neural development and homeostasis Flashcards
at 17 days old there are 3 germ layers in an embryo - what are they and what do they become?
Endoderm becomes the lining of the viscera
Mesoderm becomes bones and muscles
Ectoderm becomes the skin and nervous system
describe what happens in neurulation
Neural plate becomes a neural tube
The neural tube becomes the CNS
the neural crest surrounding the neural tube - becomes PNS (dorsal root ganglion neurons, Schwann and chromaffin cells etc…)
a part from the mesoderm - forms the two somites, that are to do with the somatic nervous system (makes sense, controls skeletal muscle)
The tube leaves space for ventricles
what happens in anencephaly and spina bifida and how are these prevented?
anencephaly = the folding of the neural plate (which occurs from the back to the forebrain) is not complete, doesn’t seal at the front
spina bifida = when the back/caudal end of the neural plate doesn’t close properly, or the meninges or spinal cord don’t form properly
folic acid supplements reduce chances of these defects
what sections arise form the neural tube?
rostral end = forebrain or prosencephalon
middle = midbrain/mesencephalon
caudal end = hindbrain/rhombencephalon
how does the forebrain differentiate using vesicles?
two telencephalic vesicles balloon out to become cerebral hemispheres
two smaller optic vesicles form, become the optic nerve and retina, made of retinal ganglion cells
explain how the forebrain differentiates further
two areas of the prosencephalon:
the telencephalon, which is the outer/dorsal part, and the diencephalon which is the lower inner ventral part
telencephalon = cerebral cortex and basal telencephalon
diencephalon = thalamus and hypothalamus
where are ventricles visible in the forebrain?
two lateral ventricles visible in telencephalon
third ventricle visible in diencephalon
what white matter develops in the forebrain? any other vesicles?
corpus callosum = biggest commissarial fibre in the brain meaning it connects the two hemispheres
internal capsule - carries all info from cerebral cortex to the thalamus
other vesicles form the olfactory bulbs
the midbrain differentiates into what?
cerebral aqueduct is easiest to spot - looks like a hole in the middle , connects third and fourth ventricles
dorsal part = tectum
ventral part has the tegmentum
these develop into the superior colliculus (eyes info) and inferior colliculus (audio processing)
also get the periaqueductal gray - pain processing
the red nucleus and substantia nigra (core and muscle control)
hindbrain - what does the rhombencephalon/hindbrain consist of and develop into?
has the rhombic lips on the top
has the fourth ventricle
the front part forms the pons, below this the medulla forms with white matter tracts to the cerebral cortex
cerebellum
describe the structure of the cerebellum and where it gets input/sends output
has its own hemispheres and grooves called folia, more neurons than the cerebral hemispheres
receives inout from the vestibular system and proprioception
90% of cerebral cortex output goes to the pontine nuclei in the cerebellum
cerebellum outputs come from deep cerebellar nuclei
why do we regulate our brain extracellular fluid?
if not controlled, it would lead to neuronal cell death/dysfunction
all the action potentials cause an increase in BECF K+, a decrease in O2 and glucose, an increase in CO2, increased H+, NT concentration changes, Ca2+ changes
not actually in Na+ as there’s so much small local changes caused by action potential don’t really have a big effect
what would the changes action potential cause in BECF result in if not regulated?
increase in K+ outside neurons bring cells closer to the the threshold for action potentials which results in oversensitive neurons and unwanted action potentials
how was the blood brain-barrier discovered?
intravenous injection dyes passed across leaky capillaries and stained soft tissues but not the brain
what is the function of the blood-brain barrier?
Protect neurons from changes in the blood that could affect changes in the BECF
For example, amino acids absorbed in digestion would interact with receptors in the brain
Increases in K+ and H+ concentration after exercise in the blood would mess with neurons
Circulating hormones in blood can affect receptors in the brain if they were allowed there (e.g. a progesterone derivative can act on GABA)
Toxins in blood we wouldn’t want to reach the brain
how is the barrier maintained?
Systemic capillaries have endothelial cells that have gap junctions allowing paracellular movement
The BBB capillaries have tight junctions that don’t allow this paracellular route
BBB has a thicker basement membrane
Astrocytic end-feet contact blood vessels to facilitate transport between BECF and blood
how do important molecules get through the blood-brain barrier?
glucose = facilitated diffusion
most things require transport proteins like exchangers and co-transporters
lots of mitochondria in capillary endothelial cells for active transport
small uncharged/lipid soluble molecules diffuse across - good for O2 and CO2, but some drugs can too
what must be considered when producing neurological drugs?
can it cross the BBB?
caffeine can, but so can some harmful substances like heroin and nicotine
some areas of the brain produce a lot of hormones, or require access to blood - how is this achieved? give examples
posterior pituitary produces hormones that need access to systemic circulation
osmoreceptors and thermoreceptors in the hypothalamus need to check the blood
this happens using leaky capillaries in specific areas of the BBB
some examples of these areas are the choroid plexuses, circumventricular organs, posterior pituitary etc…
what do the ventricles do and where are they/how do they connect?
