The concept of a tripartite synapse has been proposed, referring to the tight relationship occurring at synapses among a presynaptic element, a postsynaptic element and a glial element. Structural: They are involved in the physical structuring of the brain. Astrocytes get their name because they are "star-shaped". They are the most abundant glial cells in the brain that are closely associated with neuronal synapses. They regulate the transmission of electrical impulses within the brain. Glycogen fuel reserve buffer: Astrocytes contain glycogen and are capable of gluconeogenesis. The astrocytes next to neurons in the frontal cortex and hippocampus store and release glucose. Thus, astrocytes can fuel neurons with glucose during periods of high rate of glucose consumption and glucose shortage. A recent research on rats suggests there may be a connection between this activity and physical exercise. Metabolic support: They provide neurons with nutrients such as lactate. Glucose sensing: normally associated with neurons, the detection of interstitial glucose levels within the brain is also controlled by astrocytes. Astrocytes in vitro become activated by low glucose and are in vivo this activation increases gastric emptying to increase digestion. Blood–brain barrier: The astrocyte end-feet encircling endothelial cells were thought to aid in the maintenance of the blood–brain barrier, but recent research indicates that they do not play a substantial role; instead, it is the tight junctions and basal lamina of the cerebral endothelial cells that play the most substantial role in maintaining the barrier. However, it has recently been shown that astrocyte activity is linked to blood flow in the brain, and that this is what is actually being measured in fMRI. Transmitter uptake and release: Astrocytes express plasma membrane transporters such as glutamate transporters for several neurotransmitters, including glutamate, ATP, and GABA. More recently, astrocytes were shown to release glutamate or ATP in a vesicular, Ca2+-dependent manner. (This has been disputed for hippocampal astrocytes.) Regulation of ion concentration in the extracellular space: Astrocytes express potassium channels at a high density. When neurons are active, they release potassium, increasing the local extracellular concentration. Because astrocytes are highly permeable to potassium, they rapidly clear the excess accumulation in the extracellular space. If this function is interfered with, the extracellular concentration of potassium will rise, leading to neuronal depolarization by the Goldman equation. Abnormal accumulation of extracellular potassium is well known to result in epileptic neuronal activity. Modulation of synaptic transmission: In the supraoptic nucleus