Autonomic Nervous System: Structure, Function, and Divisions

Autonomic Nervous System (ANS)

Overview

The autonomic nervous system (ANS) is a division of the peripheral nervous system that controls involuntary body functions. It consists of motor neurons that innervate smooth muscle, cardiac muscle, and glands, making adjustments to ensure optimal support for body activities. The ANS operates largely via subconscious control and is also known as the involuntary nervous system or general visceral motor system.

• Effectors: Smooth muscle, cardiac muscle, glands

• Functions: Regulates heart rate, blood pressure, digestive processes, and more

• Control: Subconscious (involuntary)

Development of the ANS

Embryological Origins

• Preganglionic neurons derive from the neural tube (like somatic motor neurons).

• ANS structures in the PNS (postganglionic neurons, adrenal medulla, ganglia) derive from the neural crest.

• Nerve growth factor and signaling chemicals guide axons to their target organs.

• In youth, ANS impairments are usually due to injury.

Structural Organization of the Nervous System

Placement of the ANS

The nervous system is divided into the central nervous system (CNS) and peripheral nervous system (PNS). The PNS is further divided into sensory (afferent) and motor (efferent) divisions. The motor division includes the somatic nervous system (controls skeletal muscle) and the autonomic nervous system (controls smooth/cardiac muscle and glands). The ANS itself is divided into sympathetic and parasympathetic divisions.

ANS Versus Somatic Nervous System

Key Differences

• Effectors: Somatic NS innervates skeletal muscles; ANS innervates cardiac muscle, smooth muscle, and glands.

• Efferent Pathways and Ganglia:

  • Somatic NS: Single, thick, myelinated axon from CNS to muscle.

  • ANS: Two-neuron chain:

    1. Preganglionic neuron: Cell body in CNS, thin, lightly myelinated axon to ganglion.

    2. Postganglionic neuron: Cell body in autonomic ganglion (outside CNS), nonmyelinated axon to effector organ.

• Neurotransmitter Effects:

  • Somatic NS: All motor neurons release acetylcholine (ACh); effect is always stimulatory.

  • ANS: Preganglionic fibers release ACh; postganglionic fibers release norepinephrine (NE) or ACh; effect can be stimulatory or inhibitory depending on receptor type.

Overlap of Somatic and Autonomic Function

Higher brain centers regulate and coordinate both systems. Most spinal and many cranial nerves contain both somatic and autonomic fibers. Adaptations often involve both skeletal muscles and visceral organs (e.g., increased heart rate and open airways during exercise).

Comparison Table: Somatic vs. Autonomic Motor Neurons

Feature	Somatic Nervous System	Autonomic Nervous System
Number of Neurons	One (from CNS to effector)	Two (preganglionic and postganglionic)
Effector Organs	Skeletal muscle	Smooth muscle, cardiac muscle, glands
Neurotransmitter	Acetylcholine (ACh)	ACh (preganglionic), ACh or NE (postganglionic)
Effect	Always stimulatory	Stimulatory or inhibitory

Divisions of the Autonomic Nervous System

Parasympathetic vs. Sympathetic

• Parasympathetic division: Promotes maintenance functions, conserves energy ("rest-and-digest").

• Sympathetic division: Mobilizes body during activity ("fight-or-flight").

• Dual innervation: Most visceral organs are served by both divisions, which usually have opposite effects. Dynamic antagonism maintains homeostasis.

Role of the Parasympathetic Division

• Keeps body energy use low while carrying out maintenance activities (digestion, diuresis, defecation).

• Referred to as the "rest-and-digest" system.

• Example: After a meal, blood pressure, heart rate, and respiratory rates are low; gastrointestinal activity is high; pupils constricted, lenses accommodate for close vision.

Role of the Sympathetic Division

• Mobilizes the body during activity ("fight-or-flight").

• Activated by exercise, excitement, emergency, embarrassment.

• Effects: Increased heart rate, dry mouth, cold/sweaty skin, dilated pupils.

• During vigorous activity: Shunts blood to skeletal muscles and heart, dilates bronchioles, causes liver to release glucose.

Key Anatomical Differences Between Divisions

• Sites of Origin:

  • Parasympathetic: Craniosacral (brain and sacral spinal cord)

  • Sympathetic: Thoracolumbar (thoracic and lumbar spinal cord)

• Relative Lengths of Fibers:

  • Parasympathetic: Long preganglionic, short postganglionic

  • Sympathetic: Short preganglionic, long postganglionic

• Location of Ganglia:

  • Parasympathetic: In or near visceral effector organs

  • Sympathetic: Close to spinal cord

Table: Anatomical and Physiological Differences

Characteristic	Parasympathetic	Sympathetic
Origin	Craniosacral (brain stem, sacral spinal cord)	Thoracolumbar (lateral horns of thoracic/lumbar spinal cord)
Location of Ganglia	Within/near effector organ	Close to spinal cord
Fiber Lengths	Long preganglionic, short postganglionic	Short preganglionic, long postganglionic
Branching of Preganglionic Fibers	Minimal	Extensive
Neurotransmitters	All pre- and postganglionic fibers release ACh	Preganglionic: ACh; Postganglionic: NE (most), ACh (sweat glands)
Functional Role	Maintenance, rest-and-digest	Fight-or-flight

Parasympathetic Division

Craniosacral Division

• Fibers originate from brain stem and sacral regions of spinal cord.

