurogenesis & Migration of Neurons
• Neurons of the CNS– Originate in the neural tube
– Neuroblasts: immature neurons, precursor
– Neuroblast neuron = neurogenesis
– Early neural tube formation• Neurons- carry messages
• Glial Cells- no messages, 90% in brain, give physical and
nutritional support
Axon Growth
• Axons grow from CNS to distant areas
• Tip of axon has a “growth cone” that directs
axons to their destination
• In vitro– Axon avoids unfavorable surfaces, contracts
– Axon grows toward favorable surface
Axon Growth, cont.
• Final destination of neuron:
– Synaptic connections made with target cells via
chemical messages
– Some molecules can act as signals to the growth
cone
• CAM- cell adhesion molecule
– Located on cell surface in growth environment of axon
• Growth cone has receptor called CAM specific receptor
• CAM and CAM receptor recognize each other to
produce a chemical signal in neuron activation that
causes elongation of neuron.
Axon Growth, cont.
• Some growth cone receptors target cell
secretions
– Chemotrophic factors
• Can be attractive or repellant
Mammalian motor neurons begin in neural tube
of CNS, must extend out to target tissue muscles
CAM causes migration to muscle tissue
Longest neurons are motor neurons
Axon Growth, cont.
• Multiple synapses are made initially
– Neurons find “best fit” to target cell
– When connections don’t work- eliminated
– When connections do work- strengthened
• Huang- proposed mechanism for this activity:
– Mediated by Ig CAM (Immunoglobulin CAM) which
has a lock and key mechanism of effect
– CAMs form physical but reversible bond
– Some connections do not persist, but the strongest
ones will
Axon Growth, cont.
• Some connections do not persist, but the strongest
ones will
• Neuromuscular junctions have neurons
competing for innervation
• https://www.youtube.com/watch?v=4P3gj2SH
ZOw
Activity
• Create a flow chart illustrating the migration
of neurons from the CNS to the PNS.
– Pages 499-500
Neural Pruning
• From 2-3 years old, 15,000 synapses are
present for each neuron, twice as many as
adults
• Neural pruning eliminates axons not used.
– Remove simpler connections and replace with
more complex adult connections
– “Use it or lose it”
– Makes brain more efficient
Neural Pruning, cont.
• Studies using mice– Microglia prunes unused synapses
– Elimination of weak and re-inforcement of strong
synapses key to brain development
– Microglia select synapse for removal on basis of
inactivity
– https://www.youtube.com/watch?v=rxPT78F_ZVE
Plasticity of the Nervous System
• Brains have the ability to change and adapt as
a result of experience
• Baby’s brain and adult brains both have
plasticity
– Evidence of recovery following massive strokes in
adults
– Plasticity varies with age, environment, heredity
Plasticity, cont.
• Functional and Structural
– Functional:
• Ability to move functions from a damaged area to an
undamaged area
• Ex: tennis player loses use of arm due to a stroke given
the task of cleaning tables. The arm “remembers” how
to move during rehab, thus regaining function.
Plasticity, cont.
• Structural:
– Ex: Taxi drivers with more experience have larger
hippocampii (?)
• Hippocampus- memory
• Neuroplasticity
– http://www.dnatube.com/video/1302/BrainPlasticity
Stroke and Brain Function
• Stroke- also called CVA (cerebrovascular
accident)- a blood clot or rupture of a blood
vessel in the brain.
– Brain recovery• Functional and structural reorganization
• Axon sprouting
• Post- stroke neurogenesis ( migration of new neurons
to the site of injury
Stroke, cont.
• Promotion of recovery
– Exercise- primate studies show that the weakness
of hand movement due to stroke can be regained
with physical therapy (gripping exercises)
– Shoulder movement took over hand movement
showing evidence of brain reorganization
New Technology for Stroke Patients
• fMRI- functional MRI maps areas of activity in
brain tissue
• PET- positron emission tomography
• MEG- brain mapping
(magnetoenchephalography)
• Post- stroke aphasia- loss of speech after
stroke can be corrected by technology +
physical therapy
The Brain
• Nerve cells migrate to outer edge of neural
tube and cause the walls to thicken
• Neural tube becomes the entire central
nervous system (CNS- brain and spinal cord)
• Anterior endcerebral hemispheres
• Posterior endother parts of brain and spinal
cord
• First to start developing and last to finish
Roles of Parts of Brain
• Brain- jelly-like mass, 1.4kg, 100 billion
neurons, site of memory, learning, personality
• Brain regulates:
– Unconscious body processes- breathing, heart
rate, blood pressure
– Balance, muscle coordination, voluntary
movement
– Speech, emotions, problem solving, thinking,
dreaming
Roles, cont.
• Cerebral hemispheres: learning, memory,
emotions
• Hypothalamus: homeostasis, nervous and
endocrine system coordination
– Synthesizes hormones stored in posterior pituitary
and release factors controlling the anterior
pituitary
Roles, cont.
• Cerebellum- “little brain”- has 2 hemispheres
and a highly folded surface
– Unconscious functions, movement and balance
• Medulla oblongata-automatic and
homeostatic activities
– Swallowing, digestion, vomiting, breathing, heart
activity
Roles, cont.
