How does the liver respond to sympathetic nervous system arousal?

During sympathetic nervous system arousal, often associated with the body's "fight or flight" response, the liver undergoes specific physiological changes. These changes are primarily mediated by the release of norepinephrine (noradrenaline) from sympathetic nerve endings innervating the liver. Here's how the liver responds to sympathetic nervous system arousal:

1. Glycogenolysis:

- Sympathetic stimulation triggers the activation of the enzyme glycogen phosphorylase, which breaks down glycogen, the storage form of glucose in the liver, into glucose-1-phosphate.

- This action increases the concentration of glucose-1-phosphate, which is further converted into glucose-6-phosphate (G6P) and subsequently glucose for release into the bloodstream.

2. Gluconeogenesis:

- The sympathetic nervous system also promotes gluconeogenesis, the process of generating glucose from non-carbohydrate sources.

- During sympathetic arousal, gluconeogenic precursors such as lactate, pyruvate, and amino acids are mobilized from peripheral tissues and taken up by the liver. These precursors are converted into glucose through various biochemical pathways.

3. Inhibition of Glycogenesis:

- Glycogenesis, the process of synthesizing glycogen from glucose, is inhibited during sympathetic arousal.

- Phosphorylation and inactivation of enzymes involved in glycogenesis, such as glycogen synthase, prevent the liver from replenishing its glycogen stores.

4. Lipolysis:

- Sympathetic nerve activity stimulates lipolysis, the breakdown of stored triglycerides in the liver and adipose tissue.

- This results in the release of free fatty acids (FFAs) into the bloodstream, providing an alternative energy source for the body during heightened activity.

5. Vasoconstriction:

- The sympathetic nervous system also causes vasoconstriction of hepatic blood vessels, reducing blood flow to the liver.

- This diversion of blood flow allows more blood to be directed to essential organs and muscles involved in the body's immediate responses to stress.

6. Reduced Bile Production:

- Sympathetic stimulation can temporarily reduce bile production and impair gallbladder contraction, affecting the digestion and absorption of dietary fats.

In summary, sympathetic nervous system arousal induces glycogenolysis, gluconeogenesis, inhibition of glycogenesis, lipolysis, vasoconstriction, and reduced bile production in the liver, ensuring a coordinated physiological response to meet the body's energy demands and prioritize survival during challenging situations.