Chapter 15 Study Guide

What is the function of the Autonomic Nervous System (ANS)?

It maintains homeostasis by controlling involuntary (unconscious) functions.

• Responsible for controlling heart rate, digestion, respiratory rate, pupillary response, urination, vasomotor-tone, gland secretion, arousal

• Provides a balance between parasympathetic and sympathetic divisions

Parasympathetic Division (PANS) Functions

This division functions as the control center for “rest and digest” functions.

• Promotes relaxation, conserves energy, and manages involuntary processes like digestion, heart rate, and breathing to maintain balance after stress.

• “Survive later” Functions: Slows heart rate, increases digestion, constricts pupils for nearby vision, facilitates waste elimination, stimulates saliva and mucus.

Sympathetic Division (SANS) Functions

This division functions as the control center for the body's rapid-response "fight-or-flight" system

• Mobilizes energy and resources to handle stress, danger, or intense physical activity.

• It increases heart rate, dilates pupils, relaxes airways, and redirects blood flow toward muscles while suppressing digestion.

• Survive now!

What are the differences between the somatic and autonomic nervous systems?

Somatic:

• It is voluntary (conscious) functions

• This division controls skeletal muscle activation

• Uses a single neuron from the CNS to the effector

• It ONLY releases ACh

Autonomic:

• It is involuntary (unconscious) functions

• This division controls smooth muscle, cardiac muscle, and gland secretion

• It requires two neuron chain from CNS to effector

  • 1. Preganglionic neuron

  • 2. Post ganglionic neuron

• It can release either ACh or NE (these neurotransmitters have the ability to be excitatory or inhibitory)

Acetylcholine (ACh)

ACh is the "standard" messenger for internal maintenance and the initial jump-start of any ANS signal.

• All ANS pre-ganglionic neurons (both Sympathetic and Parasympathetic).

• All Parasympathetic post-ganglionic neurons.

• Sympathetic post-ganglionic neurons that lead to sweat glands.

• Somatic motor neurons (triggering skeletal muscle)

• Neurons that release ACh are known as Cholinergic Neurons

ACh binds to Nicotinic and Muscarinic receptors

Nicotinic

• Nicotinic receptors are a specific class of receptors that respond to the neurotransmitter Acetylcholine (ACh).

• Once ACh binds, it changes the shape of receptor, allowing Na+ to flood in and depolarize for an action potential.

• Always excitatory— works to stimulate the target cell

• Found in:

  • All Autonomic Ganglia: In both the Sympathetic and Parasympathetic systems → the "hand-off" from the first neuron to the second neuron

  • The Adrenal Medulla: When you get a rush of adrenaline, it's because ACh hit nicotinic receptors on the adrenal glands, telling them to dump hormones into your blood.

Muscarinic

Muscarinic receptors are the other major class of receptors that respond to Acetylcholine (ACh).

• Unlike nicotinic receptors, which are simple channels, muscarinic receptors are G-protein-coupled receptors (GPCRs)

• The Result: This G-protein then travels to other parts of the cell to open ion channels or activate enzymes. Because of this extra step, the response is slower but lasts longer than nicotinic signaling.

• Can have excitatory or inhibitory effects

Found in:

• All parasympathetic target organ such as the heart, lungs, digestive tract, and bladder.

• Sweat Glands: A "weird" exception where the sympathetic system uses ACh to trigger sweating.

Norepinephrine

A neurotransmitter released by almost all Sympathetic post-ganglionic neurons

Neurons that release NE are called adrenergic neurons.

Epinephrine

Released by the Adrenal Medulla into the bloodstream as a hormone (along side some NE) to enhance the "fight or flight" response.

Differences between cholinergic neurons and andrenergic neurons:

• ACh is the "standard" messenger for internal maintenance and the initial jump-start of any ANS signal.

• NE and E are the "specialists" of the sympathetic system, fine-tuning the body's reaction to stress by either squeezing blood vessels (alpha 1) or opening up the lungs (beta 2)

What organs are innervated by sympathetic fibers?

1. Eyes: Specifically the iris dilator muscle. Activation causes the pupils to dilate to let in more light for better peripheral vision.

2. Salivary Glands: Unlike the "watery" saliva for digestion, sympathetic activation makes saliva thick and mucus-rich, leading to "dry mouth" during stress.

3. Heart: Sympathetic fibers hit the SA node and the ventricles which allows for increased heart rate and increased force of contraction.

4. Lungs: Specifically the bronchial smooth muscle to open airways and maximize oxygen intake.

5. Stomach & Intestines: Decreases blood flow (vasoconstriction) and slows down muscle contractions (peristalsis).

6. Liver: Stimulates glycogenolysis—the breakdown of stored glycogen into glucose to flood the blood with "instant fuel."

