“Dehydration increases the osmolality of extracellular fluid in the body and decreases blood volume [Costill et al., 1976]. Both are processes that could reduce total brain volume and lead to a corresponding increase in CSF volume. Increased concentrations of solutes in extracellular fluid cause water to move from inside cells to the extracellular fluid along the osmotic gradient, causing cellular shrinkage [Gullans and Verbalis, 1993]. However, cells prevent large increases and decreases in volume by actively regulating their intracellular solute (particularly potassium ions), which is of vital importance in the brain because of the fixed volume of the cranium [Stricker and Verbalis, 2003]. Indeed, if such homeostatic processes were not active we might expect to see larger changes in brain structure following dehydration. As brain volume decreases, the empty space is filled by CSF; this may come from an increase in production or a decrease in absorption of CSF, or from the CSF filled spinal dural sack which unlike the cranium, can change its volume in response to intracranial pressure [Lee et al., 2001; Lofgren and Zwetnow, 1973].”
From Effects of acute dehydration on brain morphology in healthy humans, Matthew J. Kempton1,2,*, Ulrich Ettinger1, Anne Schmechtig1, Edward M. Winter3, Luke Smith4, Terry McMorris4, Iain D. Wilkinson5, Steven C.R. Williams1 and Marcus S. Smith4. Human Brain Mapping
Neurons store water in tiny balloon-like structures called vacuoles. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain cases they may contain solids which have been engulfed.
Water is essential for optimal brain health and function. Water is necessary to maintain the tone of membranes for normal neurotransmission. It enhances circulation and aids in removing wastes.
Microtubules in the neuron is filled with water.  Microtubules are tiny sub-components of cells. They are prominent aspects of the skeleton of all eukaryotic cells and are the structural and dynamical basis of the cells. They may participate in important quantum mechanical phenomena involving water ordering in their interior.
Microtubules contribute to the structural integrity of neurons as they maintain the semi-rigidity of neurons.
There are various substances that may interfere with microtubules:
- Acetaldehyde may inhibit the ability of tubulin to assemble into microtubules. 
- Mercury may disrupt the structural integrity of neuronal microtubules. 
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