Week 11 Physiology - CVS Autoregulation, Special Circulations Flashcards
What types of blood vessels contain smooth muscle and receive motor fibres from from SNS?
All blood vessels aside from capillaries and venules
What anatomical location of the CNS is largely responsible for sympathetic nerves controlling vasculature?
Rostral ventrolateral medulla
What factors directly stimulate Rostral ventrolateral medulla?
CO2
Hypoxia
Describe Cushing’s reflex and the role of the Rostral ventrolateral medulla?
RVLM in the setting of raised ICP will have decreased blood supply (Monroe Kelly) and therefore will have an excitatory input to increasing BP via SNS activity. This will have a feedback effect via carotid baro-receptors, causing reflex decrease in heart rate (bradycardia)
Remember, Cushing’s triad:
- Hypertension
- Bradycardia
- Decreased respiratory rate
What are baroreceptors?
“Stretch” receptors that monitor pressure, located within walls of heart and blood vessels
Where are “high pressure” baroreceptors located?
Carotid sinus
Aortic arch
Where are “low pressure” baroreceptors located?
R+L atria
Cardiac entrance of SVC + IVC
Pulmonary veins
Where are ‘cardiopulmonary baroreceptors’ located?
Endocardial surface of ventricles
In what layer of blood vessels are baroreceptors located?
Adventitia
What “stimulates” baroreceptors?
Blood vessel distension
Describe the pathway of the baroreceptor reflex:
- Afferent fibres of the carotid sinus = branch of glossopharyngeal nerve, aortic arch = vagus
- These afferents pass to medulla, with most ending in nucleus of tracts solitaries
- Results in glutamate release, stimulates excitatory projections into from the NTS to the vagal motor neurons (of various nuclei)
- Result of baroreceptor discharge = inhibition of tonic discharge of SNS at RVLM and excites vagal stimulation of the heart
What are the effects of discharging from baroreceptor vagal efferents?
vasodilation, venodilation, decreased BP, bradycardia, decreased cardiac output
What is the Bezold-Jarisch reflex?
Bradycardia, hypotension, and brief apnoea, followed by rapid shallow breathing
What is the theorised mechanism of the Bezold- Jarisch reflex?
Inhibitory mechanoreceptors in the left ventricle (particularly the inferoposterior wall). Stimulation of these inhibitory cardiac receptors by stretch (poorly filled ventricle), increases renin and vasopressin release and parasympathetic activity and inhibits sympathetic activity.
How does Bezold-Jarisch reflex explain vasovagal syncope?
I.e. venous pooling –> drop in pressure sensed in carotid sinus –> SNS stimulation –> increased contraction of underfilled heart –> paradoxical activation of parasympathetic nervous system and transient but marked hypotension and poor perfusion to CNS –> syncope
What is the valsalva manoeuvre?
Forced expiration against a closed glottis
Describe the sequence of events in the valsalva manoeuvre and how it leads to vagal stimulation?
- Increase in blood pressure at onset of straining
- BP falls as intrathoracic pressure continues to rise and compresses veins, decreasing venous return to the heart
- Decreased atrial and pulse pressure inhibits baroreceptors, causing tachycardia and rise in TPR
- Glottis is opened, intrathoracic pressure returns to normal, cardiac output restored, but peripheral vessels are constricted, causing inc. in BP and strongly stimulating baroreceptors –> bradycardia (also the mechanism of how defecation syncope happens)
What is the (simplified) components of chemoreceptor reflex arc?
- Inputs
- Integration and control centre
- Effectors
Where are peripheral chemoreceptors located?
Carotid and aortic BODIES (not sinuses)
What is the primary and secondary factors that stimulate peripheral chemoreceptors?
Primary = Decreased PaO2 (stimulated by low O2 tension)
Secondary = Decreased PaCO2 (20% response of that causes by decreased PaO2)
Where are central chemoreceptors, and how are they stimulated?
Located on the ventral medulla, and are stimulated by a fall in CSF pH
(Remembering that ions cannot cross BBB, but CO2 freely can, which then combines with H20 in CSF to form H2CO3 –> H+ and HCO3 - –> pH sensitive stimulation of effector arm
Other than increased minute volume, what other effects can chemoreceptors have?
Vasoconstriction
Increased vagal nerve activity
What substances are secreted by the endothelium, and what vasomotor activity do they display?
- Prostacyclin –> vasodilation
- Thromboxane A2 –> vasoconstriction
- Nitric Oxide/Endothelium-derived relaxing factor (EDRF) –>vasodilation (inhibited by Hb)
- Endothelins –> vasoconstrictive
What is the mechanism by which NO causes vasodilation?
Produced by endothelium (activated by ACh, bradykinin) –> diffuses from endothelium to vascular smooth muscle cells –> activates guanylyl cyclase –> cGMP –> second messenger that causes decreased Ca2+ entry, hyper polarisation and de-phosphorylation of the myosin light chains –> decreased contraction
