from Guyton and Hall textbook of medical physiology, 4th South Asia edition write a 5 mark short note on CNS Ischemic Response

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CNS Ischemic Response

[Short Note - 5 Marks | Source: Guyton and Hall Textbook of Medical Physiology]

Definition

The CNS ischemic response is an intense sympathetic reflex response elicited when blood flow to the vasomotor center in the lower brain stem falls to critically low levels, causing cerebral ischemia. The resulting arterial pressure elevation in response to this ischemia is specifically termed the CNS ischemic response.

Mechanism

Most nervous control of blood pressure operates through reflexes originating from peripheral receptors (baroreceptors, chemoreceptors, low-pressure receptors). However, when blood flow to the vasomotor center becomes severely reduced, the vasoconstrictor and cardioaccelerator neurons within that center are powerfully excited directly by the ischemia.

The primary trigger is the accumulation of CO2 - slowly flowing ischemic blood fails to carry carbon dioxide away from the brain stem vasomotor center. The resulting local hypercapnia has an extremely potent effect in stimulating the sympathetic vasomotor areas in the medulla. Additionally, buildup of lactic acid and other acidic metabolites in the vasomotor center further contributes to the intense stimulation.

Effects

The ischemic stimulation of the vasomotor center causes massive sympathetic discharge, producing:

Intense vasoconstriction throughout the peripheral circulation
Increased heart rate and cardiac contractility
Arterial pressure elevation - sometimes to as high as 250 mm Hg for up to 10 minutes
Peripheral vessels may become totally or almost totally occluded (e.g., renal arteriolar constriction may completely stop urine output)

Because of this magnitude of response, the CNS ischemic response is considered one of the most powerful activators of the sympathetic vasoconstrictor system.

Threshold and Role as a Pressure Regulator

Despite its power, this response does not become significant until arterial pressure falls below 60 mm Hg. It reaches its greatest degree of stimulation at a pressure of only 15-20 mm Hg. Therefore, it is NOT a normal day-to-day pressure regulatory mechanism. Instead, it functions as an emergency "last-ditch stand" pressure control mechanism that acts rapidly and powerfully to prevent further fall in arterial pressure when brain blood flow approaches lethal levels.

Cushing Reaction - A Special Form

The Cushing reaction is a specific type of CNS ischemic response triggered by raised intracranial pressure (e.g., raised cerebrospinal fluid pressure). When CSF pressure rises to equal arterial pressure, it compresses both the brain and its arteries, shutting off cerebral blood supply. This triggers a CNS ischemic response that raises arterial pressure above the CSF pressure, restoring cerebral blood flow. This protective mechanism prevents vital brain centers from ischemic injury when CSF pressure rises dangerously high.

Source: Guyton and Hall Textbook of Medical Physiology (9780443111013), Chapter 18 - "Nervous Regulation of the Circulation and Rapid Control of Arterial Pressure," pp. 230-231

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