Eimer's organ

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Eimer's organs are sensory organs in which the epidermis is modified to form bulbous papillae. First isolated by Theodor Eimer from the European mole in 1871, these organs are present in many moles, and are particularly common in the star-nosed mole, which bears 25,000 of them on its unique tentacled snout. The organs are formed from a stack of epidermal cells, which is innervated by nerve processes from myelinated fibers in the dermis, which form terminal swellings just below the outer keratinized layer of epidermis. They contain a Merkel cell-neurite complex in the epidermis and a lamellated corpuscle in the dermal connective tissue. [1] [2]

Contents

Discovery

Theodor Eimer described the discrete microscopic organ of touch that densely populates the tip of the nose of the European mole Talpa europaea. The organ is named in his honour. In his original publication in 1871, he examined the structure of the nose, the distribution of the touch organs on the nasal skin, and the relationship of their density with the nose's use for palpation, to examine or explore by touching. Eimer established a connection between structure and function.

Eimer recognized the importance of the mole's nose to its behaviour. He stated in 1871: "The mole's snout must be the seat of an extraordinarily well developed sense of touch because it replaces almost entirely the animal's sense of face, constituting its only guide on its paths underground." He estimated that the nose of the European mole was covered with more than 5,000 Eimer's organs, which were invested with 105,000 nerve fibres. He took the abundance of sensory innervation (stimulate a nerve or muscle) to affirm his contention that the nose's touch must represent the moles dominant facial sense. Eimer asserted that his interpretation was consistent with the common knowledge of his time. In his publication he noted that the extreme density of highly sensitive nerve fibres is the cause of a light blow to the snout being able to kill the mole instantly. Roughly 130 years after Eimer's discovery, Catania and colleagues recorded in 2004 striking behavioural evidence in favour of his conclusions, using a high-speed camera. Moles with the help of their Eimer's organs may be perfectly poised to detect seismic wave vibrations.

Structure

The organ consists of a minute skin papilla with 0.1–0.2 mm diameter. At the papilla's core, a geometric constellation of nerve fibres with free endings is embedded symmetrically in a column of epithelial cells. Eimer saw two to three single nerve fibres, rising straight in the middle of the column and ending in the fifth layer under the stratum corneum that forms the hard top of the epidermis. The fibres extend short protrusions perpendicularly into each epithelial layer they traverse, where the protrusions end in 'buttons'. They are ringed by a circle of roughly 19 evenly spaced nerve fibres, known as satellite fibres, whose protrusions point inwards. In addition, Eimer distinguished a separate set of nerve fibres with free nerve endings. By contrast to the fibres in the papilla's core, these travel obliquely toward the surface at the papilla's perimeter.

With improved histological techniques, a second touch receptor type, Merkel cell-neurite complexes, was found in the stratum germinativum at the bottom of the epidermis, and a third, lamellated corpuscles of Vater and Pacini, was discovered in the stratum papillare of the dermis underneath the Merkel cells as published by Halata in 1975.

Function

Today it is still not understood precisely how these receptors convert touch into the electrical signals that the nerve fibres transmit to the brain. Interesting are the properties of touch, e.g. frequency and force, to which the receptors respond and how their responsiveness changes with prolonged stimulation. The receptors can be functionally distinguished based on these features:

Marasco et al. attribute different functions to Eimer's two sets of free-ending nerve fibres in the star-nosed mole and the coast mole Scapanus orarius. The authors published micrographs of the organ and its innervation, depicting Eimer's free-ending fibers as well as the Merkel cell-neurite complexes and the Vater-Pacini corpuscles. Using a histochemical marker for a protein known to be involved in the processing of pain, they were able to label the nerve fibres at the perimeter of the papilla, suggesting that they are nociceptive, i.e. they respond to pain. By contrast, the fibres in papilla's core did not stain for the protein, suggesting that they are mechano-receptive. These nerve fibres as well as the Merkel cell-neurite complexes are known to respond to local touches with great sensitivity, whereas the Vater-Pacini corpuscles are highly tuned to the frequencies of dispersed vibrations. Eimer's organ, therefore, forms a receptor complex, integrating pain receptors as well as three fundamentally different types of touch receptors which preferentially respond to either skin indentations or vibrations. The follicles of whiskers, also known as vibrissae or sinus hairs, and the push rods in monotremes, as published by Proske et al., represent the only other known discrete structures in the skin that combine three mechanoreceptor types.

