Pathophysiology
P-II-31. Neurovascular coupling & fNIRS in brain imaging
神経血管カップリングと機能的脳画像（fNIRS）
Neurovascular Coupling
- = increased blood flow induced by neural activity
- Neurovascular unit: endothelial cells + neurons + glial cells (astrocytes, oligodendrocytes, microglia) + surrounding pericytes / smooth muscle cells → form one functional unit
- Many vasoactive mediators drive the vascular response (ions, metabolic by-products, neurotransmitters, NO, prostaglandin) → vasodilation when released from neurons/astrocytes
- Mechanisms linking activity → ↑blood flow:
- Glutamate → NMDA receptors on neurons → Ca²⁺ influx → production of NO + prostaglandin → relaxation of smooth muscle cells / pericytes
- Glutamate → metabotropic glutamate receptors on astrocytes → release of vasoactive mediators → vasodilation → ↑blood flow
- Impaired coupling (high BP, Alzheimer’s, ischemic stroke) → ↓nutrient & O₂ supply + ↓removal of metabolic end products → destabilizes the brain microenvironment
fNIRS (Functional Near-Infrared Spectroscopy)
- Activity-induced local flow can be imaged by PET, SPECT, fMRI, fNIRS
- fNIRS = continuous monitoring of blood oxygenation + blood volume changes from nerve activity
- Principle: light source emits near-infrared light (700–900 nm) → passes through tissue → partly absorbed + dispersed → detector reads the drop in light intensity → deduces chromophore concentrations
- Main chromophores: HbO₂ (oxygenated) + Hb (deoxygenated) → peak absorption at different wavelengths
- Beer–Lambert law: total Hb (HbO₂ + Hb) → blood volume change; their difference → oxygenation change vs baseline
Pros & Cons
| Pros | Cons |
|---|---|
| Non-invasive; no ionizing radiation; good temporal resolution; mobile | Poor spatial resolution (worse than fMRI/PET); only a few cm deep (light weakens with depth) |
Applications
- Sensory, motor, visual, auditory, frontal cortex & language centers
- Developmental psychology studies
- Epileptic seizures, schizophrenia, Alzheimer’s disease
一問一答
▶What is neurovascular coupling?
The increase in local blood flow induced by neural activity.
▶What cells make up the neurovascular unit?
Endothelial cells, neurons, glial cells (astrocytes, oligodendrocytes, microglia), and surrounding pericytes/smooth muscle cells.
▶How does glutamate acting on neuronal NMDA receptors increase blood flow?
It causes Ca²⁺ influx that drives production of NO and prostaglandin, relaxing smooth muscle cells/pericytes (vasodilation).
▶How do astrocytes contribute to neurovascular coupling?
Glutamate activates metabotropic glutamate receptors on astrocytes, which release vasoactive mediators causing vasodilation and increased blood flow.
▶Name key vasoactive mediators involved in neurovascular coupling.
Ions, metabolic by-products, neurotransmitters, nitric oxide (NO), and prostaglandin.
▶Which conditions impair neurovascular coupling?
High blood pressure, Alzheimer's disease, and ischemic stroke.
▶What are the consequences of impaired neurovascular coupling?
Reduced nutrient and O₂ supply plus reduced clearance of metabolic end products, destabilizing the brain microenvironment.
▶What does fNIRS stand for and measure?
Functional near-infrared spectroscopy; it continuously monitors changes in blood oxygenation and blood volume from neural activity.
▶What wavelength of light does fNIRS use, and what is its principle?
Near-infrared light (700–900 nm) passes through tissue, is partly absorbed/dispersed, and the detected drop in intensity is used to deduce chromophore concentrations.
▶What are the main chromophores measured by fNIRS?
Oxygenated haemoglobin (HbO₂) and deoxygenated haemoglobin (Hb), which peak at different wavelengths.
▶How does fNIRS use the Beer–Lambert law to derive blood volume and oxygenation?
Total Hb (HbO₂ + Hb) reflects blood volume change, while the difference between HbO₂ and Hb reflects oxygenation change versus baseline.
▶What are the main advantages of fNIRS?
Non-invasive, no ionizing radiation, good temporal resolution, and mobile/portable.
▶What are the main limitations of fNIRS?
Poor spatial resolution (worse than fMRI/PET) and limited penetration depth (only a few cm, as light weakens with depth).
▶Which imaging methods can detect activity-induced local blood flow?
PET, SPECT, fMRI, and fNIRS.
▶Which cortical areas/functions can fNIRS study?
Sensory, motor, visual, auditory, frontal cortex, and language centres.
▶What clinical/research applications use fNIRS?
Developmental psychology studies and investigation of epileptic seizures, schizophrenia, and Alzheimer's disease.
▶Why does neural activity require increased local blood flow?
Active neurons need more oxygen and nutrients and need metabolic waste removed, met by increased perfusion.
▶Which cells are the contractile effectors that dilate cerebral microvessels?
Pericytes and vascular smooth muscle cells, which relax in response to NO and prostaglandin.
▶Why does fNIRS have good temporal but poor spatial resolution?
It samples optical signals rapidly and continuously (good temporal), but light scattering and limited depth blur localization (poor spatial).
▶Why is impaired neurovascular coupling relevant to Alzheimer's disease?
Reduced activity-driven perfusion limits O₂/nutrient delivery and waste clearance, worsening the brain microenvironment in the disease.