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-human-embryonic-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 stages- brain 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=6Ii_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=4P3gj2SHZOw

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: Neuroplasticity Ex: Taxi drivers with more experience have larger hippocampii (?) Hippocampus- memory Neuroplasticity http://www.dnatube.com/video/1302/Brain-Plasticity

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