CHAPTER 16

1. What is the Autonomic Nervous System (ANS)?

The autonomic nervous system (ANS) is the part of the peripheral nervous system (PNS) that regulates involuntary functions of the body.

The ANS helps maintain homeostasis, which means keeping the body's internal environment stable.

The ANS controls:

• Cardiac muscle → heart rate and force of contraction

• Smooth muscle → digestive tract, blood vessels, bladder, airways, etc.

• Glands → sweat glands, salivary glands, digestive glands, etc.

• Adipose tissue

• Certain metabolic processes

You normally do not consciously decide to control these functions.

For example, you do not have to think:

"I need my heart to beat right now."

The ANS automatically regulates it.

The ANS has two major divisions:

Sympathetic division

The sympathetic division prepares the body for physical activity, stress, or emergencies.

Fight-or-flight

Parasympathetic division

The parasympathetic division supports rest, digestion, and energy conservation.

Rest-and-digest

Important concept:

The sympathetic and parasympathetic divisions often have opposite effects on the same organ, but they do not always.

2. What is the Somatic Nervous System (SNS)?

The somatic nervous system is the part of the PNS responsible for communication between the CNS and:

• Skeletal muscles

• Sensory receptors

The motor portion of the somatic nervous system controls voluntary skeletal muscle movement.

Examples:

• Walking

• Writing

• Picking up your phone

• Kicking a ball

• Moving your arm

Somatic motor pathway

The somatic nervous system generally uses ONE motor neuron:

CNS → motor neuron → skeletal muscle

The neuron releases acetylcholine (ACh) at the neuromuscular junction.

ACh binds to nicotinic acetylcholine receptors on skeletal muscle.

3. Define Receptor and Effector

Receptor

A receptor is a protein that detects or responds to a specific chemical messenger.

In the ANS, neurotransmitters bind to receptors on target cells.

Examples:

• Nicotinic receptors

• Muscarinic receptors

• Adrenergic receptors

Think:

Receptor = receives the signal

Effector

An effector is the structure that produces the response after receiving a signal.

In the ANS, effectors include:

• Smooth muscle

• Cardiac muscle

• Glands

Example:

Your heart receives sympathetic stimulation.

Neurotransmitter → receptor → heart

The heart responds by increasing its activity.

4. Main Differences Between the SNS and ANS

This is an important comparison.

Somatic pathway:

CNS → ONE neuron → skeletal muscle

Autonomic pathway:

CNS → preganglionic neuron → ganglion → postganglionic neuron → effector

🚨 Important difference:

The somatic motor neuron releases ACh, which binds to nicotinic receptors on skeletal muscle.

The ANS has different neurotransmitters and receptors depending on the pathway.

5. Define Sympathetic and Parasympathetic

Sympathetic Division

The sympathetic division is responsible for the body's fight-or-flight response.

It becomes more active when the body needs to respond quickly to a stressful situation.

Examples:

• Exercise

• Fear

• Anger

• Excitement

• Danger

Sympathetic effects:

• ↑ Heart rate

• ↑ Force of heart contraction

• ↑ Blood pressure

• Dilates pupils

• Dilates airways

• ↓ Digestive activity

• ↑ Blood flow to skeletal muscles

• ↑ Sweating

• Stimulates adrenal medulla

• Promotes energy availability

Parasympathetic Division

The parasympathetic division is responsible for rest-and-digest functions.

It is especially active when the body is relaxed.

Examples:

• Eating

• Resting

• Sleeping

• Digesting food

Parasympathetic effects:

• ↓ Heart rate

• Constricts pupils

• Constricts airways

• ↑ Digestive activity

• ↑ Salivation

• Promotes urination

• Promotes defecation

Easy way to remember:

Sympathetic = spend energy

Parasympathetic = save energy

6. Preganglionic vs. Postganglionic Neurons

The ANS uses a two-neuron chain.

Preganglionic neuron

The preganglionic neuron is the first neuron.

Its cell body is located in the CNS.

Its axon travels from:

CNS → autonomic ganglion

The preganglionic neuron releases ACh.

Postganglionic neuron

The postganglionic neuron is the second neuron.

Its cell body is located in an autonomic ganglion.

Its axon travels from:

Ganglion → target organ

The neurotransmitter released by the postganglionic neuron depends on whether the pathway is sympathetic or parasympathetic.

⭐ Memorize this:

PRE = CNS to ganglion

POST = ganglion to organ

7. Where Do Sympathetic Preganglionic Neurons Originate?

Sympathetic preganglionic neurons originate from the thoracic and lumbar regions of the spinal cord.

This is why the sympathetic division is called the:

Thoracolumbar division

The cell bodies are primarily located in the lateral horns of the spinal cord from T1–L2.

