S4) The Autonomic Nervous System Flashcards Preview

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Flashcards in S4) The Autonomic Nervous System Deck (24):

What does the autonomic nervous system do?

Regulates physiological functions 

- Heart rate

- BP

- Body temperature


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

- Sympathetic activity is increased under stress

- Parasympathetic activity is increased under basal conditions


What does the ANS exert control over?

- Smooth muscle

- Exocrine secretion

- Inotropy & chronotropy in heart


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

- 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 βadrenoreceptors 


Outline the ANS control of the cardiovascular system

The ANS controls:

- Heart rate

- Force of contraction of heart

- Peripheral resistance of blood vessels

- Amount of venoconstriction


In 4 steps, describe the parasympathetic input to the heart

⇒ 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


What is the effect of the PNS on the heart?

- Decrease heart rate (negative chronotropic effect)

- Decrease AVN conduction velocity


In 4 steps, describe the sympathetic input to the heart

⇒ Postganglionic fibres from the sympathetic trunk

⇒ Innervate SAN, AVN and myocardium

NA is released

⇒ NA mainly on β1 adrenoreceptors


What is the effect of the SNS on the heart?

- Increases heart rate (positive chronotropic effect)

- Increases force of contraction (positive inotropic effect)


Outline the opposing sympathetic and parasympathetic effects on pacemaker potentials

- 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


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

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+


What type of receptors are found in the systemic vasculature?

- Most blood vessels have α1-adrenoreceptors

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


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


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

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 


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

- 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 


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

- 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 


What are baroreceptors and what do they do?

- Baroreceptors are nerve endings in the carotid sinus and aortic arch are sensitive to stretch

- Increased arterial pressure stretches these receptors


Briefly, outline the baroreceptor reflex


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

- Sympathomimetics

- Adrenoceptor antagonists

- Cholinergics


What are the two types of sympathomimetics?

- α-adrenoceptor agonists

- β-adrenoceptor agonists


Describe the uses of sympathomimetics

- Adrenaline to restore function in cardiac arrest

- β1 agonists given in cardiogenic shock (pump failure)

- β2 agonists used for treatment of asthma 


What are the two types of adrenoreceptor antagonists?

- α-adrenoreceptor antagonists

- β-adrenoreceptor antagonists 


Describe the uses of adrenoreceptor antagonists

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)


What are the two types of cholinergics?

- Muscarinic agonists

- Muscarinic antagonists