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The autonomic nervous system (ANS) is the involuntary part of the nervous system that regulates the functions of smooth muscles, cardiac muscle, and glands.
Unlike the somatic system (which governs voluntary skeletal muscle activity), the ANS functions automatically and subconsciously, maintaining the body’s internal environment in a state of dynamic balance — homeostasis.
The ANS is divided into two complementary divisions:
Sympathetic (thoracolumbar) division
Parasympathetic (craniosacral) division
Both divisions work in opposition but often in coordinated balance — one stimulating, the other inhibiting — to maintain internal stability.
Each visceral efferent pathway involves two neurons:
Preganglionic neuron: Cell body lies in the central nervous system (CNS) — brainstem or spinal cord.
Postganglionic neuron: Cell body lies in an autonomic ganglion outside the CNS; its axon ends in the target organ (gland, smooth or cardiac muscle).
Heart: Regulates rate and force of contraction.
Lungs: Controls bronchial diameter.
Gastrointestinal tract: Modulates motility and secretions.
Pupil: Controls constriction and dilation.
Blood vessels: Governs vasoconstriction and vasodilation.
Glands: Stimulates or inhibits secretion depending on the division activated.
Also known as the thoracolumbar outflow, the sympathetic system prepares the body for “fight, flight, or fright” reactions.
Its main function is to mobilize energy resources and increase alertness in emergencies or stress.
Preganglionic neurons arise from the lateral horn (intermediolateral column) of the spinal cord segments T1 to L2.
Axons leave the spinal cord through the anterior roots, join the spinal nerve, and pass into the white rami communicantes to reach the sympathetic chain (paravertebral ganglia).
Sympathetic Chain (Paravertebral Ganglia):
A paired vertical chain of ganglia on each side of the vertebral column, extending from the base of skull to coccyx.
The two chains join in front of the coccyx to form the ganglion impar.
Each chain has 3 cervical, 11 thoracic, 4 lumbar, 4 sacral, and 1 coccygeal ganglion (variations may occur).
Connections to Spinal Nerves:
White rami communicantes: Carry preganglionic fibers from spinal nerves T1–L2 into the sympathetic chain.
Gray rami communicantes: Carry postganglionic fibers from the sympathetic chain back to all spinal nerves for distribution to blood vessels, sweat glands, and arrector pili muscles.
Distribution of Sympathetic Fibers:
Somatic distribution: To skin, blood vessels, and sweat glands of body wall and limbs.
Visceral distribution: To thoracic, abdominal, and pelvic organs through splanchnic nerves.
A preganglionic fiber entering the sympathetic chain may:
Synapse in the ganglion at the same level, and the postganglionic fiber exits via a gray ramus to a spinal nerve.
Ascend or descend to synapse in a higher or lower ganglion within the chain.
Pass through the chain without synapsing, forming splanchnic nerves, which synapse in prevertebral (collateral) ganglia such as:
Celiac ganglion
Superior mesenteric ganglion
Inferior mesenteric ganglion
These postganglionic fibers then supply abdominal and pelvic viscera.
| Target Organ | Spinal Origin | Effect |
|---|---|---|
| Pupil (via superior cervical ganglion) | T1 | Dilatation (mydriasis) |
| Heart | T1–T5 | Increases rate and force |
| Lungs | T2–T5 | Bronchodilation |
| Blood vessels (skin, skeletal muscle) | T1–L2 | Vasoconstriction or vasodilation |
| Sweat glands | T1–L2 | Secretion (only sympathetic fibers are cholinergic) |
| Abdominal viscera | T6–L2 | Inhibits motility, contracts sphincters |
| Adrenal medulla | T8–L1 | Secretes adrenaline and noradrenaline |
Preganglionic fibers: Release acetylcholine (ACh).
Postganglionic fibers: Usually release noradrenaline (norepinephrine) (except to sweat glands and some blood vessels, where ACh is used).
Horner’s Syndrome:
Results from interruption of sympathetic supply to the head and neck.
Features: Ptosis (drooping eyelid), miosis (pupil constriction), anhidrosis (no sweating), and enophthalmos (sunken eyeball).
Common causes: Pancoast tumor, cervical rib, or neck trauma.
Raynaud’s Disease:
Episodic constriction of digital arteries due to excessive sympathetic vasoconstriction → pallor and pain in fingers.
Sympathectomy:
Surgical interruption of sympathetic chain used to treat hyperhidrosis (excessive sweating) or vasospastic disorders.
