Unit 3 Flashcards
characteristics of embryo in pharyngeal stage
- Pharynx
- Central neural tube
- Notochord
- Somites
- Head region
How do species generally differ in regard to development?
all animals go through the same/similar stages of life, but the timing of these stages varies
6 general stages of development
- Fertilization
- Cleavage
- Gastrulation
- Organogenesis
- Metamorphosis
- Gametogenesis
fertilization
first stage of development that is characterized by the fusion of mature gametes
cleavage (stage of development)
second stage of development that is characterized by a series of rapid cell divisions during which the cytoplasm is split between smaller daughter cells (blastomeres), resulting in the blastula
gastrulation
third stage of development that is characterized by slower cell division and dramatic cellular rearrangements, resulting in 3 germ layers: ectoderm, mesoderm, endoderm
3 germ layers
3 distinct regions of the embryo (result of gastrulation) that give rise to differentiated cell types and specific organ systems:
1. Ectoderm
2. Mesoderm
3. Endoderm
ectoderm
outer germ layer of embryo that gives rise to:
1. Skin
2. Brain
3. Neural Crest
mesoderm
middle germ layer of embryo that gives rise to:
1. Blood
2. Heart
3. Kidney
4. Gonads
5. Bones
6. Muscles
7. Connective Tissue
endoderm
inner germ layer of embryo that gives rise to the digestive tube and its associated organs, including the lungs
organogenesis
fourth stage of development that is characterized by the formation of tissues and organs
What is unique in regard to organogenesis and the germ layers?
many organs actually contain cells from multiple germ layers; e.g., cells in the outer layer of skin (epidermis) are ectodermal while inner layers are mesodermal
notochord
rod of mesodermal cells that signals overlying ectoderm to become the posterior nervous system (essential to pattern posterior spinal cord); begins developing at 17 days and is gone by 7-10 weeks (undergoes programmed cell death)
metamorphosis (stage of development)
fifth stage of development characterized by the process of changing from immature to sexually mature organism
gametogenesis
sixth and final stage of development characterized by the process of producing gametes for reproduction; requires meiosis, which provides 4 germ cells
germ cells
gamete precursors that are set aside during very early development and critically involved in gametogenesis; different than somatic cells (all other cells of the body)
important things to remember about meiosis
- Chromosomes replicate prior to cell division so each gene is represented 4 times
- Replicated chromosomes (called chromatids) are held together by the kinetochore and all 4 chromatids pair together (recombination occurs)
- 1st meiotic division = separate chromatid pairs
- 2nd meiotic division = splits the kinetochore so each chromatid becomes a single chromosome
- Result = 4 germ cells with a haploid nucleus
von Baer’s laws (4 generalizations of vertebrate development)
- The general features of a large group of animals appears earlier in development than do the specialized features of a smaller group
- Less general characters develop from the more general, until finally the most specialized appear
- The embryo of a given species, instead of passing through the adult stages of lower (simpler anatomically) animals, departs more and more from them
- Therefore, the early embryo of a higher animal is never like a lower animal but only like its early embryo
von Baer’s first law
The general features of a large group of animals appears earlier in development than do the specialized features of a smaller group
- all developing vertebrates look very similar after gastrulation and diversity is only present later; all vertebrate embryos have gill arches, a notochord, a spinal cord, and primitive kidneys
characteristics of all vertebrate embryos after gastrulation (von Baer’s first law)
all vertebrate embryos after gastrulation have gill arches, a notochord, spinal cord, and primitive kidneys
von Baer’s second law
Less general characters develop from the more general, until finally the most specialized appear
e.g., early on, all vertebrates have a similar skin, and specializations such as scales, feathers, hair, etc. develop later
von Baer’s third law
The embryo of a given species, instead of passing through the adult stages of lower (simpler anatomically) animals, departs more and more from them
e.g., all embryonic vertebrates have gill arches. These are not the same as adult fish gills. Rather, fish elaborate and develop these structures into gills, while mammals develop these structures into the eustachian tubes (ear-mouth connection)
von Baer’s fourth law
The early embryo of a higher animal is never like a lower animal but only like its early embryo
e.g., human embryos never pass through a stage where they look like an adult fish or bird. Rather, human embryos, fish embryos, and bird embryos initially share common characteristics and look similar
fate map
identification of groups of cells in the gastrula that will become a particular tissue type in the adult; i.e., map of what tissues develop from what areas of the gastrula
2 methods of fate mapping
- Flourescent dye labeling
- Chimeric organisms (i.e., quail chick chimeras)
flourescent dye labeling fate mapping
- Inject cells of gastrula with green, flourescent tracking dye
- See where these cells go in the embryo
chimeric organism fate mapping
- Transplant cells from quail embryo into chick
- See where these cells go (indentifiable via differences in nuclear DNA condensation and antibodies via immunohistochemistry)
What did chimeric organism fate mapping reveal about the neural crest?
chimeric organism fate mapping revealed that the neural crest arises from the ectoderm and is adjacent to the developing neural tube, then delaminates and migrates away (neural crest migration)
What does the neural crest become?
- Pigment cells
- Peripheral glia
- Some peripheral neurons
- Enteric nervous system
- Craniofacial bones
What happened when the neural crest from a pigmented chicken was transplanted to an albino chicken?
transplant of neural crest from pigmented chicken embryo into albino chicken embryo resulted in the formation of black feathers in the adult albino
What arises from defects in neural crest migration?
defects in neural crest migration cause cleft palates
induction of the nervous system (general definition)
the ability of existing embryonic tissue to reprogram surrounding pluripotent cells to become the nervous system
What induces the nervous system?
Notochord and Hensen’s node/Spemann-Mangold Organizer/Shield
Hensen’s node
organizer/indicer of the nervous system in mammals; it forms at the anterior edge of the primitive streak from a primitive structure called Koller’s sickle, and can induce and pattern an embryonic axis; induces notochord and anterior nervous system
homologous to Spemann-Mangold organizer in amphibians and shield in zebrafish
discovery that Hensen’s node induces the nervous system
transplantation of Hensen’s node into a new animal results in the induction of a second neural tube (second nervous system development), proving that Hensen’s node is an organizing center
primitive streak
facet of embryo to which cells migrate during gastrulation to form the germ layers; location of Hensen’s node/Spemann-Mangold organizer/shield
Spemann-Mangold organizer
organizer/indicer of the nervous system in amphibians; it forms at the anterior edge of the primitive streak from a primitive structure called Koller’s sickle, and can induce and pattern an embryonic axis
homologous to Hensen’s node in mammals and shield in zebrafish
shield
organizer/indicer of the nervous system in zebrafish; it forms at the anterior edge of the primitive streak from a primitive structure called Koller’s sickle, and can induce and pattern an embryonic axis
homologous to Hensen’s node in mammals and Spemann-Mangold organizer in amphibians
discovery that the Spemann-Mangold organizer induces the nervous system
transplanting cells from the blastopore lip (Hensen’s node in mammals) to another embryo resulted in the formation of a second axis/nervous system in newts (frog)
steps in nervous system induction by Hensen’s node/SMO
- Hensen’s node/SMO induces notochord and anterior nervous system, also secreting Chordin and Dickkopf
- Notochord secretes Noggin
- Chordin and Noggin suppress bone morphogenetic protein (BMP) signaling to promote anterior neural fate, and Dickkopf suppresses Wnt signaling
effects of Chordin OR Noggin knockout
knockout of either Chordin OR Noggin has no effect, as one is able to compensate for the loss of the other
effects of Chordin AND Noggin knockout
knockout of both Chordin and Noggin results in a loss of facial structures and forebrain, indicating that BMP signaling must be suppressed
morphogens and anterior nervous system
anterior structures require:
1. Low BMP
2. Low Wnt
results of high Hensen’s node activity
morphogens and posterior nervous system
posterior structures require:
1. High Wnt
2. High BMP
3. High FGF
results of low Hensen’s node activity
neurulation
the process by which the neural plate becomes the neural tube
Where does neurogenesis occur?
