15. PHYSIOLOGY OF THE ANS

15. PHYSIOLOGY OF THE ANS

Autonomic Tone

•  most body organs receive innervation from both divisions of the ANS, which typically work in opposition to one another. 
• The balance between sympathetic and parasympathetic activity, called autonomic tone, 
• is regulated by the hypothalamus. 

• Typically, the hypothalamus turns up sympathetic tone at the same time it turns down parasympathetic tone, and vice versa. 

The two divisions can affect body organs differently 

• because their postganglionic neurons release different neurotransmitters
• and because the effector organs possess different adrenergic and cholinergic receptors. 

A few structures receive only sympathetic innervation—

• sweat glands,
•  arrector pili muscles attached to hair follicles in the skin, 
• the kidneys, 
• the spleen, 
• most blood vessels, 
• and the adrenal medullae .

•  In these structures there is no opposition from the parasympathetic division. 
• Still, an increase in sympathetic tone has one effect, and a decrease in sympathetic tone produces the opposite effect.

Sympathetic Responses

• During physical or emotional stress, the sympathetic division dominates the parasympathetic division. 

• High sympathetic tone favors body functions that can support vigorous physical activity and rapid production of ATP. 
• At the same time, the sympathetic division reduces body functions that favor the storage of energy.

• Besides physical exertion, various emotions—such as fear, embarrassment, or rage—stimulate the sympathetic division.
• Visualizing body changes that occur during “E situations” such as exercise, emergency, excitement, and embarrassment will help you remember most of the sympathetic responses.

• Activation of the sympathetic division and release of hormones by the adrenal medullae set in motion a series of physiological responses collectively called the fight-or-flight response, which includes the following effects:

1. The pupils of the eyes dilate.
2. Heart rate, force of heart contraction, and blood pressure increase.
3. The airways dilate, allowing faster movement of air into and out of the lungs.
4.  The blood vessels that supply the kidneys and gastrointestinal tract constrict, which decreases blood flow through these tissues. • The result is a slowing of urine formation and digestive activities, which are not essential during exercise.
5. Blood vessels that supply organs involved in exercise or fighting off danger—skeletal muscles, cardiac muscle, liver, and adipose tissue—dilate, allowing greater blood flow through these tissues.
6. Liver cells perform glycogenolysis (breakdown of glycogen to glucose), and adipose tissue cells perform lipolysis (breakdown of triglycerides to fatty acids and glycerol).
7. Release of glucose by the liver increases blood glucose level.
8. Processes that are not essential for meeting the stressful situation are inhibited. 

For example,

•  muscular movements of the gastrointestinal tract and digestive secretions slow down or even stop.

• The effects of sympathetic stimulation are longer lasting and more widespread than the effects of parasympathetic stimulation for three reasons: 

(1) Sympathetic postganglionic axons diverge
more extensively; as a result, many tissues are activated simultaneously.

(2) Acetylcholinesterase quickly inactivates acetylcholine,
but norepinephrine lingers in the synaptic cleft for a
longer period. 

(3) Epinephrine and norepinephrine secreted into
the blood from the adrenal medulla intensify and prolong the responses caused by NE liberated from sympathetic postganglionic axons. 

• These blood-borne hormones circulate throughout the body, 
• affecting all tissues that have alpha and beta receptors.
• In time, blood-borne NE and epinephrine are inactivated by enzymatic destruction in the liver.

 Parasympathetic Responses


In contrast to the fight-or-flight activities of the sympathetic division,

• the parasympathetic division enhances rest and digest activities. 
• Parasympathetic responses support body functions that conserve and restore body energy during times of rest and recovery. 

• In the quiet intervals between periods of exercise, parasympathetic impulses to the digestive glands and the smooth muscle of the gastrointestinal tract predominate over sympathetic impulses.
•  This allows energy-supplying food to be digested and absorbed. 

• At the same time, parasympathetic responses reduce body functions that support physical activity.

The acronym SLUDD can be helpful in remembering five parasympathetic responses. 

It stands for 

1. salivation (S),
2. lacrimation (L), 
3. urination (U), 
4. digestion (D), and 
5. defecation (D). 

• All of these activities are stimulated mainly by the parasympathetic division. 
• Besides the increasing SLUDD responses, other important  parasympathetic responses are “three decreases”: 

1. decreased heart rate, 
2. decreased diameter of airways (bronchoconstriction),
3. and decreased diameter (constriction) of the pupils.

Comparison of Sympathetic and Parasympathetic Divisions of the ANS
	SYMPATHETIC (THORACOLUMBAR)	PARASYMPATHETIC (CRANIOSACRAL)
Distribution	Wide regions of the body: skin, sweat glands, arrector pili muscles of hair follicles, adipose tissue, smooth muscle of blood vessels.	Limited mainly to head and to viscera of thorax, abdomen, and pelvis; some blood vessels.
Location of    preganglionic neuron cell bodies and site of outflow	Cell bodies of preganglionic neurons are located in lateral gray horns of spinal cord segments T1–L2. Axons of preganglionic neurons constitute thoracolumbar outflow.	Cell bodies of preganglionic neurons are located in the nuclei of cranial nerves III, VII, IX, and X and the lateral gray matter of spinal cord segments S2–S4. Axons of preganglionic neurons constitute craniosacral outflow.
Associated    ganglia	Two types: sympathetic trunk ganglia and prevertebral ganglia.	One type: terminal ganglia.
Ganglia locations	Close to CNS and distant from visceral effectors.	Typically near or within wall of visceral effectors.
Axon length and divergence	Preganglionic neurons with short axons synapse with many postganglionic neurons with long axons that pass to many visceral effectors.	Preganglionic neurons with long axons usually synapse with four to five postganglionic neurons with short axons that pass to a single visceral effector.
Rami communicantes	Both present; white rami communicantes contain myelinated preganglionic axons, and gray rami communicantes contain unmyelinated postganglionic axons.	Neither present.
Neurotransmitters	Preganglionic neurons release acetylcholine (ACh), which is excitatory and stimulates postganglionic neurons; most postganglionic neurons release norepinephrine (NE); postganglionic neurons that innervate most sweat glands and some blood vessels in skeletal muscle release ACh.	Preganglionic neurons release acetylcholine (ACh), which is excitatory and stimulates postganglionic neurons; postganglionic neurons release ACh.
Physiological effects	Fight-or-flight responses.	Rest-and-digest activities.

