homeostasis Flashcards
what is the neuronal microenvironment composed of (4)
glia
capillaries
other neurons
extracellular space - extracellular matric, brain extracellular fluid (BECF)
BECF - how increased neuronal activity changes BECF composition (4)
increased K+ concentration - K+ efflux from neuron
changes in Ca+, O2, glucose, CO2 concentrations
increased H+ from neurons - acidification
neurotransmitter concentrations - likely to increase
BECF - how BECF composition increases neuronal activity (2)
increased K+ → elevates resting potential of neuron, bringing cell closer to threshold for firing an action potential (membrane potential increases with potassium concentration)
increased neurotransmitter release → could be increased excitation or inhibition - unspecific neuronal activity
BECF and neuronal activity
influence each other in a loop → increased activity releases more K+ and more K+ means membrane potential is closer to the threshold to fire again
BBB - blood brain barrier function (with examples of effects of different molecules (5))
to protect neurons from fluctuations in concentrations of substances in the blood
examples:
increased amino acid concentrations after a meal = unspecific activation of receptors if in brain
increased K+ and H+ after exercise = influence membrane potential of neurons
hormones - variation in females e.g. breakdown product of progesterone binds to GABA receptors which influences mood therefore needs to be prevented
inflammatory mediators
toxins
BBB maintenance (4)
tight junctions between endothelial cells
thick basement membrane
astrocytic endfeet - processes attach to blood vessels
endothelial cells prevent paracellular diffusion so they need to move through membranes instead (transcellular route)
BBB - how important molecules get through
facilitated transport, exchangers, co-transporters
lots of mitochondria for active transport
small, uncharged, lipid soluble molecules can pass through the BBB e.g. CO2, O2, nicotine
BBB - challenges with drug development
hard to develop drugs that can pass through the BBB - difficult administration
BBB - leaky legions (2)
choroid plexuses - in ventricular system where CSF is made (to get substances into CSF)
circumventricular organs - surround ventricles
these areas have ependymal cells beneath with tight junctions
BBB - why are there leaky legions (2)
hormone release - e.g. hypothalamus and pituitary gland
osmoreceptors - e.g. OVLT and SFO, hypothalamus (water and ion levels in body)
temperature control centres and fever - cytokines - e.g. OVLT (conscious awareness of this can make us change behaviour to reduce this e.g. getting a drink when dehydrated)
CSF - ventricular system (what it is (2) function (3))
cavity which contains CSF
provides physical protection - buffer between brain and skull
maintains ion levels
removes waste
becomes central canal as it thins down the medulla
CSF composition (compared to plasma)
lower K+ than plasma
much lower amino acids than plasma
VERY low protein compared to plasma
CSF - exchange between CSF and BECF
CSF -> BECF = 3
BECF -> CSF = 2
CSF → BECF
macronutrients e.g. glucose
micronutrients e.g. vitamins
ions e.g. HCO3- (bicarbonate)
BECF → CSF
metabolic waste products e.g. CO2
neurotransmitters
CSF - flow
secreted by choroid plexus
circulates around ventricles and central canal (central canal is very small - only few micrometres across)
some CSF goes into foramens (holes that lead to outside of brain)
CSF is absorbed from subarachnoid space to the venous blood system at the superior sagittal sinus (sinus = structure that leads back into veinous system)
CSF - movement from blood to BECF (3 stages)
movement of substances from capillaries to BECF - regulates BECF composition
ultrafiltration – plasma into ECF across normal leaky capillaries
selective absorption – ECF into CSF across choroidal epithelial cells (have tight junctions)
free movement – from CSF to BECF across ependymal cells
3 meninges
3 membranes around the brain (from central to outermost layer)
pia mater = “soft mother” = clings around gyri and sulci, very thin, no tight junctions, leaky membrane, free exchange between BECF in brain tissue and fluid in subarachnoid space
arachnoid mater = selective membrane with tight junctions, more similar to choroidal epithelium
dura mater = “tough mother” = outermost layer, function to keep brain rigid (structural) and prevent movement of ions, 2 layers: one follows arachnoid and pia mater (the leptomeninges) around gyri and sulci; the other layer round the top to make a smoother surface around brain
pia and arachnoid mater = leptomeninges
CSF - arachnoid granulations
evaginations of arachnoid membrane - stick out into sinuses
large - up to 1cm
pinocytosis - engulfs large portions of the CSF in a large vesicle to deposit in the sinus
increased intracranial pressure = increased absorption (more CSF is made so more pinocytosis and more CSF moved into the sinus)
CSF - hydrocephalus
CSF not properly circulating
dilation of ventricular system so CSF cannot leave to be absorbed in sinuses so fluid builds up
increases intracranial pressure and loss of cells within brain and brainstem
role of neurons and astrocytes with glutamate at tripartite synapse
tripartite synapse = 2 neurons and an astrocyte
glutamate = main excitatory neurotransmitter
excitotoxicity = overexciting of neurons leading to death of synapses - therefore needs to be removed from synapse once used
astrocytes convert glutamate into glutamine an transport it away
processes to correct high extracellular potassium around neurons (3)
negative effect
sodium potassium pumps move K+ back into cell so it is not in fluid
increased glucose metabolism - more ATP to fuel pumps so K+ can be moved into neurons more
with astrocytes - conversion of glucose to lactate and transport it into neurons to maintain activity of pumps when needed
equilibrium potential point for K+ and resting membrane potential in neurons vs astrocytes
equilibrium potential point for K+ = the point when K+ stops moving in and starts moving out = -90mV in neurons and astrocytes
neuron resting membrane potential = -65mV
astrocyte resting membrane potential = -85mV
neuronal membranes = more permeable to Na+ than astrocytic = less effected by changes in extracellular K+
astrocytic membranes = more selective to K+ = extracellular K+ will influence Vm more
redistribution of K+ to less active areas via astrocytes
astrocytes electrically couple for form syncytium with gap junctions
syncytium = directly actively coupled group of cells
connexins make connexons to form gap junctions an cell cytoplasm’s become continuous
allows movement of K+ from astrocyte which is by a very active neuron (therefore high extracellular K+) to other astrocytes by less active neurons (lower extracellular K+)
K+ moves down concentration gradient through astrocytes before moving back into ECF at areas of lower K+
neurovascular coupling
increased neuronal firing = increased calcium in astrocytes
Ca2+ can move along syncytium (like K+)
astrocytes can sense this change and communicate it to blood vessels
more Ca2+ = release vasoactive substances into blood vessels = changes dilation/constriction of vessel –> more blood needed when more active
functional imaging techniques (2)
PET - positron emission tomography - compares high/low glucose use areas
fMRI - functional magnetic resonance imaging - compares oxyhaemoglobin and deoxyhaemoglobin in blood in areas of brain
neurovascular coupling is the concept used to take these images - more active = more blood flow needed for more oxygen