Important:

Sympathetic = T1–L2

Anatomy:

Spinal cord T1–L2

↓

Sympathetic preganglionic neuron

↓

Sympathetic ganglion

↓

Postganglionic neuron

↓

Target

8. Where Do Sympathetic Postganglionic Neurons Originate?

Sympathetic postganglionic neuron cell bodies are located in sympathetic ganglia.

There are two major types:

1. Sympathetic trunk ganglia

These form chains that run along both sides of the vertebral column.

They are also called:

• Paravertebral ganglia

• Sympathetic chain ganglia

2. Prevertebral ganglia

These are located anterior to the vertebral column, closer to abdominal organs.

Examples include ganglia associated with:

• Celiac plexus

• Superior mesenteric plexus

• Inferior mesenteric plexus

Why are sympathetic postganglionic neurons long?

Because sympathetic ganglia are generally close to the spinal cord.

Therefore:

Short preganglionic axon

Long postganglionic axon

9. Where Do Parasympathetic Preganglionic Neurons Originate?

Parasympathetic preganglionic neurons originate from:

Brainstem

Certain cranial nerves carry parasympathetic fibers.

Important cranial nerve:

CN X — Vagus nerve

The vagus nerve carries a large amount of parasympathetic output to organs in the:

• Thorax

• Abdomen

Sacral spinal cord

Parasympathetic neurons also originate from the sacral spinal cord, especially:

S2–S4

This is why the parasympathetic division is called:

Craniosacral division

⭐ Memorize:

Parasympathetic = brainstem + S2–S4

10. Where Do Parasympathetic Postganglionic Neurons Originate?

Parasympathetic postganglionic neuron cell bodies are located in:

• Terminal ganglia

• Intramural ganglia

These ganglia are located very close to or within the target organ.

Because the ganglion is near the organ:

Long preganglionic axon

Short postganglionic axon

Example:

Brainstem

↓

Long preganglionic neuron

↓

Ganglion near/in heart

↓

Short postganglionic neuron

↓

Heart

11. Are Autonomic Ganglia Inside or Outside the CNS?

Autonomic ganglia are outside the CNS.

Therefore, they are part of the PNS.

Remember:

CNS

Brain + spinal cord

PNS

Everything outside the brain and spinal cord

Ganglion vs nucleus

This distinction is important:

Ganglion = collection of neuron cell bodies in PNS

Nucleus = collection of neuron cell bodies in CNS

12. What Neurotransmitters Do Preganglionic Neurons Release?

This is one of the most important facts in the chapter.

ALL autonomic preganglionic neurons release ACh.

This applies to BOTH:

• Sympathetic

• Parasympathetic

The ACh binds to:

Nicotinic receptors

Sympathetic:

Preganglionic neuron → ACh → nicotinic receptor

Parasympathetic:

Preganglionic neuron → ACh → nicotinic receptor

🚨 MEMORIZE:

All autonomic preganglionic = ACh → nicotinic

This is true even though sympathetic and parasympathetic postganglionic neurons are different.

13. What Neurotransmitters Do Postganglionic Neurons Release?

Now we separate the two divisions.

Sympathetic Postganglionic Neurons

Most sympathetic postganglionic neurons release:

Norepinephrine (NE)

NE binds to:

Adrenergic receptors

The two major types are:

• Alpha (α)

• Beta (β)

So:

Sympathetic postganglionic

→ NE

→ Adrenergic receptor

🚨 Major Exception: Sweat Glands

Sympathetic postganglionic neurons that stimulate most sweat glands release:

ACh

instead of NE.

ACh binds to:

Muscarinic receptors

So:

Sympathetic → sweat gland

→ ACh

→ Muscarinic receptor

⭐ Memorize:

Sweat is sympathetic but uses ACh.

Parasympathetic Postganglionic Neurons

Parasympathetic postganglionic neurons release:

ACh

ACh binds to:

Muscarinic receptors

So:

Parasympathetic postganglionic

→ ACh

→ Muscarinic

🔥 THE NEUROTRANSMITTER MAP

Memorize this exact pattern:

PREganglionic — BOTH divisions

ACh → Nicotinic

↓

Sympathetic POSTganglionic

Usually NE → Adrenergic

↓

Parasympathetic POSTganglionic

ACh → Muscarinic

↓

Sympathetic sweat glands

ACh → Muscarinic

14. Sympathetic Effects — Specific Organ Effects

❤ Heart

Sympathetic stimulation:

• Increases heart rate

• Increases force of contraction

• Increases cardiac output

Why?

The body needs more blood delivered to tissues during fight-or-flight.

👁 Eye

Sympathetic stimulation causes:

Pupil dilation

This is called:

Mydriasis

Larger pupils allow more light to enter.

🫁 Lungs

Sympathetic stimulation causes:

Bronchodilation

The airways become wider.

