FMRI – Week 5 – MR Signal Scott Huettel, Duke University Physiological Basis of fMRI (and...
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Transcript of FMRI – Week 5 – MR Signal Scott Huettel, Duke University Physiological Basis of fMRI (and...
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
Physiological Basis of fMRI(and Neuroanatomy, in brief)
FMRI Undergraduate Course (PSY 181F) FMRI Graduate Course (NBIO 381, PSY
362)
Dr. Scott Huettel, Course Director
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
I.Neurophysiology
What brain processes consume energy?
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
There are two primary types of information flow in the CNS:
1) Signaling via action potentials (axonal activity) and
2) Integration via dendritic activity
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
Action potential
Depolarization opens CA2+ channels
Vesicles fuse with presynaptic membrane
Neurotransmitter release
Neurotransmitters open ion channels on postsynaptic membrane
Change in potential
IPSP or EPSP
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
Energy Demands of Integration/Signaling
Following activity, neurons require energy to restore concentration gradients of key
ions.Sodium-Potassium
pump takes sodium out of the cell while
bringing potassium into the cell.
Note that for action potentials, the
movement of ions is along gradients.
Key concept: activity of neurons does not itself require energy; restoring membrane potentials afterward does.
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
What metabolites are the sources of that energy?
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
Oxygen (via
hemoglobin)
Glucose
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
Facts about energy supply to brain
• 30-50 μmol/g/min of ATP for awake brain
• 10 μmol/g/min of ATP for comatose brain
• Information processing accounts for >75% of ATP consumption
• 54mL/min of blood for each 100 g of brain tissue
• Brain is ~3% of body weight, but demands 15-20% of blood flow and ~20% of blood oxygen
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
Data from rodent models (Attwell & Laughlin, 2001). In humans, integrative activity
may be 50% greater.
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
Why do neuroenergetics matter?
• Information reduction necessitated by energy demands!
• How could we increase information transmission?– Decrease membrane resistance
finer-resolution of dendritic activity (~200Hz)
– Increase action potential rate (~100-300Hz)
• Decreasing membrane resistance would increase maintenance costs
• Increasing action potential rate would rapidly increase signaling costs
• The energy available to the brain limits neural information processing
Attwell and Gibb, 2005
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
How are energy sources (metabolites) delivered?
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
The brain does not store glucose and oxygen in appreciable
quantities.
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
Duvernoy, H. M., Delon, S., & Vannson, J. L.
(1981). Cortical blood vessels of the human brain. Brain Research
Bulletin, 7(5), 519-579.
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
Arteries (1-25mm)Arterioles (10 - 300 microns)
precapillary sphinctersCapillaries (5-10 microns)Venules (8-50 microns)
Veins
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
Key concepts in vascular system
• Vast change in scale from largest arteries to capillaries– Small changes in diameter result in large changes in
flow (2x diameter = 16x flow)
• Pulsatile flow in arteries smoothed out by resistance vessels (arterioles)
• Surface area of capillaries is essential for O2 exchange– Neurons are usually within 20μm from a capillary
• Capillaries are not always perfused!– Blood can bypass capillaries – Saves weight, cost (in blood), etc.
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
(anastomosis of internal carotids and basilar
artery)
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
ACA – Medial cortex
MCA – Anterolateral cortex
PCA – Posterior temporal and occipital lobes
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
Sinus. n. An separation of the dura mater in
which blood drains into the venous system.
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
Distribution of vascularization across cortical layers
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
Capillary structure
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
How does function map onto blood flow?
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
Iadecola, Nature Reviews Neuroscience, 2004
“[Mosso] relates of his female subject that one day whilst tracing her brain-pulse he observed a sudden rise with no apparent
outer or inner cause. She however confessed to him afterwards that at that
moment she had caught sight of a skull on top of a piece of furniture in the room, and
that this had given her a slight emotion.”
-James Principles… (1890)
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
“These facts seem to us to indicate the existence of an automatic mechanism by which the blood supply of any part of the cerebral tissue is varied in accordance with the activity of the chemical changes which underlie the functional action of that part.
