[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Cardiovascular](https://durgical.com/en/physiology/#s-cardio) › Nervous control of the circulation and the CNS ischemic response [Physiology](https://durgical.com/en/physiology/) · [Cardiovascular](https://durgical.com/en/physiology/#s-cardio) # Nervous control of the circulation and the CNS ischemic response - Regulation - Exam priority: medium The brain raises or lowers blood pressure within seconds through the sympathetic nerves and the vagus. Updated 2026-11-01 **See it in 3D:** [Sympathetic trunk](https://durgical.com/en/anatomy/sympathetic-trunk/) · [Medulla oblongata](https://durgical.com/en/anatomy/medulla-oblongata/) · [Vagus nerve](https://durgical.com/en/anatomy/vagus-nerve/) · [Carotid bifurcation](https://durgical.com/en/anatomy/carotid-bifurcation/) · [Left adrenal gland](https://durgical.com/en/anatomy/suprarenal-gland/) · [Sinoatrial (SA) node](https://durgical.com/en/anatomy/sinuatrial-node/) ## Key points - Baseline sympathetic tone keeps vessels partly constricted; if it is blocked (high spinal anesthesia, spinal cord injury), pressure falls. - The sympathetic system acts on arterioles (resistance), veins (venous return), heart (rate and force) and adrenal medulla (epinephrine). - The parasympathetic system (vagus) acts almost only on heart rate; it barely innervates vessels. - CNS ischemic response: switches on when mean pressure falls below ≈60 mmHg and peaks around 15–20 mmHg; a last-ditch defense. - Cushing reflex: high intracranial pressure → hypertension, bradycardia and irregular breathing; a sign of impending herniation. ## What it is The vasomotor center in the medulla keeps a steady sympathetic tone on arterioles, veins and heart, while the vagus brakes heart rate. It is the fastest system for correcting blood pressure: it acts in seconds, unlike renal control, which takes hours to days. When the brainstem itself loses flow, it fires a maximal sympathetic discharge: the CNS ischemic response. ## How it works, step by step - Neurons of the rostral ventrolateral medulla send tonic impulses down the spinal cord. - Preganglionic sympathetic fibers leave at T1–L2 and synapse in the sympathetic chain. - Norepinephrine constricts arterioles and veins via α1 receptors and speeds and strengthens the heart via β1. - The nucleus of the solitary tract receives baroreceptor input and tunes that tone beat by beat. - If brainstem flow falls severely, CO₂ and local metabolites strongly excite the vasomotor center. - The maximal sympathetic discharge constricts almost every vascular bed to push pressure above intracranial pressure. ## Regulation - *+* **Sympathetic to the heart (β1)**: Raises rate, conduction velocity and contractility. - *+* **Sympathetic to arterioles (α1)**: Raises peripheral resistance and diastolic pressure. - *+* **Sympathetic to veins (α1)**: Mobilizes blood from the venous reservoir: more return and output. - *+* **Adrenal medulla (epinephrine and norepinephrine)**: Reinforces the sympathetic effects through the blood. - *−* **Vagus to the SA and AV nodes (M2)**: Lowers rate and slows AV conduction. - *−* **Stretched baroreceptors**: Inhibit the vasomotor center and stimulate the vagus. ## Circulatory effectors and their nerve supply | Effector | Sympathetic | Parasympathetic | | --- | --- | --- | | SA node | ↑ rate (β1) | ↓ rate (M2), dominant at rest | | Ventricular myocardium | ↑ force (β1) | Minimal effect | | Skin and visceral arterioles | Constriction (α1) | Almost no innervation | | Veins | Constriction (α1) | Almost no innervation | | Adrenal medulla | Releases epinephrine | No effect | ## Clinical notes - A Cushing reflex in head trauma (hypertension with bradycardia) signals critical intracranial pressure. - Neurogenic shock after high spinal cord injury gives hypotension with bradycardia, as sympathetic tone is lost and the vagus is left unopposed. - Beta blockers lower rate and