Lecture 3 - fMRI Flashcards
fMRI relies on the fact that…
deoxygenated haemoglobin disrupts the homogeneity of the magnetic field
EPSPs cause a neuron to become…
depolarized
The influx of the following ion is critical for neurotransmitter release…
calcium
The resting membrane potential of a neuron is approximately…
-60/65 mV
Which voltage-gated ion channel opens at the onset of an action potential?
Sodium channels open first
Resulting in a flood of Na+ ions
= rising phase
What two forces drive the movement of ions?
Diffusion and electrical
What does the sodium potassium pump do?
Takes 3 sodium ions from inside cell to outside
Takes 2 potassium ions from outside to inside
This pump is responsible for resting state potential (-60mv)
What energy molecule does the sodium potassium pump require?
ATP
Adenosine Triphosphate
What is hyperpolarization?
More negative
What is depolarization?
More positive
What is the threshold for triggering action potential?
-50mv
Do actions potentials vary in size?
Obvs not
“All or nothing principle”
Which forces pull Na+ into the cell once sodium channels open?
Electrical and diffusion
What happens at maximum positivity?
Sodium channels close
Potassium channels open and potassium leaves cell
What happens when potassium channels open?
Neuron becomes negative
= hyperpolarization
What is an EPSP?
Graded potential which causes depolarization
What is an IPSP?
Graded potential which causes hyperpolarization
Action potential time course
- EPSP add up
- Voltage gated sodium channels open (trigger each other)
- Sodium comes into cell
- Depolarization (reaches threshold)
- Continued depolarization (peaks at +40mv) - Sodium channels close (inactivated state)
- Potassium leaves cell through leak channels because of diffusion and electrical force
- Potassium also leaves through voltage gated potassium channels
- Falling phase/undershoot - Voltage gated potassium channels close (slowly)
- Absolute Refractory period
- cannot trigger another action potential - Relative Refractory period
- difficult to trigger another action potential
EPSP at the synapse
AP travels down axon
AP depolarizes pre-synaptic membrane causing voltage-dependent Ca2+ ion channels to open
Ca2+ influx = vesicles to fuse with pre-synaptic membrane
Neurotransmitter e.g. glutamate released into synapse
Neurotransmitter binds with receptors on post-synaptic ion channels which opens them
Ions (e.g. Na+) flow into post-synaptic cell
= EPSP/ depolarization
IPSP at the synapse
AP travels down axon
AP depolarizes pre-synaptic membrane causing voltage-dependent Ca2+ ion channels to open
Ca2+ influx = vesicles to fuse with pre-synaptic membrane
Neurotransmitter e.g. GABA released into synapse
Neurotransmitter binds with receptors on post-synaptic ion channels which opens them
Ion movement causes resting state potential of post-synaptic membrane to become more negative (i.e. further away from threshold)
- This may occur due to e.g. influx of Cl- or efflux of K+
= IPSP
What are axons insulated by?
Why is this good?
Myelin sheath
Without Myelin, conduction of APs 100 times slower
What do APs jump between?
Nodes of ranvier
What happens to the neurotransmitter released into synaptic cleft?
It is either broken down by an enzyme or ‘re-uptake’ occurs
what is an agonist?
Enhances (some prevent re-uptake)
what is an antagonist?
Blocks
Two types of summation
Temporal and spatial
What is ATP?
Currency of energy
How much energy does PSPs use?
37.5%
How much energy do APs use?
15.8%
How much energy is needed for resting potential?
15%
Energy use for housekeeping?
25%
Energy for presynaptic transmitter release
3.8%
What carries blood to body?
Arteries
What carries blood to heart?
Veins
What are capillaries?
The function of capillaries is to allow food and oxygen to diffuse to cells while waste is diffused from cells.
Capillaries have thin walls
What is the relationship between neural activity and blood flow?
Active neurons release vasoactive substance which diffuses through extracellular space to reach nearby blood vessels
= dilation
= reduced resistance
= increased blood flow
If you increase diameter of vessels by 2…
you increase flow by 16
proportional to the power of 4
Positron Emission Tomography
Measures blood flow
- Inject PS with radioactive molecule e.g. glucose
- When task is undertaken, radioactive material (e.g. fluorine 18) goes to active area
- Fluorine 18 decays and releases positron
- Positron releases two gamma rays
- PET measures these gamma rays and locates the intersection
Magnetic properties of HbO and HbO
Hb = paramagnetic = inhomogeneity to nearby magnetic field
HbO= dimagnetic = little effect on magnetic field
Effect of Hb on MRI signal
more Hb = rapid dephasing in transverse plane (T2) = drop in T2 signal
Therefore
Big difference in T2 decay between active and inactive brain area
What is the haemodynamic delay?
Time between AP and BOLD signal
At rest, there is more Na+ inside or outside the neuron?
Outside!
After Na+ has entered the neuron, K+ leaves the
neuron, because…
the outside is negative with a
lower concentration of K+
What structures in the neuronal membrane allow
selective movement of ions along their electrical
and chemical gradients?
Ion channels
transmitter recycling
3%
