Physiology Flashcards
Where is CSF made?
By the secretory epithelium of the choroid plexus in the ventricles of the brain
How much CSF is produced a day?
500-600ml/ day
What is the function of CSF?
Mechanical protection
Homeostatic function
Circulation
How does the CSF act as mechanical protection?
Shock-absorbing medium that protects brain tissue. Brain “floats” inside cranial cavity
What homeostatic function does the CSF provide?
pH of CSF affects pulmonary ventilation and cerebral blood flow
Transports hormones
What role does the CSF have in circulation?
Medium for mior exchange of nutrients and waste products between blood and brain tissue
How is CSF analysis performed?
Lumbar puncture - aids the diagnosis of brain, meninges and spinal cord
What should normal CSF fluid look like?
Clear and colourless, contains little protein (15-45 mg/dl) and little immunoglobulins (1-5 cell/ml)
How does hydrocephalus develop?
Obstruction of CSF outflow with resulting enlargements of ventricular spaces upstream of the blockage
Bones of the developing skull move apart and head will enlarge
May need CSF shunt
When does the brain begin to develop?
3 weeks post conception
What is a choroid plexus?
Arteries invaginated in the ventricle to form villi surrounded by enedymal cells
Where can choroid plexuses be found?
Lateral ventricles
3rd ventricle
4th ventricle
How is CSF produced in the choroid plexus?
CSF secretion involves the transport of ions (sodium, chloride and bicarbonate) across the epithelium from blood to CSF
What doe secretion of fluid by the choroid plexus depend on?
The active sodium transport across the cells into the CSF. The electrical gradient pulls along chloride and both drag water via osmosis
What are the differences in electrolytes between the plasma and CSF?
CSF has lower potassium, glucose and protein
CSF has higher sodium and chloride
Does CSF production depend on arterial pressure?
No, it is an active secretory process
What connects the lateral ventricle to the 3rd ventricle?
Interventricular foramina of monroe
What connects the 3rd ventricle to the 4th?
Cerebral aqueduct of sylvius
What connects the 4th ventricle to the subarachnoid space?
Foramen of magendie
Foramen of Luschka
Describe the circulation of CSF?
CSF formed in choroid plexus of lateral ventricles
Flows to 3rd ventricle via foramina of monroe
More CSF added in 3rd ventricle
Flows through cerebral aqueduct of sylvius to the 4th ventricle
4th ventricle adds more CSF
CSF enters subarachnoid space via foramen of magendie and luschka
Circulates central canal of spinal cord
Returns to venous blood via superior sagittal sinus
Where is the subarachnoid space?
Between pia and dura mater
What makes up the blood brain barrier?
Capillary endothelium, basal membrane and perivascular astrocytes
Tight junctions between brain endothelial cells prevent paracellular movement of molecules
What does the BBB protect the brain from?
Common bacterial infections and toxins
Determining factor for clinical CSF analysis
Main obstacle for drug delivery to CNS
What tumours can arise in the ventricles, choroid plexus and CSF?
Colloid cyst - interventricular foramen of monroe
Ependymomas - arises from the ependymal cells lining ventricles
Choroid plexus tumours
What bleeds can occur in the brain?
Ventricle haemorrhage - blood in the ventricles
Epidural hematoma - arterial bleed between skull and dura
Subdural hematoma - venous bleed between dura and arachnoid
Subarachnoid haemorrhage - bleed between pia and dura mater
What is hydrocephalus?
Accumulation of CSF in ventricular system or around the brain
Enlargement of ventricles and CSF pressure
What is idiopathic intracranial hypertension?
Eningmatic condition
Headache, visual disturbances due to papilloadema
What is papilloedema?
Optic disc swelling due to increased intracranial pressure transmitted to the subarachnoid space surrounding the optic nerve
What visual symptoms occur with papilloedema?
Enlarged blind spot
Blurring of vision
Visual obscurations
Loss of vision
What is aqueous humor?
A specialized fluid that bathes the structures within the eye. It provides oxygen and metabolites and contains bicarbonate
Bicarbonate buffers the protons produced in the cornea and lens via anaerobic glycolysis
What produces aqueous humor?
Ciliary body
What is the flow of aqueous humour around the eye?