contractility and blunt the sympathetic stress response. ## Memory aid "The vagus brakes, the sympathetic squeezes": the vagus only touches the heart; the sympathetic squeezes heart, arterioles and veins. ## Test yourself Think of the answer, then tap the question to see it. ## Why can high spinal anesthesia cause marked hypotension? It blocks the thoracolumbar sympathetic outflow: arteriolar and venous constrictor tone is lost, resistance falls and blood pools in the venous reservoir. ## What explains the bradycardia of the Cushing reflex if there is a sympathetic discharge? The sympathetic surge drives arterial pressure very high; baroreceptors sense it and activate the vagus, which slows the heart. ## Why does neurogenic shock come with bradycardia rather than the tachycardia of hypovolemic shock? The cord injury cuts the sympathetic path to heart and vessels, while the vagus, which leaves from the brainstem, keeps acting unopposed. ## Why is the nervous system the fastest controller but not the one that sets pressure long term? It acts within seconds, but baroreceptors reset to a new level over days; long-term pressure is set by the kidney's salt and water balance. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/physiology/nervous-control-of-the-circulation-and-the-cns-ischemic-response/) ## See it in 3D The structures of Durgical's 3D anatomy where it happens. - [Sympathetic trunk](https://durgical.com/en/anatomy/sympathetic-trunk/) - [Medulla oblongata](https://durgical.com/en/anatomy/medulla-oblongata/) - [Vagus nerve](https://durgical.com/en/anatomy/vagus-nerve/) - [Carotid bifurcation](https://durgical.com/en/anatomy/carotid-bifurcation/) - [Left adrenal gland](https://durgical.com/en/anatomy/suprarenal-gland/) - [Sinoatrial (SA) node](https://durgical.com/en/anatomy/sinuatrial-node/) ## Related topics - [The baroreceptor reflex](https://durgical.com/en/physiology/the-baroreceptor-reflex/) - [The response to standing up](https://durgical.com/en/physiology/the-response-to-standing-up/) - [Local and humoral control of blood flow](https://durgical.com/en/physiology/local-and-humoral-control-of-blood-flow/) - [Long-term blood pressure control by the kidney](https://durgical.com/en/physiology/long-term-blood-pressure-control-by-the-kidney/) - [Shock physiology](https://durgical.com/en/physiology/shock-physiology/) ## Related conditions - [Traumatic brain injury](https://durgical.com/en/conditions/traumatic-brain-injury/) - [Hypovolaemic shock](https://durgical.com/en/conditions/hypovolaemic-shock/) - [Arterial hypertension](https://durgical.com/en/conditions/arterial-hypertension/) ## In Biochemistry - [Catecholamines: synthesis and breakdown](https://durgical.com/en/biochemistry/topics/catecholamines-synthesis-and-breakdown/) - [Adrenaline](https://durgical.com/en/biochemistry/molecules/adrenaline/) ## In Pharmacology - [Beta blockers](https://durgical.com/en/pharmacology/classes/beta-blockers/) - [Propranolol](https://durgical.com/en/pharmacology/propranolol/) - [Metoprolol](https://durgical.com/en/pharmacology/metoprolol/) [← All of physiology](https://durgical.com/en/physiology/) Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given. Sources: [20.4 Homeostatic Regulation of the Vascular System](https://openstax.org/books/anatomy-and-physiology-2e/pages/20-4-homeostatic-regulation-of-the-vascular-system) (OpenStax (Rice University), 2022); [Traumatic Brain Injury](https://www.ncbi.nlm.nih.gov/books/NBK557861/) (StatPearls (NCBI Bookshelf), 2025); [Hypovolemia and Hypovolemic Shock](https://www.ncbi.nlm.nih.gov/books/NBK513297/) (StatPearls (NCBI Bookshelf), 2025); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 18). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/physiology/nervous-control-of-the-circulation-and-the-cns-ischemic-response/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/nervous-control-of-the-circulation-and-the-cns-ischemic-response/