Systemic control by nervous system

[Ref: WG21:chp31]

3 systems

1. Sympathetic system
   * Vasoconstricting
   * Chronotropic
   * Inotropic
2. Sympathetic vasodilator system
   * Cholinergic
   * Innervate resistance vessels in skeletal muscles only
   * Significance unknown
3. Parasympathetic system
   * Innervate heart (??only)
   * Small if any effect on total systemic vascular resistance

 

Most influential:
--> Sympathetic nervous system

• All blood vessels except capillaries and venules contain smooth muscles and receive motor nerve fibres from the sympathetic nervous system.
• Innervation of most veins is sparse

Exception:

• Splanchnic veins well innervated.

NB:

• Increased sympathetic activity also increases renin secretion, leading to secretion of angiotensin II and aldosterone.

Innervation

Noradrenergic fibres = vasoconstricting

Noradrenergic fibres innervate most of the blood vessels

But,

Resistance vessels of the skeletal muscles are ALSO innervated by (cholinergic) vasodilator fibres which travel with the sympathetic nerves.

Vasodilator vs vasoconstrictor nerves

No tonic activities in vasodilator fibres

Some tonic activity in vasoconstrictor fibres

--> Vasodilation is caused by decreased activity in vasoconstrictor fibres.

However, in skeletal muscles

--> Vasodilation can ALSO be caused by activation of sympathetic vasodilator system

Capacitance vessels vs resistance vessels

• Capacitance vessels are more responsive to sympathetic stimulation, but do not have beta-receptors and cannot respond to vasodilator metabolites
• Basal sympathetic tone is low in capacitance vessels