The thoracic sympathetic trunk is the continuation of the cervical part above and the lumbar part below. It lies in the posterior mediastinum, forming part of the autonomic control pathway for thoracic and abdominal viscera.
Lies along the heads of ribs on each side of the vertebral column.
Extends from the neck of the first rib (continuing from the inferior cervical ganglion) to the body of the last thoracic vertebra (T12).
It passes behind the medial arcuate ligament of the diaphragm to become continuous with the lumbar sympathetic trunk.
Usually 11 thoracic ganglia are present (sometimes 10 due to fusion of the first with the inferior cervical ganglion to form the stellate ganglion).
Each ganglion is connected to the corresponding intercostal nerve by gray rami communicantes.
The upper thoracic ganglia (T1–T5) mainly supply thoracic viscera (heart, lungs).
The lower ganglia (T6–T12) give rise to splanchnic nerves for the abdomen.
Each thoracic ganglion gives off:
Gray rami communicantes – To all thoracic spinal nerves for vasomotor, pilomotor, and sudomotor (sweat gland) fibers.
Visceral branches – To thoracic organs (heart, lungs, oesophagus).
Splanchnic nerves – Preganglionic fibers that pass through the chain to prevertebral ganglia.
Greater Splanchnic Nerve (T5–T9):
Descends obliquely on the vertebral bodies.
Pierces the diaphragm’s crus to end in the celiac ganglion.
Supplies liver, stomach, pancreas, and upper small intestine.
Lesser Splanchnic Nerve (T10–T11):
Ends in the aorticorenal ganglion.
Supplies kidneys and suprarenal glands.
Least (Lowest) Splanchnic Nerve (T12):
Smallest and most variable.
Ends in the renal plexus.
Sympathetic overactivity can cause hypertension or tachycardia.
Thoracic sympathectomy (T2–T4) is performed to reduce excessive sweating or treat vasospastic disorders like Raynaud’s disease.
Pancoast tumor at the lung apex can involve the stellate ganglion, leading to Horner’s syndrome.
The heart receives autonomic innervation from both sympathetic and parasympathetic systems, which form the cardiac plexuses — regulating heart rate, force, and conduction.
Preganglionic fibers: Originate from the lateral horn of T1–T5 spinal segments.
They synapse in the cervical and upper thoracic sympathetic ganglia.
Postganglionic fibers form the superior, middle, and inferior cardiac nerves, which join the cardiac plexus.
Effects:
Increases heart rate (positive chronotropy).
Increases force of contraction (positive inotropy).
Dilates coronary vessels (via metabolic autoregulation).
Carries pain fibers, explaining referred pain to the left arm and chest in angina.
From the vagus nerve (cranial nerve X).
Preganglionic fibers arise from the dorsal motor nucleus of the vagus.
They form cardiac branches that join the cardiac plexuses and synapse in intramural ganglia within the heart wall.
Effects:
Decreases heart rate (negative chronotropy).
Reduces force of contraction (negative inotropy).
Causes vasoconstriction of coronary arteries.
There are two interconnected cardiac plexuses:
Superficial Cardiac Plexus:
Lies below the aortic arch, in front of the right pulmonary artery.
Formed by the superior cervical sympathetic cardiac branch and the lower cervical vagal branch.
Deep Cardiac Plexus:
Lies in front of the tracheal bifurcation.
Receives fibers from both vagi and sympathetic trunks.
Sends branches to coronary plexuses, atria, and ventricles.
Referred cardiac pain: Travels via sympathetic afferents (T1–T5 dermatomes) to the left chest and arm.
Vagal stimulation: Causes bradycardia or heart block.
Sympathetic overactivity: Causes tachycardia and arrhythmias.
Stellate ganglion block: Used therapeutically to reduce refractory angina by lowering sympathetic tone.
The lungs are richly innervated by autonomic fibers that regulate bronchial tone, vascular flow, and secretions.
Two plexuses are formed by mixed sympathetic and parasympathetic fibers:
Anterior pulmonary plexus — lies in front of the root of the lung.
Posterior pulmonary plexus — lies behind the root of the lung.
Both plexuses distribute fibers along the bronchi and pulmonary vessels into the lung substance.
Origin: From T2–T5 thoracic ganglia.
Pathway: Postganglionic fibers reach the pulmonary plexuses.
Effects:
Bronchodilation (relaxation of bronchial muscles).
Vasoconstriction of pulmonary vessels.
Decreased mucus secretion from bronchial glands.
Derived from the vagus nerve.
Effects:
Bronchoconstriction.