Produced in the posterior chamber
Flows into anterior chamber
Drains to the scleral venous sinus through the canal of schlemm situated in the angle between the iris and cornea iridocorneal angle
What type of epithelium covers the ciliary body?
Two juxtaposed layers of epithelial cells, a forward continuation of the pigment epithelium of the retina and overlain by an inner nonpigmented epithelial layer
Describe the process of the exchange of bicarbonate and hydrogen for chlorine and sodium in aqueous humour?
Bicarb and H+ formed in the epithelial cells from hydration of carbon dioxide catalysed by carbonic anhydrase are transported across the basolateral membranes of PE cells into the interstitial fluid in exchange for chlorine and sodium
What happens to sodium and chlorine ions once they enter the cells?
They diffuse through gap junctions between the PE and NPE cells and are transported out of the NPE cells into aqueous humor via the Na+/K+2CL- cotransporter
Potassium ions leaving are recycled and some chlorine will also leave
What is the outcome of movement of chlorine and sodium?
Movement of sodium and chlorine from interstitial fluid to aqueous humour is accompanied by water moving through ciliary epithelial cell aquaporins and through the paracellular pathway down the osmotic gradient created by solute movement
How much aqueous humour is made?
1-ml/min of fluid is secreted and flows into anterior chamber
Vol of posterior chamber is approx 60ml whilst anterior is 250ml
What is glaucoma?
Raised intra-ocular pressure by an imbalance between the rates of secretion and removal of aqueous humour
What drugs can be used in the treatment of glaucoma?
Carbonic anhydrase inhibitors:
Dorzolamide - eye drops
Acetazolomide - oral administration
Summarise vision
Pattern of object must fall on the vision receptors (rods and cones)
Amount of light entering must be regulated
Energy form waves of photons is transduced into electrical signals
Brain receives and interprets the signals
What are the layers of the retina?
Photoreceptor cells
Bipolar cells
Ganglion cells
What is a horizontal cell?
Receives input from photoreceptors and projects to other photoreceptors and bipolar cells
What is an amacrine cell?
Receives input from bipolar cells and projects to ganglion cells, bipolar cells and other amacrine cells
What are the 4 main regions of photoreceptors?
Outer segment
Inner segment
Cell body
Synaptic terminal
What are the different types of photoreceptors?
Rods
Cones
What neurotransmitter is involved in photoreception?
Glutamate
What will happen to the membrane potential of photoreceptor cells upon exposure to light?
Hyperpolarization - there is a cGMP gated Na+ channel that is open in the dark and closes in the light
The change in Na+ with light is the signal that enables the brain to perceive objects in the visual field
What is the release of transmitter like in the dark?
Steady release
Less glutamate released in the light
What is the pigment molecule in rods?
Rhodopsin - retinal and opsin (G-coupled receptor)
What will light do to rhodopin?
Photoconvert 11-cis-retinal to all-tans-retinal setting a G-protein cascade
What occurs during the G-protein cascade in photoreceptors in response to light?
Activation of G-protein
G-protein activated cGMP phosphodiesterase (PDE)
PDE hydrolyses cGMP reducing the concentration
Closure of Na+ channels
What is visual acuity?
Ability to distinguish two nearby points
Determined largely by photoreceptor spacing and refractive power
When are cods and rods used respectively?
Rods - dim light
Cones - normal daylight
Describe the differences in convergence between rods and cones?
Many rods for one ganglion cell - high convergence
Few cones per ganglion cell - low convergence
What is the basis for colour vision?
Different opsins in cones for discrete wavelengths - short wave cone (blue), medium wave cone (green) and long wave cone (red)
Compare rods and cones
Rods: achromatic, peripheral retina, high convergence, high light sensitivity, low visual acuity
Cones: chromatic, central retina (fovea), low convergence, low light sensitivity, high visual acuity
What fibres cross the optic chiasm?
Nasal fibres cross
Temporal stays on the outside
What layer of the visual cortex is used for input of axons from the optic nerve?
Layer 4
What is the impact of having a congenital blindness in one eye?
Lasting damage
In the first few weeks/ months of life, the axons carve out their space on the target cell in the visual cortex and everytime there is correlated activity between presynaptic and post-synaptic cells, the synaptic connection strengthens
