physiology Flashcards
where is the cerebrospinal fluid produced?
ependymal cells of the choroid plexus
where does the CSF fluid circulate?
it is formed in the ventricles of the brain and then circulates in the subarachnoid space then absorbed into venous circulation.
What are the main functions of cerebrospinal fluid?
- Mechanical: shock absorbing medium that protects brain tissue. Brain “floats” inside the cranial cavity
- Homeostatic: pH of CSF affects pulmonary ventilation and cerebral blood flow. Transport hormones
- Circulation: medium for minor exchange of nutrients and waste products between blood and brain tissue
How is CSF obtained for diagnosis ?
Lumbar puncture
what does normal CSF look like?
clear and colourless fluid
contains small amount of protein ( 15-45 mg/dl)
Contains small amount of immunoglobulins
contains small amount of cells (1-5 cells/ml)
Discuss the embryonic development of the brain and ventricular system.
at 3 weeks the developing nervous system consists of a tube - neural canal
Its cavity gives rise to the adult brain’s ventricles and the spinal cord’s central canal
the choroid plexus develops from cells in the walls of the ventricles
These choroidal cells are specialised secreting cells that produce CSF
developing arteries invaginate the roof of the ventricle form the choroid fissure
The involuted ependymal cells along with the vessels enlarge into villi and form the choroid plexus, responsible for CSF production
choroid plexus in the adult brain is found in the 3rd, 4th and lateral ventricles
what are the main ions required for CSF production
Na+ - absorbed into cell and then into apical surface
CL- - absorbed into cell and then into apical surface
HCO3- - diffused into blood ( basal lateral)
this also allows water to enter ependymal cells
what co transporter is found in the apical surface of the ependymal cells which is key in the formation of CSF?
Na-K-Cl cotransporter (NKCC)
allows all three to leave cell into CSF
the production of CSF is a an active secretory process true or false?
true
Compare the contents of CSF to Plasma.
In comparison to plasma, CSF has:
Lower k+
Lower glucose
Lower protein
Higher Na+
Higher CL-
what is the role of the intraventricular foramina (of Monroe)
connects lateral ventricles to third ventricle
What is the role of the cerebral Aqueduct?
connects third ventricles to fourth
What is the role of the Foramen of magendie?
Median aperture - connects fourth ventricle to subarachnoid space
what is the role of the Foramen of Luschka?
Lateral apertures - connects fourth ventricle to subarachnoid space
How does the CSF flow from the lateral ventricles to the third ventricle?
ii. what occurs at the third ventricle?
Via the interventricular foramina
ii. more CSF is added by choroid plexus in the roof of third ventricle
which ventricles create CSF first?
lateral ventricles
How does the CSF flow from the third ventricles to the fourth ventricles?
ii. what occurs at the fourth ventricle?
via the cerebral aqueduct
ii. more CSF is added by the choroid plexus in the fourth ventricle
what occurs during CSF circulation once it enters the subarachnoid space?
Enters subarachnoid space via:
one median apertures
paired lateral apertures
it then exchanges with the venous blood via the the arachnoid villi (granulations) of the dural venous sinuses (superior sagittal sinus)
it then enters the circulatory system
what makes up the final portion of CSF?
ii. how does it reach the CSF fluid?
brain interstitial fluid
ii. Via the perivascular spaces
What does the blood brain barrier consist of?
capillary endothelium
its basal membrane
perivascular astrocytes
what is the role of the BBB?
protects the brain from infections and toxins
Which parts of the brain do not have the BBB?
circumventricular organs - points of communication between the blood, the brain parenchyma, and the CSF
pineal gland
what problems can arise from the brain having a BBB?
prevents drug delivery to CNS
what is papilloedema?
optic disc swelling due to increased intracranial pressure transmitted to the subarachnoid space surrounding the optic nerve which has an enlarged amount of CSF due to inadequate draining
The CSF in the subarachnoid space will push on the eye creating a bulging optic disc
What is the aqueous Humor?
a specialised fluid that bathes the structures within the eye.
it provides oxygen and metabolites and contains bicarbonates (HCO3)
bicarbonate buffers the H+ produced in the cornea and lens by anaerobic glycolysis.
contains ascorbate which is a powerful antioxidant
Where is Aqueous humor produced?
epithelial layer of the ciliary body
Describe the drainage of the aqueous humor.
- It is produced in the cililary body
- Drains into the posterior chamber of the eye
- Then flows into the anterior chamber
- then drains to the scleral venous sinus through a trabecular meshwork and the canal of schlemm
- Small amount diffuses through the vitreous being absorbed across the retinal pigment epithelium
where is the canal of schlemm found?
Angle between the iris and cornea iridocorneal angle
what are the two epithelial layers which cover the cililary body and posterior surface of the iris?
- Pigment epithelium of the retina (PE)
2. Inner nonpigmented epithelial layer (NPE)
How does the ionic composition of aqueous humor occur?
- HCO3- and H+ is formed in the pitherlial cells from hydration of CO2 via carbonic anhydrase
- These ions travel to the basolateral membranes of the PE cells into the interstitial fluid in exchange for Cl- and Na+
- CL- and Na+ ions which enter the PE cells diffuse through the gap junctions between the PE and NPE cells
- These ions are then transported out of the NPE cells into the aqueous humor via the Na+/K+/2cl- co transporter
- K+ ions leaving the cells on either side are recycled by the Na+/K+ pumps and some CL- leave the cells on both sides too
water:
Net movement of Cl- and Na+ through the cells from interstitial fluid to aqueous humor is accompanied by water moving through the ciliary epithelial water channels (aquaporins - AQP1 of the NPE cells) and through paracellular pathway via osmosis
Compare the contents the contents of Aqueous humor to Plasma.
