HUMAN BIOLOGY

268 Chapter 14

somatic sensation
Sensory input from the vari-
ous types of receptors that
are scattered throughout
the body, such as free nerve
endings.

somatosensory cortex
The part of the cerebrum
that receives signals from
somatic receptors.

somatic sensations

n Somatic sensations start with receptors near the body

surface, in skeletal muscles, and in the walls of soft internal

organs.

n Links to Skin structure 4.9, Inflammation 9.4,

Somatosensory cortex 13.8

Receptors for somatic senses are

scattered in different parts of the

b o dy. Somatic sensations come

about when signals from receptors

reach the somatosensory cortex

in the cerebrum. There, interneurons

are organized like maps of indi-

vidual parts of the body surface, just

as they are for the motor cortex. The

largest areas of the map correspond

to body parts where sensory recep-

tors are the most dense. These body parts, including the

fingers, thumbs, and lips, have the sharpest sensory acuity

and require the most intricate control (Figure 14.3).

receptors near the body surface sense

touch, pressure, and more

There are thousands of sensory receptors in your skin,

providing information about touch, pressure, cold,

warmth, and pain (Figure 14.4). Places with the most sen-

sory receptors, such as the fingertips and the tip of the

tongue, are the most sensitive. Less sensitive areas, such as

the back of the hand, have many fewer receptors.

There are several types of free nerve endings in the epi-

dermis and many connective tissues. As noted in Section

14.1, this form of sensory receptor detects touch, pressure,

heat, cold, or pain. Free nerve endings are simple struc-

tures. Basically, they are thinly myelinated or unmyelin-

ated (“naked”) dendrites of sensory neurons. One type

coils around hair follicles and detects the movement of the

hair inside. That might be how, for instance, you become

aware that a spider is gingerly making its way across your

arm. Free nerve endings that are sensitive to chemicals

such as histamine may be responsible for the sensation of

itching.

The dermis of your skin contains four types of encap-

sulated receptors. Each bears the name of its discoverer.

One type, Merkel’s discs, adapt slowly and are the most

important receptors for steady touch. In the lips, fingertips,

eyelids, nipples, and genitals there are many Meissner’s

corpuscles, which are sensitive to light touching. Deep in

the dermis and in joint capsules are Ruffini endings, which

respond to steady pressure.

The Pacinian corpuscles widely scattered in the dermis

are sensitive to deep pressure and vibrations. They also are

located near freely movable joints (like shoulder and hip

joints) and in some soft internal organs.

Sensing limb motions and changes in body position

relies on mechanoreceptors in skin, skeletal muscles, joints,

tendons, and ligaments. Examples include the stretch

receptors of muscle spindles described in Section 14.1.

Free nerve endings detect stimuli that we

perceive as pain and itching

Pain is perceived injury to some body region. The term for

the body’s most important pain receptors, “nociceptor,”

comes from the Latin word nocere, meaning “to do harm.”

Nociceptors are free nerve endings. Several million of them

are distributed throughout the skin and in all internal tis-

sues except the brain.

Somatic pain starts with nociceptors in skin, skeletal

muscles, joints, and tendons. One group is the source of

prickling pain, like the jab of a pin when you stick your

finger. Another contributes to itching or the feeling of

warmth caused by chemicals such as histamine. Sensations

of visceral pain, which is associated with internal organs, are

related to muscle spasms, muscle fatigue, too little blood

flow to organs, and other abnormal conditions.

When cells are damaged, they release chemicals that

activate neighboring pain receptors. The most potent of

these substances are bradykinins. They trigger the release

of histamine, prostaglandins, and other substances associ-

ated with inflammation (Section 9.4).

When signals from pain receptors reach interneurons

in the spinal cord, the interneurons release a chemical

called substance P. One result is that the hypothalamus and

14.2

nose

eye

thumb

index

middle

genitalia

toes

foot

hipleg

ringlittle

handwrist

forearm

elbowarmshoulderheadnecktrunk

face

upper lip

lips

lower lip

teeth, gums, and jaw

tongue

pharynx

intra-abdominal

Figure 14.3 The somato sensory cortex “maps” body parts. This

strip of cerebral cortex is a little wider than an inch (2.5 centimeters),

from the top of the head to just above the ear.

Dr. Colin Chumbley/Science Source

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