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Autonomic Nervous System

I. INTRODUCTION. The autonomic nervous system (ANS) is a general visceral efferent motor system that controls and regulates smooth muscle, cardiac muscle, and glands.

A. The ANS consists of two types of projection neurons:

1. Preganglionic neurons

2. Postganglionic neurons. Sympathetic ganglia have interneurons.

B. Autonomic output is controlled by the hypothalamus.

C. The ANS has three divisions:

1. Sympathetic. Figure 18-1 shows the sympathetic innervation of the ANS.

2. Parasympathetic. Figure 18-2 shows the parasympathetic innervation of the ANS. Table 18-1 compares the effects of sympathetic and parasympathetic activity on organ systems.

3. Enteric. The enteric division includes the intramural ganglia of the gastrointestinal tract, submucosal plexus, and myenteric plexus.

II. CRANIAL NERVES (CN) WITH PARASYMPATHETIC COMPONENTS include the following:

A. CN III (ciliary ganglion)

B. CN VII (pterygopalatine and submandibular ganglia)

C. CN IX (otic ganglion)

D. CN X [terminal (mural) ganglia]

III. COMMUNICATING RAMI of the ANS include:

A. White communicating rami, which are found between T-l and L-3, are myelinated.

B. Gray communicating rami, which are found at all spinal levels, are unmyelinated.

IV. NEUROTRANSMITTERS of the ANS include:

A. Acetylcholine, which is the neurotransmitter of the preganglionic neurons

B. Norepinephrine, which is the neurotransmitter of the postganglionic neurons, with the exception of sweat glands and some blood vessels that receive cholinergic sympathetic innervation

C. Dopamine, which is the neurotransmitter of the small intensely fluorescent (SIF) cells, which are interneurons of the sympathetic ganglia

Superior cervical ganglion.

Superior cervical ganglion.

Where Thesuperior Lacriminal Gland

- Lacrimal gland " Eye: dilator of pupil

Submandibular and sublingual glands

Parotid gland

Heart

Bronchial tree

Stomach

Small intestine

Large intestine

Ductus deferens

N Sympathetic trunk

Figure 18-1. The sympathetic (thoracolumbar) innervation of the autonomic nervous system. The entire sympathetic innervation of the head is through the superior cervical ganglion. Gray communicating rami are found at all spinal cord levels. White communicating rami arc found only in spinal segments T-l through L-3.

N Sympathetic trunk

Tarsal muscle

- Lacrimal gland " Eye: dilator of pupil

Submandibular and sublingual glands

Parotid gland

Heart

Bronchial tree

Stomach

Small intestine

Large intestine

Ductus deferens

Figure 18-1. The sympathetic (thoracolumbar) innervation of the autonomic nervous system. The entire sympathetic innervation of the head is through the superior cervical ganglion. Gray communicating rami are found at all spinal cord levels. White communicating rami arc found only in spinal segments T-l through L-3.

Midbrain

Pons

Medulla

White Communicating Rumi Spinal Cord

Ciliary ganglion

Pterygopalatine ganglion

Otic ganglion

Genital erectile tissue

Lacrimal nucleus >

Superior salivatory _ nucleus------

Inferior salivatory nucleus

Dorsal motor nucleus of vagal nerve

Submandibular ganglion

Pelvic splanchnic nerves

Eye: constrictor of pupil and ciliary body

Lacrimal and nasal glands

Submandibular and sublingual glands

Parotid gland

Heart

Bronchial tree

Stomach

Small intestine

Large intestine

Urinary bladder

Figure 18-2. The parasympathetic (craniosacral) innervation of" the autonomic nervous system. Sacral outflow includes segments S-2 through S-4. Cranial outflow is mediated through cranial nerves (CN) III, VII, IX, and X.

Table 18-1

Sympathetic and Parasympathetic Activity on Organ Systems

Structure

Sympathetic Function

Parasympathetic Function

Radial muscle of iris Circular muscle of iris Ciliary muscle of ciliary body Lacrimal gland Salivary glands Sweat glands Thermoregulatory Apocrine (stress) Heart Sinoatrial node Atrioventricular node Contractility Vascular smooth muscle Skin, splanchnic vessels Skeletal muscle vessels Bronchiolar smooth muscle Gastrointestinal tract Smooth muscle Walls Sphincters Secretion and motility Genitourinary tract Smooth muscle Bladder wall Sphincter

Penis, seminal vesicles Adrenal medulla

Metabolic functions Liver

Fat cells Kidney

Dilation of pupil (mydriasis)

Viscous secretion

Increase Increase

Acceleration

Increase in conduction velocity Increase

Contraction

Relaxation

Relaxation

Relaxation

Contraction

Decrease

Little or no effect

Contraction

Ejaculation*

Secretion of epinephrine and norepinephrine

Gluconeogenesis and glycogenosis Lipolysis Renin release

Constriction of pupil (miosis) Contraction for near vision Stimulation of secretion Watery secretion

Deceleration (vagal arrest) Decrease in conduction velocity Decrease (atria)

Contraction

Contraction

Relaxation

Increase

Contraction

Relaxation

Erection*

♦Note erection versus ejaculation: Remember point and shoot: p = parasympathetic, s sympathetic. Reprinted with permission from Fix J: BRS Neuroanatomy. Media. PA, Williams & Wilkins, 1991.

D. Vasoactive intestinal polypeptide (VIP), a vasodilator that is colocalized with acetylcholine in some postganglionic parasympathetic fibers

E. Nitric oxide (NO), a newly discovered neurotransmitter that is responsible for the relaxation of smooth muscle. It is also responsible for penile erection (see Chapter 22).

V. CLINICAL CORRELATION

A. Megacolon (Hirschsprung's disease, or congenital aganglionic megacolon) is characterized by extreme dilation and hypertrophy of the colon, with fecal retention, and by the absence of ganglion cells in the myenteric plexus. It occurs when neural crest cells do not migrate into the colon.

B. Familial dysautonomia (Riley-Day syndrome) predominantly affects Jewish children. It is an autosomal recessive trait that is characterized by abnormal sweating, un stable blood pressure (e.g., orthostatic hypotension), difficulty in feeding (as a result of inadequate muscle tone in the gastrointestinal tract), and progressive sensory loss. It results in the loss of neurons in the autonomic and sensory ganglia.

C. Raynaud's disease is a painful disorder of the terminal arteries of the extremities. It is characterized by idiopathic paroxysmal bilateral cyanosis of the digits (as a result of arterial and arteriolar constriction because of cold or emotion). It may be treated by preganglionic sympathectomy.

D. Peptic ulcer disease results from excessive production of hydrochloric acid because of increased parasympathetic (tone) stimulation.

E. Horner's syndrome (see Chapter 17) is oculosympathetic paralysis.

F. Shy-Drager syndrome involves preganglionic sympathetic neurons from the inter-mediolateral cell column. It is characterized by orthostatic hypotension, anhidrosis, impotence, and bladder atonicity.

G. Botulism. The toxin of Clostridium botulinum blocks the release of acetylcholine and results in paralysis of all striated muscles. Autonomic effects include dry eyes, dry mouth, and gastrointestinal ileus (bowel obstruction).

H. Lambert-Eaton myasthenic syndrome (see Chapter 22)

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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