General characteristics of the autonomic nervous system
The autonomic nervous system (ANS) is a set of central and peripheral neural structures that ensure the coordinated activity of internal organs. The main effector organs of the ANS are visceral organs, exocrine glands, the heart, vascular smooth muscle, and the smooth muscle of internal organs.
The following divisions are distinguished:
- sympathetic division;
- parasympathetic division;
- metasympathetic division (distinguished in domestic literature).
The sympathetic and parasympathetic divisions are functional antagonists: they exert opposite effects on organs, which ensures fine regulation of autonomic functions.
Two-neuron pathway of autonomic innervation
The pathway from the ANS center to the effector organ always consists of two sequentially connected neurons:
- Preganglionic neuron — the cell body lies in the CNS (spinal cord or brainstem); its axon is called the preganglionic fiber and travels to a ganglion.
- Postganglionic neuron — the cell body is located in an autonomic ganglion; its axon, the postganglionic fiber, goes directly to the effector organ.
A synapse between the preganglionic fiber and the body of the second neuron takes place in the ganglion.
Sympathetic division
The cell bodies of preganglionic sympathetic neurons are localized in the lateral horns of the spinal cord (thoracolumbar segments). Preganglionic fibers are short, run to ganglia, and synapse:
- in the sympathetic trunk (truncus sympathicus) — for the innervation of the heart, bronchi, and head organs (including the eyes);
- in prevertebral ganglia — the celiac, superior mesenteric, aortico-renal, inferior mesenteric, and hypogastric ganglia — predominantly for organs below the diaphragm.
The postganglionic fiber is long; it uses an artery as a conduit to the organ, wrapping around it in a plexus.
Parasympathetic division
The cell bodies of preganglionic parasympathetic neurons are located in two zones:
- autonomic nuclei of the cranial nerves III, VII, IX, and X, lying in the brainstem;
- lateral horns of the sacral segments of the spinal cord (S2–S4) — for pelvic organs (urinary bladder, reproductive system, distal bowel).
Preganglionic fibers are long; ganglia are located close to the organ or intramurally (within the organ wall). Therefore, postganglionic fibers are short and do not require a vascular conduit.
Neurotransmitters and Receptors
Signal transmission in the ANS is carried out via two neurotransmitter systems:
- Cholinergic — neurotransmitter acetylcholine, acts on cholinergic receptors (N-nicotinic and M-muscarinic).
- Adrenergic — neurotransmitters norepinephrine and epinephrine, act on α1, α2, β1, β2, and β3 adrenergic receptors.
Distribution of neurotransmitters:
- In the ganglionic synapse of both divisions, acetylcholine is released (acting on N-cholinergic receptors).
- The postganglionic fiber of the sympathetic division releases norepinephrine (epinephrine) acting on the organ's adrenergic receptors.
- The postganglionic fiber of the parasympathetic division releases acetylcholine acting on the organ's M-cholinergic receptors.
Main effects on target organs
Heart. The sympathetic division via β1-adrenergic receptors increases heart rate, contractility, and conductivity. The parasympathetic division via M-cholinergic receptors decreases these parameters.
Bronchi. The sympathetic division via β2-adrenergic receptors relaxes bronchial smooth muscle (bronchodilation) and inhibits glandular secretion. The parasympathetic division via M-cholinergic receptors causes bronchoconstriction and increased secretion.
Gastrointestinal tract. The sympathetic division inhibits motility and glandular secretion. The parasympathetic division enhances motility and the secretion of digestive glands.
Pupil. The sympathetic division via α1-adrenergic receptors of the iris radial muscle causes mydriasis (pupillary dilation). The parasympathetic division via M-cholinergic receptors of the iris circular muscle causes miosis (pupillary constriction).
Highest center of autonomic regulation
Above the segmental centers of the sympathetic and parasympathetic divisions stands the hypothalamus — the highest subcortical center of autonomic regulation. It conventionally comprises:
- anterior nuclei — integrative centers of the parasympathetic division;
- posterior nuclei — integrative centers of the sympathetic division;
- middle nuclei — primarily regulation of metabolism and endocrine functions.
The hypothalamus coordinates the work of both ANS divisions, aligning it with endocrine regulation and behavioral reactions.
Key points
- The ANS innervates visceral organs, glands, the heart, and smooth muscle; it has sympathetic and parasympathetic divisions, which are functional antagonists.
- The efferent pathway of the ANS is always two-neuron: a preganglionic neuron in the CNS and a postganglionic neuron in a ganglion.
- Sympathetic centers are located in the lateral horns of the thoracolumbar segments of the spinal cord; ganglia are the sympathetic trunk and prevertebral plexuses.
- Parasympathetic centers are the autonomic nuclei of cranial nerve pairs III, VII, IX, X, and the lateral horns of segments S2–S4.
- In the ganglia of both divisions, the neurotransmitter is acetylcholine (N-cholinergic receptors).
- At the effector, the sympathetic division releases norepinephrine/epinephrine (adrenergic receptors), and the parasympathetic division releases acetylcholine (M-cholinergic receptors).
- The sympathetic division increases heart activity, dilates bronchi and pupils, and inhibits the GI tract; the parasympathetic division produces opposite effects.
- The highest center of autonomic regulation is the hypothalamus: anterior nuclei — parasympathetic, posterior — sympathetic.