CHAPTER 7
Nervous Tissue
125
Figure 7-8A.
Peripheral sensory receptors.
Axons of the neuron cell bodies in the posterior root
ganglia carry information from
sensory receptors
in the
skin, muscles, joints, viscera, blood vessels, mesentaries,
and other connective tissues. Sensory receptors associ-
ated with these axons may consist of
encapsulated axon
endings
, specialized endings around
hair follicles
, endings
in conjunction with specialized cells (e.g.,
Merkel cells
),
muscle spindles
, and specialized regions of axonal mem-
branes termed
free nerve endings
(see Figs. 6-7A,B and
13-1).
Meissner corpuscles
and
Pacinian corpuscles
are
two types of encapsulated endings and are easier to see
using light microscopy than other receptors in the skin.
In these receptors, the axonal membrane is specialized to
change its permeability in response to mechanical pres-
sure (the regions of specialization are indicated by the
thicker blue lines
). The connective tissue capsules help to
focus mechanical force on the axonal membrane.
J. Lynch &T. Yang
Meissner corpuscle
Basement
membrane
Pacinian corpuscle
Epidermis
Capsule
Capsule
Myelin
Schwann
cell
A
Epidermis
Epidermis
Meissner
Meissner
corpusle
corpusle
Epidermis
Meissner
corpusle
Dermis
B
Figure 7-8B.
Meissner corpuscle, thick skin of palm.
H&E,
3
136; inset
3
360
Meissner corpuscles
are encapsulated, quickly adapting
mechanoreceptors that are sensitive to light touch and
low-frequency vibration. They are important for the
sensation of discriminative touch. Meissner corpuscles
are located in the
dermal ridges
, at the interface of the
dermis
and
epidermis
. Only the nuclei of the capsu-
lar connective tissue cells are visible after H&E stains.
Using special stains, the corpuscle looks like a stack of
pancakes (Schwann cells, Fig. 7-8A) with one or more
axons intertwining among them. The main axons associ-
ated with Meissner corpuscles are large (6–12
μ
m) and
heavily myelinated, hence their rapid conduction veloci-
ties. Meissner corpuscles are found in all parts of the skin
of the hand and foot, in the lips, and in a number of
other locations but are most concentrated in thick, hair-
less (
glabrous
) skin.
Pacinian corpuscles
Capsule
Axon
Core
Growth zone
Adipose tissue
Connective tissue
C
Figure 7-8C.
Pacinian corpuscle, thick skin of palm.
H&E,
3
68; inset
3
105
Pacinian corpuscles
are large, encapsulated structures that
detect very light touch and vibration. They are located
primarily in the
hypodermis
of the palms of the hands
and ± ngers and the soles of the feet but are also found in
other areas of skin as well as in the periostea and mes-
entery. They are much larger than Meissner corpuscles,
sometimes reaching 2 mm in length (note different mag-
ni±
cation). Pacinian corpuscles consist of a specialized
zone of axonal membrane that is exquisitely sensitive
to pressure (Fig. 7-8A) surrounded by a layered cellular
structure that consists of a
central core
immediately sur-
rounding the axon, an
intermediate growth zone
, and an
outer capsule
. Around 60 to 100 layers are formed by
very thin cells that overlap at their edges, giving the Pacin-
ian corpuscle the appearance of an onion when
sectioned.
This sample is from the hypodermis of the skin.
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