the ventricular surface of the developing brain (neocortex); during development, this region is very thin and divided into the ventricular zone and marginal zone (dorsal); progenitor division occurs in ventricular zone
progenitor
cell from which animal originates (radial glial cells in the scope of the class); forms a neuroblast in assymetrical cell division
radial glial cells
progenitor cells with soma presence in the ventricular zone with long projections extending to the superficial surface of developing neural tissue; serve as the progenitor cells for all neurons and astrocytes of the CNS
general steps of neurogenesis
- Radial glial cell extends process to reach towards the pia (top layer) at the surface of the brain
- Interkinetic nuclear migration - DNA is replicated
- Radial glial cell retracts its apically extending arm
- Cell division
- Migration of neuroblast from ventricular surface
interkinetic nuclear migration
step in neurogenesis in which the nucleus of a radial glial cell migrates away and back towards the ventricular zone; necessary for DNA replication and ultimately cell division; occurs in the intermediate zone
2 types of cell division initiated by interkinetic nuclear migration
- Symmetrical division
- Assymetrical division
symmetrical cell division
happens early in development (almost every division early is symmetrical) to expand progenitor population; no neuroblasts are formed in this process
neuroblast
neural precursor cell that has yet to be differentiated; daughter cell of progenitor (radial glial cell) division
asymmetrical cell division
happens later in development to maintain progenitor pool size and produce a neuroblast; one neuroblast and one progenitor cell from each division
When are the majority of neocortical neurons born in humans?
between 5 weeks and 5 months of gestation
peak neurogenesis rate
250,000 new neurons per minute
subventricular zone (SVZ)
zone of the embryonic neocortex that is specific to primates, contains the second layer of proliferative progenitors, and gives rise to upper cortical layers
SVZ and upper cortical layers
the subventricular zone gives rise to the upper cortical layers, which are hypothesized to increase connectivity between diverse neural areas through bridging neuronal projections that concentrate in the upper layers
What cells maintain the ability to divide in neurogenesis?
- Daughter cells from an asymmetric division will NEVER divide again; i.e., neuroblast product has its differential path already established
- Some progenitor cells are maintained into adulthood, but the vast majority of neurons you are born with are what you have to work with
adult neurogenesis and thymidine
the initial discovery that adult neurogenesis occurs in mammals was made via radioactive thymidine; another study showing adult neurogenesis in canaries also used thymidine
BrdU
uridine analog that can be incorporated instead of thymidine into replicating DNA; antibodies against BrdU can be used for immunolabeling of DNA that includes BrdU; BrdU was later used to confirm intitial studies of adult neurogenesis in mammals
How was adult neurogenesis of the human brain discovered?
researchers have used carbon dating (C-14) from nuclear bomb tests to determine that adult neurogenesis occurs in humans; ~700 neurons are turned over in the hippocampus each day (~2%/year)
What determines whether a cell is a progenitor or a neuroblast?
during division, asymmetric compartmentalization of proteins (and RNAs) leads to fate determination
How many different types of cells can one radial glial progenitor produce?
MANY! Including different types of neurons and glia; this can also be influence by intra- and extracellular factors that dictate how cells move and what they become
2 primary types of cell migration in the developing cortex
- Cortical migration from ventricular zone of dorsal telencephalon to cortex
- Tangential migration from ventricular zone of the ventral telencephalon to cortex
cortical migration in cortex
migration of neuroblasts along radial glial fibers (like a scaffold) from the ventricular zone of dorsal telencephalon to cortex, giving rise to pyramidal neurons and astrocytes; “inside out” assembly, as the subplate + outer layers are formed before layer I
tangential migration in cortex
migration from 3 ganglionic eminences in the ventricular zone of ventral telencephalon to cortex, giving rise to inhibitory interneurons and oligodendrocytes; does not use radial glia, as instead, neurons maintain a stellate shape and are highly individually motile (some evidence for chain migration)
neuroblast morphology during cortical migration
neuroblasts “climbing” radial glial fibers to the cortex have a leading process, nucleus, and trailing process; the nucleus is pulled into the leading process as the neuron migrates up the fiber
inside out cortical assembly
in cortical migration, the outer layers are formed before layer 1; the first cells to migrate from the ventricular zone are the sublate cells, next cells to migrate make up layer VI, next cells layer V, and so on until layer I forms and subplate disappears
subplate
transient, most active fetal brain structure with glutamatergic (excitatory) neurons; first cells to migrate from the ventricular zone are the subplate cells; function of the subplate is unknown, but evidence points to a role in defining cortical regions for sensory processing
life cycle of the subplate
- Activity onset 9-10 weeks post-conception in humans
- Thickest 28-34 weeks post-conception
- Subplate is gone by 3 months post-birth
reelin mutation
reelin mutant disrupts inside-out cortical assembly and results in much less organized cortical layers; has an abnormal gate
reelin
extracellular matrix glycoprotein (protein bound by oligoacchardies) that is secreted by Cajal-Retzius neurons in the marginal zone and binds to Vldlr and Apoer2 to inhibit or stimulate migration, respectively
very low density lipoprotein receptor (Vldlr)
reelin receptor that, when bound by reelin, is a stop signal for migration (inhibitory)
Apoer2
reelin receptor that, when bound by reelin, is a go signal for migration (stimulatory); specific to late born cortical neurons
How was reelin discovered?
spontaneous mutation in a mouse line, named reelin for its abnormal gate; histopathological analysis revealed deficits in cortical laminar organization
abnormal reelin expression and disorders
abnormal reelin expression is linked to a variety of psychiatric disorders:
1. Schizophrenia
2. Autism
3. Bipolar disorder
4. Alzheimer’s
3 ganglionic eminences (tangential migration)
- Medial ganglionic eminence
- Lateral ganglionic eminence
- Caudal ganglionic eminence
medial ganglionic eminence (tangential migration)
produces GABAergic interneurons that migrate to cortex
lateral ganglionic eminence (tangential migration)
produces GABAergic interneurons that migrate in the rostral migratory stream to the olfactory bulb (smell)
caudal ganglionic eminence (tangential migration)
also produces inhibitory interneurons that migrate to cortex
What occurs immediately following neuroblast migration?
neuronal differentiation; immediate in that layer IV neurons differentiate before layer III neurons even migrate through
4 steps of neuronal differentiation
- Neurite outgrowth
- Axon and dendrite specification
- Target selection and stabilization
- Synapse formation
What regulates neuronal differentiation (very general)?
intracellular and extracellular signals
green flourescent protein (GFP)
discovery that changed our ability to visualize cells and intracellular processes, namely neuronal differentiation
Radial Unit Hypothesis
conceptual theory of cortical differentiation based on the idea that progenitor cells in the ventricular zone give rise to columns of cortical neurons with connected fates, meaning a cortical “protomap” exists in the ventricular zone
How many neurons do not migrate up radial glia (cortical migration)?
