Cellular Elements

Although the BBB serves as the major regulatory site of fluid exchange in the central nervous system, multiple actions at the cellular level in the brain parenchyma further influence fluid homeostasis. These cellular elements include the neuroglia, mainly astro-cytes, and the neurons. Neuroglia were once thought to form an inactive architectural framework that supported the neuronal population. Further investigation revealed that neuroglia play an important role in the maintenance of fluid composition and volume. These cells contain numerous membrane surface ion channels that exchange molecules between the intra-cellular space and the extracellular milieu of the interstitial space and thereby adjust the electrochemical gradients influencing excitatory thresholds and the release of neurotransmitter substances. Glutamate, the main excitatory neurotransmitter of the central ner vous system, is ubiquitous in the intracellular space. Efficient uptake systems in glial and neuronal tissue clear glutamate from the extracellular space.

Volumetric changes are energy dependent and modified by fluid osmolarity, principally determined by the sodium concentration. Sodium is exchanged with potassium at the Na + /K+ ATPase channel, requiring energy from the release ofATP. This channel is oxygen dependent and therefore sensitive to hypoxic stress. As the astrocytic sodium concentration increases, cellular volume expands and vice versa. Glial swelling represents a normal component of home-ostasis, although extreme states occur in pathologic conditions such as hypoxia.

Although sodium shifts are the principal determinant of cellular volume changes, the measurement of sodium concentrations in abnormal hyperosmolar states has revealed that other substances might account for the osmolar increase. Specific identification of these substances has proved difficult, but the concept of idiogenic osmoles has been advanced. These molecules are produced under conditions of hypoosmolarity in order to compensate for volume decreases. During the resolution phase of systemic hyperosmolar states, a differential equilibration of inorganic ions such as sodium relative to idiogenic osmoles results in cellular swelling. Throughout this regulatory volume decrease of brain cells, organic osmolytes leave the cell through a molecular pore or ion channel, termed the voltage-sensitive organic anion channel (VSOAC). Osmotic swelling normally activates the VSOAC, although this pathway may fail under pathologic conditions.

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