S4) The Autonomic Nervous System

Question 1

What does the autonomic nervous system do?

Answer 1

Regulates physiological functions

• Heart rate
• BP
• Body temperature

Question 2

Describe two general ways in which the ANS can regulate physiological functions

Answer 2

• Sympathetic activity is increased under stress
• Parasympathetic activity is increased under basal conditions

Question 3

What does the ANS exert control over?

Answer 3

• Smooth muscle
• Exocrine secretion
• Inotropy & chronotropy in heart

Question 4

Describe the neurotransmitters and receptors acting on the heart in the parasympathetic nervous system and sympathetic nervous system respectively

Answer 4

• PNS action on heart rate is mediated via acetylcholine acting on M₂ muscarinic receptors
• SNS action on heart rate and contractility is mediated via noradrenaline acting on β₁ adrenoreceptors

Question 5

Outline the ANS control of the cardiovascular system

Answer 5

The ANS controls:

• Heart rate
• Force of contraction of heart
• Peripheral resistance of blood vessels
• Amount of venoconstriction

Question 6

In 4 steps, describe the parasympathetic input to the heart

Answer 6

⇒ Preganglionic fibres via vagus nerve

⇒ Synapse with postganglionic cells on epicardial surface / within heart wall of at SAN/AVN

⇒ Postganglionic cells release ACh

⇒ ACh acts on M₂ receptors

Question 7

What is the effect of the PNS on the heart?

Answer 7

• Decrease heart rate (negative chronotropic effect)
• Decrease AVN conduction velocity

Question 8

In 4 steps, describe the sympathetic input to the heart

Answer 8

⇒ Postganglionic fibres from the sympathetic trunk

⇒ Innervate SAN, AVN and myocardium

⇒ NA is released

⇒ NA mainly on β₁ adrenoreceptors

Question 9

What is the effect of the SNS on the heart?

Answer 9

• Increases heart rate (positive chronotropic effect)
• Increases force of contraction (positive inotropic effect)

Question 10

Outline the opposing sympathetic and parasympathetic effects on pacemaker potentials

Answer 10

• Sympathetic activity:

I. Mediated by β₁ receptors

II. GPCRs increase cAMP which speeds up the pacemaker potential

• Parasympathetic activity:

I. Mediated by M₂ receptors

II. GPCRs increase K⁺ conductance and decrease cAMP

Question 11

In 4 steps, explain how noradrenaline increases the force of contraction

Answer 11

NA acts on β₁ receptors in myocardium:

⇒ Increased cAMP activates PKA

⇒ Phosphorylation of Ca²⁺ channels increases Ca²⁺ entry

⇒ Increased uptake of Ca²⁺ in SR

⇒ Increased sensitivity of contractile machinery to Ca²⁺

Question 12

What type of receptors are found in the systemic vasculature?

Answer 12

• Most blood vessels have α₁-adrenoreceptors
• Some blood vessels have β₂-adrenoreceptors e.g. myocardium, liver, skeletal muscle vasculature

Question 13

Describe the effects of increased/decreased sympathetic output to the peripheral vasculature

Answer 13

Question 14

Describe the effect of circulating adrenaline on blood vessels which have both β₂ adrenoceptors and α₁ adrenoreceptors

Answer 14

Circulating adrenaline has a higher affinity for β₂ adrenoceptors than for α₁ receptors:

• At [physiological], it will preferentially bind to β₂ adrenoceptors
• At [higher], it will also activate α₁ adrenoreceptors

Question 15

Describe the opposing effects of β₂ and α₁ adrenoreceptors on vascular smooth muscle

Answer 15

• Activating β₂ adrenoreceptors causes vasodilation:

I. Increases cAMP → Opens K⁺ channels & inhibits MLCK

II. Relaxation of smooth muscle

• Activating α₁ adrenoreceptors causes vasoconstriction:

I. Stimulates IP₃ production → increase [Ca²⁺]_i

II. Contraction of smooth muscle

Question 16

Describe the cardiovascular control system established by the ANS in terms of afferent and efferent nerves

Answer 16

• Changes are communicated to the brain via afferent nerves:

I. Baroreceptors (high pressure side of system)

II. Atrial receptors (low pressure side of system)

• Alters activity of efferent nerves

Question 17

What are baroreceptors and what do they do?

Answer 17

• Baroreceptors are nerve endings in the carotid sinus and aortic arch are sensitive to stretch
• Increased arterial pressure stretches these receptors

Question 18

Briefly, outline the baroreceptor reflex

Answer 18

.

Question 19

What are the three types of drugs which act on the ANS?

Answer 19

• Sympathomimetics
• Adrenoceptor antagonists
• Cholinergics

Question 20

What are the two types of sympathomimetics?

Answer 20

• α-adrenoceptor agonists
• β-adrenoceptor agonists

Question 21

Describe the uses of sympathomimetics

Answer 21

• Adrenaline to restore function in cardiac arrest
• β₁ agonists given in cardiogenic shock (pump failure)
• β₂ agonists used for treatment of asthma

Question 22

What are the two types of adrenoreceptor antagonists?

Answer 22

• α-adrenoreceptor antagonists
• β-adrenoreceptor antagonists

Question 23

Describe the uses of adrenoreceptor antagonists

Answer 23

• α-adrenoreceptor antagonists – inhibits NA action on vascular smooth muscle α1 receptors, causing vasodilation
• β-adrenoreceptor antagonists – slows HR, reduces force of contraction (β₁) and causes bronchoconstriction (β₂)

Question 24

What are the two types of cholinergics?

Answer 24

• Muscarinic agonists
• Muscarinic antagonists