Autonomic Nervous System: Structure, Function, and Integration

Autonomic Nervous System (ANS)

Overview

The autonomic nervous system (ANS) regulates involuntary (unconscious) body functions, including the control of cardiac muscle, smooth muscle, glands, and fat cells. It is anatomically and functionally distinct from the somatic nervous system (SNS), which controls voluntary skeletal muscle movements.

• Somatic Nervous System (SNS): Uses one motor neuron from the CNS to skeletal muscle.

• Autonomic Nervous System (ANS): Uses two motor neurons from the CNS to the effector:

  • Preganglionic neuron: Synapses in a ganglion.

  • Postganglionic neuron: Carries signal to the target organ.

Somatic and Autonomic Nervous Systems

Comparison

• SNS: Direct, single-neuron pathway to skeletal muscle.

• ANS: Two-neuron chain (preganglionic and postganglionic) to smooth/cardiac muscle, glands, and fat.

Divisions of the ANS

Sympathetic Division

• Preganglionic neurons originate in the thoracic and lumbar spinal cord.

• Ganglia are near the spinal cord.

• Short preganglionic fibers, long postganglionic fibers.

Parasympathetic Division

• Preganglionic neurons originate in the brainstem and sacral spinal cord.

• Ganglia are near or within target organs.

• Long preganglionic fibers, short postganglionic fibers.

Neurotransmitters at Specific Synapses

• All preganglionic fibers: Cholinergic (release acetylcholine, ACh); always excitatory on postganglionic neuron.

• Parasympathetic postganglionic fibers: Cholinergic (release ACh); effects can be excitatory or inhibitory depending on the target cell receptor.

• Sympathetic postganglionic fibers: Mostly adrenergic (release norepinephrine, NE); effects are usually excitatory.

Neurotransmitter Receptors

• Neurotransmitters act by binding to specific receptors on target cells.

• Receptor type determines the effect (not just the neurotransmitter itself).

• The same neurotransmitter can have different effects in different tissues (e.g., excitatory vs. inhibitory).

Same Neurotransmitter, Different Effects

• Norepinephrine (NE):

  • Heart: Increases heart rate and force of contraction.

  • Blood vessels: Causes vasoconstriction, increasing blood pressure.

  • Digestive tract: Decreases motility and secretions.

• Reason: NE binds to different receptor types in different organs.

Sympathetic Pathways

• Arise from spinal cord segments T1–L2.

• Preganglionic fibers enter the sympathetic chain ganglia (just outside the spinal column).

• Fibers may synapse in the chain or pass through to collateral ganglia (anterior to the spinal cord).

• Postganglionic fibers innervate abdominopelvic organs.

Adrenal Medullae: A Special Sympathetic Pathway

• Located in the center of the adrenal glands.

• Innervated by preganglionic sympathetic fibers.

• Function as modified sympathetic ganglia.

• Secrete norepinephrine and epinephrine into the bloodstream.

• Act as an endocrine organ, mimicking sympathetic nervous system stimulation.

Sympathetic Division Effects

• Known as the "fight-or-flight" division.

• Major effects:

  • Increased alertness and metabolic rate

  • Increased heart rate and blood pressure

  • Increased blood flow to skeletal muscles

  • Bronchodilation (airways open)

  • Pupil dilation

  • Increased sweating

  • Decreased blood flow to digestive organs

• Hormonal support: Epinephrine and norepinephrine from adrenal medulla prolong and amplify effects.

Parasympathetic Division Organization

• Preganglionic neurons arise from the brainstem and sacral spinal cord.

• Preganglionic fibers travel in cranial nerves (primarily CN X, vagus nerve) and sacral spinal nerves.

• Ganglia located near or within target organs.

• Long preganglionic fibers, short postganglionic fibers.

• Effects are localized and specific to target organs.

Parasympathetic Division Functions

• Known as the "rest-and-digest" division.

• Less divergence than in the sympathetic division; effects are more localized and specific.

• Major effects:

  • Constricts pupils

  • Increases digestive secretions and digestive tract smooth muscle activity

  • Stimulates urination and defecation

  • Constricts bronchioles

  • Decreases heart rate

Relationship Between Sympathetic and Parasympathetic Nervous Systems

• Most organs have dual innervation (input from both divisions).

• Allows fine control of organ function.

• Effects are often antagonistic (one increases activity, the other decreases it).

• Some organs receive input from only one division (e.g., sweat glands, most blood vessels, adrenal glands, arrector pili muscles).

Aging and the Nervous System

• Age-related changes begin by age 30 and accumulate over time.

• May include:

  • Reduction in brain size and weight

  • Reduction in number of neurons

  • Decrease in blood flow to the brain

  • Change in synaptic organization of the brain

  • Increase in intracellular deposits and extracellular plaques

Nervous System and Other Systems

• Muscular System: Motor neurons stimulate skeletal, smooth, and cardiac muscle; allows movement, posture, and organ function.

• Cardiovascular System: ANS regulates heart rate and blood vessel diameter; maintains blood pressure and tissue perfusion.

• Respiratory System: Brainstem controls breathing rate and rhythm; ANS adjusts airway diameter.

• Digestive System: Parasympathetic activity promotes digestion; sympathetic activity inhibits digestion during stress.

• Endocrine System: Nervous system triggers hormone release (e.g., adrenal medulla).

Table: Comparison of Sympathetic and Parasympathetic Divisions

Feature	Sympathetic Division	Parasympathetic Division
Origin	Thoracic & lumbar spinal cord	Brainstem & sacral spinal cord
Ganglia Location	Near spinal cord	Near or within target organs
Preganglionic Fiber Length	Short	Long
Postganglionic Fiber Length	Long	Short
Main Neurotransmitter (Postganglionic)	Norepinephrine (adrenergic)	Acetylcholine (cholinergic)
General Function	"Fight-or-flight"	"Rest-and-digest"