1/3 of all neurons do not go through cortical migration
means of cortical patterning
- Cortical migration via radial glial fibers
- Transcriptional patterning (Pax6 and Emx2)
- Projections (e.g., LGN input to visual cortex)
Pax6 and Emx2
transcription factors expressed by the anterior cortex (Pax6) and the posterior cortex (Emx2) that are essential for cortical patterning
Emx2 mutant
expands motor and reduces visual cortex patterning, causing a somatosensory posterior shift
Pax6 mutant
expands visual and reduces somatosensory and motor cortex patterning, causing an anterior shift of auditory and somatosensory
What happens if you eliminate LGN input to visual cortex?
drastically reduce the size of the visual cortex
cortical transplants and input sufficiency experiment
in rats, somatosensory cortex arranged in “barrels” that correspond to whisker fields; peeling off somatosensory cortex and replacing it with visual cortex results in visual cortex developing barrels, evidence that cortical architecture can be shaped by thalamic projections
axon microtubule polarity
unipolar, all minus ends toward cell body and all plus ends distal to cell body
dendrite microtubule polarity
mixed polarity, 50/50 mix of minus ends toward cell body
microtubule structure
made of alpha- beta- tubulin heterodimers that alternate in a single protofilament; composed of 13 protofilaments with a slight angle that yields a helical tube; has plus ends and minus ends
microtubule plus ends
fast growing
microtubule minus ends
slow growing
GTP cap
essential for microtubule dynamics; presence of GTP cap leads to polymerization/growth, loss of cap or hydrolysis to GDP leads to depolymerization/shrinking
4 stages of microtubule dynamics
- Growth
- Shrinking
- Catastrophe
- Rescue
growth stage of microtubule dynamics
constant addition of new heterodimers (polymerization) via presence of GTP cap
shrinking stage of microtubule dynamics
hydrolysis of GTP to GDP leading to instability of the polymer and heterodimer release (depolymerization)
catastrophe stage of microtubule dynamics
direct conversion from growth to shrinking
rescue stage of microtubule dynamics
direct conversion from shrinking to growth
dendritic polarization vs. axonal polarization
Dendrites:
1. Receptors
2. Mixed polarity microtubule structure
3. Varied organelle distribution
Axons:
1. Synaptic vesicles
2. Unipolar microtubule structure
3. Varied organelle distribution
NgCAM
cell adhesion molecule (protein) that is axonally polarized, meaning it knows how to go to axons; it is “cargo” that is transported to axons by motor proteins
TfR
receptor that is dendritically polarized, meaning it knows how to go to dendrites; it is “cargo” that is transported to dendrites by motor proteins
Vamp2
synaptic vesicle protein
What motor protein moves “cargo” into axons?
kinesin motor protein; moves cargo such as NgCAM to axon (soma to synapse), as it has a preference towards microtubule plus ends
What motor protein moves “cargo” into dendrites?
both kinesin and dynein motor protein; kinesin moves toward plus ends, dynein toward minus ends
motor protein “walking”
motor proteins attach to “cargo” (proteins) and “walk” along microtubules
kinesin
motor protein that moves cargo towards microtubule plus ends
dynein
motor protein that moves cargo towards microtubule minus ends
2 models for movement of cargo
- Smart motor
- Cargo steering
smart motor model for movement of cargo in microtubules
motor protein selects axon or dendrite (kinesin/dynein decide)
cargo steering model for movement of cargo in microtubules
cargo has an address label that dictates where motor moves it; this address label is likely applied during protein processing/vesicle formation in the Golgi
Once you have an axon, how does it find its target (very general)?
the axon growth cone
axon growth cone structure
- Large, flat lamellipodia
- Spikes emerging from lamellipodia called filopodia
- Microtubules invade center of the lamellipodia
- Actin underlies filopodia
3 regions of the growth cone
- P (peripheral) domain
- T (transitional) zone
- C (central) domain
peripheral domain of growth cone
outermost region of growth cone, contains filopodia/actin networks
central domain of growth cone
innermost region of growth cone, contains bundled microtubules with dynamic ends
actin
375 amino acid polypeptide that binds ATP when not in filament, but hydrolyzes to ADP and assembles into filaments called F-actin; filaments have a + (barbed end) and - (pointed end) end
ends of actin filaments (F-actin ends)
- Plus end: fast growing, also known as barbed end
- Minus end: slow growing, also known as pointed end
What is the role of actin in the growth cone?
actin is actively treadmilling in the growth cone to extend filopodia and facilitate growth cone motility; treadmilling is the term for actin dynamics during growth cone advance
actin assembly and the need for nucleation
actin subunits can assemble spontaneously but are highly unstable and rapidly disassemble; for filamentous actin formation, stability is needed, so actin nucleation must occur
actin nucleation
initial aggregate stabilized by multiple subunit-subunit contacts that forms the stable base of an assembling filament
2 actin nucleators
- Arp2/3 complex
- Formin
Arp2/3 actin nucleation
actin related protein (Arp) that binds to minus side of filamentous actin, forming a nucleation point, allowing the plus end to grow into a branched actin network in the lamellipodia
formin actin nucleation
formins are a family of proteins that work as dimers, binding to the actin plus end and recruiting two actin monomers; nucleates straight/unbranched actin filaments in the filopodia (as opposed to Arp2/3)
What would happen if actin assembly was not regulated?
more actin monomers = more likely to have polymer growth, so a cellular concentration of actin monomers high enough would lead to uncontrolled actin filament growth
2 regulators of actin assembly
- Thymosin
- Profilin
thymosin
regulator of actin assembly that binds actin monomers and prevents their ability to incorporate into filamentous actin
profilin
regulator of actin assembly that binds to actin monomers and enhances their ability to integrate into filamentous actin; formin-mediated assembly is augmented by profilin presence
formins vs. Arp2/3
formin nucleation result in straight, filamentous bundles of actin in filopodia, whereas Arp2/3 nucleation results in cross-linking/branching in the lamellipodia
filamin
actin cross-linking protein that allows filaments to be bound at roughly right angles; forms gel-like actin essential for lamellipodia, which is crucial for cell migration
periventricular heterotopia
caused by a mutation in filamin, resulting in newly born neurons staying where they are born in the periventricular region instead of migrating into the cortex; forms nodules and is associated with epilepsy that is resistant to AEDs
2 ways actin treadmilling advances the growth cone
- Polymerization and recycling of actin filaments
- In the peripheral domain, myosin II motor walks on actin to create traction forces that physically pull the growth cone forward towards adhesion sites
actin-mediated growth cone advance (methods of actin treadmilling)
- Mix of actin polymerization in the peripheral zone
-regulated by profilin and thymosin - Actomyosin-based contraction in the T-zone
-regulated by myosin, which is an actin-based motor protein - Actin depolymerization between the peripheral and T-zone
-regulated by cofilin and actin depolymerizing factor (ADF)
“Molecular Clutch”
actin retrograde flow in treadmilling does not exert force on its own; this force requires connection to substrate (the extracellular matrix), and connection is through focal adhesions
How do microtubule dynamics affect growth cone motility?
both stabilizing and destabilizing microtubules causes loss of dynamic + ends; while axons will continue to grow (very slowly), they cannot respond to directional cues without cytoskeletal regulation (think growth cone continuing through substrate instead of turning in example in class)
What cells does cortical migration give rise to?
pyramidal neurons and astrocytes
What cells does tangential migration give rise to?
inhibitory (GABAergic) neurons and oligodendrocytes; production via ganglionic eminences
intermediate zone
area of interkinetic nuclear migration; becomes white matter area after cortical layer assembly
marginal zone
top of neocortex where Cajal Retzius neurons secrete reelin; remains at the top of the cortex until all 6 cortical layers have formed
What are the rounds of selection that guide axon outgrowth?
