Lec. 5. Cardiac innervation.

Intro.

The heart receives efferent autonomic sympathetic and parasympathetic nerves supplied by higher centers in the medulla oblongata: Cardiac Inhibitory Center (CIC) & Cardiac Accelerator Center (CAC). The afferent nerves coming from the heart follow alongside the efferent.

Efferent cardiac nerves.

Sympathetic supply of the heart.

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• CAC preganglionic axons originate from LHCs of upper 5 thoracic segments.
• They descend in the spinal cord.
• They relay in the superior, middle, and inferior ganglia.
• They supply the whole heart.

Parasympathetic supply of the heart.

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• CIC preganglionic axons originate from the dorsal motor nucleus of the vagus in the medulla.
• They join the vagus nerve.
• They relay in terminal ganglia in the right atrium and interatrial septum.
• They supply the atria, SAN, AVN, stem of AV bundle, and coronary blood vessel. (NOT the ventricles.)

Sympathetic supply of the coronary vessels.

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Increases coronary blood flow through:

1. Direct effect.
   • ${\beta }_2$ receptor stimulation -⇒ mild VD. (dominant in the heart.)
   • ${\alpha }_1$ receptor stimulation -⇒ mild VC. (dominant in peripheral BVs.)
2. Indirect effect.
   • Sympathetic stimulation -⇒ increased cardiac metabolism -⇒ accumulation of metabolites -⇒ local, strong, and dominant VD.

Parasympathetic supply of the coronary vessels.

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Decreases coronary blood flow through:

1. Direct effect.
   • M₃ receptor stimulation -⇒ mild VD. (dominant in peripheral BVs.)
2. Indirect effects.
   • Parasympathetic stimulation -⇒ decreases cardiac metabolism -⇒ less O₂ consumption -⇒ Increased O₂ tension -⇒ local VC of arteries.

(Too much O₂ = VC to redistribute it to places that need it.)

(The cardiovascular center is formed of 2 formations in the medulla oblongata: The CIC and Vasomotor center. The CAC is found in the upper 4 thoracic LHCs.)

Afferent cardiac nerves.

1. Afferent fibers carrying pain sensation run alongside sympathetic efferent nerves to the spinal cord then to the brain through the lateral spinothalamic tract. (pain receptors → afferent fibers → spinal cord → lateral spinothalamic tract → brain.)

2. Afferent fibers from stretch receptors and chemoreceptors run alongside vagus nerve up to sensory area in medulla oblongata. (stretch receptors & chemoreceptors → afferent fibers → sensory area in medulla oblongata.)

Sympathetic and parasympathetic tones.

Sympathetic tone.

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• Increases contractility by 30%. (+ inotropic.)
• Has no effect on rhythmicity. (none-chronotropic.)

Parasympathetic tone.

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• Has - chronotropic effect on heart, reducing SAN rhythm from 110 to 70 BPM.
• This effect is more dominant during rest than sympathetic tone.
• Mechanism:
  • blood pressure measured by the baroreceptors in the wall of the aortic arch and coronary sinus pass their signals through the carotid sinus nerve and aortic nerve to the CIC which “replies” through the vagus nerve by inhibiting SAN and decreasing HR.

Vagal escape phenomenon:

Continuous vagal stimulation of the heart causes severe inhibition of SAN until the heart stops completely. After that, the ventricles start beating according to their own rhythm since SAN is now “off”.