these cavities in the brain filled with cerebrospinal fluid increase buoyancy and therefore provide physical protection
they are responsible for maintaining ion levels and removing waste products
1 and 2 = lateral ventricles
3 is in the middle
4 is connected to the third by the cerebral aqueduct and lies between the pons and the cerebellum, the most ventral, as it connects to the spinal cord to become the central canal
what is in cerebrospinal fluid and what is exchanged (both ways) between it and the brain extracellular fluid?
cerebrospinal fluid has massively less K+, amino acids and proteins when compares to plasma for protective effects
CSF to BECF =
macronutrients like glucose, micronutrients like vitamins
ions like bicarbonate to neutralise spikes in H+
BECF to CSF =
metabolic waste like CO2
neurotransmitters
what are the choroid plexuses?
networks of blood vessels in each ventricle of the brain, have leakier capillaries and secrete CSF
how does CSF move around the brain etc…
secreted by the choroid plexuses, circulates ventricles and central canal
reaches outer edges of the brain through the foramen holes to fill the subarachnoid space
then it needs to drain - this is done via arachnoid granulations, to drain into the superior sagittal sinus to enter the venous system
what volume is the ventricular system?
it can hold 150ml of CSF
500mls is made a day so it’s roughly filled 3 times a day
how is secretion of CSF carried out?
choroid plexus has capillaries that allow secretion out of them to form essentially extracellular fluid
then stuff is secreted selectively, across the choroidal epithelial cells, into the ventricles to produce cerebral spinal fluid
movement of CSF outside the ventricles to the BECF is pretty free across ependymal cells
describe the location/structure/function of the dura mater
the outermost meningeal layer sitting underneath the skull
the thickest, provides the strongest protection for the brain underneath the skull
describe the location/structure/function of the arachnoid mater
named for it’s spiderweb like appearance, it cushions the brain
it is attached to the dura mater
there is a space underneath for cerebrospinal fluid
it is a selective layer
describe the location/structure/function of the pia mater
the thinnest layer, wraps tightly around the brain and prevents loss of the cerebrospinal fluid beneath
it is leaky
how is blood able to reach the brain?
arteries and veins in the dura mater and subarachnoid space bring blood between the brain and heart, and that blood can then be distributed by capillaries in the pia mater
why does the arachnoid layer have evaginations?
these are arachnoid granulations (and villi) and project through the dura mater which is hard to cross, allowing the CSF to drain into the superior sagittal sinus
pinocytosis?
pressure of the CSF drives this process???
what can happen if CSF cannot circulate properly?
hydrocephalus = inability to drain CSF due to blockage of, usually, the aqueduct
or ineffective pinocytosis leading to a build up of fluid and pressure
it eventually pushes down on the cerebellum/brainstem and affects neurons involved in unconscious processes
how does the presynaptic terminal remove glutamate?
has transporters to collect it and turn it back into glutamine once in the cell as too much glutamate can cause excitatory toxicity
how do astrocytes and neurons deal with too much K+ in the extracellular space?
Na+/k+ pump moves K+ back into the cell and Na+ out to re-establish membrane potential
K+ needs to be low in the ECF to prevent neurons from becoming oversensitive
astrocytes take in K+ from the ECF, what does this trigger in the cell?
an increase in glucose metabolism as high intracellular K+ signals that the Na+/K+ pump has been used
how do astrocyte membrane potentials differ from neurons and why does it mean they deal with extracellular K+ more than neurons?
Eq potential for K+ in neurons and astrocytes is -90mV, yet astrocyte resting potential is more negative than a neuron’s because astrocytes are much mor permeable to K+ than Na+
hence why they deal with K+
how do astrocytes get rid of any K+ they take up from ECF?
they must maintain the conc. gradient that allows K+ in should more need removing, so they couple with each other using gap junctions of connexin to form a syncytium
this is used to redistribute K+ to areas of decreased activity (can also be used to transport sugars, amino acids and other needed things)
what is neurovascular coupling?
increased neuronal firing = lots of Ca2+ inside the astrocytes, the astrocytes then release vasoactive substances to increase blood supply to active neurons
what functional imaging techniques can be used?
often exploit neurovascular coupling
examples = PET positron emission tomography which exploits glucose use
and there’s functional magnetic resonance imaging (fMRI)
from the prosencephalon?
telencephalon = cerebral cortex, cerebral nuclei/basal ganglia/amygdala/basal forebrain
lateral ventricles
diencephalon = thalamus, hypothalamus and retina
third ventricle
from the mesencephalon?
superior and inferior colliculi
red nucleus
substantia nigra
cerebral aqueduct
from the rhombencephalon?
(metencephalon) cerebellum and pons
(myelencephalon) medulla oblangata
fourth ventricles
