Chapter 13
The Peripheral Nervous System and Reflex Activity
487
13
Close your eyes and flex and extend your fingers—you can
feel exactly which joints are moving.
Check Your Understanding
1.
Your PNS mostly consists of nerves. What else belongs to
your PNS?
2.
You’ve cut your finger on a broken beaker in your A&P lab.
Using stimulus type, location, and receptor structure, classify
the sensory receptors that allow you to feel the pain.
For answers, see Appendix H.
Sensory Integration:
From Sensation to Perception
Outline the events that lead to sensation and perception.
Describe receptor and generator potentials and sensory
adaptation.
Describe the main aspects of sensory perception.
Our survival depends not only on
sensation
(awareness of
changes in the internal and external environments) but also on
perception
(conscious interpretation of those stimuli). For ex-
ample, a pebble in your shoe causes the
sensation
of localized
deep pressure, but your
perception
of it is an awareness of dis-
comfort. Perception in turn determines how we will respond: In
this case, you take off your shoe to get rid of the pesky pebble.
General Organization
of the Somatosensory System
Te
somatosensory system
—the part of the sensory system
serving the body wall and limbs—receives inputs from extero-
ceptors, proprioceptors, and interoceptors. Consequently, it
transmits information about several different sensory modali-
ties, or types of sensation.
Tree main levels of neural integration operate in the so-
matosensory (or any sensory) system
(Figure 13.2)
:
1
Receptor level:
sensory receptors
2
Circuit level:
processing in ascending pathways
3
Perceptual level:
processing in cortical sensory areas
Sensory input is generally relayed toward the head, but note that
it is also processed along the way.
Let’s examine the events that must occur at each level along
the pathway.
Processing at the Receptor Level
For sensation to occur, a stimulus must excite a receptor and
action potentials must reach the CNS (Figure 13.2,
1
). For this
to happen:
Te stimulus energy must match the
specificity
of the recep-
tor. For example, a touch receptor may be sensitive to me-
chanical pressure, stretch, and vibration, but not to light
corpuscles, lamellar corpuscles, bulbous corpuscles, muscle
spindles, tendon organs, and joint kinesthetic receptors.
Tactile corpuscles
or
Meissner’s corpuscles
are small recep-
tors in which a few spiraling sensory terminals are sur-
rounded by Schwann cells and then by a thin egg-shaped
connective tissue capsule. ±actile corpuscles are found just
beneath the epidermis in the dermal papillae and are espe-
cially numerous in sensitive and hairless skin areas such as
the nipples, fingertips, and soles of the feet. Tey are recep-
tors for discriminative touch, and apparently play the same
role in sensing light touch in hairless skin that hair follicle
receptors do in hairy skin.
Lamellar corpuscles
, also called
Pacinian corpuscles
, are
scattered deep in the dermis, and in subcutaneous tissue
underlying the skin. Although they are mechanoreceptors
stimulated by deep pressure, they respond only when the
pressure is first applied, and thus are best suited to monitor-
ing vibration (an “on/off” pressure stimulus). Tey are the
largest corpuscular receptors. Some are over 3 mm long and
half as wide and are visible to the naked eye as white, egg-
shaped bodies. In section, a lamellar corpuscle resembles a
cut onion. Its single dendrite is surrounded by a capsule con-
taining up to 60 layers of collagen fibers and flattened sup-
porting cells.
Bulbous corpuscles
or
Ruffini endings
, which lie in the der-
mis, subcutaneous tissue, and joint capsules, contain a spray
of receptor endings enclosed by a flattened capsule. Tey
bear a striking resemblance to tendon organs (which moni-
tor tendon stretch) and probably play a similar role in other
dense connective tissues where they respond to deep and
continuous
pressure.
Muscle spindles
are fusiform (spindle-shaped) propriocep-
tors found throughout the perimysium of a skeletal muscle.
Each muscle spindle consists of a bundle of modified skel-
etal muscle fibers, called
intrafusal fibers
(in
0
trah-fu
9
zal),
enclosed in a connective tissue capsule (±able 13.1). Muscle
spindles detect muscle stretch and initiate a reflex that resists
the stretch. We will consider the details of their innervation
later when we describe the stretch reflex (see p. 515).
Tendon organs
are proprioceptors located in tendons, close
to the skeletal muscle insertion. Tey consist of small bun-
dles of tendon (collagen) fibers enclosed in a layered capsule,
with sensory terminals coiling between and around the fi-
bers. When muscle contraction stretches the tendon fibers,
the resulting compression of the nerve fibers activates the
tendon organs. Tis initiates a reflex that causes the contract-
ing muscle to relax.
Joint kinesthetic receptors
(kin
0
es-thet
9
ik) are propriocep-
tors that monitor stretch in the articular capsules that enclose
synovial joints. Tis receptor category contains at least four
receptor types: lamellar corpuscles, bulbous corpuscles, free
nerve endings, and receptors resembling tendon organs. ±o-
gether these receptors provide information on joint position
and motion (
kines
5
movement), a sensation of which we are
highly conscious.
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