Effects on the heart and blood vessels
The cardiovascular system is under continuous dual control.
- The parasympathetic nervous system (PNS) acts via M-cholinergic receptors. It causes a decrease in heart rate (effect on the sinoatrial node), a reduction in atrial contractility, and slowed conduction, up to atrioventricular block.
- The sympathetic nervous system (SNS) operates via adrenergic receptors. Activation of $\beta_1$-receptors leads to an increased heart rate, as well as increased conduction and myocardial contractility.
Blood vessels also respond differently. M-receptors mediate arteriolar dilation. Adrenergic effects are more complex: $\alpha$-receptors cause constriction (e.g., in the skin, mucous membranes, brain, salivary glands, and veins), while $\beta_2$-receptors cause dilation of certain arterioles and veins.
Respiratory and digestive systems
Bronchial smooth muscle contracts (bronchospasm) upon activation of M-receptors and relaxes (bronchodilation) upon stimulation of $\beta_2$-receptors. Secretion of bronchial glands is enhanced by the PNS and inhibited by the SNS.
In the gastrointestinal tract, the PNS (via M-receptors) enhances motility (contraction and tone of the stomach and intestinal walls), relaxes sphincters, and increases secretion. Conversely, the SNS decreases wall tone ($\alpha_1, \beta_2$), contracts sphincters ($\alpha_1$), and inhibits secretion ($\beta_2$). Similarly, the PNS causes contraction of the gallbladder to release bile, while the SNS causes its relaxation.
Urinary and reproductive systems
The act of urination is coordinated by both systems:
- PNS (M-receptors): contracts the detrusor (the muscle that compresses the urinary bladder) to empty it and relaxes the sphincter.
- SNS: relaxes the detrusor via $\beta$-receptors for urine storage and contracts the sphincter via $\alpha$-receptors for continence.
Ureters respond to stimulation of both M-receptors and $\alpha$-receptors with increased motility and tone.
In the reproductive system, the effects of the SNS on the uterus depend on the phase of the cycle (pregnancy or labor). In males, the PNS mediates erection, and the SNS mediates ejaculation.
Metabolism and glands
The autonomic nervous system actively influences metabolism and glandular secretion:
- Liver: SNS ($\alpha_1, \beta_2$) stimulates glycogenolysis.
- Adipose tissue: SNS ($\beta_1$) triggers lipolysis.
- Pancreas: PNS (M) increases secretion of the exocrine portion (acini), while the SNS ($\alpha$) decreases it. Insulin and glucagon secretion are also regulated: M-receptors enhance it, while $\alpha$-receptors reduce it.
- Adrenal glands: M-receptors of the medulla stimulate the release of adrenaline and noradrenaline.
- Salivary glands: PNS causes copious watery secretion, while the SNS ($\alpha$) causes scanty, viscous secretion.
Briefly: answers to topical questions
What effects in organs and tissues are caused by the activation of beta-2 adrenergic receptors?
Activation of $\beta_2$-adrenergic receptors causes the following effects in various organs and tissues:
- Heart (myocardium) — increased contractility and conduction.
- Blood vessels (arterioles and veins) — dilation (vasodilation) and relaxation of vascular smooth muscle.
- Lungs (bronchial muscle and glands) — relaxation (bronchodilation) and inhibition of secretion.
- Stomach and intestines — decreased contraction and wall tone, inhibition of secretion.
- Gallbladder and ducts — relaxation.
- Uterus — effect depends on the phase of the reproductive cycle (pregnancy/labor).
- Spleen — contraction or relaxation of the capsule and vessels.
- Liver — glycogenolysis (breakdown of glycogen).
What effects does the autonomic nervous system have on eye structures?
The autonomic nervous system controls the internal smooth muscles and glands of the eye, producing the following effects:
- Radial muscle of the iris — contraction (mydriasis) under noradrenergic influence ($\alpha$-receptors).
- Sphincter of the iris — contraction (miosis) under parasympathetic (cholinergic, M-receptors) influence, and relaxation for distance vision under noradrenergic ($\beta$-receptors) influence.
- Ciliary muscle — contraction for near vision (accommodation) under parasympathetic (M-receptors) influence, which decreases the diameter of the muscular ring and makes the lens more convex.
- Lacrimal glands — secretion upon cholinergic stimulation (M-receptors).
What effects are caused by the activation of alpha-1 adrenergic receptors?
Activation of $\alpha_1$-adrenergic receptors causes:
- Stomach and intestines — decreased contraction and wall tone, as well as sphincter contraction.
- Uterus — effect depends on the phase of the reproductive cycle (pregnancy/labor).
- Liver — glycogenolysis (breakdown of glycogen).
- Blood vessels — vasoconstriction (narrowing).
The PNS is the "rest and digest" system (constricts the pupil, slows the heart, stimulates the GI tract and glands). The SNS is the "fight or flight" system (dilates the pupil and bronchi, accelerates the heart, inhibits digestion).
Class and exam questions
Which receptors mediate the effects of the parasympathetic nervous system?
In target tissues, these are M-cholinergic receptors. In the ganglia of the autonomic nervous system, N-cholinergic receptors operate.
How does the sympathetic nervous system affect the bronchi?
Via $\beta_2$-adrenergic receptors, it causes relaxation of the bronchial smooth muscle (bronchodilation) and inhibits glandular secretion.
Which system controls sweating?
Sweat glands are stimulated predominantly by cholinergic fibers via M-receptors (generalized secretion). Adrenergic stimulation produces only weak local secretion.
What else to study on this topic
- Effect of the ANS on eye structures (radial and sphincter muscles of the iris, ciliary muscle)
- Regulation of renal renin secretion
- Effect of adrenergic stimulation on the pineal gland (melatonin secretion)
- Skin reaction to adrenergic stimulation (pili-motor muscles)
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