• Pituitary gland-2 lobes, posterior and anterior
– Both controlled by hypothalamus
– Both secrete hormones
Role of Medulla
• Contains a “swallowing center” that
coordinates the muscles of the mouth,
pharynx, larynx, so that food does not go
down the trachea and into the lungs
• Controls breathing by monitoring carbon
dioxide levels in the blood.
– Too much? Breathing rate increases
• Cardiovascular center- regulates heart rate
Identifying the Role of Different Brain
Parts
• Brain lesions– Used to study brain function
• Right and left hemispheres
– Connected by corpus callosum- thick band of axons
– Do not have the same function
• Left hemisphere- all forms of communication
• Damage causes difficulty speaking or using hand gestures
• Deaf people with damage may lose ability to use sign
language
Identifying Roles, cont.
• Right hemisphere– No communication, but understanding of words
– Analysis of all information coming from all senses
– Lesions here• Problems identifying faces
• Problems locating objects correctly in a space
• Problems identifying melodies
Language Areas
• Broca and Wernicke- 1800s
– Left side damage- speech and language problems
– Pierre Paul Broca- Broca’s area• Damage interferes with ability to vocalize words
– Carl Wernicke- Wernicke’s area
• Damage interferes with ability to put words into
sentences
• https://www.youtube.com/watch?v=jHxyP-nUhUY
Split-Brain Surgery
• Patients with severe epilepsy sometimes have
their corpus callosum severed- called split-brain
surgery.
• Input from right visual field perceived by left
hemisphere, and vice versa.
• Visual information changed in these patients– Objects flashed for the right eye were correctly
identified (Left hemisphere is language)
– Objects flashed for the left eye were not identified.
– https://www.youtube.com/watch?v=ZMLzP1VCANo
Functional MRI
• Uses radio waves and strong magnetic fields
• Sees blood flow in real time
• Can determine which sections are active during certain
stimulation and how long they remain active.
• Used for:
–
–
–
–
–
Surgery plan
Treatment for stroke
Placement of radiation therapy
Effects of degenerative disease –Alzheimer’s
Diagnosis of how injured brains work
PET
• Slower scanner, but can tell how the brain is
activated in presence of certain NTs and drugs
Autopsy Findings
• Determines which brain parts are involved in
certain functions
• Broca
– Autopsied patient with language disorder, found
lesion in language center of brain (Broca’s Area)
Autonomic Nervous System
• Brain and spinal cord- CNS
• Everything else- PNS- Peripheral NS
• PNS has 2 parts:
– Somatic system- sensory info from sensory
receptors to CNS muscles(Reflex Arc)
– Autonomic system-involuntary- regulates glands,
smooth muscle and the heart, located in the
medulla
Autonomic, cont.
• Sympathetic NS- response is fight or flight
–
–
–
–
–
NT is NA
Exitatory
Increases HR and Stroke Volume
Dilates bronchi and pupil of eye,
Restricts blood flow to the gut
• Parasympathetic NS- relaxed, “business as usual”
–
–
–
–
–
NT is Ach
Inhibitory
Pupil returns to normal
HR slows, stroke volume reduced
Blood flow back to gut
• Systems are antagonistic- both cannot be activated at the same time.
• https://www.youtube.com/watch?v=J968Wco1u0s
The Pupil Reflex
• Pupil constricts in presence of bright light
– Cranial reflex- sensory and motor neurons connect
directly to the brain
– Controlled by Ach
– Can be inhibited by atropine, which blocks Ach
temporarily
– Pathway- optic nerve receives signals from
retinasynapse with bipolar neuronsganglion
cells connects with pretectal nucleus of brain
stemEdinger-Westphal nucleus (axons run along
oculomotor nervesciliary ganglion stimulate
circular muscle of iris contracts
http://www.tedmontgomery.com/the
_eye/reflex.html
Brain Death
• Brainstem controls heart rate, breathing rate,
blood flow to digestive system.
• Brain controls body temperature, blood
pressure and fluid retention.
– All can be maintained artificially without a
functioning brain
Brain Death, cont.
• Legal description:
– When a physician or physicians has determined that the
brain and brainstem have irreversibly lost all neurological
function.
Coma- neurological signs can be measured based on
responses to external stimuli
https://www.youtube.com/watch?v=Ffqz-vKZO5Q
Brain death, cont.
• Exams for brain death– Toxicology- to make sure drug isn’t cause of symptoms
– Movement of extremities-no hesitation when
dropped
– Eye movement-should be fixed
– Corneal reflex-must be absent (Q-tip over surface)
– Pupil reflex- absent
– Gag reflex- absent
– Respiration response- absent, cannot breathe without
support
– Still can have spinal reflexes- go to spine, not brain
Brain death, cont.
• EEG-(electroencephalogram) check for
electrical brain activity
• CBF- (cerebral blood flow)- radioactive isotope
injected, if it doesn’t make it to the brain,
there is no activity
• Can maintain patient on ventilator, but not
indefinitely