plss describe about the eschemic response of cns.. also give its mechanism

Hello Aspirant,

Thanks for your interest regarding this vital topic of Medical Science. Now I'm describing about CNS ISCHEMIC RESPONSE in a very short and simple way , hope you'll like it :

CNS ISCHEMIC RESPONSE:

Definition and  Explaination:

The CNS Ischemic reflex or the medullary ischemic reflex is a big response to a drop in blood pressure in brain particularly in the medulla, where the lack of oxygen due to decreased perfusion triggers an autonomic response from the cardiac and vasomotor centres.

Most nervous control of blood pressure is achieved by reflexes that originate in the baroreceptors, the chemoreceptors and the low-pressure receptors, all of which are located in the peripheral circulation outside the brain. However, when blood flow to the vasomotor centre, in the lower brainstem becomes decreased severely enough to cause nutritional deficiency- that is, to cause cerebral ischemia - the vasoconstrictor and cardioaccelerator neurons in the vasomotor centre respond directly to the ischemia and become strongly excited. When this excitation occurs, the systemic nervous pressure often rises to a level as high as the heart can pump. This effect is believed to be caused by failure of slowly flowing blood to carry carbon dioxide away from the brainstem vasomotor centre: at low levels of blood flow to the vasomotor centre, the local concentration of carbon dioxide increases greatly and has an extremely potent effect in stimulation the sympathetic vasomotor nervous control areas in the  brain’s medulla

It is possible that other factors, such as build-up of lactic acid and other acidic substances in the vasomotor centre, also contribute to the marked stimulation and elevation in arterial pressure. The arterial pressure elevation in response to cerebral ischemia is known as the CNS Ischemia Response.

The ischemic effect on vasomotor activity can elevate the mean arterial pressure dramatically, sometimes to as high as 250 mmHg for as long as 10 minutes. The degree of sympathetic vasoconstriction caused by intense cerebral ischemia is often so great that some of the peripheral vessel become totally or almost totally occluded. The kidneys, for instance, often entirely cease their production of urine because of the renal arteriolar constriction in response to sympathetic discharge. Therefore, the CNS Ischemic Response is one of the most powerful of all activators of sympathetic vasoconstrictor system.

Importance of CNS Ischemic Response as a Regular of Artificial Pressure :

Despite the powerful nature of the CNS Ischemic Response, it does not become significant until arterial pressure falls far below normal level, down to 60 mmHg and below, reaching its greatest degree of stimulation at a pressure of 15-20 mmHg. Therefore, the CNS Ischemic Response is not one of the normal mechanisms for regulating arterial pressure. Instead, it operates principally as an emergency pressure control system that acts rapidly and powerfully to prevent further decrease in arterial pressure whenever blood flow to the brain decreases dangerously close to lethal level. Sometimes, it is called, the “Last Ditch Stand” pressure control system.

Cushing Reaction to Increased Pressure around Brain:

The so-called Cushing reaction is a special type of CNS Ischemic Response that results from increased pressure of the cerebrospinal fluid around the brain in cranial vault. For instance, when the cerebrospinal fluid pressure rises to equal the arterial pressure, it compresses the whole brain, as well as the arteries in the brain and cuts off the blood supply to the brain. This action initiates a CNS Ischemic Response that causes the arterial pressure to rise.

Cushing Reflex Mechanism:

BP< 40 mm Hg ----> Cerebral Hypoxia, Hypercapnoea -------> Vaso Motor Centre -------> Symp Nervous System -------> Vasoconstriction ------> Increased BP

When the arterial pressure has risen to a level, higher than the cerebrospinal fluid pressure, blood will flow once again into the vessels of the brain to relieve the brain ischemia. Ordinarily, the blood pressure comes to a new equilibrium level slightly higher than the cerebrospinal pressure, thus allowing blood to flow through the brain again. The Cushing reactions helps to protect vital centres of the brain from loss pf nutrition if the cerebrospinal fluid level pressure ever rises high enough to compress the cerebral arteries.

I think all these informations are more than sufficient for you to understand this topic prominently. If still you've any question in this regard, please let me know in the comment box, I'll definitely answer.