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HOM: cardiovascular physiology > Lecture 4: integration of reflex control > Flashcards

Lecture 4: integration of reflex control Flashcards

1
Q

What is the simplest model of a reflex arc?

A

Composed of a sensor which sends information to the brainstem (integrator) via afferent pathways. The information is then intergrated and sent to effector organs via efferent pathways

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2
Q

What are the 2 types of receptors found in arteries?

A

Mechanoreceptors and chemoreceptors

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3
Q

What type of receptors are baroreceptors?

A

Mechanoreceptors

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4
Q

What do arterial baroreceptors respond to?

A

Changes in arterial blood pressure/stretch

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5
Q

What are the 2 types of arterial barpreceptors?

A

Carotid baroreceptors and aortic baroreceptors

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6
Q

Where are carotid baroreceptors located?

A

The carotid sinus at the bifurcation of the internal and external carotid arteries

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7
Q

Where are the aortic baroreceptors located?

A

The arch of the aorta

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8
Q

What is the afferent pathway of the carotid reflex loop?

A

Carotid sinus

↓

Glossopharyngeal (IXth cranial) nerve

↓

cardiovascular centre in medulla

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9
Q

What is the efferent pathway of the carotid baroreflex loop?

A

Cardiovascular centre in medulla

↓

Vagus (Xth cranial) nerve

↓

SA node

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10
Q

What is the afferent pathway of the aortic baroreflex loop?

A

Aortic sinus

↓

Aortic nerves

↓

Cardiovascular centre in medulla

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11
Q

What is the efferent pathway of the aortic baroreflex loop?

A

Cardiovascular centre in medulla

↓

Vagus (Xth cranial) nerves

↓

SA node

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12
Q

What is hypotension and what are the effects of this?

A

Low arterial blood pressure

Reduced perfusion of oxygenated blood to tissues

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13
Q

What is hypertension and what are the effects of this?

A

High arterial blood pressure

Damage to fragile circulations, particularly cerebral circulation

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14
Q

What is the role of the carotid baroreflex?

A

Maintains normal pressure in circulation perfusing the brain

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15
Q

What is the role of the aortic baroreflex?

A

Governs systemic arterial blood pressure homeostasis

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16
Q

What type of activity is shown by baroreceptor fibres?

A

Tonic

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17
Q

What does ‘tonically active’ mean in reference to baroreceptor fibres?

A

Continuous bursts of action potentials are sent to the brainstem via their respective afferent pathways

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18
Q

What is tonic baroreceptor discharge in phase with?

A

Aterial blood pressure pulse

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19
Q

What is mechanotransduction?

A

The process by which a mechanical stimulus is converted intoa neural code which the brainstem can perceive

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20
Q

How does an increase in arterial blood pressure result in a change in the heart rate?

A
  • High blood pressure stretches walls of aorta and carotid sinus, stimulating baroreceptors
  • Increased frequency of action potentials triggered in the afferent nerve so increase in AP received by cardiovascular centre
  • Increased efferent parasympathetic discharge
  • Decreased efferent sympathetic discharge
  • Decreased rate and force of contraction so decrease in CO
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21
Q

Which part of the cardiovascular centre is responsible for parasympathetic discharge?

A

Cardiac inhibitory centre

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22
Q

Which part of the cardiovascular centre is responsible for sympathetic discharge?

A

Cardiac acceleratory centre

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23
Q

What are the 2 regions of the cardiovascular centre in the medulla?

A

Cardiac centre and vasomotor centre

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24
Q

What is the result of an increased rate of action potentials to the vasomotor centre?

A
  • Reduced sympatetic impulses, causing peripheral vasoconstriction
  • Reduced arteriolar tone
  • Reduced venomotor tone
  • Reduced CO and TPR
  • Reduces blood pressure
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25
Q

What may cause a sudden decrease in arterial blood pressure

A

Haemorrhage

26
Q

What is the physiological response to a fall in blood pressure?

A
  • Decrease in frequency of carotid and aortic baroreceptor afferent discharge
  • Decreased parasympathetic discharge from cardiac inhibitory centre
  • Increased sympathetic discharge from cardiac acceleratory centre
  • Increased rate and force of heart contraction
  • Increased BP
27
Q

Name a vasoconstricting drug

How does it work

A

Phenylephrine (synthetic form of noradrenaline)

Acts on α1-adrenergic receptors (like noradrenaline) to cause vasoconstriction

28
Q

What is the effect of phenylephrine on heart rate?

A

Decrease

Increased blood pressure (due to vasoconstriction) causes fall in heart rate via cardiac baroreflex

29
Q

Name a drug that is a vasodilatator

A

Sodium nitroprusside

30
Q

What is the effect of sodium nitroprusside on heart rate?

A

Increases

decreased blood pressure (due to vasodilatation) causes a rise in HR via the cardiac baroreflex

31
Q

What is the shape of a baroreflex curve?

