Physiology of Sympathetic NS

Question 1

What happens to the heart under a sympathetic response?

Answer 1

Heart rate and contractility increase - to increase cardiac output, so more blood goes to muscles. NA and A act on B₁ adrenoreceptors, found at:

• SAN: increases freq of pacemaker potentials -> inc HR (tachycardia)
• Conduction system: inc rate of impulses through atria to ventricles
• Muscle cells: inc force of contraction of atria/ventricles -> inc SV

CO = SV x HR
Helps CO rise from a resting 5 l/m to over 20 l/m during exercise

Question 2

What happens in the lungs under a sympathetic response?

Answer 2

Lungs don’t receive sympathetic innervation - bronchioles contain B₂-adrenoreceptors which are activated by circulating adrenaline to produce bronchodilation.

So increased air into lungs for greater O₂ uptake/CO₂ removal

Question 3

Describe the central control of the sympathetic nervous system

Answer 3

Hypothalamus receives danger/fight or flight signal, it then stimulates the rostral ventral lateral medulla (RVLM), whcih sends impulses down the spinal cord to preganglionic nerves and intermeriolateral medulla of spinal cord -> stimulating sympathetic nerves

Question 4

What is the effect of sympathetic activity on blood vessels?

Answer 4

NA and A act on a₁-adrenoreceptors

• Constriction of arterioles (vasoconstriction) -> increase in TPR
• Constriction of veins -> inc blood to the heart, so increase in Cardiac Output

Increase in CO x TPR will mean an increase in BLOOD PRESSURE

There are also B₂-adrenoreceptors on coronary and skeletal muscle arteries -> these dilate increasing blood flow to heart and sk muscle.

Question 5

What is the effect of the sympathetic stimulation on the kidney?

Answer 5

Sympathetic nerves act on B₁ adrenoreceptors found on granular cells in the kidney -> this stimulates release of renin

Renin -> Ang I –ACE–> Ang II -> Increased BP + CO (how?):

• Vasoconstriction of arterioles -> inc TPR
• Release of aldosterone -> Na+/H2O retention -> inc BP + CO

Therefore B₁ blockers are successful at decreasing BP.

Question 6

What is the action of sympathetic innervation in the liver and skeletal muscle in terms of metabolism?

Answer 6

Stimulation of a/b-adrenoreceptors

• Breaks down glycogen into glucose (glycogenolysis)
• Promotes glucose synthesis (gluconeogenesis)

Increasing breakdown of sugars -> forming glucose -> more resp -> more ATP

Question 7

What is the effect of sympathetic innervation on the pancreas in terms of metabolism?

Answer 7

Stimulation of a-adrenoreceptors

• Decrease release of insulin from Beta-cells* - also stops insulin-induced reductions in glycogenolysis, gluconeogenesis, lipolysis - so you get more of these!

*Less insulin being taken up means less glucose is taken up, so you want glucose to go to areas where energy is needed.

• Glucagon released from alpha-cells

Question 8

What is the effect of sympathetic innervation on adipose tissue in terms of metabolism?

Answer 8

Stimulation of B-adrenoreceptors

• Increase lipolysis : triglycerides into free fatty acids + glycerol
• Free fatty acids undergo B-oxidation into Acetyl CoA, enter Krebs
• Glycerol - recycled into glycolysis pathway
• Both increase ATP yield

Question 9

What happens to the GI tract/bladder upon sympathetic stimulation?

Answer 9

Stimulation of a_1/2 and B₂ adrenoreceptors induces INHIBITION of GI motility

Activation of a₁-adrenoreceptors evokes contraction of bladder sphincter, B₂-adrenoreceptors induces relaxation of smooth muscle allowing filling.

Bowel/bladder function non-essential during ‘fear-fight-flight’

Question 10

What happens to the eye under sympathetic innervation?

Answer 10

Stim of a₁ on dilator pupillae causes dilation of pupil -> more light in retina -> more acute sight

Question 11

The sweat glands are unique in terms of their sympathetic innervation, how is this and what is the action?

Answer 11

Release of ACh acting at MUS receptors (not NA like normal post-ganglionic) -> induces SWEATING - temperature control during FFF