Ch 16: The Autonomic Nervous System (ANS): Structure, Function, and Divisions

The Autonomic Nervous System (ANS)

Overview of the ANS

The Autonomic Nervous System (ANS) is a critical component of the peripheral nervous system that regulates involuntary physiological processes, including heart rate, blood pressure, respiration, digestion, and sexual arousal. The ANS operates largely outside conscious awareness and is essential for maintaining homeostasis.

• Two main divisions: Sympathetic and Parasympathetic

• Control center: Medulla oblongata in the brainstem

• Neural pathway: Each ANS pathway consists of two neurons (preganglionic and postganglionic) connecting the CNS to the target organ

• Response modulation: ANS responses are graded (ramp up/down), not all-or-nothing

Divisions of the ANS

Sympathetic Division

The sympathetic division prepares the body for stressful or energetic activity, often referred to as the "fight or flight" response.

• Origin: Thoracolumbar region of the spinal cord

• Neural structure: Short preganglionic fibers, long postganglionic fibers

• Neurotransmitters: Acetylcholine (ACh) at the first synapse (nicotinic receptors), norepinephrine (NE) at the second synapse (adrenergic receptors: alpha or beta)

• Ganglia location: Near the spinal cord (sympathetic chain ganglia)

• Key effects: Increases heart rate, blood pressure, respiratory rate, metabolic rate; dilates respiratory passageways; mobilizes energy reserves; reduces digestive and urinary functions

Parasympathetic Division

The parasympathetic division supports "rest and digest" activities, conserving energy and promoting routine maintenance functions.

• Origin: Craniosacral region (brainstem and sacral spinal cord)

• Neural structure: Long preganglionic fibers, short postganglionic fibers

• Neurotransmitters: Acetylcholine (ACh) at both synapses (nicotinic and muscarinic receptors)

• Ganglia location: Near or within target organs

• Key effects: Decreases heart rate and blood pressure, increases digestive activity, stimulates urination and defecation, promotes sexual arousal

Sympathetic vs. Parasympathetic: Key Differences

Feature	Sympathetic	Parasympathetic
Origin	Thoracolumbar	Craniosacral
Preganglionic Fiber Length	Short	Long
Postganglionic Fiber Length	Long	Short
Neurotransmitters	ACh (preganglionic), NE (postganglionic)	ACh (both synapses)
Receptors	Nicotinic (first), Adrenergic (second)	Nicotinic (first), Muscarinic (second)
Main Functions	"Fight or Flight"	"Rest and Digest"

Neurotransmitters and Receptors in the ANS

Sympathetic Neurotransmitters

• Preganglionic neurons: Release ACh, which binds to nicotinic receptors (excitatory)

• Postganglionic neurons: Release NE (and sometimes epinephrine), which binds to adrenergic receptors (alpha and beta types)

• Adrenergic receptors: Activate intracellular enzymes via G proteins; alpha receptors are more sensitive to NE, beta receptors respond to both NE and epinephrine

Parasympathetic Neurotransmitters

• All neurons: Release ACh

• Receptors: Nicotinic (on ganglion cells, always excitatory) and muscarinic (on target tissues, can be excitatory or inhibitory)

• Inactivation: ACh is rapidly broken down by acetylcholinesterase (AChE) at the synapse

Physiological Responses of the ANS

Sympathetic Activation: Seven Key Responses

1. Heightened mental alertness (pupil dilation, focused vision)

2. Increased metabolic rate (more ATP production)

3. Reduced digestive and urinary functions

4. Activation of energy reserves (mobilization of fats)

5. Increased respiratory rate and dilation of airways

6. Increased heart rate and blood pressure (cardiac output increases)

7. Activation of sweat glands (thermoregulation)

Parasympathetic Activation: Five Key Responses

1. Decreased metabolic rate

2. Decreased heart rate and blood pressure (cardiac output normalizes)

3. Increased secretion by salivary and digestive glands

4. Increased motility and blood flow in the digestive tract

5. Stimulation of urination and defecation

• Other effects: Pupil constriction, lens focusing for near vision, secretion of digestive hormones, sexual arousal, increased smooth muscle activity in the digestive tract

Cholinergic Receptors: Nicotinic vs. Muscarinic

• Nicotinic receptors: Found on all postganglionic neurons (sympathetic and parasympathetic); always excitatory

• Muscarinic receptors: Found on target tissues of parasympathetic neurons and some sympathetic targets; effects can be excitatory or inhibitory, mediated by G proteins and longer-lasting than nicotinic effects

The Enteric Nervous System (ENS)

Structure and Function

The Enteric Nervous System (ENS) is sometimes considered a third division of the ANS. It is an extensive network of neurons embedded in the walls of the digestive tract, capable of coordinating complex reflexes independently of the CNS.

• Contains approximately 100 million neurons

• Regulates digestive processes such as motility, secretion, and blood flow

• Uses all major neurotransmitters found in the brain

Summary Table: Sympathetic vs. Parasympathetic

Division	Origin	Preganglionic Fiber	Postganglionic Fiber	Neurotransmitters	Main Effects
Sympathetic	Thoracolumbar	Short	Long	ACh (preganglionic), NE (postganglionic)	"Fight or Flight"
Parasympathetic	Craniosacral	Long	Short	ACh (both synapses)	"Rest and Digest"

Additional info: The ANS is essential for maintaining homeostasis and adapting the body to changing internal and external environments. Understanding the differences between sympathetic and parasympathetic divisions is crucial for interpreting physiological responses and pharmacological interventions.