NEUR 2100 Midterm

gastrulation

the process in which an embryo transforms from a single layer of cells (the blastula) into the three germ layers

three germ layers

ectoderm, mesoderm, endoderm

neurulation

the process in which a subset of cells within the ectoderm differentiate into neuroectodermal precursor cells that form the neural plate, this process is dependent on the formation of the notochord (from mesoderm) which defines the midline axis

neural tube

formed at the midline of the neural plate, consists of neural stem cells, the floor plate, the roof plate, and the neural crest, when neural tube formation is complete the mesoderm forms somites

somites

precursors of axial musculature and skeleton

neural induction

generation of neural cell identity, occurs via the spatial and temporal control of gene expression by signaling factors, these factors come from the roofplate, floorplate, notochord, somites, and neuroectoderm; multipotent neural precursor cells and radial glial cells populate the neural tube - the precursor cells undergo mitosis near the ventricular surface, eventually postmitotic neuroblasts are produced by asymmetric division

spatial vs temporal control

spatial = signaling factors turn specific genes on/off in the correct location

temporal = signaling factors turn specific genes on/off at the right time

neural inductive signaling factors (6)

retinoic acid, fibroblast growth factors, bone morphogenetic proteins, Wnts, sonic hedgehog, delta/notch

retinoic acid (RA)

released by many inductive structures (roofplate, notochord, floorplate), drives cellular differentiation of neural stem cells, binds to retinoid receptors and creates a complex that interacts with transcription cofactors (coactivators or corepressors) to modulate gene expression (e.g., hox genes, hif1a, sonic hedgehog), also involved in neural patterning and axonal growth following early embryogenesis

result of vitamin a excess/deficiency

vitamin a = retinoic acid, excess/deficiency can cause birth defects due to neural tube malformation

two superfamilies of peptide hormones

fibroblast growth factors, transforming growth factors (e.g., bmps)

fibroblast growth factors (FGFs)

23 ligands (in the family), secreted into the extracellular matrix and bind to receptor tyrosine kinases to activate ras-MAP kinase pathway, fgf8 is particularly important for forebrain and midbrain development

bone morphogenetic proteins (BMPs)

6 ligands, released by the roofplate and somites, important for guiding differentiation of the dorsal spinal cord and hindbrain and neural crest, also key for the initial induction of the neural ectoderm, act on receptor serine kinases that complex with SMAD (transcription factors), the action of bmps is regulated by noggin and chordin (endogenous antagonists) - when bmps are bound to these they are prevented from binding receptors and neuralization continues (default for ectodermal cells)

Wnts

19 ligands, can act on the non-canonical and the canonical pathway

canonical pathway

activation of frizzled receptor (by wnts) and stabilization of B-catenin, which translocates to the nucleus and interacts with transcription factors to induce gene expression

non-canonical pathway

regulates the cell movements and fate leading to the lengthening of the neural plate and tube via activation of frizzled and dishevelled (by wnts) causing changes to intracellular calcium and protein kinase c

sonic hedgehog (Shh)

important for the closure of the neural tube and driving differentiation of neurons within the ventral neural tube (e.g., motor neurons), expression is high in the notochord and floorplate during early embryogenesis, acts on two surface receptors (patched and smoothened) that leads to translocation of gli1 or gli2 (transcriptional regulators), when shh isn’t present an inhibitory complex forms and only gli3 is active in the nucleus and it represses expression of target genes

delta and notch

involves the interaction of transmembrane ligands (delta) and surface receptors (notch) - this interaction must occur between neighbouring cells, when bound the notch intracellular domain (nicd) is cleaved and translocates to the nucleus, nicd binds to the transcription factors that repress the expression of neurogenic factors basic helix-loop-helix (bhlh), relative notch and bhlh transcription factors also regulate the generation of glial cells (oligodendrocytes and astrocytes)

spatial scale

signals work together to create gradients of effects that regulate neural tube formation and differentiation during different phases of development

glial patterning: temporal scale

macroglia are derived from neural progenitors (radial glial cells) and are regulated by inductive factors and gene expression following neurogenesis

peripheral migration

migratory path of neural crest cells are first influenced by the anterior-posterior (rostral-caudal) position of the neural tube, neural crest cells start as neuroectodermal cells and then undergo epithelial to mesenchymal transition, cells upregulate snail 1 and 2 (bhlh family members) which inhibit protein expression for intercellular junctions and cellular adhesion, neural crest cells are guided into the periphery by extracellular signals expressed by non-neuronal structures in the periphery (e.g., somites), precursor cells of the neural crest follow different pathways (some become sensory and autonomic ganglia, some chrommafin cells of the adrenal gland, some non-neuronal)

what do neural crest cells that migrate further away from the ectoderm layer give rise to, where do they come from

sensory and autonomic neurons and glial cells, these come from the mid-posterior trunk region of the neural tube (anterior trunk forms the cardiac crest)

development of schwann cells from neural crest cells (3 steps)

