BIO 004 · Human Anatomy
The Autonomic Nervous System
Block 5 · Module 6: The Autonomic Nervous System
A reference for the autonomic nervous system video and lab. This page covers the difference between the somatic and autonomic systems, the two-neuron motor pathway, the named autonomic ganglia, the sympathetic and parasympathetic divisions, and the autonomic and enteric plexuses. The focus is on the structures and the job each one does.
How to use this sheet Toggle the toolbar above. Notes prints the full reference for review. Study prints as a fill-in-the-blank worksheet. Print it, then write each definition while you watch the video or read your book. Quiz me is on-screen typing practice: type the term, click Reveal to check yourself. The comparison grids respond to Study and Quiz too, with a Reveal button on each row.
The Foundations video gives you a complete foundational understanding of this topic, enough on its own for a foundational course. Learn it first, then move on to the Deep dive, which adds the majors-level depth for this course.
- Contrast the somatic and autonomic motor systems.
- Describe the two-neuron motor pathway and name the autonomic ganglia.
- Compare the sympathetic and parasympathetic divisions and the plexuses.
Your pre-work
Work through these the evening before class. None of it is turned in. It is how you learn the material and build your spaced recall.
This is more than a checklist. Ticking these boxes is the start, not the finish. Committing this material to memory and being able to apply it takes considerable time and repeated effort. You are not done when the boxes are checked. Put in the real hours, and keep coming back for frequent recall and review until the material is genuinely yours.
The two divisions
Add a diagram contrasting the sympathetic and parasympathetic outflow from the CNS.
The two-neuron pathway
Add a labeled view of the preganglionic neuron, the ganglion, and the postganglionic neuron.
The autonomic ganglia
Add a view showing the sympathetic trunk, the prevertebral ganglia, and the terminal ganglia.
The Autonomic Nervous System, an Overview
The autonomic nervous system is the involuntary motor division. It quietly regulates the organs, adjusting them without conscious effort or awareness.
- Autonomic nervous systemthe involuntary division of the motor system; it controls cardiac muscle, smooth muscle, and glands
- Autonomic effectorsthe targets of the ANS: cardiac muscle, smooth muscle, and glands
- Autonomic sensory neuronsneurons that carry input from receptors in the viscera and blood vessels to the CNS
- Autonomic motor neuronsneurons that carry commands out to the visceral effectors, either exciting or inhibiting them
- Two divisionsthe sympathetic and the parasympathetic divisions, which usually act in opposite directions on the same organ
- Involuntary controlmost autonomic activity cannot be consciously altered, although emotion and pain can influence it
Somatic Versus Autonomic
The somatic and autonomic systems are both motor, but they differ in what they control, how many neurons they use, and what effect they have. Compare them.
| Feature | Somatic nervous system | Autonomic nervous system |
|---|---|---|
| Effector | skeletal muscle | cardiac muscle, smooth muscle, and glands |
| Control | voluntary | involuntary |
| Motor neurons in the pathway | one, from the CNS straight to the muscle | two, with a synapse in a ganglion |
| Effect on the effector | always excitation; it causes contraction | either excitation or inhibition |
| Awareness | normally conscious | normally not conscious |
The Two-Neuron Motor Pathway
Where the somatic system reaches its muscle with a single neuron, the autonomic system always uses two neurons in a row, with a synapse in a ganglion between them.
- Preganglionic neuronthe first neuron; its cell body sits in the brain or spinal cord, and its myelinated axon runs out to an autonomic ganglion
- Autonomic ganglionthe cluster of cell bodies where the two neurons synapse
- Postganglionic neuronthe second neuron; its cell body sits in the ganglion, and its unmyelinated axon runs from the ganglion to the visceral effector
- The pathwaythe preganglionic neuron synapses in the ganglion with the postganglionic neuron, which then reaches the effector
The Autonomic Ganglia
The autonomic ganglia fall into two groups by division. The sympathetic ganglia sit close to the spinal cord; the parasympathetic ganglia sit close to or within the organ.
