homeostasis in the nervous tissue

Question 1

what is the relationship between brain extracellular fluid (BECF) and activity in a neuron?

Answer 1

they influence each other - change in BECF influences activity in a neuron and vice versa
therefore BECF composition must be tightly controlled

Question 2

what happens if BECF is not tightly controlled?

Answer 2

neuronal dysfunction / neuronal death

Question 3

how could a change in BECF composition change neuronal activity?

Answer 3

• increased K+ concentration in the BECF could elevate resting potential bringing the cell closer to threshold for firing an action potential
• increased neurotransmitter release leading to unspecific receptor activation

Question 4

what is the function of the blood brain barrier?

Answer 4

to protect neurons from fluctuations in concentrations of substances in the blood

Question 5

how is the blood brain barrier maintained?

Answer 5

• tight junctions between endothelial cells
• thick basement membrane
• astrocytic endfeet (ends of astrocytes)

Question 6

how do important molecules get through BBB?

Answer 6

• facilitated transport
• exchangers
• co-transporters
• increased numbers of mitochondria = active transport
• small, uncharged, lipid soluble can pass through

Question 7

what are the leaky regions of the BBB?

Answer 7

• choroid plexuses - part of ventricular system where cerebrospinal fluid is made
• circumventricular organs

Question 8

why are there leaky areas of the BBB?

Answer 8

• hormone release (from hypothalamus and pituitary gland)
• osmoreceptors (brain needs to know fluid levels)
• temperature control centers and fever

Question 9

how do we regulate neuronal microenvironment, including BECF?

Answer 9

• blood brain barrier
• cerebrospinal fluid in ventricular system
• neurons
• glial cells (esp astrocytes)

Question 10

what is exchanged from CSF to BECF?

Answer 10

• macronutrients e.g. glucose
• micronutrients e.g. vitamins
• ions

Question 11

what is exchanged from BECF to CSF?

Answer 11

• metabolic waste products e.g. CO2
• neurotransmitters

Question 12

what is the flow of cerebrospinal fluid?

Answer 12

1. secreted by choroid plexus (leaky part of BBB)
2. circulates around ventricles and central canal
3. absorbed from the subarachnoid space to the venous blood system at the superior sagittal sinus

Question 13

what are the 3 types pf meninges?

Answer 13

• pia mater - innermost membrane, very thin, leaky
• arachnoid mater - tight junctions so selective
• dura mater - tough, outermost layer

Question 14

how is CSF absorbed?

Answer 14

absorbed through arachnoid granulations and arachnoid villi

Question 15

what happens if CSF cannot circulate properly?

Answer 15

hydrocephalus
- obstruction in ventricular system causes build up of CSF in brain
- pushes down on brain stem causing death of cells and loss of brainstem reflexes

Question 16

what are the two main functions of the CSF?

Answer 16

• protection
• regulation of ions

Question 17

what is the function of neurons and astrocytes in regulating neuronal microenvironment?

Answer 17

• terminating neurotransmission at the tripartite synapse
• remove K+ from the extracellular space

Question 18

how do neurons and astrocytes terminate neurotransmission at the tripartite synapse?

Answer 18

transporters which absorb neurotransmitters into the cell which are then recycled

Question 19

how do neurons and astrocytes remove K+ from the extracellular space?

Answer 19

Na+/K+ ATPase pump moves K+ inside the cell against conc. gradient

Question 20

how do astrocyte membrane potentials vary from neurons?

Answer 20

neurons resting potential = -65mc
glia resting potential = -85mv

Question 21

why do astrocytes and neurons have different resting membrane potentials?

Answer 21

• neuronal membranes are more permeable to Na+
• astrocytes have higher K+ selectivity

Question 22

how do astrocytes get rid of potassium to maintain conc. gradient?

Answer 22

astrocytes couple to each other and form syncytium (gap junctions)
- redistributes K+ to areas of decreased activity

Question 23

what does syncytium do?

Answer 23

• allow redistribution of K+
• neurovascular coupling

Question 24

what is neurovascular coupling?

Answer 24

coupling of neurons to vascular system (arterioles)
1. increased neuron firing
2. increased Ca2+ in astrocytes
3. release of vasoactive substances from astrocyte
4. blood vessel diameter changes

Question 25

two types of functional imaging techniques to view the brain?

Answer 25

positron emission tomography (PET) - exploits glucose use
functional magnetic resonance imaging (fMRI) - exploits oxygen use