homeostasis in the nervous system Flashcards
what is the The neuronal microenvironment composed of
-Glia
-Capillaries
-Other neurons
-Extracellular Space:
=Extracellular matrix
=Brain extracellular fluid (BECF)
how can BECF influence and be influenced by neurons
- increase in activity in neurone -> change in BECF composition - > increase in activity in neurone so on
-Uncontrolled:
=neuronal dysfunction
=neuronal death
-BECF composition must be tightly regulated
How could neuronal activity change the composition of the BECF
- increase in K+ concentration
-Changes in O2, glucose and CO2 concentrations
-Increased H+, thus acidification
-Neurotransmitter concentrations
How could a change in BECF composition change neuronal activity
-Increased K+ concentration in the BECF could elevate resting potential bringing the cell closer to threshold for firing an action potential
-Increased neurotransmitter release.
-Increased neurotransmitter concentrations could lead to unspecific receptor activation and thus unspecific neuronal activity.
How do we regulate the neuronal microenvironment, including BECF
1)Blood brain barrier
2)Cerebrospinal fluid (CSF) in ventricular system
3)Neurons
4)Glial cells – focus on astrocytes
whats the blood brain barrier
-Revealed by the intravenous injection of dyes
-Dyes pass across leaky capillaries
-Stain soft tissues
-But not in the brain - no staining
-capillaries in brain have a tighter junction in them to stop dye from leaking out
whats the function of a blood brain barrier
-Function - to protect neurons from fluctuations in concentrations of substances in the blood
-For example:
=increased amino acid concentrations after a meal
=increases in K+ and H+ concentration after exercise
=circulating hormones
=inflammatory mediators
=toxins
how is the blood brain barrier maintained
-Tight junctions between endothelial cells prevents the paracellular route
-Thick basement membrane
-Astrocytic endfeet
-Astrocytes can induce formation of tight junctions between endothelial cells and may facilitate transport between BECF and blood
what route do the important molecules move through
-transcellular route
-Facilitated transport
-Exchangers
-Co-transporters
-Increased numbers of mitochondria = active transport
-Small, uncharged and/or lipid soluble can pass the BBB more easily E.g. CO2, O2, nicotine, heroin, caffeine (dont want the last 3 )
what are the leaky regions of the blood brain barrier
1)Choroid plexuses – ventricular system
2)Circumventricular organs
-In these areas ependymal cells beneath have tight junctions
why have leaky areas of the BBB
-Hormone release e.g. hypothalamus and pituitary gland
-Osmoreceptors e.g. OVLT and SFO, hypothalamus
-Temperature control centres and fever – cytokines e.g. OVLT
key points of the ventricular system
-Cavity
-Cerebrospinal fluid
-Provides physical protection - buffer
-Maintains appropriate levels of ions
-Removes waste products
why are Amino acids and potassium lower in the CSF
-for protection
What is exchanged between CSF and BECF
-CSF to BECF :
=Macronutrients e.g. Glucose
=Micronutrients e.g. vitamins
=Ions e.g. HCO3-
-BECF to CSF:
=Metabolic waste products e.g. CO2
=Neurotransmitters
whats The flow of cerebrospinal fluid (CSF)
-Secreted by choroid plexus (made here)
-Circulates around the ventricles and central canal
-Absorbed from the subarachnoid space to the venous blood system at the superior sagittal sinus
-150 mls CSF in total, 30 in ventricles, 120 in SAS
-Reduces effective weight from 1400 g to <50 g. Decreases risk of accelerating and decelerating injuries
what occurs with the Secretion of CSF
-500mls produced a day, CSF is very active considering this much volume is produced from a small thing
1)Ultrafiltration of plasma into ECF across normal ‘leaky’ capillaries
2)Selective absorption of substances into CSF across choroidal epithelial cells
3)Free movement of substances from CSF to BECF across ependymal cells
-CSF replaced 3 times in a day
-30% made my capillaries
-Ependymal cells - specialised glial cells that line ventricles
how many types of meninges are there
-3: Leptomeninges:
=Pia mater- closest to neuronal tissue
=Arachnoid mater
-dura matter
-thin tissue layer that surrounds outside of brain and spinal chord
-dura matter= thickest, Pia mater= thinest
-dura matter normally follows curves but can fold in
-subarachnoid space is below the arachnoid matter
-arachnoid matter= not leaky, things can’t move past it
-Pia - covers surface of brain and blood vessels and allows diffusion between CSF and BECF
-Arachnoid mater - cells linked by tight junctions, preventing diffusion between CSF and plasma
-Dura mater:
=Think, inelastic membrane
=2 layers - split to form intracranial sinuses
how does absorption of CSF occur
-Evaginations of arachnoid membrane (how CSF moves through arachnoid layer):
=Arachnoid granulations (up to 1 cm)
=Arachnoid villi
-Increased absorption with increased intracranial pressure
What if CSF cannot circulate prope
-Hydrocephalus:
=Dilation of ventricular system
=bstruction in ventricular system
=Or interrupted CSF absorption
=Increased intracranial pressure
=Loss of cells within the brain
=Loss of brainstem reflexes
-blocked tiny tube from 3rd to 4th ventricle
-blockage further down leading to an increase in expansion of 4 th ventricle= very critical
what do Neurons and astrocytes terminate
-neurotransmission at the tripartite synapse
-Can recycle neurotransmitters to presynaptic terminals
-glutamine released by exocytosis-> neurones take up the glutamate
-astrocytes take up glutamate, breaks it down and recycles it back to pre-synaptic terminal
what do Neurons and astrocytes remove
-K+ from the extracellular space
what does Ana increase win extracellular K+ effect
-astrocyte function
-Increased glucose metabolism
-Increased K+ uptake
what does Astrocytic syncytium allow
-spatial buffering
-move K+ from one area to another which is less active by forming the syncytium
-Gap junctions create a syncytium
-Redistributes K+ to areas of decreased activity
-Can also transport sugars, amino acids, cAMP, Ca2+
whats Neurovascular coupling
-neuron firing rate -> astrocyte Ca2+ -> release of vasoactive substances from astrocyte -> blood vessel diameter changes