7. Adrenal Medulla: This is a special connection; sympathetic fibers trigger it to release epinephrine (adrenaline) directly into the blood.

8. Bladder: Relaxes the bladder wall but constricts the internal sphincter to prevent urination.

9. Kidneys: Triggers the release of renin, which helps increase blood pressure.

10. Blood Vessels: Most blood vessels are under constant sympathetic "tone." Increased firing causes them to constrict (raising blood pressure).

11. Sweat Glands: Triggered to help cool the body during exertion.

Where are the three locations of sympathetic ganglia?

1. Sympathetic Trunk (Paravertebral) Ganglia: Running from the base of the skull to the coccyx

2. Prevertebral (Collateral) Ganglia: Celiac, Superior Mesenteric, and Inferior Mesenteric

3. Adrenal Medulla

Alpha 1: The constrictors

A type of adrenergic receptor found in smooth muscle. They are generally excitatory.

• Primary Location: Blood vessels and radial muscle of the eye.

• Action: They cause vasoconstriction to redirect blood to the heart and brain and pupillary dilation to let in more light.

• Mnemonic: "A-1" constriction.

Alpha-2 Receptors: The "Brakes"

Alpha-2 Receptors: The "Brakes"

These are unique because they are often found on the presynaptic nerve terminal itself. They are generally inhibitory.

• Primary Location: Adrenergic nerve endings and the pancreas.

• Action: They act as a feedback loop to inhibit the release of more Norepinephrine, preventing the sympathetic system from over-firing. In the pancreas, they inhibit insulin secretion.

Beta-1 Receptors: The "Heart" Receptors

These are the primary receptors for cardiac stimulation. They are excitatory.

• Primary Location: The heart (SA node, AV node, and cardiac muscle) and the kidneys.

• Action: In the heart, they increase heart rate and in the kidneys, they trigger the release of renin, which eventually raises blood pressure.

• Mnemonic: You have 1 heart, so beta affects the heart.

Beta-2 Receptors: The "Dilators"

Beta-2 receptors are often inhibitory to smooth muscle.

• Primary Location: Lungs (bronchioles), blood vessels leading to skeletal muscle and the heart, and the wall of the digestive tract.

• Action: They cause bronchodilation (opening the airways) and vasodilation (opening blood vessels) specifically in the muscles you need for running or fighting.

• Mnemonic: You have 2 lungs, so beta-2 affects the lungs

5. Beta-3 Receptors: The "Fuel Burners"

These have a very specific metabolic role.

• Primary Location: Adipose tissue (brown fat) and the bladder.

• Action: They stimulate lipolysis (breaking down fat for energy).

Sympathetic Nervous System Characteristics

• Thoracolumbar (T1–L2)

• Short pre-ganglionic; Long post-ganglionic

• High Divergence: In the sympathetic system, there is high divergence. One pre-ganglionic fiber exits the spinal cord and can synapse with 20 or more post-ganglionic neurons across several different ganglia in the sympathetic chain.

Parasympathetic Nervous System

• Craniosacral (CN III, VII, IX, X + S2–S4).

• Long pre-ganglionic; Short post-ganglionic

• Where are PSNS ganglia located?

  Back: Terminal (near organ) or Intramural (inside organ walls)

• The effects of the parasympathetic nervous system (PSNS) are primarily local and discrete.

• Low divergence: One pre-ganglionic fiber travels almost all the way to the target and synapses with only one or very few post-ganglionic neurons.

What composes the Parasympathetic Nervous System?

• Cranial Nerves: Fibers originate from the brainstem and travel via four specific cranial nerves:

  • III (Oculomotor) 3: Controls pupils and lens.

  • VII (Facial) 7: Controls lacrimal (tear) and salivary glands.

  • IX (Glossopharyngeal) 9: Controls the parotid salivary gland.

  • X (Vagus) 10: The "powerhouse" nerve that carries ~90% of all parasympathetic fibers to the heart, lungs, and most digestive organs.

• Sacral Nerves: Fibers originate from the lateral gray matter of spinal segments S2 through S4 (often called the pelvic splanchnic nerves). These target the lower colon, rectum, bladder, and reproductive organs.

Parasympathetic PRE-ganglionic fibers

Origin: Brainstem or Sacral spinal cord.

Relative Size: Very Long. They must travel from the CNS all the way to the target organ.

Neurotransmitter: ACh

Receptor Type: Nicotinic (at the ganglion).

Parasympathetic POST-ganglionic fibers

Origin: Terminal or intramural ganglia.

Relative Size: Very Short. They only need to travel a few millimeters from the ganglion to the target cells.

Neurotransmitter: ACh

Receptor Type: Muscarinic (at the organ).

