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Effects of the Autonomic Nervous System on Organs and Tissues

Q: What effects are caused by alpha-1 adrenergic receptor activation?

Activation of $\alpha_1$-adrenergic receptors causes: Stomach and intestines — decreased contraction and wall tone, alongside sphincter contraction.Uterus — effect depends on the reproductive cycle phase (pregnancy/labor).Liver — glycogenolysis (breakdown of glycogen).Blood vessels — vasoconstriction.

Systema nervosum autonomicum

For medical students2 min readUpdated 2026-10-10

The autonomic nervous system regulates the functions of internal organs, adapting them to the body's current physiological needs. The specific effect depends on the branch (sympathetic or parasympathetic) and the type of activated receptors in the target tissue.

CholinergicM- and N-cholinergic receptors

Adrenergicα- and β-adrenergic receptors

EyeMiosis (M-receptors) and mydriasis (α-receptors)

BronchiBronchospasm (M) and bronchodilation (β₂)

Effects on the Cardiovascular System

The cardiovascular system is under continuous dual control.

Parasympathetic nervous system (PNS) acts via M-cholinergic receptors. It causes a decrease in heart rate (effect on the sinuatrial node), decreased atrial contractility, and slowed conduction, potentially leading to atrioventricular block.
Sympathetic nervous system (SNS) operates via adrenergic receptors. Activation of $\beta_1$-receptors leads to increased heart rate, conduction velocity, and myocardial contractility.

Blood vessels also respond differently. M-receptors mediate vasodilation of certain arterioles. Adrenergic influences are more complex: $\alpha$-receptors cause vasoconstriction (e.g., in skin, mucosa, brain, salivary glands, and veins), while $\beta_2$-receptors induce vasodilation in specific arterioles and veins.

Respiratory and Digestive Systems

Bronchial smooth muscle contracts (bronchospasm) upon M-receptor activation and relaxes (bronchodilation) upon $\beta_2$-receptor stimulation. Bronchial gland secretion is stimulated by the PNS and inhibited by the SNS.

In the gastrointestinal tract, the PNS (via M-receptors) increases motility (contraction and tone of the stomach and intestinal walls), relaxes sphincters, and stimulates secretion. Conversely, the SNS decreases wall tone ($\alpha_1, \beta_2$), contracts sphincters ($\alpha_1$), and inhibits secretion ($\beta_2$). Similarly, the PNS causes gallbladder contraction for bile release, whereas the SNS causes relaxation.

Urinary and Reproductive Systems

Micturition is coordinated by both systems:

PNS (M-receptors): contracts the detrusor muscle of the urinary bladder for voiding 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 with increased motility and tone to stimulation of both M- and $\alpha$-receptors.

In the reproductive system, the effects of the SNS on the uterus depend on the cycle phase (pregnancy vs. labor). In males, the PNS mediates erection, while 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 exocrine secretion (acinar cells), while the SNS ($\alpha$) decreases it. Insulin and glucagon secretion are also regulated: M-receptors enhance it, whereas $\alpha$-receptors reduce it.
Adrenal medulla: M-receptors stimulate the release of epinephrine and norepinephrine.
Salivary glands: PNS induces profuse watery secretion, while the SNS ($\alpha$) causes scant, viscous secretion.

Mnemonic

PNS is the "rest and digest" system (constricts pupils, slows heart, stimulates GI tract and glands). SNS is the "fight or flight" system (dilates pupils and bronchi, accelerates heart, inhibits digestion).

Frequently asked questions

What effects in organs and tissues are caused by beta-2 adrenergic receptor activation?

Activation of $\beta_2$-adrenergic receptors causes the following effects across various organs and tissues:

Heart (myocardium) — increased contractility and conduction velocity.
Blood vessels (arterioles and veins) — vasodilation and relaxation of vascular smooth muscle.
Lungs (bronchial smooth muscle and glands) — relaxation (bronchodilation) and inhibition of secretion.
Stomach and intestines — decreased wall contraction and tone, inhibition of secretion.
Gallbladder and biliary ducts — relaxation.
Uterus — effect depends on the reproductive cycle phase (pregnancy/labor).
Spleen — contraction or relaxation of the capsule and blood vessels.
Liver — glycogenolysis (breakdown of glycogen).

What effects does the autonomic nervous system exert on ocular structures?

The autonomic nervous system controls intraocular smooth muscles and glands, producing the following effects:

Iris dilator muscle (radial muscle) — contraction (mydriasis) via noradrenergic input ($\alpha$-receptors).
Iris sphincter muscle — contraction (miosis) via parasympathetic (cholinergic, M-receptors) input and relaxation for distance vision via noradrenergic ($\beta$-receptors) input.
Ciliary muscle — contraction for near accommodation via parasympathetic (M-receptors) input, which decreases the diameter of the muscular ring and causes the lens to round up.
Lacrimal glands — secretion via cholinergic stimulation (M-receptors).

What effects are caused by alpha-1 adrenergic receptor activation?

Activation of $\alpha_1$-adrenergic receptors causes:

Stomach and intestines — decreased contraction and wall tone, alongside sphincter contraction.
Uterus — effect depends on the reproductive cycle phase (pregnancy/labor).
Liver — glycogenolysis (breakdown of glycogen).
Blood vessels — vasoconstriction.

Which receptors mediate the effects of the parasympathetic nervous system?

In target tissues, these are M-cholinergic (muscarinic) receptors. In autonomic ganglia, N-cholinergic (nicotinic) receptors are utilized.

How does the sympathetic nervous system affect the bronchi?

Through $\beta_2$-adrenergic receptors, it induces relaxation of bronchial smooth muscle (bronchodilation) and inhibits glandular secretion.

Which system controls sweating?

Sweat glands are primarily stimulated by cholinergic fibers via M-receptors (generalized thermoregulatory secretion). Adrenergic stimulation produces only localized, scant secretion.

Go deeper

Effects of the ANS on ocular structures (iris dilator and sphincter muscles, ciliary muscle)
Renal regulation of renin secretion
Effects of adrenergic stimulation on the pineal gland (melatonin secretion)
Skin response to adrenergic stimulation (arrector pili muscles)