This allows more air to enter the lungs.

🍔 Digestive system

Sympathetic stimulation generally:

• Decreases digestive activity

• Decreases intestinal movement

• Decreases digestive secretions

• Reduces blood flow to the digestive tract

Why?

Digestion is not the body's priority during an emergency.

💦 Sweat glands

Sympathetic stimulation:

Increases sweating

Remember:

Sympathetic but ACh → muscarinic

🩸 Blood vessels

Sympathetic stimulation can cause:

• Vasoconstriction in many blood vessels

• Changes in blood distribution

• Increased blood pressure

The exact effect depends on the receptor and tissue.

🧠 Adrenal medulla

Sympathetic stimulation activates the adrenal medulla.

The adrenal medulla releases:

• Epinephrine

• Norepinephrine

into the bloodstream.

These hormones help create a widespread fight-or-flight response.

15. Parasympathetic Effects — Specific Organ Effects

❤ Heart

Parasympathetic stimulation:

↓ Heart rate

The vagus nerve is especially important for parasympathetic control of the heart.

👁 Eye

Parasympathetic stimulation causes:

Pupil constriction

This is called:

Miosis

🫁 Lungs

Parasympathetic stimulation generally causes:

Bronchoconstriction

The airways become narrower.

🍔 Digestive system

Parasympathetic stimulation:

• ↑ Digestive secretions

• ↑ Intestinal movement

• ↑ Blood flow associated with digestion

• Promotes digestion

💧 Salivary glands

Parasympathetic stimulation increases salivation.

This helps prepare the body for digestion.

🚽 Urinary bladder

Parasympathetic activity promotes:

Urination

💩 Large intestine

Parasympathetic activity promotes:

Defecation

16. Nicotinic, Muscarinic, and Adrenergic Receptors

You NEED to know the difference.

Nicotinic Receptors

Activated by:

ACh

Found in:

• Autonomic ganglia

• Neuromuscular junctions of skeletal muscle

For the ANS:

Preganglionic neuron → ACh → nicotinic receptor

Think:

Nicotinic = ganglion

Muscarinic Receptors

Activated by:

ACh

Found on many parasympathetic target organs.

Also found on sweat glands controlled by sympathetic neurons.

Think:

Muscarinic = organ

Adrenergic Receptors

Activated by:

• Norepinephrine

• Epinephrine

Types include:

Alpha

α₁

α₂

Beta

β₁

β₂

β₃

⭐ HIGH-YIELD RECEPTOR MEMORY

NICOTINIC

ACh

Autonomic ganglia

MUSCARINIC

ACh

Parasympathetic target organs

Sympathetic sweat glands

ADRENERGIC

NE/Epinephrine

Sympathetic target organs

17. Agonists and Antagonists

Agonist

An agonist is a chemical that binds to a receptor and activates or stimulates its normal response.

Example:

A cholinergic agonist can mimic the effects of ACh.

Think:

AGONIST = ACTIVATE

Antagonist

An antagonist binds to a receptor and blocks its activation.

It prevents the receptor from producing its normal response.

Think:

ANTAGONIST = BLOCK

Drug Terminology

You may see these terms:

Cholinergic

Related to acetylcholine.

Adrenergic

Related to norepinephrine and epinephrine.

Cholinergic agonist

Mimics or enhances ACh effects.

Cholinergic antagonist

Blocks ACh effects.

Adrenergic agonist

Mimics sympathetic adrenergic effects.

Adrenergic antagonist

Blocks adrenergic effects.

18. Autonomic Reflex Arc

A reflex is an automatic response to a stimulus.

An autonomic reflex helps regulate things like:

• Blood pressure

• Heart rate

• Digestion

• Bladder function

• Pupil size

Five Components of a Reflex Arc

1. Receptor

Detects the stimulus.

↓

2. Sensory neuron

Carries information toward the CNS.

↓

3. Integration center

Located in the CNS.

The CNS processes the information and determines an appropriate response.

↓

4. Motor neuron

Carries the response away from the CNS.

For an autonomic reflex, this involves:

Preganglionic neuron → ganglion → postganglionic neuron

↓

5. Effector

Produces the response.

Examples:

• Cardiac muscle

• Smooth muscle

• Glands

⭐ Reflex Arc Memory Trick

R → S → I → M → E

Receptor
Sensory neuron
Integration center
Motor neuron
Effector

Think:

"Really Smart Individuals Make Everything."

🧠 AUTONOMIC TONE

Autonomic tone means that the ANS maintains a continuous level of activity in organs.

The sympathetic and parasympathetic divisions are not simply:

ON vs OFF

Instead, they constantly adjust their activity.

This allows the body to make small changes to maintain homeostasis.