1) neural crest cells form schwann cell precursors

2) schwann cell precursors generate immature schwann cells

3) immature schwann cells form myelinating or non-myelinating cells that ensheath large and small axons

signals and factors that determine cell fate in the pns

sex determining region y box 10 (sox10) is expressed by all neural crest cells and is required for glial lineage differentiation, early neurons and schwann cells precursors rely on each other for survival, sox10 encodes for erbb2 (a neuroregulin receptor), neuroregulin 1 (nrg1) is an axon-derived survival factor for schwann cell precursors and drives proliferation of these cells, bmps inhibit glial differentiation of neural crest cells (stimulate formation of sympathetic neurons), myelin associated transcription factor krox20 prepares immature schwann cells for myelination, c-jun signals drive de-differentiation

neuroregulin 1 (nrg1)

autocrine survival factor in immature schwann cells, inhibits neurogenesis

immature → mature schwann cells

the transition of immature to mature schwann cells is a reversible process, determination of the role of mature schwann cells is dependent on the large or small axon it associates with, schwann cells secrete growth factors (e.g., fgf, ntf-3) that can support their own survival and are no longer dependent on neurons, in late embryogenesis immature schwann cells undergo radial sorting and begin to ensheath single large axons, number of schwann cells is controlled by cell death mediated by the p75 receptor and transforming growth factor B (tgfb)

myelination

provides insulating sheath on neurons to enable saltatory conduction, schwann cells myelinate axons in pns, oligodendrocytes myelinate axons in cns, myelin has a high proportion of lipid (70-80%) and a lower proportion of protein (15-30%) bc lipids are good electrical insulators (less permeable to ions), myelin protein 0 (p0) and peripheral myelin protein 22 (pmp22) are important for initiation and compaction of myelin in pns - alterations of the expression of these proteins results in several peripheral de myelinating neuropathies

guillian-barre syndrome

inflammatory disorder of the pns, can be caused by antibody response to p0 (immune system attacks pns myelin proteins), afflicts any age (most common <40), progression over days to weeks, 80-90% recover with no lasting effects, spontaneous recovery 2-4 weeks after progression (time it takes the schwann cells to de-differentiate, proliferate, then mature into myelinating schwann cells again)

four stages of myelination (schwann cells)

1) schwann cells surround axon

2) membrane fusion of the plasma membrane in one area

3) layers beginning to form due to schwann cell cytoplasm rotation

4) layers compact to form a mature sheath and the cytoplasm is squeezed to the outside

mesaxon

a double membrane that spirals around the axon, the inner mesaxon (im) is the origin of the myelin sheet and is continuous with the outer mesaxon (om)

myelination double membrane

formed by the apposition of external surfaces that form the major dense line (mdl) and internal surfaces that form the intraperiod line (ipl), these surfaces are scaffolded by specific myelin proteins

myelin compaction

occurs via direct interactions between extracellular p0 proteins on opposing external membrane and via electrostatic interactions of mbp with the phospholipid internal membrane (mbp is +, internal membrane is -) and interactions with the cytoplasmic tail of p0 (pulls tail causing compaction)

internodes

location of compact myelin between the nodes of ranvier

paranodal junction

at the lateral edge of the myelin layers, myelin consists of cytoplasm filled channels that spiral around the paranodal junction, they provide a physical and electrical barrier between the voltage-gated na+ channels at the node and the delayed rectifier potassium channels in the juxtaparanode, they adhere to the axon via adhesion molecules and to each other with tight junctions and gap junctions which a spatially distinct

cns myelination

cns myelination by oligodendrocytes occurs similarly but with some distinctions (e.g., number (myelinates many axons), diameter (thinner), myelin proteins involved in compaction)

non-myelinating schwann cells (nmscs)

arise from schwann cell precursors and retain the capacity to myelinate, provide growth and survival factors to the axons and are essential for normal pns development and function

