HUMAN BIOLOGY

SenSory SyStemS 269

What are somatic sensations?

• Somatic sensations include touch, pressure, heat and cold, pain,
  itching, limb motions, and changes in body positions. They are
  detected by free nerve endings and encapsulated receptors.


• The phenomenon of referred pain arises when the brain
  misidentifies the bodily origin of a pain signal.

taKe-Home message

midbrain send signals that call for the release of endorphins
and enkephalins. These are natural opiates (morphine-like
substances) that, like morphine derived from opium pop-
pies, reduce our ability to perceive pain. Morphine, hypno-
sis, and natural childbirth techniques may also stimulate
the release of these natural opiates.

Referred pain is a matter of perception

A person’s perception of pain often depends on the brain’s
ability to identify the affected tissue. Get hit in the face

Ruffini endings

(pressure, touch)

free nerve

endings

(pain) hair

Meissner’s

corpuscle

(light touch)

Merkel’s discs

(steady touch)

hair follicle

receptor

(hair displacement)

Pacinian

corpuscle

(deep pressure,

vibrations)

epidermis

subcutaneous

layer

dermis

Pacinian

corpuscle

Meissner’s

corpuscle

Ruffini

endings

Merkel’s discs

Figure 14.5 In referred pain, the brain projects a sensation
from an internal organ to an area of the skin. (© Cengage Learning)

lungs,

diaphragm

heart

liver, gallbladder

stomach

small intestine

ovaries

colon

appendix

urinary

bladder

kidney

ureter

Figure 14.4 The skin contains a variety of sensory receptors. (© Cengage Learning)

with a snowball and you “feel” the contact on facial skin.

However, sensations of pain from some inter nal organs

may be wrongly projected to part of the skin surface. This

response, called referred pain, is related to the way the ner-

vous system is built. Sensory information from the skin

and from certain internal organs may enter the spinal

cord along the same nerve pathways, so the brain can’t

accurately identify their source. For example, as shown in

Figure 14.5, a heart attack can be felt as pain in skin above

the heart and along the left shoulder and arm, or between

the shoulder blades.

Referred pain is not the same as the phantom pain

reported by amputees. Often people who have lost a body

part sense the presence of the missing part, as if it were

still there. In some undetermined way, sensory nerves that

were cut during the amputation continue to respond to the

trauma. The brain projects the pain back to the missing

part, past the healed region.

Frederic Cirou/PhotoAlto/Corbis

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).