2018B Question 09

Describe the autonomic innervation of the heart (60%), and the effects of autonomic stimulation on cardiac function (40%).

Examiner Report

67% of candidates achieved a pass in this question.

To pass the first part of this question (60%), a description of the anatomy of cardiac autonomic supply was required. This was best done in a systematic manner starting from the origins of the cardiac sympathetic and parasympathetic fibres to the course that these nerves traverse to the heart. Answers that included a discussion of the differential cardiac distribution of the right and left branches of both divisions scored higher marks.

The second part of the question (40%) called for a discussion of the physiological effects of stimulation of these nerves on cardiac function. Better answers avoided the use of clichés such as “rest and digest” to describe the parasympathetic system since the beating heart performs neither of these functions. Although both divisions of the autonomic system function in a reciprocal manner, their actions are not merely the opposite of each other. An example is the effect of parasympathetic stimulation on contractility of the heart, which is not due to muscarinic stimulation but to interneuronal interactions at the level of nerve endings.

Candidates who gave correct reasons for the brief latency and rapid decay of vagal stimulation as well as the slow onset and gradual offset of sympathetic stimulation were awarded extra marks. Those who mentioned the different effects of right and left sided stimulation of both divisions on cardiac function were given bonus marks. Many candidates spent too much time on unnecessary biochemical details on receptor function. A full discussion of the baroreceptor reflex was not required for a pass in this section.

Model Answer


  • General features
  • Main actions
  • Autonomic efferents
  • General visceral afferents

General Features

Resting tone
Onset speed

Effects via 2nd messenger cAMP

ACh directly activates special KACh channel

Offset speed Slower (Reuptake of NAD)

(AChE +++ at synapse)

Beat-to-beat regulation No Yes
Baroreceptor reflex effect MAP → ↓ Stretch → Disinhibited MAP → ↓ Stretch → Inhibited

Main Actions

Inotropy ↔ ↓

Autonomic Efferents

Pre-motor - Rostral ventrolateral medulla -

- Lateral horn

- C7-T6; mostly T1-4; bilateral

- (nicotinic)

- Nucleus ambiguus

- Dorsal motor nucleus of CNX

- (nicotinic)


- Stellate and middle cervical ganglia


- Epicardial ganglion


Cardiac Plexus - After synapse - Before synapse

- ?L SNS to myocardium

- ?R SNS to SA and AV nodes

- L CNX to AV node

- R CNX to SA node

- (mostly)

Myocyte Effect

- β1 adrenoceptor 85%, β2 15%

- (Gs GPCR)

- Ventricules > atria

- ↑ CAMP

- ↑ PKA

- Phosphorylation of

- L-Ca2+ channel → ↑ Inotropy

- Myosin → ↑ Rate cross-bridge cycling

- Phospholamban, TNI → ↑ Rate relaxation

- m2AChR

- (Gi GPCR)

- Atria > ventricles

- Minimal direct mAChR effect

- ↓ Inotropy via interneuronal effect

Pacemaker Cell

- β1 and β2

- ↑ CAMP

- ↑ Activity HCN channel

- ↑ If = ↑ ICa > ↑ IK

- ↑ Slope all phases (mostly phase 4)

- m2AChR

- ↓ CAMP

- ↑ IK

- ↓ Slope phase 4

- ↑ Max negative potential

Coronary Arteries

- β1, α1

- Direct constrict

- Indirect ↑ work → Dilatation (autoregulation

- ?Receptor

- Direct dilatation

- Indirect ↓ work → Constriction (autoregulation)

Adrenal Medulla

- ~80% adrenaline

- ~20% noradrenaline

- Nil

General Visceral Afferents

Factor Detail
Primary Afferent

- Unmyelinated C fibre

- Piggybacks onto autonomic efferents

 - With SNS: Via white ramus communicans to spinal cord T1-L12

 - With PSNS: Via sacral nerves to S2-4 (also cranial)

- Direct synapse with wide dynamic range 2° afferents in deeper layers

- Convergence onto somatic nociceptive-specific 2° afferents in superficial layers
(hence referred pain)

Secondary Afferent

- Decussation via anterior commissure

 - Incomplete for older pathways (hence bilateral pain but L>R)

- Ascent via multiple spinothalamic tracts

Last updated 2021-08-23

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