Ganong's Review of Medical Physiology, 23rd Edition

262
SECTION III
Central & Peripheral Neurophysiology

ANATOMIC ORGANIZATION

OF AUTONOMIC OUTFLOW

GENERAL FEATURES

Figure 17–1 compares some fundamental characteristics of
the innervation to skeletal muscles and innervation to smooth

muscle, cardiac muscle, and glands. As discussed in earlier

chapters, the final common pathway linking the central ner-

vous system (CNS) to skeletal muscles is the

α

-motor neuron.

Similarly, sympathetic and parasympathetic neurons serve as

the final common pathway from the CNS to visceral targets.

However, in marked contrast to the somatomotor nervous

system, the peripheral motor portions of the ANS are made up

of two neurons:

preganglionic

and

postganglionic neurons.

CLINICAL BOX 17–1

Multiple System Atrophy & Shy–Drager Syndrome

Multiple system atrophy (MSA)

is a neurodegenerative dis-

order associated with autonomic failure due to loss of

preganglionic autonomic neurons in the spinal cord and

brain stem. In the absence of an autonomic nervous system,

it is difficult to regulate body temperature, fluid and electro-

lyte balance, and blood pressure. In addition to these auto-

nomic abnormalities, MSA presents with cerebellar, basal

ganglia, locus coeruleus, inferior olivary nucleus, and pyrami-

dal tract deficits. MSA is defined as “a sporadic, progressive,

adult onset disorder characterized by autonomic dysfunc-

tion, parkinsonism, and cerebellar ataxia in any combina-

tion.”

Shy–Drager syndrome

is a subtype of MSA in which

autonomic failure dominates. The pathological hallmark of

MSA is cytoplasmic and nuclear inclusions in oligodendro-

cytes and neurons in central motor and autonomic areas.

There is also depletion of monoaminergic, cholinergic, and

peptidergic markers in several brain regions and in the cere-

brospinal fluid. Basal levels of sympathetic activity and

plasma norepinephrine levels are normal in MSA patients, but

they fail to increase in response to standing or other stimuli

and leads to severe

orthostatic hypotension.

In addition to

the fall in blood pressure, orthostatic hypotension leads to

dizziness, dimness of vision, and even fainting. MSA is also ac-

companied by parasympathetic dysfunction, including uri-

nary and sexual dysfunction. MSA is most often diagnosed in

individuals between 50 and 70 years of age; it affects more

men than women. Erectile dysfunction is often the first symp-

tom of the disease. There are also abnormalities in barorecep-

tor reflex and respiratory control mechanisms. Although auto-

nomic abnormalities are often the first symptoms, 75% of

patients with MSA also experience motor disturbances.

FIGURE 17–1
Comparison of peripheral organization and transmitters released by somatomotor and autonomic nervous systems
(NS).
ACh, acetylcholine; DA, dopamine; NE, norepinephrine; Epi, epinephrine.
(From Widmaier EP, Raff H, Strang KT:
Vander’s Human Physiology.
McGraw-Hill,
2008.)

Autonomic nervous system:

Parasympathetic division

Somatic nervous system

CNS

CNS

Effector

organ

Effector

organ

Effector

organ

Effector

organ

ACh

ACh Ganglion ACh NE

Adrenal

medulla

Epi (also NE, DA, peptides)

(via bloodstream)

Ganglion ACh

CNS

Autonomic nervous system:

Sympathetic division