W4 Homeostasis in the Nervous Tissue

Question 1

The neuronal microenvironment is composed of

Answer 1

Glia cells, Capillaries, other neurons, extracellular space (= extracellular matrix, brain extracellular fluid BECF)

Question 2

Brain extracellular fluid BECF

Answer 2

is used to distinguish from the ECF that is found within parts of the ventricular system. BECF can both influence neurons and be influenced by neurons.
Uncontrolled: neuronal dysfunction, neuronal death. BECF composition must be tightly regylated.

Question 3

How could neuronal activity change the composition of the BECF?

Answer 3

Change in the concentration of ions (Cl+, K+, Na+) and ph. Increased K+ concentration. Changes in Ca2+ concentration. Changes in O2 glucose and CO2 concentration. Increased H+ thus acidification.

Question 4

How could a change in BECF composition change neuronal activity?

Answer 4

Increased K+ concentration in the BECF could elevate resting potential bringing the cell cloer to threshold for firing an action potential.

Question 5

How do we regulate the neuronal microenvironment, (including BECF) ?

Answer 5

1-Blood brain barrier
2-Cerebrospinal fluid (CSF) in ventricular system
3-Neurons
4-Glial cells- focus on astrocytes.

Question 6

Blood-brain barrier

Answer 6

Function, to protect neurons from fluctuations in concentrations of substances in the blood. (example = increase amino acid concentrations after a meal, increases in K+ adn H+ concentration after exercice, circulating hormones, inflammatory mediators, toxins)

Question 7

How is the Blood Brain Barrier maintained

Answer 7

Tigh junctions between endothelial cells = prevent things from moving paracellularly between cells,
thick basement membranes,
astrocytic end-feet = glial cells send little process and surround the cappilaries around the brain, provide extra barrier.

Question 8

How do importnat molecules get throught the blood brain barrier?

Answer 8

Facilitated, 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.

Question 9

Leaky regions of the blood brain barrier

Answer 9

Choroid plexuses = ventricular system, circumventricular organs, ependymal cells beneath have tigh junctions in this areas.

Question 10

Cerebrospinal fluid (CSF) in ventricular system

Answer 10

The ventricular system = Cavity, Cerebrospinal fluid, Provides physical protection (buffer), Maintains appropriate levels of ions. Removes waste products.
CSF is similar to blood but has lower levels of proteins, amino acids and potassium than in blood.

Question 11

What is exchanged betweeen CSF and BECF

Answer 11

CSF to BECF = macronutrients (glucose), microntrients (vitamins), Ion.
BECF to CSF = metabolic waster prodcuts, neurotransmitters.

Question 12

The flow of cerebrospinal fluid (CSF)

Answer 12

Secreted by choroid plexus. Circulates around the ventricles and central canal. Absorbed from the subarachnoid space to the venous blood system at the superior sagittal sinus. Tissue where CSF is produces (red)

Question 13

Secretion of CSF

Answer 13

1. Ultrafiltration of plasma into ECF across normal ‘leaky’ capillaries.
2. Selective absorption of substances into CSF across choroidal epithelial cells = tight junctions.
3. Free movement of substances from CSF to BECF across ependymal cells.

Question 14

The Meninges = 3 types

Answer 14

Pia Mater, Arachnoid mater, Dura mater

Question 15

Pia Mater [Meninges]

Answer 15

covers surface of brain and blood vessels and allows diffusion between CSF and BECF

Question 16

Arachnoid matter [Meninges]

Answer 16

cells linked by tight junctions, preventing diffusion between CSF and plasma.

Question 17

Dura matter [Meninges]

Answer 17

think, inelastic membrane, 2 layers = split to form intracranial sinuses, helpful for structure.

Question 18

Absorption of CSF

Answer 18

Evaginations of arachnoid membrane:
- Arachnoid granulations (up to 1 cm)
- Arachnoid villi
Increased absorption with increased intracranial pressure

Question 19

Hydorcephalus

Answer 19

Dilation of ventricular system.Obstruction in ventricular system. Or interrupted CSF absorption. Increased intracranial pressure.
Loss of cells within the brain. Loss of brainstem reflexes.

Question 20

Neurons and Glial cells (focus on astrocytes)

Answer 20

Neurons and astrocytes terminate neurotransmission at the tripartite synapse. Can recycle neurotransmitters to presynaptic terminals. Astrocytes have glutamate transporters that can take glutamate out of the synapse.
Neurons and astrocytes can remove K+ from the extracellular space.

Question 21

Increase in extracellular K+ affects astrocyte function.

Answer 21

Astrocytes can sense the increase K+ and this increase their glucose metabolism. Increased K+ uptake

Question 22

Why do neurons have resting membrane potentials of ~-65 mV and glia have resting membrane potentials of ~-85 mV?

Answer 22

Astrocytes have a greater selectivity than neurons.
Neuronal membranes are more permeable to Na+ than astrocytic membranes.
The more selective a membrane is to K+ the more extracellular K+ will influence Vm. Astrocytes are more sensitive to extracellular K+ changes.

Question 23

Astrocytic syncytia allow spatial buffering

Answer 23

Astrocytes are capable of communicating between each other without a chemical synapse. Gap junctions create a syncytium. Redistributes K+ to areas of decreased activity. Can also transport sugars, amino acids, cAMP, Ca2+

Question 24

Neurovascular coupling

Answer 24

= coupling the activity of our neurons to the activity of our vascular system.

Active neurons realise neurotransmitter in post-synaptic neuron, causin gactional potentials. The astrocyte can sense the activity, as it’s attached to the atrial vessel (lot of muscle around). It modulates the activity, so the blood vessel relaxes (more blood being transported with glucose) or contracts (smaller vessel, less blood)

Question 25

Magnetic resonance imaging (MRI) allow 3D structural images of the brain…but what about techniques that measure activity?

Answer 25

Active neurons need more glucose and oxygen. More blood is directed to these areas.
Two techniques detect changes the subsequent changes in blood flow.

Question 26

Positron emission tomography (PET)

Answer 26

exploits glucose use

Question 27

Functional magnetic resonance imaging (fMRI)

Answer 27

exploits oxygen use

Question 28

Functional Magnetic resonance imaging (fMRI)

Answer 28

MRI = Uses an electromagnetic wave to disrupt hydrogen atom state
Blood Oxygenation Level Dependent (BOLD)
Oxyhaemoglobin and deoxyhaemoglobin distort the magnetic resonances properties of hydrogen atoms differently.

Question 29

BECF

Answer 29

Brain Extracellular fluid

Question 30

CSF

Answer 30

Cerebrospinal fluid