Effects of Sympathetic and Parasympathetic Divisions of the ANS
VISCERAL EFFECTOR	EFFECT OF SYMPATHETIC STIMULATION (α OR β )ADRENERGIC RECEPTORS, EXCEPT AS NOTED)*	EFFECT OF PARASYMPATHETIC STIMULATION (MUSCARINIC ACh RECEPTORS)
GLANDS
Adrenal medullae	Secretion of epinephrine and norepinephrine (nicotinic ACh receptors).	No known effect.
Lacrimal (tear)	Slight secretion of tears (α).	Secretion of tears.
Pancreas	Inhibits secretion of digestive enzymes and the hormone insulin (α2); promotes secretion of the hormone glucagon (β2).	Secretion of digestive enzymes and the hormone insulin.
Posterior pituitary	Secretion of antidiuretic hormone (ADH) (β1).	No known effect.
Pineal	Increases synthesis and release of melatonin (β).	No known effect
Sweat	Increases sweating in most body regions (muscarinic ACh receptors); sweating on palms and soles (α1).	No known effect.
Adipose tissue†	Lipolysis (breakdown of triglycerides into fatty acids and glycerol) (β1); release of fatty acids into blood (β1 and β3).	No known effect.
Liver †	Glycogenolysis (conversion of glycogen into glucose); gluconeogenesis (conversion of noncarbohydrates into glucose); decreased bile secretion (α and β2).	Glycogen synthesis; increased bile secretion.
Kidney, juxtaglomerular cells†	Secretion of renin (β1).	No known effect.
CARDIAC (HEART) MUSCLE
	Increased heart rate and force of atrial and ventricular contractions (β1).	Decreased heart rate; decreased force of atrial contraction.
SMOOTH MUSCLE
Iris, radial muscle	Contraction → dilation of pupil (α1).	No known effect
Iris, circular muscle	No known effect.	Contraction → constriction of pupil.
Ciliary muscle of eye	Relaxation for distant vision (β2).	Contraction for close vision.
Lungs, bronchial muscle	Relaxation → airway dilation (β2).	Contraction → airway constriction
Gallbladder and ducts	Relaxation (β2).	Contraction → increased release of bile into small intestine.
Stomach and intestines	Decreased motility and tone(α1, α2, β2); contraction of sphincters (α1).	Increased motility and tone; relaxation of sphincters.
Spleen	Contraction and discharge of stored blood into general circulation (α1).	No known effect.
Ureter	Increases motility (α1).	Increases motility (?).
Urinary bladder	Relaxation of muscular wall (β2); contraction of sphincter (α1).	Contraction of muscular wall; relaxation of sphincter.
Uterus	Inhibits contraction in nonpregnant women (β2); promotes contraction in pregnant women (α1).	Minimal effect.
Sex organs	In males : contraction of smooth muscle of ductus (vas) deferens, seminal vesicle, prostate → ejaculation of semen (α1).	Vasodilation; erection of clitoris (females) and penis(males).
Hair follicles, arrector pili muscle	Contraction → erection of hairs (α1).	No known effect.
VASCULAR SMOOTH MUSCLE
Salivary gland arterioles	Vasoconstriction, which decreases secretion (α1).	Vasodilation, which increases K+ and water secretion.
Gastric gland arterioles	Vasoconstriction, which inhibits secretion (α1).	Secretion of gastric juice.
Intestinal gland arterioles	Vasoconstriction, which inhibits secretion (α1).	Secretion of intestinal juice.
Coronary (heart) arterioles	Relaxation → vasodilation (β2); contraction → vasoconstriction (α1, α2); contraction → vasoconstriction (muscarinic ACh receptors).	Contraction → vasoconstriction.
Skin and mucosal arterioles	Contraction → vasoconstriction (α1).	Vasodilation, which may not be physiologically significant.
Skeletal muscle arterioles	Contraction → vasoconstriction (α1); relaxation : vasodilation (β2); relaxation → vasodilation (muscarinic ACh receptors).	No known effect.
Abdominal viscera arterioles	Contraction : vasoconstriction (α1, β2).	No known effect
Brain arterioles	Slight contraction → vasoconstriction (α1).	No known effect.
Kidney arterioles	Constriction of blood vessels → decreased urine volume (α1).	No known effect.
Systemic veins	Contraction → constriction (α1); relaxation → dilation (β2).	No known effect.

*Subcategories of and receptors are listed if known. 

†Grouped with glands because they release substances into the blood.