Physiology Flashcards
What is CSF mainly composed of?
Water
What is CSF produced by?
Secretory epithelium of the choroid plexus
What rate is CSF produced at?
~0.4ml/min/g tissue, 500-600ml/day
What is the total volume of CSF?
150ml volume, replaced 3-4xdaily
Where is CSF formed and subsequently circulate?
Ventricles and then circulates in the subarachnoid space then absorbed into venous circulation
What does the CSF supply to tissue?
Water, aa, ions
What does the CSF remove?
Metabolites
What are the 3 major functions of CSF?
Mechanical protection:shock absorbing medium that protects brain tissue. Homeostatic function: pH of CSF affects pulmonary ventilation and cerebral blood flow. Transports hormones. Circulation: medium for minor exchange of nutrients and waste products between blood and brain tissue
How is CSF obtained for clinical analysis?
LP
What is the composition of CSF?
Clear, colourless. Little protein (15-45mg/dl), little Ig and only 1-5 cells/ml
What does the developing nervous system consist of at 3 weeks?
A Tube-neural canal
What does the cavity of the neural canal give rise to?
Adult brain’s ventricles and spinal cords central canal
What does the choroid plexus develop from?
Cells in the walls of the ventricles
How is the choroid fissure and plexus formed?
Developing arteries invaginate the roof of the ventricle. The involuted ependymal cells along with vessels enlarge into villi and form the plexus
Where is the choroid plexus found in the adult brain?
In the 3rd, 4th and lateral ventricles
What is the choroid plexus?
Network of capillaries in walls of ventricles
What ion transport is involved in CSF secretion and in what direction?
Na+, Cl-, HCO3 (Plus H20) from blood to CSF
How can secretion occur in the epithelial cells of the plexus?
Because of the polarised distribution of specific ion transporters in the apical or basolateral membrane of the epithelial cells
What is the constitution of CSF compared to plasma?
Lower K+ and glucose, and much lower protein than blood, and higher concentrations of Na+ and Cl-
Which ventricles are connected by the intraventricular foramina (of Monroe)?
Lateral’s to 3rd
Which ventricles are connected by the cerebral aqueduct (of Sylvius)?
3rd to 4th
Which ventricles are connected by the foramen of mengendie?
Median aperture-4th to subarachnoid space
Which ventricles are connected by the foramen of Luschka?
Lateral apertures-4th to subarachnoid space
What is the circulation of CSF in the brain?
Formed in choroid plexus of each lateral ventricle. Flows to 3rd through interventricular foramina. More added by plexus in roof of 3rd. Flows through aqueduct of midbrain and into 4th. Another plexus in 4th adds. Enters subarachnoid space through 3 openings in roof of 4th (single medium/paired lateral apertures). Then circulates in central canal of cord
How does CSF return to venous blood?
Through arachnoid granulations in the superior sagittal sinus (SSS)
Through what does the brain interstitial fluid drain through to the CSF?
Perivascular spaces
What are the site of the BBB?
Endothelial cells in brain capillaries
What does the BBB consist of?
Capillary endothelium, its basal membrane and perivascular astrocytes
What prevents paracellular movement of molecules in the BBB?
Tight junctions between brain endothelial cells
What parts of the brain do not have a BBB?
Circumventricular organs or pineal gland
What is the main obstacle for drug delivery to the CNS?
BBB
What tumours can occur of the ventricles, choroid plexus and CSF?
Colloid cyst (often at interventricular foramen), ependymomas (arising from ependymal cells lining ventricles), choroid plexus tiumours
What haemorrhage and hematoma pathologies of the ventricles, choroid plexus and CSF can occur?
Ventricular haemorrage: accumulation of blood in ventricles. Epidural haematoma: arterial bleed between skull and dura. Subdural haematoma: venous bleed between dura and arachnoid. Subarachnoid haemorrhage
What is hydrocephalus?
Accumulation of CSF in the ventricular system or around brain (obstruction/overproduction), subsequent enlargement of one or more ventricles and increase in CSF pressure
What is idiopathic intracranial HT/pseudotumour cerebri?
Enigmatic condition. Symtoms-headache an visual disturbances (field disturbances to blind) due to papilloedema.
What is papilloedema?
Optic disk swelling due to increased intracranial pressure transmitted to the subarachnoid space surrounding optic nerve
What visual symptoms occur in papilloedema?
Enlarged blind spot, blurring of vision, visual obscurations and loss of vision
What is aqueous humor?