A

Sigmoidal

32
Q

What does the gradient of the baroreflex curve represent?

A

ΔHR/ΔP = sensitivity or gain

33
Q

What is the point on a baroreflex function curve which the reflex strives to maintain?

A

The set point

34
Q

What is central re-setting of baroreceptors and how does it occur?

A

Rise in arterial blood pressure not accompanied by fall in heart rate

(Part of fight/flight reaction)

Stimulation of the hypothalamus ‘gates out’ baroreceptor sends afferents to the cardiovascular centre. Baroreflex is therefore reset to operate around a higher pressure

35
Q

What is peripheral resetting of baroreceptors?

A

Pressure is raised in a sustained manner so that the stimulus response curve shifts to the right so that the set point of the baroreflex increases

36
Q

What is the advantage of peripheral resetting of baroreceptors?

A

Allows greater resting blood pressure without a sustained increase in baroreceptor discharge which conserves energy

37
Q

What is a disadvantage of peripheral resetting of baroreceptors?

A

May lead to hypertension

38
Q

Give an example of when peripheral re-setting occurs

A

At birth to shift ABP from the fetal set-point to the post-natal set point

Fetal set point = 40mmHg

Post-natal set point = 60-80mmHg

39
Q

How does baroreceptor sensitivity change from fetal to post-natal circulation?

A

Decreases

40
Q

What is the afferent pathway of the afferent carotid chemoreflex?

A

Carotid bodies

↓

Carotid sinus nerve

↓

Glossopharyngel (IXth) nerve

↓

Cardiovascular centre in medulla

41
Q

What is the afferent pathway of the aortic chemoreflex?

A

Aortic bodies

↓

Aortic nerves

↓

Cardiovascular centre

42
Q

What is the efferent pathway of the carotid chemoreflex?

A

Cardiovascular cente

↓

Vagus (Xth) nerve

↓

SA node

43
Q

What is the efferent pathway of the aortic chemoreflex?

A

Cardiovascular centre

↓

Vagus nerve

↓

SA node

44
Q

What is peripheral chemoreceptor tissue composed of?

A

Islands of glomus cells

45
Q

What is the function of glomus cells?

A

Oxygen sensors stimulated by a fall in partial pressures of oxygen (PO2) and arterial blood (PaO2)

46
Q

What are the adaptations of glomus cells?

A

Contain many mitochondria and dark vesicles containing peptides needed for chemotransduction

47
Q

What is hypoxia?

What is hypoxaemia?

A

Decreased oxygen in the environment

Decreased partial pressure of oxygen in the arteries

48
Q

How does hypoxaemia affect carotid and aortic chemoreceptors?

A

Increased discharge from both receptors

49
Q

What is represented by the set point of the chemoreflex function curve?

A

The PO2 at which peripheral chemoreceptor discharge starts

50
Q

What does the gradient of the chemoreflex function curve represent?

A

Δdischarge/ΔPaO2 = Sensitivity

51
Q

What is peripheral resetting of baroreceptors?

A

A shift of peripheral chemoreceptor discharge towrads a higher or lower PO2

52
Q

Give an example of peripheral chemoreceptor resetting

A

Chemoreceptor discharge curve shifts right after birth since adult PO2 is higher than fetal

Fetal PO2 = 25-40mmHg

Adult PO2 = 90-100mmHg

53
Q

What is a consquence of faulty peripheral chemoreceptor resetting at birth?

A

Cot death (Sudden infant death syndrome - SIDS)

54
Q

What is the physiological reponse to hypoxia?

A
  • Increased blood flow to most circulations
  • Increased HR
  • Vasodilatation (increased flow/ decreased vasular resistance)
55
Q

Describe the experiment by Daly and Scott in 1962

What did they observe?

A

Induced hypoxia in dogs that wer allowed to either breath spontaneously or via mechanical ventilation

  • Spontaneous breathing: secondary chemoreflex cardiovascular response (inc HR….)
  • Mechanical ventilation: primary response (dec HR…..)

Hypoxia elicits a primary response which becomes modified by hyperventilation to give a secondary response. During hyperventilation, stetch receptors in the lungs increase afferent discharge to the brainstem which inhibits vagal discharge to the heart

56
Q

When is the primary chemoreflex cardiovascular response to hypoxia observed in the adult mammal?

A

During diving

57
Q

What is the primary chemoreflex cardiovascular response?

A
  • Decreased HR
  • Increased vascular resistance
58
Q

What is the secondary chemoreflex cardiovascular response?

A
  • Increased HR
  • Decreased vascular resistance (vasodilatation)
59
Q

Why does the fetus make breathing movements in utero despite not being dependent on pulmonary ventilation?

A

Develops intercostal muscles and alveoli and allows the fetus to practice breathing movements

60
Q

How does the fetus respond to hypoxia?

A

Breathing movements stop to conserve energy

Heart rate decreases

Femoral vascular resistance increases

(secondary response)

HOM: cardiovascular physiology (7 decks)

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