- Pathway selection - choosing the correct path (e.g., ipsiateral/contralateral at optic chiasm)
- Target selection - choosing the correct area to innervate (e.g., axons in optic tract innervate LGN)
- Address selection - choosing the correct cells to synapse with (e.g., selecting appropriate LGN layer)
growth cone function
interacts with extracellular components to guide growth; if signals are permissive, filopodia are stabilized and growth cone advances, if signals are repulsive, filopodia retract and growth cone does not advance
permissive signals
extracellular signals that stabilize filopodia and allow growth cone to advance
repulsive signals
extracellular signals that cause filopodia to retract and prevent growth cone advance
3 stages of growth cone advance
- Protrusion
- Engorgement
- Consolidation
protusion stage of growth cone advance
characterized by extension of filopodia and lamellipodia
engorgement stage of growth cone advance
microtubules in the C domain extend closer to the P domain, fixing direction of growth
consolidation stage of growth cone advance
actin filaments in the growth cone neck depolymerize and the membrane shrinks to form a cylindrical shaft
From where do permissive signals originate?
- Extracellular matrix
- Other axons
- Cells long distances away (chemoattractants or chemorepellents)
glycoprotein
protein that has an oligosaccharide chain covalently attached to an amino acid side chain; compose the ECM along with proteoglycans
proteoglycan
subclass of glycoprotein that has a polysaccharide chain covalently attached to an amino acid side chain (side chain is an amino sugar); compose the ECM along with glycoproteins
laminin
heterotrimeric glycoprotein composed of alpha, beta, and gamma subunits that takes on a cruciform shape; plays major role in growth cone pathfinding in CNS but NOT PNS, mutations cause ultimate prevention of axon outgrowth
What are the results of laminin mutations?
mutations in the laminin alpha 1 gene result in defects in CNS (but normal PNS) axon pathways due to growth cone pathfinding errors; neurons without laminin can’t have axon outgrowth in CNS
affects retinal ganglion cell axons, early forebrain axons, and hindbrain reticulospinal axons
fibronectin
glycoprotein in the ECM that can promote proliferation and migration; i.e., critical for axon outgrowth and also plays a role in neuron migration
Where is fibronectin expressed?
- Expressed in dynamic patterns in regions of active morphogenesis (spinal cord and subplate)
- In ventricular zone during early CNS development
- Distributed along radial glial processes in association with preplate neurons and is produced by migrating neurons that target specific cortical domains
How do laminin and fibronectin interact with ephrin and netrin?
both laminin and fibronectin can alter the effect of classic guidance cues (ephrin and netrin) on axon direction; the effect of ephrin and netrin on axonal outgrowth (inhibition vs. attraction) depends on whether the ECM is primarily laminin or fibronectin
integrin receptors
alpha and beta integrin receptors (heterodimers) on the growth cone membrane bind to laminin and fibronectin to transduce signal; highest expression in the brain during development, and expression can promote axon extension in neurons that are normally not able to extend neurites
potent way to modulate axon outgrowth
What factors regulate the upstream and downstream activities of integrin receptor function?
- Talin (upstream)
- FAK and Src (downstream)
- Vinculin and paxillin (downstream)
talin
upstream regulator of integrin function that binds to beta subunit of integrin receptors, altering the angle of the transmembrane segment, allowing ECM components sich as laminin and fibronectin to bind it; loss of talin prevents integrin receptor activation
FAK and Src
downstream regulators of integrin function that are activated by growth factor signaling and integrin receptors to phosphorylate (they are kinases) actin and integrin receptors, modulating axon pathfinding by affecting actin stability
vinculin and paxillin
downstream regulators of integrin function that are focal adhesion proteins involved in the direct or indirect linkage between actin filaments and integrin receptors; i.e., create physical link between ECM components and actin
pioneer neurons
have axons with stereotypical growth cones (pathfinding role) that stretch as the nervous system expands using receptors that can respond to either chemoattractants or chemorepellents; guide neighboring axons (follower axons) to same targets
chemoattractant
diffusible molecule that acts over a distance to attract a growing axon; e.g., netrin
netrin
first chemoattractant identified, secreted by neurons in the ventral midline of the spinal cord and attracts axons of neurons from the dorsal horn to join the spinothalamic tract
chemorepellent
a diffusible molecule that acts over a distance repels a growing axon; e.g., slit and ephrin
slit
chemorepellent that binds to Robo receptors on growing axons; Robo is upregulated after growth cones cross midline of spinal cord, binding leads to continued growth away from the midline
How do chemoattractants/chemorepellents pattern the visual system (temporal retina)?
there is an ephrin gradient in the temporal retina across the tectum/superior colliculus with high levels in the posterior region, and axons that have the ephrin receptor will not grow where there are high levels of ephrin; that is, ephrin is high in posterior tectum and low in anterior tectum, so axons only grow in the anterior tectum (areas without ephrin)
ephrin
chemorepellent that plays a major role in the patterning of the visual system (temporal retina); ephrin is high in posterior tectum and low in anterior tectum, so axons only grow in the anterior tectum (areas without ephrin)
What are the ways in which chemoattractants and chemorepellents modulate directional growth cone advance through regulation of the cytoskeleton?
- Activation/inactivation of small GTPases such as Rho, Rac, and Cdc42
- Receptor-mediated phosphorylation of cytoskeletal regulators
- Direct binding to microtubules or microtubule binding proteins
cell adhesion molecules (CAMs)
bridge axons, allowing fasciculation (axon linkage)
fasciculation
adhesion of axons together by the action of surface CAMs; serves as another way of pathfinding by allowing follower axons to extend using a pioneer axon scaffold
How can other axons be an origin of permissive signals for axon outgrowth?
pioneer axons serve as a scaffold for follower axon extension; this process is carried out by fasciculation via CAMs
2 types of fasciculation
- Homotypic CAM interactions - between same CAM
- Heterotypic CAM interactions - between different CAMs
What is the result of L1CAM inactivation?
abnormal muscle innervation; blocking L1CAM function in a developing chick muscle resulted in axon “sprouting” (instead of controlled branching) due to loss of adhesion
L1CAM
gene that expresses proteins that are able to perform both homotypic and heterotypic CAM interactions; L1CAM is critical for controlled axon branching, as loss of L1CAM causes sprouting
NCAM
CAM that is essential for retina and tectum axon pathfinding via fasciculation; loss of NCAM results in axon misrouting and no fasciculation in retina and axon “sprouting” (due to loss of fasciculation) in the tectum/superior colliculus
What is the result of NCAM loss?
