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Opthamology > Physiology > Flashcards

Physiology Flashcards

1
Q

what is CSF and what are its functions?

A 
cerebrospinal fluid
clear colourless liquid composed mainly of water
supplies water, amino acids and ions
removes metabolites
acts as a shock absorber for the brain
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2
Q

where is CSF produced and how much?

A

produced by the secretory epithelium of the choroid plexus in the ventricles
CSF then circulates in the subarachnoid space and is then absorbed into venous circulation
0.4 ml/min/g of tissue (usually around 500-600ml per day)

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3
Q

how much CSF is present in the body at any one time?

A

around 150ml

replaced 3-4 times daily

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4
Q

what are the 3 main functions of CSF?

A 
mechanical protection (shock absorber that protects brain)
homeostatic function (pH of CSF affects pulmonary ventilation and cerebral blood flow, transports hormones)
circulation (medium for exchange of nutrients and waste products between blood and brain tissue
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5
Q

how can CSF be analyzed and when might this be done?

A

lumbar puncture

aids diagnosis of the brain, meninges and spinal cord

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6
Q

what are the normal contents of CSF?

A

usually clear and colourless
contains little protein (15-45mg)
little immunoglobulins
only 1-5 cells per ml of CSF

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7
Q

how does the choroid plexus develop?

A

neural canal develops at 3 weeks
cavity of neural canal gives rise to brain ventricles
developing arteries invaginate the roof of the ventricle to form the choroid fissure
vessels continue to develop and branch as they grow into the ventricle causing formation of villi
this collection of villi form the choroid plexus

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8
Q

where is the choroid plexus found in the adult brain?

A

3rd, 4th and lateral ventricles

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9
Q

which cells in the choroid plexus secrete CSF?

A

ependymal cells

- line the villi of the choroid plexus (like epithelium)

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10
Q

how is CSF produced in the ependymal cells?

A

ependymal cells absorb plasma
plasma is modified via transport of Na+, Cl- and HCO3 across the epithelium from the blood to the CSF between the basolateral and apical ends of the cell
this increased flow of ions also drives the flow of water across the cell and out into the apical space
secretion can occur because of the polarized distribution of specific ion transporters in the apical or basolateral membrane of the epithelial cells

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11
Q

what is the secretion of CSF by the choroid plexus dependent on?

A

active transport of Na+ across the cells into the CSF
the electrical gradient pulls along Cl-
both ions drag water by osmosis
the CSF has lower K+, glucose and protein than blood plasma and higher Na+ and Cl-

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12
Q

is production of CSF directly dependent on arterial blood pressure?

A

no

it is an active secretory process

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13
Q

what 3 ventricles is CSF produced from and how are they connected?

A

lateral, 3rd and 4th ventricles
foramen of Monroe (intraventricular foramina) = between lateral and 3rd ventricle
aqueduct of sylvius = between 3rd and 4th
foramina of Luschka = 4th ventricle to arachnoid space
foramina of Magendie = 4th to subarachnoid space

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14
Q

what do the foramen/aqueducts in the ventricles do?

A

act as pores to allow passage and secretion of CSF

CSF mainly secreted via 4th ventricle through foramen of Magendie/Luschka into the subarachnoid space

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15
Q

describe the pathway of CSF circulation

A

CSF formed in choroid plexus of each lateral ventricle
flows into 3rd ventricle through interventricular foramina
More CSF added by choroid plexus in 3rd ventricle
CSF flows through aqueduct of sylvius/midbrain into 4th ventricle
another choroid plexus in 4th ventricle adds more CSF
CSF enters the subarachnoid space via 3 openings in the roof of 4th ventricle (single median aperture + paired lateral apertures)
then circulates in the central canal of spinal cord
CSF returns to venous blood through arachnoid granulations into the superior sagittal sinus (SSS)
the brain interstitial fluid makes up the final portion of CSF and drains to the CSF through the perivascular spaces

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16
Q

is the volume and pressure of CSF constant?

A

yes

as the same amount is produced and absorbed every day

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17
Q

what is the blood brain barrier (BBB)?

A

a barrier between the blood and the brain

consists of the endothelial cells in brain capillaries, their basal membrane and perivascular astrocytes

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18
Q

what does the BBB do?

A

tight junctions between brain endothelial cells prevent paracellular movement of molecules
this served to protect the brain from many common bacterial infections and toxins

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19
Q

what is the clinical significance of the BBB?

