CNS Ischemic Response

Gelo and the Brain's Emergency Alarm

ischemia

Gelo shrinks and enters a dog in severe hemorrhagic shock. Blood loss has driven arterial pressure below about 60 mmHg, and blood flow to the brainstem has slowed to a trickle. This is ___ (inadequate blood flow that deprives tissue of oxygen and nutrients).

hyopoxia and hypercapnia

Gelo becomes a molecule of carbon dioxide in the medulla. With so little blood moving, nothing washes her away, so CO2 builds up. Lactic acid accumulates too, because starved cells switch to anaerobic metabolism. The medulla is now suffering ___ (low oxygen), ___ (high CO2 ), and a falling pH, along with a shortage of nutrients such as glucose.

vasomotor center

Gelo becomes a neuron in the ___ (the medullary region that controls sympathetic outflow to the heart and vessels). Here there are no stretch receptors: the neurons themselves are the sensors.

CNS ischemic response

They are directly excited by the extra CO2 , H+, and lack of oxygen. This is the ___ (an emergency reflex in which ischemia of the brainstem strongly excites the vasomotor center and drives a maximal sympathetic discharge). It does little at normal pressures, becomes strong below roughly 60 mmHg, and is most powerful at pressures around 15-20 mmHg.

vasoconstriction

Gelo now becomes a sympathetic fiber and fires at full strength. Her signals tell arterioles throughout the body to clamp down in intense ___ (narrowing of blood vessels), especially in the skin, kidneys, and gut, so that blood is diverted toward the brain and heart. Veins constrict, raising venous return.

adrenal medulla

The ___ (the inner adrenal gland that releases epinephrine) pours out epinephrine, and the heart beats faster and harder. Total peripheral resistance and cardiac output both climb, and arterial pressure can skyrocket to as high as 250 mmHg.

negative feedback

That surge in pressure drives blood back into the ischemic brainstem. The accumulated CO2 and lactic acid are washed away, oxygen and glucose are restored, and the vasomotor neurons stop being excited. Sympathetic outflow eases, and the brain environment returns toward normal. This is ___ (a response that removes the stimulus that caused it). But it is a last-ditch effort: if ischemia lasts several minutes, the neurons begin to fail and the response fades.