Bearing in mind that strong evidence exists of localisation of function in the brain, we are of opinion that an automatic mechanism, of the kind just referred to, is well fitted to provide for a local variation of the blood supply in accordance with local variations of the functional activity.”
[Roy and Sherrington, 1890, emphasis added]
“Blood very likely may rush to each region of the cortex according as it is most active, but of this we know nothing.”
[James, 1890]
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
Facts about blood flow
• Aorta peak flow: 90 cm/s• Internal carotid flow: ~ 40 cm/s• Smaller arteries: ~10-250 mm/s• Capillaries: ~ 1 mm/s• Venules and small veins: ~10-250
mm/s
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
Stimulation of the sciatic nerve (in a rat) results in
arteriole dilation in somatosensory cortex.
There is a parallel change in blood velocity .
But, blood pressure remains relatively constant.
(This is a good thing.)
Adapted from Ngai et al., 1988
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
Change in diameter of arterioles following sciatic (hindlimb) stimulation
Adapted from Ngai et al., 1988
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
Change in arteriole dilation as a function of distance from active neurons
Iadecola, Nature Reviews Neuroscience, 2004
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
What triggers changes in blood flow?
• K+ : after synaptic activity• Adenosine : follows metabolic activity• Nitric oxide : released by active
neurons– Causes smooth muscles surrounding
arterioles to relax– NO inhibitors attenuate CBF, BOLD
• Neuronal activity ?
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
Iadecola, Nature Reviews Neuroscience, 2004
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
How does the vascular system respond to neuronal activity?
Iadecola, Nature Reviews Neuroscience, 2004
Physiological data suggests that blood flow
changes may be associated with
preponderance of dendritic activity, but
disconnections are possible.
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
Krimer, Muly, Williams, Goldman-Rakic, Nature Neuroscience, 1998
Pial Arteries (i.e., larger vessels) 10 m
Dopamine
Direct neuronal influences?
2 m
2 m
400 nm
400 nmNoradrenergic
On small capillaries, there are terminals of dopamine neurons. These appear to have slower influences than necessary for fMRI.
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
Challenges to Neurogenic Control
• Slow time scale: DA effects = minutes
• DA receptor blockade does not modulate CBF increases w/activation (e.g., Esaki et al., 2002)
• Lack of spatial specificity of blood flow responses
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
Summary of Physiology
• Information processing requires (substantial) energy– Energy is needed for restoring membrane
potentials
• Energy comes from Oxygen and Glucose– Minimal local availability
• Metabolites supplied by vascular system• Changes in blood flow with activity
– Changes may be disproportionate
• Next week: Can we identify some aspect of this process that is measurable using MRI?
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
II. Neuroanatomy
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
Terminology: Planes of Section
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
Terminology: Labels
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
Brain in skull
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
Brain covered with dura mater
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
Gyri (bumps)
Sulci (valleys)
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
cerebellum
corpus callosum
spinal cord
pons
sinus
frontal lobeoccipital lobe
thalamus
skull
falx
medulla
midbrain
hypothalamus
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
A midsagittal MRI of the human head
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
frontal lobe
precentral gyrus
central sulcusparietal lobesuperior parietal
lobule
parieto-occipitalsulcus
occipital lobe
cerebellum
temporal lobe
Sylvian fissure
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
frontal lobe
occipital lobe
Parahippocampal gyrus
fusiform gyrus
inferiortemporal
gyrus
brain stem
Optic chiasma
substantia nigra
olfactory nerves
circle ofWillis
spinal cord
basilar artery
vertebral arteries
Fig 2.15
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
posterior corpus callosum
anterior corpus callosum
ventricle
caudate
thalamus
frontal lobe
occipital lobe
Fig 2.17
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
Corpus Callosum
Internal Capsule
Caudate
Putamen
Globus PallidusAnterior
Commissure
(Collectively, these are known
as the basal ganglia)
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
Insula
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
Corpus Callosum and Indusium Griseum
FMRI – Week 5 – MR Signal Scott Huettel, Duke University
FMRI – Week 5 – MR Signal Scott Huettel, Duke University