• Long preganglionic fibers extend from CNS almost to target organs, synapsing in terminal ganglia close to or within target organs.

• Short postganglionic fibers synapse with effectors.

Cranial Part of Parasympathetic Division

• Oculomotor nerves (III): Control smooth muscle of eye (pupil constriction, lens bulging); postganglionic cell bodies in ciliary ganglia.

• Facial nerves (VII): Stimulate large glands in head; fibers synapse in pterygopalatine (nasal/lacrimal) and submandibular (salivary) ganglia.

• Glossopharyngeal nerves (IX): Stimulate parotid salivary glands; fibers synapse in otic ganglia.

• Vagus nerves (X): ~90% of all preganglionic parasympathetic fibers; serve thoracic and abdominal viscera; synapse in intramural ganglia in walls of target organs.

  • Cardiac plexus: Slows heart rate

  • Pulmonary plexus: Serves lungs and bronchi

  • Esophageal plexus: Forms vagal trunks to stomach, liver, gallbladder, pancreas, intestines

Sacral Part of Parasympathetic Division

• Originates from neurons in S2–S4; serves pelvic organs and distal half of large intestine.

• Axons travel in ventral root of spinal nerves, branch to form pelvic splanchnic nerves.

• Synapse with ganglia in pelvic floor (inferior hypogastric plexus) or intramural ganglia in walls of target organs (distal large intestine, bladder, ureters, reproductive organs).

Sympathetic Division

Thoracolumbar Division

• Preganglionic neurons in spinal cord segments T1–L2; form lateral horns of spinal cord.

• More complex, innervates more organs than parasympathetic division.

• Some structures (sweat glands, arrector pili, blood vessel smooth muscle) are innervated only by sympathetic fibers.

Sympathetic Trunk and Ganglia

• Preganglionic fibers pass through white rami communicantes to enter the sympathetic trunk (chain or paravertebral) ganglia.

• There are 23 paravertebral ganglia: 3 cervical, 11 thoracic, 4 lumbar, 4 sacral, 1 coccygeal.

• Upon entering the trunk, preganglionic fibers may:

  1. Synapse in trunk ganglia (same, higher, or lower level)

  2. Pass through trunk to synapse in collateral ganglia (anterior to vertebral column, only in abdomen/pelvis)

Summary Table: Autonomic Ganglia

Name	Division	Location
Terminal ganglia	Parasympathetic	Within wall of organ served or close to organ
Sympathetic trunk ganglia	Sympathetic	Paired, beside spinal cord
Collateral (prevertebral) ganglia	Sympathetic	Unpaired, anterior to spinal cord

Sympathetic Pathways

• Gray rami communicantes: Nonmyelinated postganglionic fibers

• White rami communicantes: Myelinated preganglionic fibers

• These fibers innervate sweat glands, arrector pili muscles, and vascular smooth muscle via pathways to the head and thorax.

Three Pathways of Sympathetic Innervation

1. Synapse in trunk ganglion at same level

2. Synapse in trunk ganglion at higher or lower level

3. Pass through trunk to synapse in collateral ganglion (e.g., splanchnic nerves to abdominal organs)

Pathways with Synapses in Trunk Ganglia

• To the head: Fibers from T1–T4 synapse in superior cervical ganglion; innervate skin, blood vessels, dilator muscles of iris, inhibit nasal/salivary glands, innervate upper eyelid, branch to heart.

• To the thorax: Preganglionic fibers from T1–T6 synapse in cervical trunk ganglia; postganglionic fibers enter nerves C4–C8; innervate heart (cardiac plexus), thyroid, skin, lungs, esophagus.

Pathways with Synapses in Collateral Ganglia

• Most fibers from T5–L2 synapse in collateral ganglia outside trunk, forming splanchnic nerves (greater, lesser, least thoracic; lumbar; sacral).

• Splanchnic nerves form abdominal aortic plexuses containing celiac and superior/inferior mesenteric ganglia; postganglionic fibers travel to abdomen and pelvis.

Example: Pathways to the Abdomen

• Preganglionic fibers from T5–L2 travel through thoracic splanchnic nerves, synapse in celiac and superior mesenteric ganglia.

• Postganglionic fibers serve stomach, intestines, liver, spleen, kidneys.

Example: Pathways to the Pelvis

• Preganglionic fibers from T10–L2 travel to lumbar and sacral ganglia, synapse with postganglionic fibers that run in lumbar and sacral splanchnic nerves or pass directly to plexuses in collateral ganglia.

• Postganglionic fibers serve distal large intestine, bladder, reproductive organs.