Somatosensory Receptors

614 Merkel Nerve Endings

Free nerve endings are found in the dermis, the locomotor system and the walls of hollow organs. In addition to these nerve endings, the dermis features Merkel mechanoreceptors (Merkel’s disks) as specialized nerve endings. They are located in the stratum basale of the epidermis, among other locations. This figure describes Merkel’s nerve endings in the epidermis of the nasal planum of a cat. The nerve endings extend from the ellipsoid Merkel cell 1 . The longitudinal cell axis measures about 9–20 μm. The axon terminates in the Merkel disk 2 . The Merkel cell nucleus is large and lobed. Merkel cells are connected via desmosomes 4 with the neighboring keratinocytes 3 of the epidermal stratum basale. Finger-shaped Merkel cell processes can reach across the intercellular space as far as to the stratum spinosum. The cytoplasm is light. It contains microfilaments and many osmiophilic granules with diameters of 80–120 nm. The granules are mostly found in the cytoplasmic area, which establishes contact with the nerve endings. The afferent nerve fiber is initially myelinated (not shown). Shortly before the papillary layer and close to the stratum basale of the epidermis, myelination discontinues and the axon forms a disk-like bulge, which connects to the basal surface of the Merkel cell. There may be synapse-like interactions at this interface.

The right side of this figure represents the basal part of the Merkel nerve ending.

1 Merkel cell

2 Nerve disk with mitochondria

3 Keratinocytes of the epidermal stratum basale

4 Desmosomes

Electron microscopy; magnification: × 10 000

615 Meissner’s Corpuscles

Meissner’s corpuscles (tactile corpuscles) are part of a group of lamellar bodies. There are lamellar bodies with or without a perineural capsule. Meissner’s corpuscles 1 occur singular or in small groups directly underneath the epidermis 2 in the dermal connective tissue papillae of the papillary layer. The oval Meissner corpuscles are 100–150 μm long and about 40–70 μm wide bodies. A loosely structured connective tissue capsule envelops them. Up to seven afferent nerve fibers 3 approach the tactile Meissner corpuscle, lose their myelin sheath and branch several times inside the corpuscle. The fibers progress as spirals and end as wide nerve terminals (see Fig. 616). The nucleated part of the Schwann cells and their stacked cytoplasmic processes (lamellae) cannot be recognized in this figure.

1 Meissner’s corpuscle

2 Epidermis

3 Afferent nerve fibers

4 Dermal connective tissue papilla

5 Reticular layer of the dermis

Stain: Bodian silver impregnation; magnification: × 500

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616 Meissner’s Corpuscles

Meissner’s tactile corpuscles occur singular or in small groups in the dermal papillae immediately underneath the epidermis. They are long oval and positioned vertical to the skin surface. Meissner’s corpuscles are about 40–70 μm wide and 100–150 μm long and consist of myelinated afferent nerve fibers with wide, plate-like nerve terminals 1 . Apposed cytoplasmic processes of Schwann cells and a connective tissue capsule encase the terminals. Approximately seven myelinated afferent axons are found per corpuscle. The axons lose their myelin sheaths only upon entering the corpuscle. Inside the corpuscle, the individual axons branch several times. The branches form helices and end with wide terminals. The longitudinal axis of these wide, plate-like terminals are arranged parallel to the skin surface. The nuclei of the Schwann cells 3 are found at the periphery of the corpuscle. Schwann cells send narrow, flat cytoplasmic processes (lamellae) toward the center of the corpuscle. The processes envelop the nerve terminals. The sheaths consist of several cytoplasmic processes (Schwann cell lamellae) 2 . Collagen fibrils occur between the processes and continue in the connective tissue of the dermal papillary layer 5 .

The figure shows a Meissner corpuscle from the fingertip of a rhesus monkey. The Meissner tactile corpuscle is located in the dermal papilla next to the basal membrane of the epidermal stratum basale 4 (cf. Fig. 600). Note the cells of the basal layer, which extend delicate cell processes (root pedicles, cf. Fig. 93) into the connective tissue of the papillary layer 5 . Meissner’s tactile corpuscles are mechanoreceptors, which react to pressure and adapt with medium speed.

Somatosensory Receptors

1 Nerve terminals

2 Cytoplasmic processes of Schwann cells

3 Nuclei of Schwann cells

4 Cells of the stratum basale with root pedicles

5 Dermal papillary layer

Electron microscopy; magnification: × 3800

617 Vater-Pacini Corpuscles

Vater-Pacini corpuscles (Pacinian corpuscles, large lamellated corpuscles) are up to 4 mm long and 2 mm thick. Their giant size makes them easy to detect, e.g., in the subcutaneous tissue of the palms of the hand and the soles of the feet and proximal phalanxes as well as in the vicinity of fasciae, periosteum and tendons, also in mesenteric tissue and the pancreas. The sheaths of these terminal nerve systems are a characteristic attribute. They consist of 40–60 concentric layers of cytoplasmic processes (perineural lamellae) in an onionlike arrangement 1 . The nerve fiber is found in the center (inner afferent axon) 2 . The connective tissue capsule around the corpuscle contains a meshwork of elastic fibers.

