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Ophthalmology > Physiology > Flashcards

Physiology Flashcards

1
Q

How much CSF is in the body at one time and how often is it replaced?

A

In CNS amounts to 150ml volume and is replaced 3-4 x daily

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2
Q

Describe the three main functions of CSF

A
  1. Mechanical protection: shock-absorbing medium that protects brain tissue. Brain “floats” inside the cranial cavity
  2. Homeostatic function: pH of CSF affects pulmonary ventilation and cerebral blood flow. Transports hormones.
  3. Circulation: medium for minor exchange of nutrients and waste products between blood and brain tissue
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3
Q

Give some characteristic of normal CSF obtained for lumbar puncture

A

Normal CSF is clear and colorless, contains little protein (15-45mg/dl), little immunoglobulins and only 1-5 cells/ml.

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4
Q

What are choroidal cells?

A

Choroidal cells are specialized secreting cells that produce cerebrospinal fluid CSF.

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5
Q

Where is the choroid plexus found?

A

Choroid plexus in the adult brain is found in the 3rd, 4th and lateral ventricles.

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6
Q

What is the choroid plexus?

A

These are networks of capillaries in walls of ventricles.

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7
Q

What does CSF secretion involve?

Why can secretion occur?

A

CSF secretion involves the transport of ions (Na+, Cl¯ and HCO3¯) across the epithelium from blood to CSF.
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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8
Q

What does secretion of fluid by the choroid plexus depend on?
How is Cl transported?

A

Depends on the active Na+-transport across the cells into the CSF.
The electrical gradient pulls along Cl-, and both ions drag water by osmosis.

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9
Q

How does the constituents of CSF compare to that of plasma?

A

The CSF has lower [K+], [glucose], and much lower [protein] than blood plasma, and higher concentrations of Na+ and Cl-.

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10
Q

How does secretion of CSF depend on arterial blood pressure?

A

The production of CSF in the choroid plexuses is an active secretory process, and not directly dependent on the arterial blood pressure.

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11
Q

What connects the two lateral ventricles to the third ventricle?

A 
Intraventricular foramina (of Monroe) 
- note these are bilateral
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12
Q

What connects the 3rd and 4th ventricles?

A

Cerebral aqueduct

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13
Q

What connects the fourth ventricle to the subarachnoid space?

A

Foramen of Magendie - Median aperture

Foramina of Luschka: Lateral apertures

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14
Q

Describe the path of CSF from lateral ventricles to fourth ventricle

A
  • CSF formed in choroid plexuses of each lateral ventricle.
    Flows to third ventricle through two narrow openings called the interventricular foramina.
  • More CSF added by choroid plexus in roof of third ventricle
  • Then flows through aqueduct of midbrain (cerebral aqueduct)
  • And into the fourth ventricle
  • Another choroid plexus in fourth ventricle adds more CSF
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15
Q

How does CSF enter the arachnoid space?

A

CSF then enters the subarachnoid space through 3 openings in roof of fourth ventricle
- Single median aperture
- Paired lateral apertures
Then circulates in the central canal of spinal cord.

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16
Q

Describe ventricular circulation and outflow of CSF

A
  • Directional flow though the ventricular system into the subarachnoid space (SAS) between pia and dura mater
  • CSF returns to venous blood through arachnoid granulations into the superior sagittal sinus (SSS)
  • The blood supply to the majority of the brain is restricted by the blood-brain barrier (BBB), which is quite tight
  • The brain interstitial fluid makes up the final portion of CSF and drains to the CSF though the perivascular spaces.
17
Q

What is the exact site of the blood brain barrier (BBB)?

  • What does this consist of?
  • Which parts of the brain do not have this?
  • What is the role of BBB?
  • What is the BBB a barrier to clinically?
A
  • Endothelial cells in brain capillaries
  • Some parts of the brain do not have a BBB such as circumventricular organs or pineal gland
  • BBB protects the brain from many common bacterial infections and toxins
  • BBB is the main obstacle for drug delivery to the CNS
18
Q

What is papilloedema?

A

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

19
Q

What are some visual symptoms of papilloedema?

