ANS innervation of the heart

[Ref: BL8: chp4]

Parasympathetic pathway to heart

Via vagus nerve
* Originate in medulla oblongata

• Right vagus nerve affect SA node predominantly
• Left vagus nerve affect AV node predominantly

 

Vagal pathway

• Neurons giving rise to vagal fibres are in:
  * Dorsal motor nucleus of the vagus, AND
  *Nucleus ambiguus
• Innervation is to the heart only
• Inhibited by neural input from NTS

[WG21:p608(fig31-8)]

Timing of vagal stimulation on heart

• Brief in duration of action (nodes are rich in cholinesterase)
• Very fast in onset
  * Because muscarinic receptors (M2) act via Gi protein which is coupled to a K+ channel

Effect of parasympathetic stimulation

[BL8:p88(fig4-4)]

Stimulates muscarinic acetylcholine receptors (M2)

--> Coupled with Gi protein

--> Inhibits adenylyl cyclase

--> Decrease cAMP

--> Decrease conductance of Ca2+

--> Decrease myocardial contractility

Also,

--> Gi protein is coupled to a K+ channel

--> Increased K+ conductance

--> Decreased RMP

--> Decrease HR (when acting on nodes)

Also,

• ACh released from vagal endings can inhibit release of norepinephrine from nearby sympathetic nerve endings
  --> Reciprocal inhibition

Sympathetic pathway to heart

Originate in intermediolateral column of upper 5-6 thoracic and lower 1 or 2 cervical segements of the spinal cord

Timing of sympathetic stimulation on heart

• Slower onset than vagal (mediated by cAMP)
• Effects decay very gradually

Effect of catecholamines

Stimulates beta-1 receptor

--> via Gs protein

--> Activate adenylyl cyclase

--> Increase cAMP

--> Activate protein kinase A

--> Phosphorylation of Ca2+ channels on membrane

--> Channels open for longer

--> Greater Ca2+ influx

--> Greater contractility

Also,

Protein kinase A

--> Phosphorylation of phospholamban

--> Stimulation of calcium pump on SR

--> Increased active transport of Ca2+ to SR

--> Faster relaxation

[BL8:p105]

Plus,

Protein kinase A

--> Phosphorylation of Troponin I

--> Inhibition of Ca2+ binding by Troponin C

--> Relaxation

--> Faster relaxation

NB:

• Catecholamines also increase sensitivity of myofilaments to Ca2+

Normal heart rate

At rest, there is a certain level of vagal tone at rest.

When vagus is cut, HR increase from 70 to 150-180 because sympathetic tone is unopposed.