2018B09 Describe the autonomic innervation of the heart (60%),
and the effects of autonomic stimulation on cardiac function (40%).

SNS

PSNS

Resting tone

↓

↑

Exercise

↑

↓

Onset speed

↓

Effects via 2^nd messenger cAMP

↑

ACh directly activates special K_ACh 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

SNS

PSNS

Inotropy

↑

↔↓

Chronotropy

↑

↓

Dromotropy

↑

↓

Lusitropy

↑

↔

SNS

PSNS

Pre-motor

·  Rostral ventrolateral medulla

-

Pre-ganglionic

·  Lateral horn

·  C7-T6; mostly T1-4; bilateral

·  (nicotinic)

·  Nucleus ambiguus

·  Dorsal motor nucleus of CNX

·  (nicotinic)

Post-ganglionic

·  Stellate and middle cervical ganglia

(adrenergic)

·  Epicardial ganglion

(muscarinic)

Cardiac plexus

·  After synapse

·  Before synapse

Asymmetry

·  ? 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%

·  (G_s GPCR)

·  Ventricules > atria

·  ↑cAMP

·  ↑PKA

·  Phosphorylation of

·  L-Ca²⁺ channel -> ↑inotropy

·  Myosin -> ↑rate cross-bridge cycling

·  Phospholamban, TNI -> ↑rate relaxation

·  m₂AChR

·  (G_i GPCR)

·  Atria > ventricles

·  Minimal direct mAChR effect

·  ↓Inotropy via interneuronal effect

Pacemaker cell

·  β₁ and β₂

·  ↑cAMP

·  ↑Activity HCN channel

·  ↑I_f = ↑I_Ca > ↑I_K

·  ↑Slope all phases (mostly phase 4)

·  m₂AChR

·  ↓cAMP

·  ↑I_K

·  ↓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

Primary afferent

·  Unmyelinated C fibre

·  Piggybacks onto autonomic efferents

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

o 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

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

·  Ascent via multiple spinothalamic tracts