- Axons are normally in a clear, fasciculated track in the retina of chicken embryo, but blocking NCAM causes axon misrouting in the retina
- Loss of NCAM causes sprouting due to a lack of fasciculation in the tectum/superior colliculus; normally there is tight fasciculation
cadherins
another type of cell adhesion protein that regulates axon fasciculation; protocadherin17 is important for homotypic fasciculation of amygdala axons as they extend to the hypothalamus and ventral striatum
What is the result of protocadherin17 (cadherin) loss?
loss of tight fasciculation and sprouted axon growth of amygdala axons en route to the hypothalamus
3 axon guidance cues that govern targeted axon outgrowth
- cell-cell contacts via adhesion proteins (CAMs)
- cell-extracellular cue via ECM proteins and their receptors (laminin/fibronectin)
- cell-diffusible signal via chemoattractants and chemorepellents (ephrin, netrin, slit)
general steps of synapse formation in the CNS
- Dendritic filopodium contacts axon
- Synaptic vesicles and active zone proteins recruited to presynaptic membrane
- Receptors accumulate at postsynaptic membrane
steps of neuromuscular junction formation
- Motor neuron axon secretes protein called agrin into the basal lamina
- Agrin receptor on muscle receives agrin signal and activates MuSK (muscle specific kinase)
- MuSK activates Rapsyn
- MuSK and rapsyn together cluster ACh receptors into plaques; size of cluster is also dictated by neuregulin
neuregulin
secreted by axon during NMJ formation, dictates size of ACh receptor cluster
NMJ reciprocal signaling
basal lamina can trigger calcium influx into axon terminal, causing the release of more neurotransmitter (example of positive feedback); reciprocal signaling is necessary for synapse formation
Is the basal lamina an active participant in NMJ formation?
Yes, the basal lamina is an ative participant in NMJ formation; it plays a role in reciprocal signaling and agrin is released here
Are cell adhesion molecules (CAMs) involved only in axon guidance?
No, CAMs can regulate synapse formation and stabilization; the best characterized synaptic CAMs are neurexin and neuroligin
neurexin
synaptic CAM that is typically presynaptic and makes a connection with neuroligin (which is postsynaptic); there are 3 neurexin genes and multiple isoforms in the brain
neuroligin
synaptic CAM that is typically postsynaptic and makes a connection with neurexin (which is presynaptic); there are 4 neuroligin genes and multiple isoforms in the brain
What is the result of neurexin deletion?
- Decreased presynaptic calcium influx
- Decreased synaptic release probability
- Decreased synapse number (only in some brain regions; differences may be due to only partial redundancy between neurexins)
4 types of neuroligins
- Neuroligin 1 - excitatory synapses
- Neuroligin 2 - inhibitory, dopaminergic, and cholinergic synapses
- Neuroligin 3 - excitatory and inhibitory synapses
- Neuroligin 4 - glycinergic synapses
What happens as a result of changing neuroligin levels (increasing/decreasing)?
- Increasing neuroligin protein levels can increase synaptic density and synaptic transmission
- Loss of neuroligin function causes decreased synaptic transmission but NO change in synapse number
this inconsistency likely means that neuroligins have diverse functions at the synapse important for synaptic activity
What neuropsychiatric disorders are linked to neurexin mutations?
Large genomic deletions that remove neurexin 1 are linked to:
1. Schizophrenia (0.18% of cases)
2. Tourette syndrome (0.5% of cases)
3. Intellectual disability
4. Epilepsy
5. Autism spectrum disorders
What neuropsychiatric disorders are linked to neuroligin mutations?
mutations in neuroligin 3 and neuroligin 4 are linked to autism spectrum disorder; expression of an autism-lined neuroligin 3 mutant gene in mice casuses synaptic and behavioral abnormailites, such as deficits in social behavior (which is a proxy for autism-like behavior)
What is required for synaptic refinement?
synaptic pruning (cutting down synapses)
4 types of synaptic refinement
- Changes in synaptic capacity (# of target cells a neuron innervates)
- Synaptic rearrangement
- Synaptic segregation
- Programmed cell death
What is an example of changes in synaptic capacity at the NMJ?
- Start with an alpha motor neuron that innervates multiple muscle fibers
- Maturation - refines so each motor neuron innervates 1 muscle fiber
How are changes in synaptic capacity at the NMJ related to muscle activity?
Synaptic loss requires muscle activity
1. Silencing muscle (block all ACh receptors) = retain polyneuronal innervation
2. Activating muscle = accelerates removal of all but 1 innervating neuron
Note that if just a subset of ACh receptors are silenced, removal of innervating neurons still occurs
ACh and motor neuron capacity refinement
Blocking a subset of ACh receptors leads to…
1. Loss of postsynaptic ACh receptors on the muscle fiber
2. Disassembly of the presynapse
3. Axon retraction
Not that if there is a loss of activity at ALL receptors, there is maintained polyneuronal innervation
activity dependent synaptic rearrangement
change in how many synapses individual input neurons have on receiving neuron due to neural activity and synaptic transmission; receiving neuron maintains same total number of synapses but how many come from each input varues
activity dependent synaptic segregation
change in which neurons axons synapse on due to neural activity; classic example is segregation of eye-specific inputs in LGN
example of “neurons that fire together, wire together” in the visual system (synaptic segregation)
neurons from one retina fire simultaneously, which strenghtens their synapses on target neurons in the LGN; also called Hebbian synapses; eye 1 has simultaneous firing, strengthening their LGN synapse, while eye 2 has different simulataneous firing, strengthening their own LGN synapse
synaptic plasticity
strengthening or weakening synaptic connections; 2 rules:
1. Fire together, wire together (LTP)
2. Fire out of sync, lose their link (LTD)
What is the impact of a single synapse on firing rate of postsynaptic neuron (plasticity)?
a single synapse has little influence on firing rate of postsynaptic neuron; rather, activity of a synapse must be correlated with activity of many other inputs converging on the same postsynaptic neuron
receptors involved in synaptic transmission in the immature visual system (plasticity)
Receptors can be either metabotropic (GPCR) or ionotropic; ionotropic glutamate receptors can be further classified as…
1. AMPA receptors - glutamate-gated ion channels
2. NMDA receptors - ion channels with unique properties
What is required for NMDA receptor activation?
Both depolarization AND glutamate needed to activate and allow Ca2+ influx; Mg2+ blocks the channel at certain negative Vm, even if glutamate binds
How does the NMDA receptor act as a co-incidence detector (Hebbian molecule)?
Ca2+ entry though NMDA receptor specifically signals that pre- and postsynaptic neurons are active at the same time; neurons that fire together, wire together (LTP)
ocular dominance columns
stripes of neurons in the visual cortex of certain mammals (including humans) that respond preferentially to input from one eye or the other; discovered via injection of radioactive proline into one eye of cat, showing that eyes innervate alternating stripes in the visual cortex
What happens if you permanently close one eye?
ocular dominance columns from that eye are reduced and those from the opposite eye are expanded; this can be reversed if eye is opened, but ability to modify columns is only during critical period (up to 6 weeks of age)
critical period
developmental time during which innervation patterns can be modified
SMO critical period in development
Spemann Mangold organizer transplantation could induce a second axis in the host, but only during a certain period in development before the host cells had already been committed to a different fate
example of critical period in behavior
Konrad Lorenz found that social attachment formed within a certain period of time for a graylag geese; within 2 days of hatching, goslings would imprint on a moving object and treat that object as their mother
critical period at the synapse
early in development, large scale changes of innervation patterns can change, but this is not possible at later/adult stages; in adult, plasticity is restricted to local changes in synaptic efficacy (LTP/LTD)
hypotheses on critical period endings for synaptic plasticity
- Plasticity diminishes when axon growth stops
- Plasticity decreases when synaptic transmission matures
- Plasticity decreases when cortical activation is constrained
How is LTP acheived?
Ca2+ enters postsynaptic terminal through NMDA receptors and activates CaMKII, which acheives LTP by:
1. Increasing effectiveness of existing AMPA receptors via phosphorlyation
2. Upregulating AMPA receptors
How is LTD acheived?