Example:

Your heart has a normal resting rate because of the balance between:

• Sympathetic stimulation

• Parasympathetic stimulation

If you suddenly exercise:

Sympathetic activity increases

If you relax:

Parasympathetic influence becomes more dominant

🧠 HYPOTHALAMUS

The hypothalamus is one of the most important control centers for the ANS.

It helps coordinate:

• Autonomic activity

• Endocrine activity

• Body temperature

• Water balance

• Hunger

• Thirst

• Emotional responses

• Circadian rhythms

• Homeostasis

How does it control the ANS?

The hypothalamus receives information about the body's internal and external conditions.

It then sends signals that influence:

Sympathetic division

and

Parasympathetic division

This allows the hypothalamus to adjust autonomic tone.

Example: Body Temperature

If your body gets too hot:

Temperature receptors detect change

↓

Hypothalamus receives information

↓

Hypothalamus activates appropriate autonomic responses

↓

Sweating increases

↓

Heat is lost

↓

Body temperature decreases

This is an example of maintaining homeostasis.

🧠 SYMPATHETIC vs PARASYMPATHETIC — FULL TABLE

🔥 THE MOST IMPORTANT ANATOMY COMPARISON

SYMPATHETIC

Thoracolumbar

T1–L2

↓

Short preganglionic

↓

Ganglion close to spinal cord

↓

Long postganglionic

↓

Organ

Neurotransmitter:

ACh → nicotinic

at ganglion

Then usually:

NE → adrenergic

at organ

PARASYMPATHETIC

Craniosacral

Brainstem + S2–S4

↓

Long preganglionic

↓

Ganglion close to/in organ

↓

Short postganglionic

↓

Organ

Neurotransmitter:

ACh → nicotinic

at ganglion

Then:

ACh → muscarinic

at organ

🚨 EXAM TRAPS TO WATCH FOR

Trap #1

"Sympathetic preganglionic neurons release norepinephrine."

❌ FALSE

They release:

✅ ACh

Trap #2

"Parasympathetic preganglionic neurons release ACh."

✅ TRUE

Trap #3

"All sympathetic postganglionic neurons release norepinephrine."

❌ FALSE

Sweat glands are the important exception.

Trap #4

"All autonomic receptors are muscarinic."

❌ FALSE

Autonomic receptors include:

• Nicotinic

• Muscarinic

• Adrenergic

Trap #5

"Sympathetic means only stress."

❌ Not exactly.

The sympathetic division is also active during normal activities such as:

• Exercise

• Standing

• Maintaining blood pressure

• Regulating blood flow

Trap #6

"Parasympathetic means the body completely shuts down."

❌ FALSE

It maintains normal body functions such as:

• Digestion

• Heart regulation

• Urination

• Defecation

🧠 MASTER MEMORIZATION MAP

If you want to memorize this chapter efficiently, memorize this sequence:

STEP 1 — DIVISIONS

ANS

↓

Sympathetic = fight/flight

Parasympathetic = rest/digest

STEP 2 — ORIGINS

Sympathetic = Thoracolumbar

T1–L2

Parasympathetic = Craniosacral

Brainstem + S2–S4

STEP 3 — AXON LENGTH

Sympathetic

Short PRE → Long POST

Parasympathetic

Long PRE → Short POST

STEP 4 — PREGANGLIONIC

BOTH:

ACh → Nicotinic

STEP 5 — POSTGANGLIONIC

Sympathetic:

NE → Adrenergic

Parasympathetic:

ACh → Muscarinic

Exception:

Sympathetic sweat glands = ACh → Muscarinic

STEP 6 — ORGAN EFFECTS

Sympathetic = ↑ heart, ↑ breathing, ↓ digestion

Parasympathetic = ↓ heart, ↑ digestion, ↑ elimination

STEP 7 — REFLEX

Receptor → Sensory → Integration → Motor → Effector

STEP 8 — CONTROL

Hypothalamus → regulates autonomic activity → maintains homeostasis

🏆 WHAT YOU SHOULD BE ABLE TO EXPLAIN WITHOUT NOTES

For an exam, don't just memorize individual definitions. You should be able to explain a complete pathway.

For example, if your professor asks:

"Describe sympathetic stimulation of an organ."

You should be able to think:

The sympathetic preganglionic neuron begins in the thoracolumbar spinal cord, T1–L2. It releases ACh onto nicotinic receptors in a sympathetic ganglion. The sympathetic postganglionic neuron then travels to the target organ and usually releases norepinephrine onto adrenergic receptors. The response depends on the organ and receptor involved.

For parasympathetic:

The parasympathetic preganglionic neuron originates in the brainstem or sacral spinal cord. It is usually long because the ganglion is near the target organ. It releases ACh onto nicotinic receptors in the ganglion. The short postganglionic neuron then releases ACh onto muscarinic receptors at the target organ.