The Eimer's organs on the nose may be the mole's main tool with which the animal can capture a refined picture of its underground habitat. Catania and Kaas have shown that the nose of the star-nosed mole is mapped in multiple topographic representations on an extraordinarily large swath of cerebral cortex that processes touch. Discrete morphological modules of nerve cells that are clearly discernible in histologically stained sections represent each ray in the same order as they surround the nose. This topographic morphological representation of the sensory periphery is similar to that of the facial whiskers by cytoarchitectonic modules called barrels in the rodent cerebral cortex.

To date, two complete cortical maps of the nose with its rays have been found in the brain of the star-nosed mole. There may be more. The nose's disproportionate representation in cerebral cortex is suggestive of a fovea for nose touch in the mole's somatic sensory system, as published by Catania.

Related Research Articles

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Trigeminal nerve Cranial nerve responsible for sensory perception and motor functions of the face

The trigeminal nerve (the fifth cranial nerve, or simply CN V) is a nerve responsible for sensation in the face and motor functions such as biting and chewing; it is the most complex of the cranial nerves. Its name ("trigeminal" = tri-, or three, and - geminus, or twin: thrice-twinned) derives from each of the two nerves (one on each side of the pons) having three major branches: the ophthalmic nerve (V1), the maxillary nerve (V2), and the mandibular nerve (V3). The ophthalmic and maxillary nerves are purely sensory, whereas the mandibular nerve supplies motor as well as sensory (or "cutaneous") functions. Adding to the complexity of this nerve is that autonomic nerve fibers as well as special sensory fibers (taste) are contained within it.

Stimulus modality, also called sensory modality, is one aspect of a stimulus or what is perceived after a stimulus. For example, the temperature modality is registered after heat or cold stimulate a receptor. Some sensory modalities include: light, sound, temperature, taste, pressure, and smell. The type and location of the sensory receptor activated by the stimulus plays the primary role in coding the sensation. All sensory modalities work together to heighten stimuli sensation when necessary.

A cutaneous receptor is the type of sensory receptor found in the skin. They are a part of the somatosensory system. Cutaneous receptors include mechanoreceptors, nociceptors (pain), and thermoreceptors (temperature).

A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are innervated by sensory neurons that convert mechanical pressure into electrical signals that, in animals, are sent to the central nervous system.

Dorsal column–medial lemniscus pathway

The dorsal column–medial lemniscus pathway (DCML) is a sensory pathway of the central nervous system that conveys sensations of fine touch, vibration, two-point discrimination, and proprioception (position) from the skin and joints. It transmits information from the body to the primary somatosensory cortex in the postcentral gyrus of the parietal lobe of the brain. The pathway receives information from sensory receptors throughout the body, and carries this in nerve tracts in the white matter of the dorsal columns of the spinal cord to the medulla, where it is continued in the medial lemniscus, on to the thalamus and relayed from there through the internal capsule and transmitted to the somatosensory cortex. The name dorsal-column medial lemniscus comes from the two structures that carry the sensory information: the dorsal columns of the spinal cord, and the medial lemniscus in the brainstem.

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Tactile corpuscle

Tactile corpuscles or Meissner's corpuscles are a type of mechanoreceptor discovered by anatomist Georg Meissner (1829–1905) and Rudolf Wagner. This corpuscle is a type of nerve ending in the skin that is responsible for sensitivity to light touch. In particular, they have their highest sensitivity when sensing vibrations between 10 and 50 hertz. They are rapidly adaptive receptors. They are most concentrated in thick hairless skin, especially at the finger pads.

Sensory neuron Nerve cell that converts environmental stimuli into corresponding internal stimuli

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Pacinian corpuscle Type of mechanoreceptor cell in hairless mammals

Pacinian corpuscle or lamellar corpuscle or Vater-Pacini corpuscle; is one of the four major types of mechanoreceptors found in mammalian skin. This type of mechanoreceptor is found in both glabrous (hairless) and hirsute (hairy) skins, viscera, joints and attached to periosteum of bone, primarily responsible for sensitivity to vibration. Few of them are also sensitive to quasi-static or low frequency pressure stimulus. Most of them respond only to sudden disturbances and are especially sensitive to vibration of few hundreds of Hz. The vibrational role may be used for detecting surface texture, e.g., rough vs. smooth. Most of the Pacinian corpuscles act as rapidly adapting mechanoreceptors. Groups of corpuscles respond to pressure changes, e.g. on grasping or releasing an object.

Star-nosed mole Species of Mole

The star-nosed mole is a small mole found in moist, low areas in the northern parts of North America. It is the only member of the tribe having a touch organ with more than 25,000 minute sensory receptors, known as Eimer's organs, with which this hamster-sized mole feels its way around. With the help of its Eimer's organs, it may be perfectly poised to detect seismic wave vibrations.