A

BBB is the determining factor for clinical CSF analysis

BBB is the main obstacle for drug delivery to the CNS

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20
Q

name 4 pathologies of the ventricles, choroid plexus and CSF

A

tumours
ventricular haemorrhages
hydrocephalus
idiopathic intracranial hypertension/pseudotumour cerebri

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21
Q

what tumours can affect CSF?

A

colloid cyst
ependymomas
choroid plexus tumours

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22
Q

how can a ventricular haemorrhage cause problems?

A

accumulation of blood in the ventricles
epidural haematoma (between skull and dura)
subdural haematoma (venous bleed between dura and arachnoid)
subarachnoid haemorrhage

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23
Q

what is hydrocephalus?

A

accumulation of CSF in the ventricular system or around the brain (due to obstruction or overproduction)
subsequent enlargement of one or more ventricles and increase in CSF pressure

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24
Q

what is idiopathic intracranial hypertension/pseudotumour cerebri and how does it present?

A

enigmatic condition
headache and visual field disturbance due to papilloaedema
no imaging features of hydrocephalus despite increased CSF pressure

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25
Q

what does hydrocephalus look like on imaging?

A

enlargement/swelling of ventricles

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26
Q

what is papilloaedema?

A

optic disc swelling due to increased intracranial pressure transmitted to the subarachnoid space surrounding the optic nerve

27
Q

what are the symptoms of papilloaedema?

A 
enlarged blind spot
blurring of vision
visual obscurations
loss of vision
bulging optic disc on fundoscopy
28
Q

what does aqueous humour do?

A

specialized fluid that bathes the structures within the ey
provides oxygen and metabolites
contains HCO3 which buffers the H+ produced in the cornea and lens by anaerobic glycolysis
contains ascorbate which is a powerful antioxidant

29
Q

describe circulation of aqueous humor

A

produced in epithelial layer ciliary body into the posterior chamber of the eye
then flows into the anterior chamber and then drains into the scleral venous sinus through a trabecular meshwork and the canal of schlemm at the iridocorneal angle
small amount diffuses through vitreous and is absorbed across the retinal pigment epithelium

30
Q

describe the organization of ciliary epithelium and where aqueous humor is produced

A

2 layers of ciliary epithelium
pigmented epithelium faces the blood (forward continuation of pigment epithelium of the retina
inner non-pigmented epithelium forms the apical surface

31
Q

how is aqueous humor produced in the ciliary body?

A

CO2 is catalyzed by carbonic anhydrase (CA) producing HCO3 and H+
HCO3 and H+ leave the cell via basolateral membrane in exchange for Cl- and Na+
Cl- and Na+ diffuse through gap junctions between PE and NPE and are transported out of the NPE cells into the aqueous humor via Na+/K+/2Cl-
net movement of Cl- and Na+ through cells from interstitial fluid to aqueous humor creates an osmotic gradient which allows water to move through aquaporins and through the paracellular pathway

32
Q

how can carbonic anhydrase be used clinically?

A

CA inhibitors can be used to inhibit production of aqueous humor

  • can be used to treat glaucoma
    e. g
  • dorzolamide
  • acetazolomide
33
Q

what is glaucoma?

A

raised intra-ocular pressure is caused by an imbalance between the rates of secretion and removal of aqueous humor

34
Q

how do the anterior and posterior chamber differ in terms of volume?

A

posterior chamber holds 0.06ml
anterior chamber holds 0.25 ml
means that aqueous in posterior chamber is replaced faster (30mins) while anterior is slower (120mins)

35
Q

what is the normal intraocular pressure and what causes this?

A

17mmHg above atmospheric

dependent on balance between secretion and drainage of aqueous

36
Q

describe the inside out laminar structure of the retina

A

photoreceptors pick up the light signals
this signal is passed on to bipolar cells and then ganglion cells which travel to the visual cortex as the optic nerve
however photoreceptors are the innermost layer so light must pass through ganglion cells and bipolar cells to reach photoreceptors

37
Q

what are the 2 types of lateral connections in the cells of the retina and what do they do?

A

horizontal cells = ecieve input from photorecpeotrs and project to other photoreceptors and bipolar cells
amacrine cells = receive input from bipolar cells and project to ganglion cells, bipolar cells and other amacrine cells
allow processing of signal from the eye to begin outwith the brain

38
Q

what are the 2 types of photoreceptor and what are the 4 regions?

A 
rods and cones
outer segment
inner segment
cell body
synaptic terminal
39
Q

what do photoreceptors do?

