Ch 15- The Autonomic Nervous System and Visceral Reflexes

• ANS is the motor system controlling glands, cardiac, and smooth muscle.

• Targets include thoracic and abdominal viscera, and some body wall structures (sweat glands, cutaneous vessels, arrector muscles). Acts involuntarily and adjusts organ activity to the body’s needs.

What is the autonomic nervous system (ANS) and its primary targets?

Denervation hypersensitivity occurs—exaggerated responses because visceral effectors can function without ANS but lose regulation.

What happens if ANS nerves to an organ are severed?

visceral reflex

Unconscious, automatic response to stimulation involving visceral receptors/effectors.

Denervation hypersensitivity—exaggerated responses.

What happens if ANS nerves to an organ are severed?

1. Receptors 

2. afferent neurons 

3. CNS integrating center 

4. efferent neurons 

5. effectors.

What is the sequence of a visceral reflex arc?

Sympathetic and parasympathetic.

Name the two divisions of the ANS.

• Prepares the body for fight-or-flight

• increases heart rate, BP, glucose

• decreases digestive & skin blood flow

What is the function of the sympathetic division?

Rest-and-digest: conserves energy, aids digestion and waste elimination.

What is the function of the parasympathetic division?

Glands, cardiac muscle, and smooth muscle.

What does the autonomic nervous system (ANS) control?

Thoracic and abdominal viscera, and some body wall structures (blood vessels, sweat glands, arrector muscles).

Which organs are primary targets of the ANS?

Involuntary; it adjusts organ activity to the body’s needs.

Is ANS activity voluntary or involuntary?

autonomic tone

Baseline activity representing the balance of sympathetic and parasympathetic activity.

Maintains intestinal smooth muscle tone; keeps resting heart rate ~70–80 bpm.

Example of parasympathetic tone?

Keeps blood vessels partially constricted, maintaining blood pressure.

Example of sympathetic tone?

Hypothalamus and brainstem.

Where are ANS control nuclei located?

• ANS uses two neurons (preganglionic → postganglionic) with a synapse in a ganglion

• somatic motor uses one neuron to reach skeletal muscle

How does the ANS motor pathway differ from somatic motor pathway?

Acetylcholine (ACh).

Neurotransmitter of preganglionic neuron?

Parasympathetic: ACh; Sympathetic: norepinephrine (NE) or ACh.

Neurotransmitter of postganglionic neuron?

Thoracic and lumbar spinal cord (thoracolumbar division).

Where does the sympathetic division arise?

Short preganglionic, long postganglionic.

Length of sympathetic fibers?

Lateral horns and nearby gray matter of T1–L2 spinal cord segments.

Where are preganglionic cell bodies of sympathetic neurons?

sympathetic chain (paravertebral ganglia)

Longitudinal ganglia next to vertebral column, connecting to spinal nerves via rami.

Via spinal nerves, sympathetic nerves, or splanchnic nerves.

How do postganglionic fibers exit the sympathetic chain?

collateral ganglia

Ganglia outside the chain that form the abdominal aortic plexus (celiac, superior mesenteric, inferior mesenteric).

neuronal divergence in the sympathetic division

One preganglionic neuron can excite 10–20 postganglionic neurons → widespread effects.

Two glands in one: cortex (steroid hormones) and medulla (sympathetic-like, secretes catecholamines).

Structure of adrenal glands?

Releases 85% epinephrine, 15% norepinephrine into bloodstream; part of sympathoadrenal system.

Function of adrenal medulla?

Brainstem and sacral spinal cord (craniosacral division).

Where does parasympathetic division arise?

Long preganglionic, short postganglionic.

Length of parasympathetic fibers?

Highly selective; little neural divergence.

How selective is parasympathetic stimulation?

• CN 3 (oculomotor)

• CN 7 (facial)

• CN 9 (glossopharyngeal)

• CN 10 (vagus)

Cranial nerves carrying parasympathetic fibers?

Distal colon, rectum, bladder, reproductive organs.

Sacral parasympathetic fibers target which organs?

enteric plexus

Nervous network in digestive tract walls with ~500 million neurons; regulates motility and secretion.

Hirschsprung disease

Absence of enteric plexus in sigmoid colon/rectum → megacolon; constipation and bowel dilation.

Acetylcholine (ACh).

Which neurotransmitter is used by all preganglionic neurons?

• Muscarinic (on target cells, excitatory or inhibitory) 

• nicotinic (on postganglionic neurons, always excitatory)

Types of cholinergic receptors?

Norepinephrine (NE).

Which neurotransmitter is used by most sympathetic postganglionic neurons?

• Alpha (usually excitatory, α1 & α2) 

• beta (usually inhibitory, β1 & β2).

Types of adrenergic receptors?

Sympathetic effects last longer; NE lingers, ACh is broken down quickly.

How do sympathetic and parasympathetic effects differ in duration?

dual innervation

Most organs receive both sympathetic and parasympathetic fibers.

One division increases activity while the other decreases it (e.g., heart rate).

Antagonistic effects of dual innervation?

Divisions act on different effectors to produce a unified effect (e.g., saliva production).

Cooperative effects of dual innervation?

Yes; some organs receive only sympathetic fibers (e.g., blood vessels, adrenal medulla, sweat glands).

Can ANS control organs without dual innervation?

Cerebral cortex, limbic system, hypothalamus, midbrain, pons, medulla, spinal cord.

Which CNS regions influence the ANS?

Limbic system connects to hypothalamus → emotional influence on autonomic function.

How do emotions affect the ANS?

• Sympathomimetics enhance sympathetic activity

• sympatholytics suppress it

Sympathomimetics vs sympatholytics?

• Parasympathomimetics enhance parasympathetic effects

• parasympatholytics suppress them.

Parasympathomimetics vs parasympatholytics?

• SSRIs (Prozac) prolong serotonin effect

• MAO inhibitors slow monoamine breakdown

• caffeine blocks adenosine receptors

Examples of CNS-acting drugs?