Transcranial doppler machines8/16/2023 ![]() In the physiological range the hemodynamic response does not depend on variations of blood pressure, heart rate or orthostatic position. Because of its limited spatial resolution, most investigations were done on the occipital cortex using visual stimuli insonating the P2 segment of the posterior cerebral artery. To assess the fast hemodynamic aspects of neurovascular coupling, many clinical studies have been performed with the transcranial Doppler technique. In the last few years, many studies have been conducted on the neurovascular coupling mechanism in experimental or clinical settings with different techniques. The most recent hypothesis assumes that the excitatory transmitter glutamate can be sensed by astrocytic receptors which then mediate the local vascular tone. ![]() Alternatively, it was shown that activity-related changes in local mediators can influence the vascular tone: changes in the extracellular ion content (potassium and hydrogen ions), depletion products of energy molecules (adenosine) or nitric oxide can mediate the vascular tone. nucleus basalis of Meynert) to modulate the cerebral blood flow regulation since they have direct endings on arterioles. One hypothesis assumes local interneurons or projection neurons from specialized brain stem nuclei (i.e. Although the detailed mechanisms still remain to be elucidated, there are three main hypotheses how the coupling might be governed. ![]() In 1890, Angelo Mosso presented data from a patient with an open skull defect in whom he found a functionally related change in cerebral perfusion. Historically, one of the first reports on the functional dependency of the cerebral blood flow in relation to the brain activity goes back to an Italian physician. In what follows, we will concentrate on the neurovascular coupling mechanism. Because of this dependency on the momentary blood flow, we can distinguish two effective, fast-regulated and fine-tuned vasoregulative mechanisms in the brain: cerebral autoregulation compensates for cerebral perfusion pressure changes, whereas neurovascular coupling adapts local cerebral blood flow in accordance with the underlying neuronal activity. A reduction of only 10% from the ideal perfusion leads to protein synthesis disturbance in neurons if it lasts for several hours or days. While it can compensate even a strong decline in blood flow for a short time period, it reacts very sensitively to a chronic mismatch of adequate perfusion. From the stroke research of the last few years it is known that the brain becomes more sensitive to inappropriate perfusion with increased duration of impaired blood flow. The high energy needs can only be assured by aerobic glycolysis in which 1 mol glucose results in 38 mol adenosine triphosphate, whereas anaerobic glycolysis only produces 2 mol of the energy molecule. This explains why the brain, although only representing 2% of the body weight, receives 15% of the cardiac output consuming 20% of the total body oxygen and 25% of total body glucose. The brain is highly dependent on adequate perfusion because it has a high energy consumption of approximately 23 J/s = 23 W, but lacks adequate stores of oxygen or energy substrate.
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