Sympathetic ganglia
- Sympathetic trunk gangliaalso called paravertebral ganglia; a vertical chain on each side of the vertebral column, from the base of the skull to the coccyx
- Prevertebral gangliaalso called collateral ganglia; ganglia anterior to the vertebral column, near the large abdominal arteries
- The five prevertebral gangliathe celiac, superior mesenteric, inferior mesenteric, aorticorenal, and renal ganglia
Parasympathetic ganglia
- Terminal gangliathe parasympathetic ganglia, located near or within the wall of the target organ
- The four named terminal ganglia of the headthe ciliary, pterygopalatine, submandibular, and otic ganglia
The Two Divisions
The two divisions are told apart first by where they leave the CNS, and then by the work they do.
The sympathetic division
- Sympathetic divisionthe fight or flight division; also called the thoracolumbar division
- Originpreganglionic cell bodies in the lateral gray horns of the T1 to L2 spinal segments
- Roleit takes over during exercise, excitement, emergency, and embarrassment
- Sample effectsdilates the pupils, speeds the heart, opens the airways, and slows digestion
The parasympathetic division
- Parasympathetic divisionthe rest and digest division; also called the craniosacral division
- Originpreganglionic cell bodies in the nuclei of four cranial nerves (III, VII, IX, and X) and in the S2 to S4 spinal segments
- Roleit conserves energy and runs routine housekeeping: digestion, defecation, and urination
- Sample effectsconstricts the pupils, slows the heart, constricts the airways, and stimulates digestion
Comparing the Two Divisions
Side by side, the two divisions differ in their origin in the CNS, the location of their ganglia, and their overall job. Compare them.
| Feature | Sympathetic | Parasympathetic |
|---|---|---|
| Other name | thoracolumbar division | craniosacral division |
| Origin in the CNS | the T1 to L2 spinal segments | the brainstem and the S2 to S4 spinal segments |
| Location of ganglia | close to the spinal cord | close to or within the effector organ |
| Overall role | fight or flight; prepares the body for action | rest and digest; conserves energy |
The Autonomic and Enteric Plexuses
Autonomic axons and ganglia travel together in networks called plexuses. A separate set of networks, the enteric plexuses, runs the wall of the gut.
Autonomic plexuses
- Autonomic plexusa network of autonomic axons and ganglia in the thorax, abdomen, and pelvis
- Cardiac plexussupplies the heart
- Pulmonary plexussupplies the bronchial tree
- Celiac (solar) plexusthe largest; it surrounds the celiac trunk and supplies most of the abdominal organs
- Superior and inferior mesenteric plexusessupply the small and large intestine
- Hypogastric plexussupplies the pelvic organs
- Renal plexussupplies the kidneys and ureters
Enteric plexuses
- Enteric plexusesthe neural networks within the wall of the GI tract, the structural core of the enteric nervous system
- Myenteric plexuslies between the longitudinal and circular muscle layers; it controls the movement of the gut
- Submucosal plexuslies between the circular muscle layer and the muscularis mucosae; it controls the secretions of the gut
Tracing an Autonomic Pathway
Every autonomic motor command travels the same four-step route from the central nervous system to the organ.
- Preganglionic neuronthe cell body in the brain or cord sends its axon out toward a ganglion
- Autonomic ganglionthe preganglionic axon synapses here with the postganglionic neuron
- Postganglionic neuronits axon carries the signal the rest of the way out
- Effector tissuethe cardiac muscle, smooth muscle, or gland receives the command
See also: The Peripheral Nervous System for the nerves these fibers travel in, and Functional Organization and Nervous Tissue for the neurons themselves. This is the final page of Block 5.
Study questions
Work on answering these in writing, in your own words. They are the questions to bring to class, and good practice for the reasoning the exams ask for.
- Compare the sympathetic and parasympathetic divisions by where their fibers leave the central nervous system.
- For one organ, describe what the sympathetic and parasympathetic inputs each do, and why rest-and-digest versus fight-or-flight captures the difference.
- Trace a two-neuron autonomic pathway from the CNS to an effector, naming the ganglion where the synapse occurs.
- Why is the adrenal medulla described as a modified sympathetic ganglion?
Step 2 . Retrieval check
Now explain it back, in your own words.
In 60 words or more, pull together what the video just taught you. Include the key concepts. This is the point where the learning actually sticks. After you submit, your spaced-recall cards for this topic unlock.