Which part of the autonomic nervous system is most important for regulating the circulation?
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Sympathetic nervous system — it is the primary autonomic regulator of vascular tone and blood distribution.
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Which part of the autonomic nervous system is most important for regulating the circulation?
Sympathetic nervous system — it is the primary autonomic regulator of vascular tone and blood distribution.
What is the principal circulatory role of the parasympathetic nervous system?
Parasympathetic system (vagus nerves) primarily controls heart rate, causing a marked decrease in rate and a slight decrease in contractility.
Through which spinal nerves do sympathetic vasomotor fibers leave the spinal cord?
They leave via the thoracic spinal nerves and the first one or two lumbar spinal nerves, then pass into the sympathetic chain.
Name the two main routes by which sympathetic fibers reach the circulation.
(1) Through specific sympathetic nerves to internal viscera and the heart; (2) into peripheral portions of spinal nerves distributed to peripheral vasculature.
Which blood vessels are innervated by sympathetic fibers?
Most vessels except capillaries — especially small arteries, arterioles, and veins; pre-capillary sphincters are innervated in some tissues.
What and where is the vasomotor center?
The vasomotor center is an area in the reticular substance of the medulla and lower third of the pons that controls sympathetic and parasympathetic outputs to the circulation.
Identify the three functional areas within the vasomotor center.
What happens to arterial pressure when vasoconstrictor tone is blocked (example: total spinal anesthesia)?
Loss of vasomotor tone causes a large fall in arterial pressure (example: arterial pressure fell from ~100 to ~50 mm Hg after total spinal anesthesia).
What is the primary neurotransmitter released by sympathetic vasoconstrictor nerve endings?
Norepinephrine — acts mainly on alpha-adrenergic receptors of vascular smooth muscle to cause vasoconstriction.
How do the adrenal medullae interact with the sympathetic vasoconstrictor system?
Sympathetic stimulation causes the adrenal medullae to secrete epinephrine and norepinephrine into the blood, producing widespread vascular effects (usually vasoconstriction; epinephrine can cause vasodilation in some tissues via β-receptors).
What mediates sympathetic vasodilation in skeletal muscle?
In some animals acetylcholine is released by vasodilator fibers; in primates, vasodilation is thought to result from epinephrine acting on β-adrenergic receptors.
What is vasovagal syncope?
Vasovagal syncope (emotional fainting) — intense emotional stimuli activate muscle vasodilator pathways and the vagal cardioinhibitory center, causing sudden hypotension and bradycardia leading to fainting.
List the three major simultaneous changes that rapidly increase arterial pressure.
How rapid is nervous control of arterial pressure?
Extremely rapid — pressure can double within 5–10 seconds during strong sympathetic activation; conversely it can fall to half within 10–40 seconds when stimulation stops.
By how much does arterial pressure typically rise during heavy exercise?
Usually rises about 30–40% (e.g., from mean ~100 mm Hg to ~130–140 mm Hg), aiding increased muscle blood flow.
What brain activation contributes to the rise in arterial pressure during exercise?
Activation of the reticular activating system including increased stimulation of vasoconstrictor and cardioacceleratory areas of the vasomotor center.
Where are the most abundant arterial baroreceptors located?
In the carotid sinus (internal carotid just above bifurcation) and in the aortic arch.
Which nerves transmit signals from carotid and aortic baroreceptors to the brainstem?
Carotid baroreceptors → Hering's nerve → glossopharyngeal nerve; aortic baroreceptors → vagus nerve; both project to the tractus solitarius in the medulla.
How do baroreceptors respond to changes in arterial pressure within the normal operating range?
They respond rapidly and sensitively; small pressure changes around ~100 mm Hg produce large changes in firing rate, and they respond more strongly to rapid changes than to steady pressures.
What reflex effects follow baroreceptor excitation (high arterial pressure)?
Baroreceptor excitation inhibits the vasoconstrictor center and excites the vagal parasympathetic center, producing vasodilation, decreased heart rate, and decreased contractility, which lower arterial pressure.
What stimuli activate carotid and aortic chemoreceptors?
Low oxygen (hypoxia), high carbon dioxide (hypercapnia), and high hydrogen ion (acidosis) levels — located in carotid bodies and aortic bodies.
When do chemoreceptors become important for arterial pressure control?
They are most active when arterial pressure falls below ~80 mm Hg, at which point they stimulate the vasomotor center to raise pressure.
What are the low-pressure receptors and what do they do?
Low-pressure receptors are stretch receptors in the atria and pulmonary arteries that detect blood volume changes and trigger reflexes to minimize arterial pressure changes during volume shifts.
Describe the Bainbridge reflex.
The Bainbridge reflex: increased atrial pressure stretches atrial receptors → afferent signals via vagus to medulla → efferent autonomic signals increase heart rate and contractility, helping prevent venous/atrial blood pooling.
What triggers the CNS ischemic response?
Severe reduced blood flow to the vasomotor center (brainstem ischemia) leading to local buildup of CO2 and acidic metabolites that powerfully stimulate sympathetic outflow.
How large can the arterial pressure rise during the CNS ischemic response?
It can rise dramatically — sometimes up to ~250 mm Hg for short periods — as the sympathetic outflow maximizes.
What is the Cushing reaction?
A special CNS ischemic response caused by elevated cerebrospinal fluid pressure compressing cerebral vessels, which elicits a marked increase in arterial pressure to restore brain perfusion.
At what arterial pressures is the CNS ischemic response most significant?
It becomes significant when arterial pressure falls below ~60 mm Hg, with greatest activation at 15–20 mm Hg — it is primarily an emergency mechanism.
What is the abdominal compression reflex and its circulatory effect?
Reflex contraction of abdominal muscles (during baroreceptor/chemoreceptor activation) compresses abdominal venous reservoirs, shifting blood to the heart and increasing cardiac output and arterial pressure.
How do skeletal muscle contractions contribute to increased cardiac output during exercise?
Muscle contractions compress blood vessels, translocating blood toward heart and lungs — this helps achieve the large (5–7×) increases in cardiac output during heavy exercise.
What causes respiratory waves in arterial pressure?
Respiratory waves arise from: (1) spillover of respiratory signals into vasomotor center; (2) thoracic pressure changes affecting venous return during inspiration; (3) excitation of vascular/atrial stretch receptors.
What are vasomotor (Mayer) waves and why do they occur?
Vasomotor (Mayer) waves are slower oscillations (periods ~7–26 s) in arterial pressure caused by reflex oscillation of baroreceptor/chemoreceptor or CNS ischemic control loops when feedback gain and delays allow cyclical behavior.