remak cells

nmscs that ensheath small diameter peripheral axons, axons with remak bundle have their own mesoaxon with one schwann cell providing support to many axons

terminal schwann cells/teloglial cells

nmscs that support presynaptic terminals at neuromuscular junctions

satellite glial cells (sgcs)

wrap around neuronal cell bodies within the pns (sensory and sympathetic ganglia), can envelope one or more cell bodies by adhering to one or more other sgcs within the ganglia, connect to each other via gap junctions, adherens, and tight junctions, sgc processes can also extend to cover not only the cell body but also the synapses, dendrites, and axons

astrocytes

make up 50% of cells in the brain, two classifications: fibrous (white matter) and protoplasmic (grey matter), high density (e.g., 200,000 astrocytes/mm² in the anterior commissure)

fibrous astrocytes

in white matter, arranged in rows between axon bundles

protoplasmic astrocytes

have fine processes that cover all areas of grey matter including dendrites, axons, synapses, and vasculature, have distinct domain coverage, a single astrocytes can cover upwards of 140,000 synapses and 300-600 dendrites

astrocytes and myelination in the cns

supports myelination by: aligning oligodendrocyte processes with axons, releasing gliotrophic factors that promote oligodendrocyte survival, increasing the rate of myelin wrapping in response to electrical activity

roles of astrocytes in the cns

maintaining physiological homeostasis of cns - k+ buffering and ph balancing, recycling of neurotransmitters, alternative energy source, production of antioxidants

formation and support of synaptic processes

maintenance and formation of the blood brain barrier

roles of astrocytes in the cns: ph balancing

carbonic anhydrase in astrocytes converts co2 to hco3- and h+, hco3- is released and can buffer the h+ from neurons

roles of astrocytes in the cns: k+ buffering

excess k+ removed to lower concentration gradient

role of astrocytes in the cns: glutamate-glutamine cycle

astrocytes remove glutamate from synaptic cleft, glutamine synthase converts it to glutamine, glutamine is transported back to neuron where it’s converted back to glutamate by glutaminase

role of astrocytes in the cns: lactate shuttle

lactate is shuttled to neurons via astrocytes as alternative energy substrate

role of astrocytes in the cns: antioxidant production

astrocytes produce/release glutathione (gsh) and precursor, precursor (cysgly) is taken in by neurons to produce gsh, gsh binds reactive oxygen species caused by neuronal activity

roles of astrocytes in the cns: formation and support of synaptic processes

secrete synaptogenic factors (e.g., thrombospondin-1, hevin) to promote the formation of synapses during development, also secretes glypicans which induce the insertion of ampa receptors of the post-synaptic membranes, astrocytes express glutamate transporters that remove glutamate from the synapses preventing excitotoxicity, astrocyte ampa-mediated ca+ signaling helps monitor neuronal activity and glutamate levels which promotes growth and synaptogenic signaling

roles of astrocytes in the cns: maintenance and formation of the bbb

regulate cerebral blood flow (induce vasoconstriction and vasodilation) and the permeability of the bbb, astrocytic endfeet ensheath the brain vascular in a network called the glial limitans

ependymal cells

derived from neural precursors, form a single layer of ciliated cells that help to circulate csf throughout the ventricular system, have approx 50 motile cilia/cell and project 20 micrometers into the csf-filled cavities, specialized ependymal cells form the choroid plexus which is responsible for the production of csf

microglia

derived from the yolk sac and later from bone marrow, don’t form from the neuroectoderm, enter the cns early in development (gestation week 5, same time neurons start to form), make up 7-10% of cns cells, support many processes of the developing brain (neurogenesis, gliogenesis, differentiation, axonal synaptic pruning, myelination), resident immune cells of the brain - provide the first line of defence by releasing cytokines that have pro or anti inflammatory effects, phagocytic cells that rapidly clear debris, proteins, toxins, or dying cells within the brain

nerve-glial antigen 2 cells (ng2 cells)

make of 5% of cns glia, aka polydendrocytes or oligodendrocyte precursor cells (opcs), these cells retain their proliferative potential into adulthood and can either remain as ng2 cells or differentiate into oligodendrocytes, up to 50% of population found to divide every 3 days, unlike any other glial cell ng2 cells have synaptic connections with neurons (excitatory and inhibitory), electrical activity of the circuit may drive production of oligodendrocytes and promote myelination

boundary cap cells (bccs)

