Final answer:
The appearance of an anionic gap in blood reaching the brain stem indicates a chemical imbalance that can manifest in neuronal dysfunction, particularly during a stroke. Excessive calcium entry through specific channels can lead to neuronal damage. The blood-brain barrier regulates blood constituents entering the CNS and maintains the composition of cerebrospinal fluid.
Step-by-step explanation:
When blood reaches the brain stem with an anionic gap, commonly referred to in the context of metabolic disturbances such as acid-base imbalances, it indicates an altered chemical composition that may affect neuronal function. Particularly during a stroke, the blood flow to part of the brain is disrupted due to either a blockage or rupture of blood vessels. This leads to neuronal depolarization and subsequent neuronal damage due to the failure of ion transport mechanisms. Excessive calcium can enter neurons through NMDA receptors and Transient Receptor Potential (TRP) channels, triggering excitotoxicity and potentially causing further damage to the brain tissue.
Moreover, the presence of an abnormal anionic gap could reflect the accumulation of unmeasured anions often associated with metabolic acidosis, which disrupts cerebral metabolism and impairs the brain's ability to regulate vascular tone and blood flow. This could exacerbate any brain injury by further impairing the delivery of oxygen and nutrients to the brain. It is crucial that the cerebrospinal fluid (CSF) remains compositionally balanced, as it circulates through the CNS and is involved in the removal of metabolic waste.
Lastly, the brain's blood supply is protected by the blood-brain barrier (BBB), which regulates the passage of substances into the CNS to maintain a stable internal environment. An anionic gap could affect the BBB's ability to filter blood into CSF, hence further disrupting the nervous tissue's environment.