A

Visual symptoms may include enlarged blind spot, blurring of vision, visual obscurations and loss of vision.

20
Q

What is aqueous humour and what is its function?

A

Specialized fluid that bathes the structures within the eye.
It provides oxygen and metabolites and contains bicarbonate (HCO3).
Bicarbonate buffers the the H+ produced in the cornea and lens by anaerobic glycolysis.
Ascorbate is a powerful antioxidant.

21
Q

What causes glaucoma?

A

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

22
Q

How is glaucoma treated?

A

Carbonic anhydrase inhibitors will reduce production of aqueous humor - used to reduce ocular pressure in glaucoma:

  • Dorzolamide - is administered as eye drops- avoiding systemic side effects
  • Acetazolomide - oral administration -> also targets kidney -> acidosis.
23
Q

What is the pathway from light hitting the retina to an AP in the optic nerve?
What are the two horizontal connections involved?

A
  1. Photoreceptors
  2. Bipolar cells
  3. Ganglion cells
  4. Optic nerve
    - Horizontal cells - receive input from photoreceptors 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
24
Q

What is the function of photoreceptors?

A

Converts electromagnetic radiation to neural signals (transduction!)

25
Q

What are the four main regions of a photoreceptor?

A
  • Outer segment
  • Inner segment
  • Cell body
  • Synaptic terminal
26
Q

What are the two types of photoreceptor?

A

Rods

Cones

27
Q

Are photoreceptors usually depolarized or hyperpolarized?
Are they more positive or negative compared to other neurons?
Why is there a resting positive Vm?

A

Vertebrate Photoreceptors have a depolarized rmp (Vm).
Compared to other neurons, resting Vm is more positive
This positive Vm is because of the “dark current” - a cGMP-gated Na+ channel that is open in the dark and closes in the light.

28
Q

What happens to the photoreceptors when light is shone on them?

A

They hyperpolarize

29
Q

The dark current is a cGMP-gated Na+ channel that is open in the dark and closes in the light.
What is it that changes with light, allowing the brain to perceive objects in the visual field?

A

This change in Na+ with light is the signal that enables the brain to perceive objects in the visual field

30
Q

How is the dark current modulated?

A

In the dark,
- PNa = PK (Na channels in the outer segment)
- Vm therefore between ENa and EK
In response to light,
- PNa is reduced (outer segment channels close), PK > PNa
- Therefore, Vm -> EK, hyperpolarizes
- Change is local and graded

31
Q

What is rhodopsin?

What does light do to it?

A

Rhodopsin = opsin + 11-cis-retinal
It is a biological pigment found in the rods of the retina and is a G-protein-coupled receptor (GPCR). Rhodopsin is extremely sensitive to light, and thus enables vision in low-light conditions. When rhodopsin is exposed to light, it immediately photobleaches.
Light converts this to all-trans-retinal
This leads to closure of cGMP-gated Na+ channel.
Lowered Na entry results in hyperpolarization.

32
Q

Describe how the dark current forms the basis of phototransduction

A
  • Open in the dark
  • Closes in response to light
  • Nucleotide-gated channel (opened by cGMP)
  • Permeable to Na+
  • Keeps photoreceptor Vm more positive than most neurons
  • > Steady release of neurotransmitter
33
Q

What two things allow sharp vision (acuity)?

A
  1. Photoreceptor spacing - determines ability to distinguish two near points
  2. Distribution of rods and cones
    - Rods – seeing in dim light
    - Cones – seeing in normal daylight
    - More convergence in rod system, increasing sensitivity while decreasing acuity
34
Q

Give five differences between rods and cones

A 
Rods
- Achromatic 
- Peripheral retina
- High convergence 
- High light sensitivity
- Low visual acuity 
Cones
- Chromatic 
- Central retina (fovea)
- Low convergence
- Low light sensitivity 
- High visual activity
35
Q

What is Hebb’s postulate?

A

When an axon of cell A is near enough to excite a cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A’s efficiency, as one of the cells firing B, is increased.”
Correlated activity between presynaptic and postsynaptic cells strengthens synaptic connections between them.

Ophthalmology (9 decks)

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