neural development and homeostasis

Question 1

at 17 days old there are 3 germ layers in an embryo - what are they and what do they become?

Answer 1

Endoderm becomes the lining of the viscera
Mesoderm becomes bones and muscles
Ectoderm becomes the skin and nervous system

Question 2

describe what happens in neurulation

Answer 2

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

Question 3

what happens in anencephaly and spina bifida and how are these prevented?

Answer 3

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

Question 4

what sections arise form the neural tube?

Answer 4

rostral end = forebrain or prosencephalon
middle = midbrain/mesencephalon
caudal end = hindbrain/rhombencephalon

Question 5

how does the forebrain differentiate using vesicles?

Answer 5

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

Question 6

explain how the forebrain differentiates further

Answer 6

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

Question 7

where are ventricles visible in the forebrain?

Answer 7

two lateral ventricles visible in telencephalon
third ventricle visible in diencephalon

Question 8

what white matter develops in the forebrain? any other vesicles?

Answer 8

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

Question 9

the midbrain differentiates into what?

Answer 9

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)

Question 10

hindbrain - what does the rhombencephalon/hindbrain consist of and develop into?

Answer 10

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

Question 11

describe the structure of the cerebellum and where it gets input/sends output

Answer 11

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

Question 12

why do we regulate our brain extracellular fluid?

Answer 12

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

Question 13

what would the changes action potential cause in BECF result in if not regulated?

Answer 13

increase in K+ outside neurons bring cells closer to the the threshold for action potentials which results in oversensitive neurons and unwanted action potentials

Question 14

how was the blood brain-barrier discovered?

Answer 14

intravenous injection dyes passed across leaky capillaries and stained soft tissues but not the brain

Question 15

what is the function of the blood-brain barrier?

Answer 15

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

Question 16

how is the barrier maintained?

Answer 16

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

Question 17

how do important molecules get through the blood-brain barrier?

Answer 17

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

Question 18

what must be considered when producing neurological drugs?

Answer 18

can it cross the BBB?
caffeine can, but so can some harmful substances like heroin and nicotine

Question 19

some areas of the brain produce a lot of hormones, or require access to blood - how is this achieved? give examples

Answer 19

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…

Question 20

what do the ventricles do and where are they/how do they connect?

Answer 20

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

Question 21

what is in cerebrospinal fluid and what is exchanged (both ways) between it and the brain extracellular fluid?

Answer 21

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

Question 22

what are the choroid plexuses?

Answer 22

networks of blood vessels in each ventricle of the brain, have leakier capillaries and secrete CSF

Question 23

how does CSF move around the brain etc…

Answer 23

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

Question 24

what volume is the ventricular system?

Answer 24

it can hold 150ml of CSF
500mls is made a day so it’s roughly filled 3 times a day

Question 25

how is secretion of CSF carried out?

Answer 25

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

Question 26

describe the location/structure/function of the dura mater

Answer 26

the outermost meningeal layer sitting underneath the skull
the thickest, provides the strongest protection for the brain underneath the skull

Question 27

describe the location/structure/function of the arachnoid mater

Answer 27

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

Question 28

describe the location/structure/function of the pia mater

Answer 28

the thinnest layer, wraps tightly around the brain and prevents loss of the cerebrospinal fluid beneath

it is leaky

Question 29

how is blood able to reach the brain?

Answer 29

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

Question 30

why does the arachnoid layer have evaginations?

Answer 30

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

Question 31

pinocytosis?

Answer 31

pressure of the CSF drives this process???

Question 32

what can happen if CSF cannot circulate properly?

Answer 32

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

Question 33

how does the presynaptic terminal remove glutamate?

Answer 33

has transporters to collect it and turn it back into glutamine once in the cell as too much glutamate can cause excitatory toxicity

Question 34

how do astrocytes and neurons deal with too much K+ in the extracellular space?

Answer 34

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

Question 35

astrocytes take in K+ from the ECF, what does this trigger in the cell?

Answer 35

an increase in glucose metabolism as high intracellular K+ signals that the Na+/K+ pump has been used

Question 36

how do astrocyte membrane potentials differ from neurons and why does it mean they deal with extracellular K+ more than neurons?

Answer 36

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+

Question 37

how do astrocytes get rid of any K+ they take up from ECF?

Answer 37

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)

Question 38

what is neurovascular coupling?

Answer 38

increased neuronal firing = lots of Ca2+ inside the astrocytes, the astrocytes then release vasoactive substances to increase blood supply to active neurons

Question 39

what functional imaging techniques can be used?

Answer 39

often exploit neurovascular coupling
examples = PET positron emission tomography which exploits glucose use

and there’s functional magnetic resonance imaging (fMRI)

Question 40

from the prosencephalon?

Answer 40

telencephalon = cerebral cortex, cerebral nuclei/basal ganglia/amygdala/basal forebrain

lateral ventricles

diencephalon = thalamus, hypothalamus and retina

third ventricle

Question 41

from the mesencephalon?

Answer 41

superior and inferior colliculi
red nucleus
substantia nigra

cerebral aqueduct

Question 42

from the rhombencephalon?

Answer 42

(metencephalon) cerebellum and pons
(myelencephalon) medulla oblangata

fourth ventricles