CVS Reflexes

Question 1

What is the pressor response? What receptors are involved?

What is the depressor response? What receptors are involved?

Answer 1

• When excitatory inputs are stimulated, the reflexes are SWITCHED ON to ↑CO/TPR/BP - arterial chemoreceptors, muscle metaboreceptors
• When inhiBitory inputs are stimulated, the reflexes are SWITCHED OFF to ↓CO/TPR/BP - arterial Baroreceptors

Question 2

What are the roles of the arterial baroreceptors?

Where are they found?

What occurs when there’s an increase in BP?

What’s the effect of Chronic HT on the baroreceptors?

Answer 2

• Maintain blood flow to the brain and heart
• Carotid and aortic bodies
• Baroreceptors are stimulated to send AP’s to the brain - impulse frequency starts off high and then slows down
• They adapt to the change - become used to the high BP

Question 3

What is Loading of the Baroreceptors?

What is the response of loading?

How does it carry out this response?

Answer 3

• The effect an increased BP has on the Baroreflex e.g. stress
• Responses work to ↓HR, BP, and TPR - DEPRESSOR reflex
• ↓Sympathetic activity and ↑Parasympathetic activity

Question 4

What is UNloading of the Baroreceptors?

What is the response of Unloading?

How does it carry out this response?

Answer 4

• The effect a decrease BP has on the Baroreflex e.g. haemorrhage
• Responses work to ↑Sympathetic and ↓Parasympathetic activity - ↑HR and contractive force = ↑CO. Vasoconstriction = ↑TPR. Venoconstriction = ↑CVP, SV, and CO - PRESSOR reflex.
• A large drop in BP causes:
  • Adrenaline secretion
  • ADH secretion
  • ↑RAAS = ↑Renin = ↑Ang II

Ang II = Vasoconstriction = ↓Capillary pressure = ↑Reabsorption = ↑Blood volume

Ang II = ↑Aldosterone = ↑Na reabsorption = ↑Blood volume

Question 5

What is the OVERALL effect of the Cardiac receptors?

What are the 3 types of Cardiac receptors? What stimulates them? What is their effect?

Answer 5

• Depressor Effect
• 1. Nociceptive sympathetic afferents:
     • Chemo-sensitive fibres in ventricle
     • Stimulated by K+, H+, Bradykinin (released during ischaemia)
     • Nerve fibres converge into same neurons in spinal cord as somatic afferents - basis for referred pain
     • Cause an ACUTE PRESSOR reflex = ↑Sympathetic activity

2. Veno-atrial mechanoreceptors:
   • Go to NTS
   • Stimulated by ↑Cardiac filling/CVP.
   • Have an INITIAL PRESSOR REFLEX = ↑Sympathetic activity
   • Have a LONGER DEPRESSOR REFLEX = changes in ADH, ANP, RAAS to ↑Diuresis = ↓Blood volume
3. Ventricular mechanoreceptors:
   • Go to NTS
   • Stimulated by over-distension of ventricles
   • Cause the DEPRESSOR REFLEX = Bradycardia, Vasodilation, ↓BP

Question 6

Where are the Arterial Chemoreceptors found?

What are they stimulated by?

What is their role?

What is their effect?

Answer 6

• Carotid and Aortic bodies
• Stimulated by low O2, High CO2, H+, K+.
• Regulate ventilation and drives the cardiac reflexes during Asphyxia/Shock/Haemorrhage
• Cause a PRESSOR REFLEX = ↑Sympathetic activity, Tachycardia, Selective vaso/venoconstriction = ↑CO and BP.

They preserve cerebral blood flow during low O2.

Question 7

Where are the Muscle Metaboreceptors found?

What are they stimulated by?

What is their effect?

Answer 7

• Skeletal muscle
• Metabolites e.g. ATP, K+, lactic acid, Adenosine
• Cause a PRESSOR REFLEX = ↑Sympathetic activity, Tachycardia, ↑Vaso/venoconstriction = ↑CO and BP.

They preserve blood flow to muscles during isometric exercise.

Question 8

What’s the effect of an increase in signalling from the baroreceptors, arterial chemoreceptors and muscle mechanoreceptors?

LOOK AT DIAGRAM!

Answer 8

↑Baroreceptors:
Excitatory NTS = ↑CVLM = Inhibition of RVLM = SWITCH OFF sympathetic nerves - DEPRESSOR RESPONSE.

↑Arterial chemoreceptors, Muscle mechanoreceptors:
Inhibitory NTS = ↓CVLM = Less inhibition of RVLM = SWITCH ON sympathetic nerves - PRESSOR RESPONSE.

Question 9

What’s the affect of a drop in BP in the signalling pathways?

LOOK AT DIAGRAM!

Answer 9

↓BP will stimulate baroreceptor excitatory pathway to NTS = Activation of Nucleus Ambiguus (NA). This will make Parasympathetic > Sympathetic activity = Vagal Bradycardia to ↓HR and TPR.

Inspiratory centre is in the medulla. Inspiration will ↓NA to temporarily switch off vagal nerves = Sympathetics dominate = ↑HR during inspiration, called Sinus Tachycardia.

HR then drops back down during expiration, called Respiratory Sinus Arrhythmia.