Sympathetic PRE-ganglionic fibers

Origin: Lateral Horn of Spinal Cord (T1–L2)

Relative Size: Short

Myelination: Myelinated

Neurotransmitter: ACh

Receptor Type: Nicotinic (on the next neuron)

Sympathetic POST-ganglionic fibers

Origin: Sympathetic Ganglia (Chain or Prevertebral)

Relative Size: Long

Myelination: Unmyelinated

Neurotransmitter: NE (Norepinephrine) *Unless ACh to Sweat glands

Receptor Type: Adrenergic → Alpha and Beta receptors on the organ

Are the effects of the parasympathetic system local or widespread?

Local due to low divergence— This limited branching keeps the signal confined to a specific organ or a small group of tissues.

Ionotropic vs Metabotropic Receptors

Ionotropic (fast): These are ligand-gated ion channels that allow for an opening of the channel and flooding of ions in or out the cell due to neurotransmitter (the ligand) binding.

• Nicotinic Receptors synapse from preganglionic fiber to postganglionic fiber very quickly.

Metabotropic (slow):

• Muscarinic Receptors found parasympathetic target organs allows PANS to fine tune heart rate or digestion smoothly rather than just flicking them on and off.

• All Adrenergic Receptors (Alpha and Beta) are Metabotropic: Because they use G-proteins and second messengers, the "fight or flight" signals can linger and coordinate complex metabolic changes like breaking down fat or glucose. Explains why adrenaline rushes don’t fade quickly.

What are the effects of parasympathetic nervous system stimulation?

• Eyes: It controls the muscles that cause pupillary constriction and near-vision focusing.

• Glands: It stimulates the lacrimal glands (tears) and the salivary glands to produce profuse, watery saliva rich in enzymes for digestion.

• Vagus Nerve (CN X) is the main driver here, acting as a "brake" for the heart and lungs.

  • Heart: It decreases the heart rate by acting on the SA and AV nodes.

  • Lungs: It causes bronchoconstriction (narrowing the airways) and increases mucus secretion, as high-volume airflow isn't needed during rest.

• The Abdominal and Pelvic Organs shifts blood flow and energy toward the viscera.

• Digestive Tract: It increases motility (peristalsis) and stimulates the secretion of digestive juices (gastric acid, bile, and enzymes).

  • It also relaxes sphincters to allow food to pass through.

• Liver: It promotes glycogenesis (storing glucose as glycogen) to save energy for later.

• Bladder: It contracts the detrusor muscle (bladder wall) and relaxes the internal urethral sphincter, facilitating urination (micturition).

• Genitalia: It triggers vasodilation, which is responsible for the erection of the clitoris and penis.

What does the Parasympathetic Nervous system NOT DO?

• Sweat Glands (Sympathetic only).

• Most Blood Vessels (They are controlled by increasing or decreasing sympathetic "tone").

• Adrenal Medulla: Sympathetic only.

What is autonomic tone? Why is it important?

Autonomic tone is the constant, low-level background activity (firing of nerve impulses) from both the sympathetic and parasympathetic divisions.

Autonomic tone is crucial because it allows for rapid, precise adjustments in either direction.

Examples:

Organ	Dominant Tone at Rest	Result of Tone
Heart	Parasympathetic	Keeps resting heart rate lower than the natural pace.
Blood Vessels	Sympathetic	Keeps vessels partially constricted to maintain BP.
Digestive Tract	Parasympathetic	Keeps digestion moving smoothly at a slow pace.
Urinary Bladder	Sympathetic	Keeps the sphincter closed to prevent leakage.

Visceral Reflex

A visceral reflex is an automatic, involuntary response to a stimulus within the internal organs (viscera).

1. The Five Components of a Visceral Reflex Arc

1. Receptor: Sensory nerve endings that detect changes (e.g., stretch in the stomach, high blood pressure in arteries, or chemical changes).

2. Sensory Neuron: Carries the signal from the receptor toward the Central Nervous System (CNS).

3. Integration Center: Usually located in the spinal cord or brainstem. It may involve an interneuron.

4. Motor Output (Two-Neuron Path): This is the key difference from somatic reflexes. It involves a pre-ganglionic and a post-ganglionic neuron.

5. Effector: The target organ (e.g., the heart slows down, or the bladder contracts).

Example of a Visceral Reflex

A Classic Example: The Baroreceptor Reflex

This reflex is your body’s primary way of maintaining stable blood pressure.

• Stimulus: High blood pressure stretches the walls of the carotid arteries.

• Receptor: Baroreceptors (stretch receptors) detect the stretch.

• Afferent (sensory) Path: Signals travel to the glossopharyngeal nerve.

• Integration: The medulla oblongata in the brainstem processes the "too high" signal.

• Efferent (motor) Path: The Vagus Nerve (parasympathetic) sends a signal to the heart.

• Effector/Response: The heart rate slows down, causing blood pressure to drop back to normal.