Lecture 4: integration of reflex control

Question 1

What is the simplest model of a reflex arc?

Answer 1

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

Question 2

What are the 2 types of receptors found in arteries?

Answer 2

Mechanoreceptors and chemoreceptors

Question 3

What type of receptors are baroreceptors?

Answer 3

Mechanoreceptors

Question 4

What do arterial baroreceptors respond to?

Answer 4

Changes in arterial blood pressure/stretch

Question 5

What are the 2 types of arterial barpreceptors?

Answer 5

Carotid baroreceptors and aortic baroreceptors

Question 6

Where are carotid baroreceptors located?

Answer 6

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

Question 7

Where are the aortic baroreceptors located?

Answer 7

The arch of the aorta

Question 8

What is the afferent pathway of the carotid reflex loop?

Answer 8

Carotid sinus

↓

Glossopharyngeal (IXth cranial) nerve

↓

cardiovascular centre in medulla

Question 9

What is the efferent pathway of the carotid baroreflex loop?

Answer 9

Cardiovascular centre in medulla

↓

Vagus (Xth cranial) nerve

↓

SA node

Question 10

What is the afferent pathway of the aortic baroreflex loop?

Answer 10

Aortic sinus

↓

Aortic nerves

↓

Cardiovascular centre in medulla

Question 11

What is the efferent pathway of the aortic baroreflex loop?

Answer 11

Cardiovascular centre in medulla

↓

Vagus (Xth cranial) nerves

↓

SA node

Question 12

What is hypotension and what are the effects of this?

Answer 12

Low arterial blood pressure

Reduced perfusion of oxygenated blood to tissues

Question 13

What is hypertension and what are the effects of this?

Answer 13

High arterial blood pressure

Damage to fragile circulations, particularly cerebral circulation

Question 14

What is the role of the carotid baroreflex?

Answer 14

Maintains normal pressure in circulation perfusing the brain

Question 15

What is the role of the aortic baroreflex?

Answer 15

Governs systemic arterial blood pressure homeostasis

Question 16

What type of activity is shown by baroreceptor fibres?

Answer 16

Tonic

Question 17

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

Answer 17

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

Question 18

What is tonic baroreceptor discharge in phase with?

Answer 18

Aterial blood pressure pulse

Question 19

What is mechanotransduction?

Answer 19

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

Question 20

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

Answer 20

• 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

Question 21

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

Answer 21

Cardiac inhibitory centre

Question 22

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

Answer 22

Cardiac acceleratory centre

Question 23

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

Answer 23

Cardiac centre and vasomotor centre

Question 24

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

Answer 24

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

Question 25

What may cause a sudden decrease in arterial blood pressure

Answer 25

Haemorrhage

Question 26

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

Answer 26

- 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

Question 27

Name a vasoconstricting drug How does it work

Answer 27

Phenylephrine (synthetic form of noradrenaline) Acts on α₁-adrenergic receptors (like noradrenaline) to cause vasoconstriction

Question 28

What is the effect of phenylephrine on heart rate?

Answer 28

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

Question 29

Name a drug that is a vasodilatator

Answer 29

Sodium nitroprusside

Question 30

What is the effect of sodium nitroprusside on heart rate?

Answer 30

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

Question 31

What is the shape of a baroreflex curve?

Answer 31

Sigmoidal

Question 32

What does the gradient of the baroreflex curve represent?

Answer 32

ΔHR/ΔP = sensitivity or gain

Question 33

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

Answer 33

The set point

Question 34

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

Answer 34

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

Question 35

What is peripheral resetting of baroreceptors?

Answer 35

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

Question 36

What is the advantage of peripheral resetting of baroreceptors?

Answer 36

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

Question 37

What is a disadvantage of peripheral resetting of baroreceptors?

Answer 37

May lead to hypertension

Question 38

Give an example of when peripheral re-setting occurs

Answer 38

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

Question 39

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

Answer 39

Decreases

Question 40

What is the afferent pathway of the afferent carotid chemoreflex?

Answer 40

Carotid bodies ↓ Carotid sinus nerve ↓ Glossopharyngel (IXth) nerve ↓ Cardiovascular centre in medulla

Question 41

What is the afferent pathway of the aortic chemoreflex?

Answer 41

Aortic bodies ↓ Aortic nerves ↓ Cardiovascular centre

Question 42

What is the efferent pathway of the carotid chemoreflex?

Answer 42

Cardiovascular cente ↓ Vagus (Xth) nerve ↓ SA node

Question 43

What is the efferent pathway of the aortic chemoreflex?

Answer 43

Cardiovascular centre ↓ Vagus nerve ↓ SA node

Question 44

What is peripheral chemoreceptor tissue composed of?

Answer 44

Islands of glomus cells

Question 45

What is the function of glomus cells?

Answer 45

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

Question 46

What are the adaptations of glomus cells?

Answer 46

Contain many mitochondria and dark vesicles containing peptides needed for chemotransduction

Question 47

What is hypoxia? What is hypoxaemia?

Answer 47

Decreased oxygen in the environment Decreased partial pressure of oxygen in the arteries

Question 48

How does hypoxaemia affect carotid and aortic chemoreceptors?

Answer 48

Increased discharge from both receptors

Question 49

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

Answer 49

The PO₂ at which peripheral chemoreceptor discharge starts

Question 50

What does the gradient of the chemoreflex function curve represent?

Answer 50

Δdischarge/ΔPaO₂ = Sensitivity

Question 51

What is peripheral resetting of baroreceptors?

Answer 51

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

Question 52

Give an example of peripheral chemoreceptor resetting

Answer 52

Chemoreceptor discharge curve shifts right after birth since adult PO₂ is higher than fetal Fetal PO₂ = 25-40mmHg Adult PO₂ = 90-100mmHg

Question 53

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

Answer 53

Cot death (Sudden infant death syndrome - SIDS)

Question 54

What is the physiological reponse to hypoxia?

Answer 54

- Increased blood flow to most circulations - Increased HR - Vasodilatation (increased flow/ decreased vasular resistance)

Question 55

Describe the experiment by Daly and Scott in 1962 What did they observe?

Answer 55

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

Question 56

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

Answer 56

During diving

Question 57

What is the primary chemoreflex cardiovascular response?

Answer 57

- Decreased HR - Increased vascular resistance

Question 58

What is the secondary chemoreflex cardiovascular response?

Answer 58

- Increased HR - Decreased vascular resistance (vasodilatation)

Question 59

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

Answer 59

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

Question 60

How does the fetus respond to hypoxia?

Answer 60

Breathing movements stop to conserve energy Heart rate decreases Femoral vascular resistance increases (secondary response)