Merkel nerve endings are mechanoreceptors, a type of sensory receptor, that are found in the basal epidermis and hair follicles. They are nerve endings and provide information on mechanical pressure, position, and deep static touch features, such as shapes and edges.

Tactile corpuscles of Grandry

The tactile corpuscles of Grandry or Grandry corpuscles are mechanoreceptors found in the beak skin and oral mucosa of aquatic birds. They were first described by Grandry in 1869 in the bill skin of ducks and geese. Their general structure includes the flattened endings of an afferent nerve fiber sandwiched between two or more somewhat flattened sensory cells called Grandry cells, all surrounded by a layer of satellite cells and a partial capsule of collagen protein. Electrophysiological studies have shown that Grandry corpuscles function as rapidly adapting velocity detectors. In birds, Grandry and Merkel corpuscles share many morphological similarities, which has led to some confusion in the literature over their classification.

Cutaneous innervation refers to the area of the skin which is supplied by a specific cutaneous nerve.

Type II sensory fiber is a type of sensory fiber, the second of the two main groups of touch receptors. The responses of different type Aβ fibers to these stimuli can be subdivided based on their adaptation properties, traditionally into rapidly adapting (RA) or slowly adapting (SA) neurons. Type II sensory fibers are slowly-adapting (SA), meaning that even when there is no change in touch, they keep respond to stimuli and fire action potentials. In the body, Type II sensory fibers belong to pseudounipolar neurons. The most notable example are neurons with Merkel cell-neurite complexes on their dendrites and Ruffini endings. Under pathological conditions they may become hyper-excitable leading to stimuli that would usually elicit sensations of tactile touch causing pain. These changes are in part induced by PGE2 which is produced by COX1, and type II fibers with free nerve endings are likely to be the subdivision of fibers that carry out this function.

Pallesthesia, or vibratory sensation, is the ability to perceive vibration. This sensation, often conducted through skin and bone, is usually generated by mechanoreceptors such as Pacinian corpuscles, Merkel disk receptors, and tactile corpuscles. All of these receptors stimulate an action potential in afferent nerves found in various layers of the skin and body. The afferent neuron travels to the spinal column and then to the brain where the information is processed. Damage to the peripheral nervous system or central nervous system can result in a decline or loss of pallesthesia.

Somatosensory system Widely distributed parts of the sensory nervous system

The somatosensory system is a part of the sensory nervous system that is associated with the sense of touch, but includes parallel receptors and nerve pathways for the sensations of temperature, body position and movement, and pain. This complex system of sensory neurons, and neural pathways responds to changes at the surface of, or inside, the body. The axons of sensory neurons connect with, or respond to, various receptor cells. These sensory receptor cells are activated by different stimuli such as heat and nociception, giving a functional name to the responding sensory neuron, such as a thermoreceptor which carries information about temperature changes. Other receptor types include mechanoreceptors, chemoreceptors, and nociceptors which send signals along a sensory nerve to the spinal cord, where the signals may be processed by other sensory neurons, and then relayed to the brain for further processing. Sensory receptors are found all over the body including the skin, epithelial tissues, muscles, bones and joints, internal organs, and the cardiovascular system.

A sense is a biological system used by an organism for sensation, the process of gathering information about the world and responding to stimuli. Although traditionally around five human senses were known, it is now recognized that there are many more. Senses used by other non-human organisms are even greater in variety and number. During sensation, sense organs collect various stimuli for transduction, meaning transformation into a form that can be understood by the brain. Sensation and perception are fundamental to nearly every aspect of an organism's cognition, behavior and thought.

Hydrodynamic reception

Hydrodynamic reception refers to the ability of some animals to sense water movements generated by biotic or abiotic sources. This form of mechanoreception is useful for orientation, hunting, predator avoidance, and schooling. Frequent encounters with conditions of low visibility can prevent vision from being a reliable information source for navigation and sensing objects or organisms in the environment. Sensing water movements is one resolution to this problem.

Group A nerve fibers are one of the three classes of nerve fiber as generally classified by Erlanger and Gasser. The other two classes are the group B nerve fibers, and the group C nerve fibers. Group A are heavily myelinated, group B are moderately myelinated, and group C are unmyelinated.

References

  1. ↑ Catania, K.C. (1995). "Structure and innervation of the sensory organs on the snout of the star-nosed mole". Journal of Comparative Neurology . 351 (4): 536–548. doi:10.1002/cne.903510405. PMID   7721982.
  2. ↑ Catania, K.C. (June 2000). "A Star is Born". Natural History . Retrieved 2015-07-07.

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