Haemodynamic response

Haemodynamic response refers to the change in blood flow that occurs in response to neural activity in the brain. This response is the basis for functional magnetic resonance imaging (fMRI), a type of brain imaging that measures brain activity by detecting changes in blood flow.

Overview

The haemodynamic response is a physiological reaction to changes in neural activity. When neurons in the brain are active, they consume more oxygen and glucose, which are delivered by the blood. This increased demand for oxygen and glucose triggers a response in the blood vessels, which dilate to increase blood flow to the active area. This increase in blood flow is what is measured by fMRI.

Mechanism

The mechanism of the haemodynamic response is complex and involves several steps. First, the active neurons release glutamate, a neurotransmitter that signals the nearby blood vessels to dilate. This dilation increases the blood flow to the area, bringing more oxygen and glucose. At the same time, the active neurons also produce more carbon dioxide, which further dilates the blood vessels.

The increased blood flow not only delivers more oxygen and glucose, but also washes away the carbon dioxide and other waste products produced by the active neurons. This process is known as neurovascular coupling.

Measurement

The haemodynamic response can be measured using fMRI. This technique uses magnetic fields and radio waves to create images of the brain. The images show changes in blood flow, which reflect neural activity.

The haemodynamic response is not instantaneous. It takes a few seconds for the blood vessels to dilate and for the increased blood flow to be detectable by fMRI. This delay is known as the haemodynamic lag.

Clinical significance

Understanding the haemodynamic response is important for interpreting fMRI data. It can also provide insights into brain disorders that affect blood flow, such as stroke and dementia.

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