Human fingertip. Vater-Pacini corpuscles are pressure and vibration receptors.

1 Lamellae of the perineural capsule

2 Central nerve fiber

3 Dense connective tissue capsule

4 Adipose tissue

Stain: alum hematoxylin-eosin; magnification: × 30

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Somatosensory Receptors

618 Vater-Pacini Corpuscles

Vater-Pacini lamellated corpuscle from the fingertip of a rhesus monkey. The afferent axon is visible in the center. The lamellae of the cytoplasmic Schwann cell processes form the inner bulb around the axon (bulbus internus). The outer layers are part of the perineural sheath 2 (cf. Fig 617). Very fine connective tissue fibers traverse the lamellar cell processes. The lower right corner of the figure displays a myelinated axon 3 , which is attached to the Vater-Pacini corpuscle. An eccrine sweat gland is visible in the right upper corner.

Receptors of the Vater-Pacini corpuscles recognize vibration.

1 Inner bulb with afferent axon

2 Layers of the perineural sheath

3 Myelinated axon

4 Capillary

Electron microscopy; magnification: × 350

619 Vater-Pacini Corpuscles

Several oval Vater-Pacini lamellated corpuscles from the mesenteric tissue of a cat are shown in this whole-mount preparation (not sectioned). This preparation was stained using methylene blue. It was then bleached. The center consists of axon and inner bulb, the surrounding layers form the perineural sheath. The thin connective tissue capsule on the surface of the lamellar corpuscle is also visible. All Vater-Pacini corpuscles in this figure are arranged around a mesenteric vessel 1 .

1 Mesenteric vessel

Stain: methylene blue; magnification: × 500

620 Golgi-Mazzoni Corpuscles

The sensory corpuscles resemble Vater-Pacini lamellated corpuscles (cf. Figs. 617–619). There are fewer perineural lamellae in Golgi-Mazzoni corpuscles. As in Vater-Pacini corpuscles, an afferent axon 1 and inner bulb of Schwann cell lamellae are found in the center. However, there are often two or more axons with inner bulbs in one corpuscle. The outer layers build the perineural sheath 2 .

The figure also shows arterioles 3 and a vein 4 (upper right). Golgi-Mazzoni corpuscles are found in the skin of the fingers, in the nail bed and in the skin of the outer genitals (genital nerve corpuscles). They contain tactile mechanoreceptors, which respond to skin movement due to pressure and touch.

1 Central afferent axon and inner bulb

2 Lamellae of the perineural sheath

3 Arterioles

4 Vein

Stain: alum hematoxylin-eosin; magnification: × 300

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621 Ruffini’s Corpuscles

Ruffini’s corpuscles are Stretch receptors. They occur, for example, in the skin, in joint capsules and in the periodontium. The perineural sheath 1 of the corpuscle forms an open cylinder. Both the ends of the cylinder are either rounded or pointed. Dermal collagen fiber bundles traverse the cylinder from end to end. Helical nerve terminals are interspersed between the fibers. The myelinated afferent axon has a diameter of 5 μm. The collagen fiber bundles 3 in this preparation are stained light brown. The nerve terminals appear black.

1 Perineural sheath

2 Nerve fiber bundle

3 Collagen fiber bundle

4 Connective tissue of the glans penis

Stain: Bodian silver stain; magnification: × 170

Somatosensory Receptors

622 Ruffini’s Corpuscles

Cross-section of the cylinder of Ruffini’s corpuscle from the knee joint capsule with perineural sheath 1 . Several nerve terminals are visible between the collagen fiber bundles inside the cylinder 2 . There are also Schwann cells 4 .

1 Perineural sheath

2 Collagen fiber bundles inside the cylinder of Ruffini’s corpuscle 3 Nerve terminals

4 Schwann cell

5 Collagen fibril bundle outside the cylinder of Ruffini’s corpuscle Electron microscopy; magnification: × 2000

623 Neuromuscular Spindle—Fusus Neuromuscularis

Neuromuscular spindles (Stretch receptors) are 2–10 mm long and 0.2–0.5 mm thick. They are embedded in perimysium internum, which consists of muscular connective tissue. The spindle consists of the equatorial region and the tapered pole regions. Concentric layers of strong connective tissue fibers (perineurium) form the capsule 1 . Five to 10 thin intrafusal muscle fibers 2 with a reduced number of fibrils are found in the center. Endomysial cells surround them. There are two groups of intrafusal muscle fibers, dependent on the configuration of their nuclei in the equatorial segment (central segment). The nuclei either form short, thin rows (nuclear chain fibers) or occur in piles (static nuclear bag fibers). Sensory and vegetative nerves as well as motor nerve fibers penetrate the corpuscular capsule. The helical nerve fibers circumscribe and innervate the central intrafusal muscle fiber. There are nerve terminals. Several small rounded cell nuclei are found in the center. They are part of the neurolemma.

Cross-section of a neuromuscular spindle.

1 Perineural sheath

2 Intrafusal nerve fibers

3 Axonal terminals

4 Skeletal muscle fibers

Stain: Heidenhain iron hematoxylin; magnification: × 500

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