neural crest derived cells, present at the entry point for sensory axons (dorsal root entry zone) or the exit point of motor axons, multipotent, can give rise to several cell types in both pns and cns, can form neurons, glia, and smooth muscle cells, can self-renew, also help regulate growth of sensory axons into the cns and prevent motor neurons and central glial cells to exit the cns during development

cns interface

dorsal root entry zone and motor exit points appear by e10.5 in mice and is fully formed by birth, boundary cap cells are transient and proliferate throughout embryogenesis and disappear by early postnatal stages, following birth the transition zones are comprised largely of glial limitans and a basal lamina layer

basal lamina

thick (50-100 micrometers) network of collagen molecules and other extracellular matrix molecules produced by epithelial cells

pns:cns interface

interface at the dorsal root, sensory neuron central axons can regenerate within the pns but fail to enter to dorsal root entry zone into the cns

cns regeneration

extremely limited, axonal regrowth largely fails, glia inhibit axon growth

pns regeneration

1 mm/day, neurons can sprout collaterals and regenerate, glia produce growth factors, macrophages remove debris, not all peripheral nerve regeneration is successful and can result in permanent loss of function, neurons dont regenerate equally, sensory neuron populations undergo axonal regeneration differentially dependent on growth factors they are responsive to (e.g., nerve growth factors promote small diameter neurons)

pns injury: wallerian degeneration

if the axon is severed the distal ends are cut off from the neuronal cell body and the distal axon degenerates

steps of pns regeneration following injury

wallerian degeneration, proximal axon degenerates up to the first node closest to the injury site, denervated muscle starts to atrophy, infiltrating macrophages phagocytose dead cells and debris within the nerve, schwann cells de-differentiate and proliferate around the distal axon forming bands of bungner, schwann cells then release neurotrophic factors that promote axonal regrowth

bands of bungner

bands that form within the injury site by schwann cells

pns injury: crush vs cut

regeneration within the pns is more successful and faster if the perineurium or epineurium is intact (crush - axons are damage but remain intact)

pns injury: surgical reapposition

results in more favourable regeneration, the neural tube assist in creation of bands of bungner, contain immune cells, and an array of extracellular matrix molecules that help guide axons

henry head

severed his own radial nerve in 1903, return to pressure and touch by 6 weeks (non-localized), 2-6 months later he regained some pain, temperature, and light touch sensation in his hand but not all, 2 years later he had not recovered some proprioception or mechanoreception

cns injury overview

currently approx 86,000 canadians living with spinal cord injury with 4,300 new cases per year, most common causes are traffic accidents and falls, changing demographic for those most afflicted (under 30 to over 60 years old), treatment options are very limited and often result in permanent and extensive deficits of sensory, motor, and autonomic function

cns following injury

following axonal injury in the cns the distal axon degenerates and demyelinates, death of oligodendrocytes leads to expansion of injury site (secondary injury), microglia become reactive and phagocytose axon and myelin debris (occurs much more slowly than in pns), astrocytes also become reactive and migrates to the injury site, they begin to line up to create a physical and chemical barrier around the injury site - this becomes the boundary of the glial scar, following local damage reactive astrocytes and microglia release damage-associated molecular patterns (damps), cytokines, and chemokines that further activate and attract glial cells, the glial scar is formed to limit immune infiltration but creates a physical and chemical barrier to axonal regeneration

cns injury: blood vessels

when blood vessels are damaged it causes edema in the injury site, the immune cells infiltrate the cns tissue (e.g., neutrophils early, leukocytes late) and create further damage to tissue, this is prolonged in the cns

cns injury: inhibitory factors

the inhibitory factors expressed by all glial cells and fibroblasts within the glial scar produce the chemical barrier for axon growth, oligodendrocytes express myelin proteins (e.g., nogo-a) that inhibit axon growth when they interact with their receptor (nogo receptor 1 and 2), astrocytes and fibroblasts secrete inhibitory factors (e.g., chondroitin sulfate proteoglycans (cspgs) that interact with NgRs, microglial cells and astrocytes secrete immune mediators (e.g., interleukins) that cause leukocyte (b and t cells) proliferation and activation

meninge layers

dura, arachnoid, pia

dura mater

firmly attached to skull, meningeal veins, arteries and nerves are plastered between the dura and skull, contains two layers - periosteal and meningeal dura that are typically adhered to each other, dura consists of several cell layers of different elongated fibroblasts and collagen fibrils, more flattened fibroblasts make up the border cell layer