In comparison to plasma, Aqueous humor has:
More Na+
More Cl-
More Ascorbate
Less HCO3-
Less Glucose
when would you use Carbonic anhydrase inhibitors to reduce production of aqueous humor?
To reduce ocular pressure in glaucoma
give examples of carbonic anhydrase inhibitors
Dorzolamide
Acetazolomide
what 4 actions need to occur for us to see an object?
- Pattern of the object must fall on the vision receptors ( rods and cones in the retina)
- The amount of light entering the eye must be regulated (too much light will “bleach out” the signals)
- The energy from the waves of photons must transduced into electrical signals
- Brain must receive and interpret the signals
what splits the retina into the nasal and temporal hemitretinas?
Fovea
What does the monocular visual field refer to?
what each eye sees of the visual space ( its own part)
What does the binocular visual field refer to?
When both eyes have their visual fields to overlap extensively
which parts of the eyes view the left visual field?
Left nasal hemiretina
Right temporal hemiretina
Which parts of the eyes view the right visual field?
Right nasal hemiretina
Left temporal hemiretina
what occurs at the optic chiasm?
Nerve fibres from the nasal half of each retina cross over
this forms two separate optic tracts allowing them to reach separate visual cortex
which cortex does the right optic tract go to?
Left visual cortex
which cortex does the left optic tract go to?
right visual cortex
which neurons in the retina will perceive upper portions of the visual field?
ii. which cortex will perceive upper portions of the visual field?
Lower neurons
ii. Lower visual cortex
which neurons in the retina will perceive lower portions of the visual field?
ii. which cortex will perceive lower portions of the visual field?
Upper neurons
ii. Upper visual cortex
which layer of the primary visual cortex receive the light signals from one eye only?
layer 4c - contains ocular dominance column so input is dominated by one eye
e.g. Left layer 4c receive input from right eye signals only
How many layers are there in the visual cortex?
ii. what happens to light signals in the other layers of the cortex?
6
ii. they receive input from both eyes
what is Amblyopia/ cortical blindness?
term refers to a variety of visual disorders where one eye has better vision than the other
even though the optics and retina of both eyes are fine
what influences the arborization of axons in the LGN?
Visual activity
lack of visual activity leads to less branching
Correlated activity between presynaptic and postsynaptic cells strengthens synaptic connections between them.
true or false?
true
what happens if there is no competition between the axons from the right eye and left eye and is dominated by one of them?
One eye takes more neural space in layer 4c
this leads to the other eye axons’ branches to regress
and leads to permanent visual loss
what is the role of the photoreceptors in the retina?
Carries out Transduction - conversion of electromagnetic radiation neural signals
Describe the direct pathway for signal transmission of the eye.
- Photoreceptor
- Bipolar cells
- Ganglion cells
Compare the direct vertical pathway for signal transmission of the eye to the direction of the light.
Both move in opposite directions:
Photoreceptor is the most posterior aspect of the eye
light travels first through ganglion cells then bipolar cells to reach the photoreceptor
it is the opposite once light has been converted into electricity
what is the role of horizontal cells?
Receive input from photoreceptors and project to other photoreceptors and bipolar cells
what is the role of amacrine cells?
Receive input from bipolar cells and project to ganglion cells, bipolar cells and other amacrine cells
what are the four main regions of the photoreceptors?
Outer segment
inner segment
cell body
synaptic terminal
what are the two main types of photoreceptors?
Rods
Cones
what is the resting potential of the photoreceptors of the neurons?
-20mv
what effect does light exposure have on the photorecptors?
causes hyperpolarisation - cyclic GMP Gated Na+ channel in the photoreceptor closes in the light
Change in Na+ with light is the signal that enable s the brain to perceive objects
what is the dark current?
During the dark it Refers to balance between
Permeability of Na+ and K+ in the photoreceptors
influx of Na+ = efflux of K+
Membrane potential is between equilibrium potentials of Na+ and K+
what happens to the Modulation of the dark current when the photoreceptor detects light?
Permeability of Na+ decreases
therefore K+ leaves via Na+/K+ pump
it becomes hyperpolarised
what is the name of visual pigment molecules in the rods?
Rhodospin
they are found in the disks in the outer segment
what effect does light have on rhodospin?
converts it to all-trans-retinal
what are the effects of all-trans-retinal on phototransduction?
- Activates transducin (G protein)
- Transducin activates cGMP phosphodiesterase (PDE)
- PDE hydrolyses cGMP reducing its concentration
- This leads to closure of Na+ channels
- Hyperpolarisation of Photorecptors
More glutamate is released in the by hyperpolarised photoreceptors true or false?
false - they release less
When is glutamate released by photoreceptors?
in the dark
what is visual acuity?
ii. what determines it?
ability to distinguish between two nearby points
determined by photoreceptor spacing and refractive power
when are rod photoreceptors used?
in dim light
when are cone photoreceptors used?
normal daylight
compare the convergence levels of rods and cones?
there is high convergence in rods
Low convergence in cones
what does more convergence in rod system mean?
there is increased sensitivity to light
there is decreased acuity to light
how can we see different colours?
Different opsins are used for different wavelengths
what are the 4 types of photoreceptors in the retina?
A. Cones:
- Short-wavelength
- Medium wavelength
- Long wavelength
B. Rods