Low levels of Ca2+ causes:
1. Increase of protein phosphatase activity
2. Dephosphorlyation of AMPA receptors on the membrane
3. Removal of existing AMPA receptors
long term potentiation (LTP)
consistent with high levels of Ca2+, increasing the number of AMPA receptors at the synapse, leading to an increased likelihood and amplitude of firing
long term depression (LTD)
consistent with low levels of Ca2+, decreasing the number of AMPA receptors at the synapse, leading to decreased likelihood and amplitude of firing
apoptosis
programmed cell dealth that occurs largely after axons have reached their targets; there is a noticeable decline in neuron and axon number due to apoptosis
How many neurons undergo apoptosis during brain development?
estimated that ~1/3 of the neurons that differentiate during development ultimately die before adulthood (estimates vary from 20%-50%)
Where does apoptosis of progenitor cells occur?
progenitor cells in the ventricular zone show high levels of apoptosis during late development
Where does apoptosis occur?
programmed cell death during development occurs in every part of the nervous system
apoptosis of frog hindlimb and rat retinal ganglion cells
in frogs, peak motor neuron population is ~4000 during development of the limb, but by adulthood, ~1200 motor neurons exist (60% are eliminated)
~50% of rat retinal ganglion cells also die during development
Why does apoptosis occur?
apoptosis is essential for neural development; when apoptosis is blocked, lethal overgrowth occurs
trophic factors
regulators of apoptosis (survival factors) that are provided in limited quantities and necessary for the maintenance of neuronal connections and neuronal survival
5 primary sources of trophic factors
Neurons receive trophic factos from:
1. Target tissues they innervate (retrograde); most common
2. Synaptic inputs
3. Neighboring neurons that do not synapse directly on them; via paracrine interactions
4. Distant cells through circulatory system
5. Glial cells
nerve growth factor (NGF)
retrograde trophic factor (from target tissue) that is produced by target tissues of sympathetic neurons
discovery of NGF
It was discovered that manipulation of limb bud number directly dictates size of dorsal root ganglia sensory neurons (removal –> fewer neurons), leading to this experimental procedure:
Hypothesis: The target tissue (limb) secretes a factor that stimulates neuron proliferation
Experiment: Remove limb and count neurons
Results: No change in number of differentiated neurons, BUT neurons die when axons reach amputated stump
New Hypothesis: The target tissue secretes a factor that is essential for neuron survival
Experimentation showed that application of snake venom and tumors caused axon survival at stump (and outgrowth), showing that a specific protein is sufficient
Necessity for this protein proved via injection of antibodies into rabbit; ganglia are reduced with antibody injection (survival inhibited)
How long after NGF withdrawal does apoptosis occur?
sympathetic neurons undergo programmed cell death within 24-48 hours after NGF withdrawal
internal cell death program
after NGF withdrawal, the mitochondria releases Cytochrome C, which activates caspases that initiate cell death
neurotrophins
subset of neurotrophic factors that have similar structures; NGF is one
neurotrophin structure
produced as pro-peptides (~250 amino acid long protein) that are processed post-translationally and cleaved (~120 amino acid peptide is the final product), and peptides holodimerize to create a biologically active molecule
What neurotrophins share conserved protein seqeunces?
NGF, BDNF, NT-3, and NT-4 share 50% protein homology and have a unique domain necessary to bind specific receptors
neurotrophin receptors
bind 1 or 2 neurotrophins; there is a high homology between receptors, ~50% homology in the extracellular domain, and each receptor is highly spliced, leading to an array of variability in receptor sequence
To what receptor does NGF bind?
TrkA
How does neurotrophin signaling affect other (non-tagret) neurons?
neurotrophin signaling is critical to the survival of different neurons
Where in the neuron do neurotrophin signals function?
signals received from target tissue (e.g., NGF) can function locally in the axon terminal or distantly in the cell body
What happens when neurotrophins are applied to axons?
target-dependent neuronal survival, meaning survival of both axons and soma; this differs from application to soma, in which just the soma survives
What happens when neurotrophins are applied to soma?
survival of soma but degradation of axons; this differs from application to axons, in which both axons/soma survive (target-dependent neuronal survival); this supports a role for neurotrophin signaling in the cell body for neuronal survival AND a role for local neurotrophin activity in the axon terminal for axon maintenance
What is the role of neurotrophins from target tissues?
- Local processes in the axon terminal that support axon maintenance
- Distant processes in the cell body that support neuron survival (circumvented by direct application of the neurotrophin to the cell body in cultured neurons)
What does neurotrophin signaling in the cell body actually mean?
changes in transcriptional programs; e.g., NGF activates CREB and NFAT, two powerful transcription factors
What does neurotrophin signaling in the cell body actually do?
produces proteins essential for axon maintenance and cell survival (changes in transcription); prevents mitochondrial induced apoptosis
retrograde transport of signaling endosomes
means by which activation of a neurotrophin receptor in the axon termnal causes changes in nuclear transcription in the cell body; for this, ligands (NGF) and receptors (TrkA) must be internalized into a signaling endosome
retrograde transport of signaling endosomes cascade
- Target tissue secretes NGF, which binds to TrkA on axon terminal
- Binding facilitates budding of vesicles on plasma membrane that are cleaved into endosomes that contain activated TrkA receptor + ligand
- Motor proteins carry endosome from axon terminal to cell body
What transcriptional factors are activated by NGF?
CREB and NFAT
What cells do laminin mutations affect?
affects retinal ganglion cell axons, early forebrain axons, and hindbrain reticulospinal axons; PNS not affected
similarities and differences of mammalian brains
look very different and are different sizes, but actually just slight modifications of the same theme; there is similar structure in all mammalian brains and common subcortical areas amongst vertebrates
2 divisions of nervous system
- Central nervous system (CNS) - cell bodies in brain and spinal cord
- Peripheral nervous system (PNS) - cell bodies outside the brain and spinal cord
rostral
anterior
caudal
posterior
dorsal
top; latin for back
ventral
bottom; latin for belly
cerebrum
largest, most anterior part of the brain that is divided into two hemispheres separated by sagittal fissure; hemispheres mediate sensation and movement in contralateral body
cerebellum
“little brain” that contains as many neurons as the cerebral hemispheres despite being smaller; it is the movement control center and has extensive connections rostrally to cerebrum and caudally to spinal cord; control in the cerebellum is ipsilateral
control in cerebrum vs. cerebellum
control in the cerebrum is contralateral while control in the cerebellum is ipsilateral
brain stem
most primitive part of the brain that has a complex nexus of fiber tracts and nuclei; fiber tracts connect cerebrum to spinal cord and cerebellum and nuclei control basal body functions such as breathing, consciousness, body temp, etc.; damage to brain stem is typically fatal
spinal cord
major conduit between body and brain that is enclosed by bony vertebral column; nerves enter and exit the central nervous system from the body in the spinal cord
2 primary regions of spinal cord
- Dorsal horns (have dorsal roots)
- Ventral horns (have ventral roots)
dorsal roots
where sensory information enters the spinal cord; there are cell bodies of sensory neurons situated outside the spinal cord in dorsal root ganglia (DRGs), and central, afferent projections leave cell body and enters dorsal horn
ventral roots
where motor information leaves the spinal cord; there are cell bodies in the ventral horn of the spinal cord, and axons of these neurons exit the spinal cord to innervate muscles in the periphery (efferent projections)
dorsal roots vs. ventral roots
dorsal roots involve afferent projections in which sensory information enters the spinal cord (afferent projections), while ventral roots involve efferent projections in which motor information leaves the spinal cord (efferent projections)
When do the neural plate/neural tube develop in humans?
neural plate develops at ~17 days in humans and neural tube about ~22 days in humans
primary neurulation
cells surrounding the neural plate direct the plate cells to proliferate, invaginate, and pinch off from the surface, forming a hollow tube
secondary neurulation
neural tube arises from cells that coalesce into a solid cord that subsequently hollows, eventually forming a hollow tube; only the posterior part of the neural tube develops from secondary neurulation in mammals
steps of primary neurulation
- After neural plate formation, edges thicken and move up from neural folds
- Neural groove appears in center of plate
- Neural folds migrate towards the embryonic midline
- Neural folds fuse creating a hollow tube
In how many places does neural tube closure begin in humans?
in humans, neural tube closure begins in ~3 places
What are the open ends of the neural tube at the end of neurulation?