Vasodilation of pulmonary vessels.
Increased mucus secretion from glands in the bronchial wall.
From both vagus and sympathetic nerves.
Vagal afferents mediate cough reflex and stretch sensation.
Sympathetic afferents carry pain fibers from bronchi and lungs.
Asthma: Results from overactivity of parasympathetic system → bronchospasm and mucus hypersecretion.
Bronchodilators (β₂ agonists): Mimic sympathetic stimulation.
Vagal stimulation: Can induce cough and bronchoconstriction.
Sympathectomy: May relieve certain cases of bronchial spasm.
Definition:
A typical intercostal nerve refers to the 3rd to 6th thoracic spinal nerves, which run entirely within the intercostal spaces and supply the thoracic wall.
Origin:
Each arises from the anterior (ventral) ramus of the corresponding thoracic spinal nerve after the dorsal ramus is given off.
Course:
Enters the posterior part of the intercostal space, passing behind the posterior intercostal vessels.
Lies in the neurovascular bundle along the costal groove in the order: Vein–Artery–Nerve (VAN) from above downward.
Initially lies between posterior intercostal membrane and subcostalis, then between internal and innermost intercostal muscles.
Near the sternum, it passes in front of internal thoracic vessels, pierces the intercostal muscles and anterior intercostal membrane, and ends as the anterior cutaneous branch.
Branches:
Communicating branches – Connect to sympathetic ganglia through white and gray rami communicantes.
Collateral branch – Runs along the upper border of the rib below, supplying intercostal muscles, pleura, and periosteum.
Lateral cutaneous branch – Arises near the midaxillary line; divides into anterior and posterior branches to supply thoracic wall skin.
Muscular branches – To intercostal, subcostalis, and transversus thoracis muscles.
Anterior cutaneous branch – Terminal branch near the sternum; divides into medial and lateral divisions to supply anterior thoracic skin.
Definition:
Intercostal nerves that deviate from the typical pattern in distribution or course.
Examples:
First intercostal nerve:
Joins the brachial plexus completely; gives no branches to the first intercostal space.
Skin of this space is supplied by supraclavicular nerves (C3–C4).
Second intercostal nerve:
Gives a large lateral cutaneous branch — the intercostobrachial nerve, which supplies the skin of axilla and medial arm.
7th–11th intercostal nerves:
Extend beyond the thoracic wall to supply abdominal wall muscles and overlying skin; also provide branches to parietal peritoneum.
12th thoracic nerve (subcostal nerve):
Runs below the 12th rib, supplies anterolateral abdominal wall and buttock skin.
The main arteries of the thorax include:
Internal Thoracic Artery:
Origin: 1st part of the subclavian artery.
Course: Descends behind the costal cartilages about 1 cm from the sternum.
Termination: Divides in the 6th intercostal space into superior epigastric and musculophrenic arteries.
Distribution: Anterior thoracic wall, mammary gland, diaphragm, and anterior abdominal wall.
Pericardiacophrenic Artery:
Branch of internal thoracic artery; accompanies the phrenic nerve.
Supplies the pericardium and diaphragm.
Mediastinal Arteries:
Small branches from internal thoracic artery; supply thymus and mediastinal fat.
Posterior Intercostal Arteries:
1st and 2nd arise from superior intercostal artery (a branch of costocervical trunk).
3rd to 11th arise from descending thoracic aorta.
Run in costal grooves with veins and nerves (VAN), supplying intercostal muscles, skin, and spinal cord branches.
Subcostal Artery:
Lowest pair from the thoracic aorta below the 12th ribs; continue into abdominal wall.
Site of Pericardial Tapping (Pericardiocentesis):
Performed in the left 4th or 5th intercostal space just next to the sternum, where pleura deviates laterally, exposing pericardium.
Alternate approach: Left xiphicostal angle, directing needle upward and backward toward pericardial cavity.
Precaution: Avoid injuring the internal thoracic artery, which lies about 1 cm lateral to the sternum.
Foreign Bodies in Trachea:
Commonly enter the right main bronchus due to its wider, shorter, and more vertical course.
Typically settle in posterior basal segments of the right lower lobe.
Site of Bone Marrow Puncture:
Manubrium sterni is preferred for sternal marrow aspiration due to accessibility and thickness.
The intercostal nerves and thoracic arteries together form the vital neurovascular network of the chest wall, ensuring effective motor, sensory, and vascular supply for respiration and protection — while the clinical applications highlight the precision required in thoracic procedures.
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