arachnoid mater

lines inner surface of dura, contains a thin cellular layer that is avascular and have tight junctions, the subarachnoid contains fibroblasts that span the membrane (arachnoid trabeculae), subarachnoid space contains csf

pia mater

covers surface of the brain, 2-5 cells thick and important for formation of the basement membrane (between meninges and neural cells)

falx cerebri

inward extension of dura in between left and right cerebral hemispheres

tentorium cerebelli

inward extension of dura in between cerebellum and cerebrum

function of dural extensions (e.g., falx cerebri and tentorium cerebelli)

structurally important to stabilize the brain, extreme movements of the cns may be restricted by the dural extensions which may result in damage to the brain or cranial nerves (e.g., herniation, coning)

herniation

tissue moves to a region it’s not supposed to be

coning

change in pressure that impacts the brainstem

meningeal nerves: headaches

headaches are caused by the brain interpreting pain from a region it isn’t, e.g., brain freeze is caused by the excess amount of cold received by the trigeminal nerve that innervates the face as well as the meninges, this causes referred pain

meningeal artery

extends from the external carotid artery up between the dura and skull where the skull is the thinnest (side of the head), damage to this part of the skull can cause a brain bleed

superior sagittal sinus

drains cerebral blood via bridging veins, a sinus is a region where two layers of dura are separated

hematoma classifications: epidural

blood between dura and skull, caused by blunt force to skull causing rupture of meningeal vessel

hematoma classifications: subdural

blood in csf space (between dura and arachnoid matter), caused by sudden movement of head causing brain to move inside skull and thus a rupture of bridging vein

hematoma classifications: subarachnoid

blood in csf space (in subarachnoid space), aneurysm causes congenital weakening of artery wall causing rupture of cerebral artery

meningitis

infection of the meninges caused primarily by bacterial infections, causes infiltration of immune cells into the subarachnoid space, this causes an increase in pressure (increased immune cells = increased pressure), approx 1000 people/year diagnosed in canada, children <5 and adults >60 are mainly affected, death is rare in canada but long-term effects can occur

epidural fat space

in between the vertebrae and dura mater of the spinal cord, easily compressed during bending, location of an epidural lumbar puncture

lumbar puncture

performed before the level of the spinal cord as to not damage it (below l1)

conus medularis

end of the spinal cord

cauda equina

horses tail, bundle of nerves extending from the spinal cord

spinal tap

injects into the subarachnoid space to collect csf

ventricles

sites of csf production

2 lateral ventricles → interventricular foramina → third ventricle → cerebral aqueduct → forth ventricle → ventral canal

regions of the lateral ventricles

frontal, body, occipital, temporal

cerebral spinal fluid

produced by the choroid plexus in the fourth, third, and lateral ventricles, it is interconnected with the interstitial fluid that surrounds cns cells to provide nutrients, hormones, and metabolites, it also provides buoyancy and protection from mechanical insult, it is circulated by hydrostatic pressure from constant production, beating ependymal cilia, and arterial pulsations, adults have a csf reservoir of 90-150 ml, rate of production is 500-600 ml per day (turnover rate 4x/day)

csf drainage

drains from the subarachnoid space into the superior sagittal sinus (venous blood), arachnoid granulations (villi) help with this, leakage from ventricular system to subarachnoid space occurs through foramina in the fourth ventricle, drainage also occurs to a lesser extent via the cerebral lymphatic system, dural sinuses and lymphatics (e.g., cervical lymph nodes) drain csf, blood, and wastes

choroid plexus

consists of a layer of ependymal cells that surround fenestrated capillaries, pericytes around the endothelial cells provide structural support, easy access for small molecules and water, larger molecules must be transported (e.g., glucose), choroid plexus is also considered a gateway for immune cell entry to the brain, ependymal cells express chemokines that facilitate passage of monocytes and leukocytes into the csf

csf composition

filtered plasma, sodium, chloride, potassium, calcium, proteins (no clotting factors), glucose (2/3), very few white blood cells (lymphocytes and monocytes), no red blood cells

ventricle stenosis

narrowing of the ventricles, usually rate of csf production and drainage are matched - if rates are unmatched pathology ensues, if the lateral and third ventricles are enlarged the stenosis is in the cerebral aqueduct

external carotid artery

supplies scalp, face, and meningeal arteries

internal carotid

supplies arteries to front and middle of brain