- Anterior neuropore
- Posterior neuropore
2 major neurulation defects
- Anencephaly - anterior closure fialure
- Spina bifida - posterior closure failure
What is thought to be the primary cause of neurulation defects?
1 in 500 births has neurulation defects due to nutritional deficits; it is estimated that 90% of the birth defects are due to lack of folic acid (role of folic acid in this process is unknown)
3 primary brain vesicles
The entire brain develops from these primary vesicles that from from swelling of the anterior neural tube:
1. Prosencephalon (forebrain)
2. Mesencephalon (midbrain)
3. Rhombencephalon (hindbrain)
What is the first step of neural differentiation (vesicles)?
swelling of the anterior neural tube leading to the formation of the primary vesicles; the entire brain develops from these primary vesicles
What are the first steps in differentiation of the prosencephalon (forebrain)?
2 telencephalic vesicles and 2 optic vesicles sprout off the sides of the prosencephalon, leaving the diencephalon as the residual structure
How does the retina develop from the optic vesicle?
- Optic vesicles grow and invaginate, forming the optic stalks and the optic cups
- The optic stalks become the optic nerves and the optic cups become the retinas in the adult
retinas and optic nerves are derived from the neural tube (anterior after swelling forms primary vesicles)
Where does the telencephalon come from and what does it give rise to?
2 telencephalic vesicles sprout off the sides of the prosencephalon, giving rise to…
1. Cerebral hemispheres
2. Olfactory bulbs
3. Basal telencephalon
grows lateral and posterior to cover diencephalon
Where does the diencephalon come from and what does it give rise to?
the diencephalon is the residual structure after vesicles sprout off the sides of the prosencephalon, giving rise to…
1. Thalamus
2. Hypothalamus
What does the cavity in the center of the developing brain develop into?
the cavity in the center of the developing brain develops into the fluid filled ventricular system; 2 lateral ventricles are in the telencephalon and 1 ventricle is in the diencephalon
3 major white matter systems
axons extend from developing prosencephalon (forebrain) to other parts of the nervous system and bundle into 3 main systems:
1. Cortical white matter
2. Corpus callosum
3. Internal capsule
cortical white matter
all axons that run to and from the neurons within the cerebral cortex
corpus callosum
white matter system that is continuous with the cortical white matter, forms a bridge that links the two hemispeheres
internal capsule
white matter system that is continuous with the cortical white matter and links cortex with brain stem
What does information pass through to get to the cortex?
thalamus
What does the cerebral cortex do?
analyzes sensory input and commands motor output
thalamus
gateway to the cortex; sensory neuron connections all pass through the thalamus en route to the cortex; axons from the thalamus to cortex are in the internal capsule, and information is relayed contralaterally
2 main cortex to spinal cord routes
- Direct route from cortex to brain stem is the corticospinal tract
- Other path first synapses on basal ganglia
basal ganglia
critical structures for voluntary movement that are made up by multiple nuclei and implicated in alternate pathway from cortex to spinal cord; basal ganglia are lost in neurodegenerative diseases such as Huntington’s and Parkinson’s
With what diseases do basal ganglia have implications?
basal ganglia are lost in…
1. Huntington’s disease - loss of BG in caudate (posterior) nucleus and putamen of prosencephalon
2. Parkinson’s disease - loss of basal ganglia in substantia nigra
What does the mesencephalon (midbrain) give rise to?
- Dorsal surface becomes the tectum
- Floor becomes the tegmentum
- Center remains open as the cerebral aqueduct - connected to ventricular system
tectum
derived from the dorsal surface of the mesencephalon (midbrain), contains the superior and inferior colliculus, each critical relays for sensory information en route to the thalamus
superior colliculus
part of tectum that receives input from the eye; also called the optic tectum
inferior colliculus
part of tectum that receives input from the ear
tegmentum
derived from the floor of the the mesencephalon (midbrain), contains the substantia nigra and red nucleus
substantia nigra
part of tegmentum that degenerates in Parkinson’s disease
red nucleus
part of tegmentum that plays a role in limb movement and is critical for infant crawling
symptoms of Parkinson’s disease
symptoms are due to a loss of dopamine signaling from substantia nigra to striatum and other brain areas and include:
1. Tremors
2. Rigid movement
3. Mask-like face
4. Stooped posture
5. GI
6. Dementia and hallucinations (late stage)
What does the rhombencephalon (hindbrain) give rise to?
- Pons - from metencephalon (rostral (anterior) hindbrain)
- Medulla oblongata - from myelencephalon (caudaul (posterior) hindbrain)
- Cerebellum - from metencephalon (rostral (anterior) hindbrain)
- 4th ventricle sits between the dorsal and ventral hindbrain derivatives
metencephalon
rostral (anterior) hindbrain (rhombencephalon) that gives rise to the pons and cerebellum
myelencephalon
caudal (posterior) hindbrain (rhombencephalon) that gives rise to the medulla oblongata
stages of anterior hindbrain development
- Initially, the rostral hindbrain is a simple tube
- Dorsal-lateral wall of the tube grows dorsally and medially (called the rhombic lips)
- This then expands into the cerebellum
- The ventral wall (below the 4th ventricle) becomes the pons
pons
switchboard connecting cerebral cortex to cerebellum that derived from the ventral wall of the metencephalon (rostral (anterior) hindbrain), has nuclei that deal with…
1. Sleep
2. Respiration
3. Swallowing
4. Bladder control
5. Hearing
6. Equilibrium
7. Taste
8. Eye movement
9. Facial expressions
10. Facial sensation
11. Posture
90% of all axons passing through the midbrain synapse on a nucleus in the pons (~20,000) axons
differentiation of the caudal hindbrain
forms the medulla, and the ventral and lateral walls swell, leaving only a thin layer of non-neuronal cells; medullary pyramids are present on the lateral edges of the ventral medulla (white matter tracts)
medullary pyramids
white matter tracts that form on the lateral edges of the ventral medulla after the ventral and lateral walls swell
medulla oblongota
carries axons of the corticospinal tract, providing a direct connection from brain to spinal cord; where the medulla joins the spinal cord, these tracts cross (pyramidal decussation); derived from the myelencephalon (caudaul (posterior) hindbrain), has medullary pyramids on the lateral edges; critical for…
1. Heartrate
2. Respiration
3. Blood Pressure
4. Reflexes (vomiting, coughing, sneezing, swallowing)
How many axons act at the pons?
