Essentials of Anatomy and Physiology

sympathetic; it is inactivated by cholinesterase in
postganglionic neurons. Parasympathetic postgan-
glionic neurons all release acetylcholine at the
synapses with their visceral effectors. Most sympa-
thetic postganglionic neurons release the transmitter
norepinephrineat the synapses with the effector cells.
Norepinephrine is inactivated by either catechol-O-
methyl transferase (COMT) or monoamine oxidase
(MAO), or it may be removed from the synapse by
reuptake.

AGING AND THE

NERVOUS SYSTEM

The aging brain does lose neurons, but this is only a
small percentage of the total and not the usual cause of
mental impairment in elderly people. (Far more com-
mon causes are depression, malnutrition, hypotension,
and the side effects of medications.) Some forgetful-
ness is to be expected, however, as is a decreased abil-
ity for rapid problem solving, but most memory
should remain intact. Voluntary movements become
slower, as do reflexes and reaction time. Think of driv-

ing a car, an ability most of us take for granted. For

elderly people, with their slower perceptions and reac-

tion times, greater consciousnessof driving is necessary.

As the autonomic nervous system ages, dry eyes and

constipation may become problems. Transient hypo-

tension may be the result of decreased sympathetic

stimulation of vasoconstriction. In most cases, how-

ever, elderly people who are aware of these aspects of

aging will be able to work with their physicians or

nurses to adapt to them.

SUMMARY

The nervous system regulates many of our simplest

and our most complex activities. The impulses gener-

ated and carried by the nervous system are an example

of the chemical level of organization of the body.

These nerve impulses then regulate the functioning of

tissues, organs, and organ systems, which permits us to

perceive and respond to the world around us and the

changes within us. The detection of such changes is

the function of the sense organs, and they are the sub-

ject of our next chapter.

The Nervous System 191

Table 8–5 FUNCTIONS OF THE AUTONOMIC NERVOUS SYSTEM

Organ Sympathetic Response Parasympathetic Response

Heart (cardiac muscle)

Bronchioles (smooth muscle)

Iris (smooth muscle)

Salivary glands

Stomach and intestines (smooth muscle)

Stomach and intestines (glands)

Internal anal sphincter

Urinary bladder (smooth muscle)

Internal urethral sphincter

Liver

Pancreas

Sweat glands

Blood vessels in skin and viscera

(smooth muscle)

Blood vessels in skeletal muscle

(smooth muscle)

Adrenal glands

• Increase rate


• Dilate


• Pupil dilates


• Decrease secretion


• Decrease peristalsis


• Decrease secretion


• Contracts to prevent defecation


• Relaxes to prevent urination


• Contracts to prevent urination


• Changes glycogen to glucose


• Secretes glucagon


• Increase secretion


• Constrict


• Dilate


• Increase secretion of epineph-
  rine and norepinephrine
  
  
  • Decrease rate (to normal)
  
  
  • Constrict (to normal)
  
  
  • Pupil constricts (to normal)
  
  
  • Increase secretion (to normal)
  
  
  • Increase peristalsis for normal digestion
  
  
  • Increase secretion for normal digestion
  
  
  • Relaxes to permit defecation
  
  
  • Contracts for normal urination
  
  
  • Relaxes to permit urination
  
  
  • None
  
  
  • Secretes insulin and digestive enzymes
  
  
  • None
  
  
  • None
  
  
  • None
  
  
  • None