A

convert electromagnetic radiation to neural signals (transduction)

40
Q

how do photoreceptors differ to other neurones?

A

they have a depolarized resting membrane potential (resting voltage is more positive)
means that photoreceptors hyperpolarize with light exposure

41
Q

what causes this more positive resting membrane potential in photoreceptors?

A

dark current
a cGMP gated Na+ channel which is open in the dark and closed in the light
this change in Na+ with light is the signal which enables the brain to perceive objects in the visual field

42
Q

what happens in the dark as a result of the dark current?

A

PNa = PK

Vm therefore between ENa and EK

43
Q

what happens to the dark current in response to light?

A

PNa is reduced (as outer segment channels close)
PK > PNa
therefore, Vm leads to EK hyperpolarizes
change is local and graded

44
Q

what is the visual pigment molecule in rod cells, what are its components are where is it found?

A

rhodopsin
components = retinal (Vit A derivative) + opsin (G-protein coupled receptor)
present in membrane folds (called discs in the outer segment)

45
Q

how does light affect rhodopsin?

A

converts 11-cis retinal (inactive form of retinal) to all-trans retinal (active form)

46
Q

what does all-trans retinal do?

A

all-trans retinal activates transducin (type of G protein)
activated transducing activates cGMP phosphodiesterase (PDE)
PDE hydrolyses cGMP, reducing its concentration
this leads to closure of Na+ channels
lowered Na+ entry results in hyperpolarization

47
Q

how sensitive is the phototransduction system?

A

very sensitive high gain mechanism
1 opsin leads to activation of 1000 transducin
1PDE leads to 1000 cGMP

48
Q

describe how the dark current channel works in the light and dark

A

open in the dark
closed in the light
nucleotide gasted channel opened by cGMP
permeable to Na+
keeps photoreceptor Vm more positive than most neurons
leads to steady release of neurotransmitter

49
Q

what neurotransmitter is used in the dark current?

A

glutamate

50
Q

what is visual acuity and what can affect this?

A

ability to distinguish two nearby points

determined largely by photoreceptor spacing and refractive power

51
Q

how do cones and rods differ in terms of function and distribution?

A

rods = seeing in dim light
cones = seeing in normal daylight
high concentration of cones in the fovea
rods are more numerous and highly distributed

52
Q

what leads to the difference in function of cone and rod cells?

A

cone have low convergence
- 1/2/3 cones will contact a single ganglion leading to high visual acuity
rods have high convergence
- many rods will contact a single ganglion leading to low acuity vision and high sensitivity in low light as many rods pull together small signals from dull light to create a picture

53
Q

are photoreceptors activated by all wavelengths of light?

A

no

only the visible spectrum

54
Q

what is the basis for colour vision?

A

there are 3 different types of cones which correspond to different opsins
different opsins for different wavelengths
short wave cone = short wave opsin = blue light
middle wave cone = middle wave opsin = green
long wave cone = long wave opsin = red

55
Q

do rods produce colour vision?

A

no

achromatic

56
Q

what problems can occur in colour vision?

A

optical illusions

visual system geared towards detecting differences in light rather than absolute light

57
Q

monocular vs binocular visual field?

A

monocular = each eye sees a part of the visual space (around 45 degrees)
monocular visual fields overlap extensively to create a binocular visual field (around 45)

58
Q

what are the 2 halves of the retina and how do they work?

A

retina divided in half relative to the fovea into a nasal and a temporal hemiretina
nerve fibres from nasal half cross over at the optic chiasm and the resulting 2 optic tracts allow right and left visual fields to reach separately the left and right hemispheres

59
Q

what is retinotopy?

A

the mapping of visual input from the retina to neurons
point of light (image in space which eyes are looking at) activates many cells in the retina due to overlapping visual fields
neurons then travel through optic nerve, optic tract (through left and right lateral geniculate nuclei) to the superior colliculus and then finally to the striate cortex

60
Q

how many layers are in the primary visual cortex and which layer does sensory info from the eyes enter?

A

4 layers

eye specific inputs are segregated in layer 4 (striate cortex)

61
Q

what is amblyopia?

A

where one eye works better than the other in the absence of any problem in the eye, optics or retina

62
Q

how does early monocular deprivation affect binocular vision later in life?

A

early deprivation of one eye (depriving one eye of visual stimulus) causes underdevelopment in the deprived eye and reduced function
other eye is highly developed

63
Q

how does monocular deprivation in adulthood affect binocular vision?

A

function of both eyes is in a normal pattern but just depressed function

Opthamology (18 decks)

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