Astrocytes A Anatomy

Astrocytes were originally described as filling the gaps between neurons. However, Golgi described in detail the morphological distinction between cell types in the CNS. Astrocytes can be divided into three groups: fibrous and protoplasmic astrocytes and radial glia. The latter cells project from the ventricular surface to the pial surface and are mainly prominent during early development. Fibrous astrocytes are located almost exclusively in white matter and have long, radially distributed processes that often terminate as end feet on capillaries. Protoplasmic astrocytes are located in gray matter and are characterized by short bushy processes; these processes also form vascular end feet. The three different types of astrocyte share common properties, including the synthesis of glial fibrillary acidic protein (GFAP). This protein is easily recognized in electron micrographs because it forms paired helical filaments, so-called intermediate filaments.

Astrocytes form connections with other astrocytes via gap junctions, and this allows intercellular communication that directly links the cytoplasm of the astrocytes. Neuronal communication, by comparison, is primarily mediated by chemical synapses. Gap junctions have a pore about 2 nm in diameter and couple astrocytes together into a syncitium, which allows the passage of all inorganic ions and small organic molecules up to 1 kDa. The use of dye injection into astrocytes allows an estimation of the extent to which the cells are coupled. The channels comprising gap junctions are large pores that extend from one cell across the extracellular space (ECS) into an adjacent cell. Each connexon hemichannel is composed of six symmetrical subunits (connexins). There are several different types of connexin, but the most common type in astrocytes is connexin 43. A gap junction is formed when a connexon from one cell aligns with a connexon from another cell. Gap junctions open and close abruptly, and the conductance varies between 50 and 150 pS depending on the type of connexin. Gap junction openings are affected by transjunctional voltage and are decreased by increases in intracellular pH, Ca2+, or octanol.

A striking feature of astrocyte morphology is the end feet surrounding capillaries. This morphology initially suggests that all substances in the blood must pass through astrocytes before being passed on to neural elements via the ECS. The astrocytic end feet do not represent a tight barrier, however, and substances that have passed the capillary endothelium (which is a tight barrier) can diffuse directly into brain extracellular space without passing through astrocytes. This notwithstanding, there is growing evidence that glucose and perhaps other blood-borne substances are taken up by astrocytes. Astrocytes possess glucose transporters, which would allow the movement of glucose from blood vessels directly into the capillaries. In the extreme, this would require that all neuronal nutrition be mediated via astrocytes (i.e., all glucose, the principal fuel of the brain, would go first to astrocytes). In actuality, some glucose undoubtedly bypasses the astrocytic end feet and diffuses directly to neurons, but astrocytes do provide fuel to neurons, probably in the form of lactate (see later discussion).

Astrocytes also seem to surround synapses and have been proposed as the main site of neurotransmitter uptake after synaptic activity. Astrocytes contain a variety of neurotransmitter transport mechanisms that are not found on neurons, in addition to enzyme systems that are capable of metabolizing neurotrans-mitters to more manageable inert compounds for transport back to neurons, where they are recycled back into the active neurotransmitter.

Breaking Bulimia

Breaking Bulimia

We have all been there: turning to the refrigerator if feeling lonely or bored or indulging in seconds or thirds if strained. But if you suffer from bulimia, the from time to time urge to overeat is more like an obsession.

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