The Autonomic Nervous System | Part 2: Divisions of the ANS

Divisions of the ANS

The ANS has two divisions: the sympathetic ("fight or flight") and parasympathetic ("rest and digest") systems.

Dual Innervation

Most visceral organs receive signals from both sympathetic and parasympathetic divisions.

Parasympathetic Division Function

Conserves body energy and maintains normal body functions at rest.

Parasympathetic Nickname

"Rest and Digest" system.

Sympathetic Division Function

Prepares the body for stressful or emergency situations.

Sympathetic Nickname

"Fight or Flight" system.

Parasympathetic Division Activities

Called the “D activities”: digestion, defecation, and diuresis.

Sympathetic Division Activities

Called the “E activities”: exercise, excitement, emergency, and embarrassment.

Parasympathetic Example

Active after eating a meal: promotes digestion, decreases heart rate, conserves energy.

Sympathetic Example

Activated when threatened or exercising: increases heart rate, breathing, and redirects blood to muscles.

Parasympathetic Physiological Effects

Low heart rate, low blood pressure, low respiration rate, active digestion, warm skin, constricted pupils.

Sympathetic Physiological Effects

Increased heart rate, rapid breathing, cold sweaty skin, dilated pupils, reduced digestive activity.

Blood Flow Changes During Sympathetic Activation

Blood flow to organs decreases, while flow to skeletal muscles increases.

Sympathetic Adjustment During Exercise

Heart rate and respiratory rate increase, skin blood flow decreases, pupils dilate.

Parasympathetic Energy Use

Low energy use — maintains homeostasis during rest.

Sympathetic Energy Use

High energy use — mobilizes stored energy during stress.

Anatomy of ANS Differences

Two key differences: 1) location of ganglia, and 2) where nerves exit the spinal cord.

Parasympathetic Ganglia Location

On or near the target organ.

Sympathetic Ganglia Location

Near the spinal cord, often forming sympathetic chain ganglia.

Sympathetic Chain Ganglia

Paired structures running along the vertebral column; contain postganglionic neurons.

Sympathetic Neuron Pathway

Preganglionic neuron leaves lateral horn → enters sympathetic chain → synapses → postganglionic axon travels to effector.

Parasympathetic Neuron Pathway

Preganglionic neuron exits via cranial nerves or sacral spinal cord → synapses on or near target organ.

Parasympathetic Outflow Origins

Exits from cranial nerves III, VII, IX, X and sacral spinal segments S2–S4.

Sympathetic Outflow Origins

Exits spinal cord from thoracolumbar region: T1–L2.

Cranial Outflow (Parasympathetic)

Includes cranial nerves III (Oculomotor), VII (Facial), IX (Glossopharyngeal), and X (Vagus).

Sacral Outflow (Parasympathetic)

Includes spinal nerves S2–S4; targets large intestine, bladder, ureters, and reproductive organs.

Oculomotor Nerve (III) Parasympathetic Function

Synapses in ciliary ganglion; constricts iris and controls lens for focusing.

Facial Nerve (VII) Parasympathetic Function

Synapses in pterygopalatine and submandibular ganglia; stimulates salivary, nasal, and lacrimal glands.

Glossopharyngeal Nerve (IX) Parasympathetic Function

Synapses in otic ganglion; stimulates parotid salivary glands.

Vagus Nerve (X) Parasympathetic Function

Synapses in ganglia within target organs; innervates heart, lungs, and most visceral organs.

Sacral Parasympathetic Targets

Large intestine, urinary bladder, ureters, and reproductive organs.

Sympathetic Preganglionic Neuron

Originates in lateral horn of T1–L2 spinal cord segments.

Sympathetic Preganglionic Axon

Pathway: exits spinal cord, enters sympathetic chain, synapses with postganglionic neuron near spinal cord.

Sympathetic Postganglionic Neuron

Axon extends from chain ganglion to effector organs throughout the body.

Sympathetic Chain Function

Allows preganglionic neurons to synapse and send signals to multiple effectors simultaneously.

Splanchnic Nerves

Sympathetic nerves that pass through chain ganglia to synapse in collateral ganglia; supply abdominal organs.

Cardiac and Pulmonary Plexuses

Sympathetic nerve networks that regulate heart rate, airway diameter, and respiratory function.

Adrenal Medulla Role

Functions as a modified sympathetic ganglion that releases epinephrine and norepinephrine into the bloodstream.

Effect of Adrenal Medulla Release

Produces widespread, long-lasting sympathetic responses throughout the body.

Parasympathetic System Activation

Typically produces discrete, localized responses near individual organs.

Sympathetic System Activation

Produces diffuse, long-lasting effects due to widespread NE and epinephrine release.

Sympathetic vs Parasympathetic Activation Speed

Sympathetic acts rapidly and diffusely; parasympathetic acts slowly and locally.

Balance Between ANS Divisions

Both divisions work together to maintain homeostasis by counterbalancing each other.

Overall Effect of Parasympathetic Activation

Calms the body, restores energy, supports routine maintenance functions.

Overall Effect of Sympathetic Activation

Prepares body for physical exertion, stress, or danger.