90% of all axons passing through the midbrain synapse on a nucleus in the pons (~20,000)
pyramidal decussation
descending axons that pass through the midbrain/pons into the spinal cord pass through the medullary pyramids; at these medullary pyramids, near where the medulla meets the spinal cord, axons in the corticospinal tract cross from ipilateral to contralateral, explaining contralateral control of movement
gyri and sulci
part of human brain that allows expansion of cortical size while fitting in the skull; the central sulcus divides the frontal and parietal lobes
human vs. rat brain
similar basic arrangement of major structures, but the rat has a larger olfactory bulb and humans have gyri and sulci (ridges); human brain also has defined cortical regions based on sulci landmarkers
4 lobes of the human brain
- Frontal
- Parietal
- Temporal
- Occipital
frontal lobe
role in reasoning, impulse control, integration; also contains motor and premotor cortex for higher order motor control/voluntary movement
parietal lobe
role in processing somatosensory information
occipital lobe
vision/visual processing
temporal lobe
role in language/auditory processing, memory, visual perception
common features of cerebral cortex in vertebrates (features that distinguish cortex from other brain regions)
- Cell bodies of cortical neurons are always arranged in layers or sheets that lie parallel to the surface of the brain
- Most superficial layer (Layer I) has no neurons, and is instead made of neuronal processes; lies beneath pia mater (meninges)
- At least one cell layer has pyramidal cells that have large apical dendrites that extend up to layer I
What is unique about the neocortex?
it is unique to mammals
What does the neocortex contain?
- Hippocampus (seahorse)
- Olfactory cortex
hippocampus
part of neocortex, shaped like seahorse, critical for learning and memory
olfactory cortex
part of neocortex, contiguous with olfactory bulb for smell
What are Brodmann’s areas?
Brodmann divided the cortex into regions based on similarities in cytoarchitecture, but it was later determined that his divisions reflected functional differences between cortical regions
Brodmann area 17
visual cortex that receives information from retina after traversing thalamus
Brodmann area 4
motor cortex that projects axons through to ventral horn in the spinal cord for voluntary movement
Brodmann area 44/45 (Broca’s area)
language production
Brodmann area 42/22 (Wernicke’s area)
language comprehension
neocortical evolution
the cortex amount has evolved, but not the structure (e.g., surface area increase from monkey to human); that is, there have always been primary sensory areas, secondary sensory areas, and motor areas, but association areas of cortex is recent evolution; this is why we can use “model” organisms to study neurobiology
structures of the human brain
What is Alzheimer’s Disease?
progressive neurodegenerative disease that impairs memory and other mental functions; most common type of dementia (60-80% of all dementia cases)
discovery of Alzheimer’s
patient had symptoms such as loss of memory, delusions, and even vegetative states; brain autopsy revealed thinning of the cerebral cortex, formation of senile plaques, and tangles found in nerve fibers
2 aggregates of Alzheimer’s
- Senile plaques
- Neurofibrillary tangles
senile plaques
aggregates of Alzheimer’s that are extracellular and composed of cleaved Beta Amyloid
neurofibrillary tangles
aggregates of Alzheimer’s that are intracellular and composed of Tau
beta amyloid (Abeta)
originates from the transmembrane protein APP, which is cleaved to from beta amyloid; abnormal aggregates of Abeta make up senile plaques
amyloid precursor protein (APP)
transmembrane protein that is cleaved by secretases to form beta amyloid, which makes up senile plaques; function of APP unknown, but APP or secretase mutations cause early-onset (hereditary) AD
tau
microtubule associated protein that is important for microtubule stability; in AD, tau forms aggregates which make up neurofibrillary tangles
tau structure
tau is a natively unfolded protein with very little structure that binds and provides structure to microtubules in its normal state; hyperphosphorlyation of tau inhibits its activity and leads to microtubule instability, as well as promotes aggregation with other tau molecules
How does hyperphosphorylation impact tau?
inhibits its activity and leads to microtubule instability, as well as promotes aggregation with other tau molecules
amyloid cascade hypothesis
Hypothesis: amyloid aggregation causes damage to neurons
1. Improper APP processing
2. Increased amyloid beta aggregation
3. Impaired neuronal function, inflammation, and oxidative injury
4. Disruption of tau and aggregation
5. Neuronal death
support/problems of the amyloid cascade hypothesis
Support:
1. Greatly increased Abeta levels in AD
2. Mutations associated with AD promote amyloid aggregation
3. Mutant APP in mice causes plaque formation and some mice show cognitive decline
Problems:
1. Abeta accumulation does not correlate with neuronal loss and cognitive decline (senile plaques can form in healthy people)
2. Treatments designed to target amyloid plaques have had little to no effect on Alzheimer’s symptoms
toxic oligomer hypothesis of AD/evidence that supports it
Hypothesis: small amyloid beta oligomers have a toxic effect on neurons
There is evidence that oligomers can…
1. Bind receptors and disrupt cell signaling
2. Disrupt synapses
3. Impair tau function
4. Disrupt calcium homeostasis
tau hypothesis of AD
Hypothesis: tau is the primary cause of Alzheimer’s pathology
Supporting evidence:
1. Tau has a known function in neurons that could cause harm if disrupted
2. Tau aggregates correlate with dementia severity/type of impairment
3. Mutant tau animal models show neuronal loss that can be rescued with drugs that prevent tau aggregation
example research on the tau hypothesis
Question: What causes tau hyperphosphorylation?
Hypothesis: The kinase GSK-3beta is found in neurofibrillary tangles and may be the cause of aggregate-promoting phosphorylation
Experiment: Add GSK-3beta to tau filaments in vitro
Result: tangle-like aggregates form
Conclusion: GSK-3beta is a potential inducer of tau hyperphosphorylation and aggregation
Why is the substantia nigra lost in Parkinson’s?
biggest link is defects in mitophagy, or the degradation of damaged mitochondria
mitophagy
the degradation and clearance of damaged mitochondria; defects in this process are the biggest link to the loss of the substantia nigra that occurs in Parkinson’s disease
Mutations in what mitophagy proteins cause Parkinson’s?
PINK1 and PARKIN
mitochondrial functions in neurons
- ATP-dependent ion pumps
- Signaling molecules
- Axon branching
- Regulation of cell death
- Calcium homeostasis
3 innerconnected mechanisms of mitochondrial maintenance
- Biogenesis - birth of new mitochondria
- Fission/fusion dynamics - exhange of mitochondrial materials
- Mitophagy - degradation of damaged organelles
How are mitochondria damaged?
making ATP causes reactive oxygen species production, and ROS damages proteins and lipids in the mitochondria; mitophagy degrades these damaged organelles
2 types of mitophagy
- Axonal mitophagy - in the axon, damaged mitochondria are non-selectively packaged for degradation with other proteins and organelles
- Cell body mitophagy - in the cell body, selective mitophagy occurs (only mitochondria, not proteins and other organelles)
PINK1-PARKIN mitophagy pathway
- Under basal (healthy) conditions, PINK1 is cleaved by mitochondrial protease and degraded
- When mitochondria are unhealthy, PINK1 is not cleaved and accumulates on the outer mitochondrial membrane
- PINK1 then activates PARKIN
- PARKIN ubiquinates proteins on the mitochondria to start process of autophagy, with step 1 being to engulf the organelle in an autophagosome membrane
- Degrad the organelle
How does loss of PINK1/PARKIN affect neurons in animal models?
minimal/no effects in mouse models, loss of dopamine neurons in rat, fish, and fly models
