Neurovascular unit
The neurovascular unit (NVU) comprises the components of the brain that collectively regulate cerebral blood flow in order to deliver the necessary nutrients to activated neurons. The NVU addresses the brain's unique dilemma of having high energy demands yet low energy storage capacity. In order to function properly, the brain must receive glucose for energy metabolism in specific areas, quantities, and times. Unlike muscle cells, which can deplete and later replenish their energy reserves, neurons require a continuous, real-time supply of energy. The neurovascular unit facilitates this delivery as needed, ensuring that cerebral metabolism is sustained and neuronal activity can continue seamlessly.
The neurovascular unit was formalized as a concept in 2001, at the inaugural Stroke Progress Review Group of the National Institute of Neurological Disorders and Stroke (NINDS). In prior years, the importance of both neurons and cerebral vasculature was well known; however, their interconnected relationship was not. The two were long considered distinct entities which, for the most part, operated independently. Since 2001, though, the rapid increase of scientific papers citing the neurovascular unit represents the growing understanding of the interactions that occur between the brain's cells and blood vessels.
The neurovascular unit consists of neurons, astrocytes, vasculature (endothelial and vascular mural cells), the vasomotor apparatus (smooth muscle cells and pericytes), and microglia. Together, these function in the homeostatic haemodynamic response of cerebral hyperaemia. Cerebral hyperaemia is a fundamental central nervous system homeostatic mechanism that increases blood supply to neural tissue when necessary. This mechanism regulates local perfusion through a multidimensional process involving the various cells of the neurovascular unit and signaling molecules. By interacting, these components of the NVU sense the neurons' needs for oxygen and glucose and trigger the appropriate vasodilatory or vasoconstrictive responses. Through this process, known as neurovascular coupling, neurons and astrocytes can modulate cerebral blood flow. Thus, the NVU provides the structural and cellular framework underlying neurovascular coupling, linking neuronal activity to cerebral blood flow and reflecting the interdependence of their development, structure, and function.
Due to the tight temporal and spatial coupling of cerebral blood flow to neuronal activity, measuring blood flow serves as an accurate proxy for brain function. Neuroimaging techniques that directly or indirectly monitor blood flow, such as fMRI and PET scans, can thus measure and locate activity in the brain with precision. Imaging of the brain also allows researchers to better understand the neurovascular unit and its many complexities.
The neurons' dependence on continuous blood flow additionally makes them highly vulnerable to vascular disruptions. Any impediments that prevent neurons from receiving the appropriate nutrients can cause an array of neurological pathologies. For example, a complete stoppage for only a few minutes, potentially caused by arterial occlusion or heart failure, can result in permanent neuronal damage and cell death. Dysfunction in the NVU is also associated with neurodegenerative diseases including Alzheimer's and Huntington's disease.