Nervous System

Extracellular fluid in the brain provides a highly regulated environment for CNS neurons

Everything that surrounds individual neurons is a part of the neuronal environment which includes:

ECF

Capillaries

Glial cells

Adjacent Neurons

The concentration of solute in Brain ECF (BECF) fluctuates with neural activity, and changes in ECF composition can influence nerve cell behavior.

The brain controls the composition of this important compartment in three ways:

Brain uses the BBB to protect the BECF from fluctuations in blood composition

The CSF strongly influences the composition of the BECF

The surrounding glial cells condition the BECF

The brain is also metabolically fragile due to its:

High rate of energy consumption

Absence of significant stored fuel in the form of glycogen

Rapid development of cellular damage when ATP is depleted

The brain receives ~15% of resting flow, ~20& of total resting oxygen and ~50% of glucose utilization.

The brain’s high metabolic demands arise from the need of its neurons to maintain the steep ion gradients on which neuronal excitability depends.

Neuroglial cells also maintain steep transmembrane ion gradients.

Energy consumption by the brain is directed to maintain ion gradients thru Na-K pump.

Falling ATP levels in the brain causes loss of consciousness w/in 10 seconds.

Irreversible nerve cell injury can occur after only 5 minutes of interrupted blood flow.

Cerebrospinal Fluid

Colorless, watery fluid

Secreted w/in the brain by the choroid plexuses

Choroid plexuses can be thought of as the brain’s kidney

Circulates to sites in the subarachnoid space where it enters the venous blood system

CSF is in slow diffusional equilibrium w/ BECF which help regulate the composition of BECF.

CSF fills the ventricles and subarachnoid space

The ventricles:

4 small compartment located within the brain

Contains choroid plexuses and is filled w/ CSF

Linked together by channels or foramina

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