Nervous System

THE NEURONAL ENVIRONMENT (Chapter 11)

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