A specialised fluid that bathes the structures within the eyes
What does aqueous humor provide to the eye?
Oxygen and metabolities (and contains bicarb)
What does bicarb do in the eye?
Buffers the H+ produced in the cornea and lens by anaerobic glycolysis
How is aquemous humor produced?
By an energy dependent process in the epithelial layer of the ciliary body into the posterior chamber of the eye.
Where does aqueous humor flow to after production?
Into anterior chamber and then drains to scleral venous sinus through a trabecular meshwork and the caal of Schlemm, situated in the angle between iris and cornea iridocorneal angle.
What happens to a small volume of aqueous humor?
Diffuses through the vitreous being absorbed across the retinal pigment epithelium
What are the two juxtaposed layers of epithelial cells that cover the ciliary body and posterior surface of iris?
A forward continuation of pigmental epithelium of retina (PE) overlain by an inner nonpigmented epithelial (NPE) layer
What transports into the aqueous humour via the Na+/K+/2Cl- cotransporter and how?
Cl- and Na+ ions, by diffusion through the gap junctions between the PE and NPE cells and are transported out of the NPE cells into the aqueous humour
Movement of Cl- and Na+ through the cells from interstitial fluid to aqueous humor is accompanied by what?
Water moving through ciliary epithelial cell water channels, and aquaporins through the paracellular pathway down the osmotic gradient created by solute movement
What are the approximate volumes of the anterior and posterior chambers?
250ml and 60ml respectively
Which chamber has a longover turnover time in terms of aqueous humor?
Anterior (~120mins), whereas posterior ~30mins
A balance between secretion and drainage maintains an intraocular pressure of what?
Around 17mmHg above atmospheric
What factors must be met in order to see an object?
- Pattern of object must fall on vision receptors>accommodation
- Amount of light entering eye must be regulated
- Energy from the waves of photons must be transduced into electrical signals
- Brain must receive and interpret signals
What is the direct pathway for signal transmission?
Photoreceptors, bipolar cell, ganglion cells
What do horizontal cells do?
Receive input from photoreceptors and project to other photoreceptors and bipolar cells
What do amacrine cells?
Receive input from bipolar cells and project 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 do photoreceptors do?
Convert electromagnetic radiation to neural signals
What is the resting membrane potential of a vertebrate photoreceptor?
~-20mV
What happens to the rmp of photoreceptors when they are exposed to light?
Hyperpolarisation
What is the dark current?
A cGMP-gated Na+ channel that is open in the dark and closes in the light
In the dark what is the permeability to Na+ and K+ in photoreceptors?
Roughly equal, therefore Vm is between Ena and Ek
What happens to Pna and Pk in response to light?
Pna is reduced (outer segment channels close), Pk>Pna. Vm becomes closer to Ek, therefore hyperpolarises
What are the visual pigment molecules in rods?
Rhodopsin- Retinal +Opsin
Where is rhodopsin present in the rods?
Disks in the outer segment
What does light do to retinal?
Converts 11-cis-retinal to all-trans-retinal
What does all-trans-retinal do?
Activates transducin, which in turns causes a molecular cascade, which decreased cGMP and increases GMP. Leads to closure of cGMP-gated Na+ channel
What does lowered Na+ entry result in?
Hyperpolarisation
What makes phototransduction a high gain mechanism?
1 opsin molecule can lead to activation of 1000 transducin, and 1 PDE can break down 1000cGMP
Is the dark current channel open or closed in the dark?
Open
What is visual acuity and what is it determined by?
Ability to distinguish two nearby points. Determined largely by photoreceptor spacing and refractive power
What does more convergence in the rod system allow?
Increased sensitivity while decreasing acuity
What allows colour vision?
Different opsins for discrete wavelengths
What are the 4 photoreceptor types in humans?
Short wave (blue) cone, middle wave (green) cone, long wavelength (red) cone, and rods
What are the properties of rods?
Achromatic, located in peripheral retina, high convergence and light sensitivity, low visual acuity
What are the properties of cones?
Chromatic, located in central retina (fovea), low convergence and light sensitivity, high visual acuity
How is the retina divided?
Nasal and temporal hemiretina
What visual fields does each eye have?
Monocular field (+-45’), binocular field (+-45’)
Nerve fibres from which retinal half cross over at the optic chiasm?
Nasal half
Where is the visual field mapped?
Retina, LGN, superior colliculus and striate cortex
In the primary visual cortex where are eye specific inputs segregated?
Layer 4
What input do cells outside of layer 4 in the PVC receive?
Input from both eyes
