Homeostasis in the nervous system Flashcards
What is the neuronal microenvironment composed of ?
Glia
Capillaries
Neurons
Extracellular space
What is extracellular space composed of?
Extracellular matrix
Brain extracellular fluid (interstitial fluid)
What can uncontrolled neuronal influence of brain extracellular fluid lead to?
Neuronal dysfunction
Neuronal death
How can neuronal activity change the composition of the BECF?
Increase K+ conc
Changes in Ca+ conc
Changes in O2, glucose and CO2 conc
Increase in H+,, leading to acidifcation
How could a change in BECF composition change neuronal activity?
Increase in K+ conc can elevate resting potential
-> Brings the cell closer to ap firing potential, increasing neurotransmitter release
How is the neuronal microenvironment regulated?
Blood brain barrier
CSF in ventricular system
Neurons
Glial cells
How was the blood brain barrier identified?
Intravenous injection of dyes
- Dyes pass across leaky capillaries
- Stain soft tissue
- No staining in the brain
Why does the brain not get stained?
The capillaries are less leaky than those in the spinal cord
Function of brain blood barrier
Protect neurons from fluctuations in concentrations of substances in the blood
How is the brain blood barrier maintained?
Tight junctions between endothelial cells
Thick basement membrane
Astrocytic endfeet
How do endothelial cells maintain the brain blood barrier?
Prevents paracellular diffusion of materials
How does the thick basement membrane maintain the bbb?
Acts as a physical barrier
How do astrocytic end feet maintain the bbb?
Interact with blood vessels and form a capsule
What is the difference between an exchanger and a cotransporter?
Exchanger is in opposite directions, cotransporters are in the same direction
What molecules pass the bbb easily?
Small, uncharged or lipid soluble molecules
What is an example of a facilitated transporter in bbb?
Glut1
What is an example of an exchanger in bbb?
Na - H exchanger
Where is cerebrospinal fluid produced?
Choroid plexuses
What are the circumventricular organs function?
Form tight junctions (in the form of ependymal cells) beneath the blood vessels
Why are there leaky areas of the bbb?
To allow secretion of hormones
Osmoreceptors
Temperature control centres
Function of the ventricular system?
Increased buoyancy
Physical protection
Maintains appropriate level of ions
Removes waste products
How does the ventricular system increase the buoyancy of the brain?
The brain doesn’t move with full weight when moving around
What does the cerebral aqueduct lead to?
The 4th ventricle where it thins through the medulla and becomes the central canal
What is in CSF?
Electrolytes
Amino acids
Proteins
Osmolality
What is exchanged between CSF and BECF?
Macronutrients
Micronutrients
Ions
What is exchanged between BECF and CSF?
Metabolic waste products
Neurotransmitters
What areas does the CSF circulate around?
- 3rd ventricle
- 4th ventricle
- central canal
- foramens
- outer brain
- sinuses
- venus system
What happens to CSF at the superior sagittal sinus?
CSF is absorbed from the subarachnoid space into the venous blood system
How much CSF is produced a day?
500 mls
What is an ependymal cell?
A specialised glial cell that line the ventricles
What are the stages of CSF secretion?
- Ultrafiltration of plasma into ECF across normal ‘leaky’ capillaries
- Selective absorption of substances into csf across choroidal epithelial cells
- Free movement of substances from CSF to BECF across ependymal cells
What are the 3 types of meninges?
Pia mater
Arachnoid mater
Dura mater
Describe pia mater
Covers the surface of brain and blood vessels and allow diffusion between CSF and BECF
Describe arachnoid mater
Cells linked by tight junctions preventing diffusion between CSF and plasma
Describe dura mater
Thick inelastic membrane with 2 layers split to form intercranial sinuses
Prevents too much movement
How do things enter the dura mater?
Arachnoid granulations push through dura mater
What is the mechanism of csf absorption?
Arachnoid granulations push up through the dura matter into the venous sinus
- The csf is transported in vesicles driven by pressure build up
What is hydrocephalus a result of ?
Dilation of ventricular system
Obstruction in ventricular system
Interrupted csf absorption
What are the symptoms of hydrocephalus?
Increased intercranial pressure
Less of brain cells
Loss of brainstem reflexes
Where do neurons and astrocytes terminate transmission?
Tripartite synapse
How is neurotransmission terminated?
Neurotransmitters are removed from synaptic cleft and recycled
Via neurons and astrocytes
How is glutamate neurotransmission terminated?
EEAT transports glutamate back into the cell for recycling
Astrocytes convert glutamate to glutamine
What is the main excitatory neurotransmitter?
Glutamate
What occurs if glutamate is left in the cleft?
Excitotoxicity
How is K+ removed from extracellular space?
Astrocytes and neurons
Sodium potassium ATPase pump, moves against gradient back into the cell
What happens when astrocytes sense high K+ in ECF?
Convert glucose into lactate to excite neurons
How does extracellular K+ affect astrocyte function?
Astrocytes drive K+ into ECF
Can drive K+ to be taken up in the cell, increasing glucose metabolism
More glucose allows better K+ uptake via ATPases
Why do neurons have a resting membrane potential of -65mV and glia have -85mV?
Neuronal membranes are more permeable to Na+ than astrocytic membranes
Astrocytes are more sensitive to extracellular K+ changes
What creates a syncytium?
Astrocytes coupling to each other via gap junctions
How is a gap junction formed?
Connexins form a connexon on 1 membrane
2 connexons in different membranes form a gap junction.
What is spatial buffering?
Regulation of extracellular potassium concentration
What is neurovascular coupling?
Neurons are coupled to vasculature
Describe the process of neurovascular coupling
Increased neuron firing rate leads to an increase in astrocyte Ca2+ levels ->
Leads to a release of vasoactive substances from the astrocyte ->
Blood vessels and diameter change
What do MRI allow imaging of ?
3d structural images of the brain
What is the function of a PET scan?
Exploits the use of glucose
What is the function of an fMRI scan?
Exploits the use of oxygen
What is BOLD in terms of fMRI?
Blood
Oxygenation
Level
Dependent
What state is the person undergoing fMRI in?
Unconscious state
