S4) The Autonomic Nervous System Flashcards
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 β1 adrenoreceptors
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
.
