Eimer's Organ
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Eimer's organs are
organ Organ and organs may refer to: Biology * Organ (biology), a group of tissues organized to serve a common function * Organ system, a collection of organs that function together to carry out specific functions within the body. Musical instruments ...
s for the sense of touch, shaped like bulbous papillae, formed from modified
epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermal layer provides a barrier to infection from environmental pathogens and regulates the ...
. First isolated by Theodor Eimer from the
European mole The European mole (''Talpa europaea'') is a mammal of the order Eulipotyphla. It is also known as the common mole and the northern mole. This mole lives in a tunnel system, which it constantly extends. It uses these tunnels to hunt its prey. ...
in 1871, these organs are present in many moles, and are particularly dense on the
star-nosed mole The star-nosed mole (''Condylura cristata'') is a small semiaquatic mole (animal), mole found in moist, low elevation areas in the northeastern parts of North America. It is the only Extant taxon, extant member of the tribe Condylurini and genu ...
, 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
myelinated Myelin Sheath ( ) is a lipid-rich material that in most vertebrates surrounds the axons of neurons to insulate them and increase the rate at which electrical impulses (called action potentials) pass along the axon. The myelinated axon can be lik ...
fibers from the
dermis The dermis or corium is a layer of skin between the epidermis (skin), epidermis (with which it makes up the cutis (anatomy), cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from s ...
, which form terminal swellings just below the
keratinized Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. It is the key structural material making up scales, hair, nails, feathers, horns, claws, hooves, and the outer layer of skin in vertebrates. Kera ...
outer surface of the epidermis. They contain a complex of
Merkel cell Merkel cells, also known as Merkel–Ranvier cells or tactile epithelial cells, are oval-shaped mechanoreceptors essential for light touch sensation and found in the skin of vertebrates. They are abundant in highly sensitive skin like that of th ...
and neurite in the epidermis, and a lamellated corpuscle in the dermal
connective tissue Connective tissue is one of the four primary types of animal tissue, a group of cells that are similar in structure, along with epithelial tissue, muscle tissue, and nervous tissue. It develops mostly from the mesenchyme, derived from the mesod ...
.


Discovery

Theodor Eimer described the
discrete Discrete may refer to: *Discrete particle or quantum in physics, for example in quantum theory * Discrete device, an electronic component with just one circuit element, either passive or active, other than an integrated circuit * Discrete group, ...
microscopic The microscopic scale () is the scale of objects and events smaller than those that can easily be seen by the naked eye, requiring a lens or microscope to see them clearly. In physics, the microscopic scale is sometimes regarded as the scale betwe ...
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 Palpation is the process of using one's hands to check the body, especially while perceiving/diagnosing a disease or illness. Usually performed by a health care practitioner, it is the process of feeling an object in or on the body to determine ...
, 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 A seismic wave is a mechanical wave of acoustic energy that travels through the Earth or another planetary body. It can result from an earthquake (or generally, a quake), volcanic eruption, magma movement, a large landslide and a large ma ...
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 ending A free nerve ending (FNE) or bare nerve ending, is an unspecialized, afferent nerve fiber sending its signal to a sensory neuron. ''Afferent'' in this case means bringing information from the body's periphery toward the brain. They function as cu ...
s. 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: * The nerve fibers with free nerve endings * The nerve fibers which end on
Merkel cell Merkel cells, also known as Merkel–Ranvier cells or tactile epithelial cells, are oval-shaped mechanoreceptors essential for light touch sensation and found in the skin of vertebrates. They are abundant in highly sensitive skin like that of th ...
s adapt their responses to touch rapidly * The nerve fibers which end in the lamellated corpuscles and which are considered slowly adapting 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 A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are located on sensory neurons that convert mechanical pressure into action potential, electrical signals tha ...
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 The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. It is the largest site of Neuron, neural integration in the central nervous system, and plays ...
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.


References

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Sources

* Peter Melzer:
The Beautiful Eimer's Organ
' 1 May 2010. * K C Catania: 'Magnified cortex in star-nosed moles.' Nature 375, (1995): 453–454. * K C Catania, F E Remple: 'Tactile foveation in the star-nosed mole.' Brain Behav Evol 63 (2004): 1–12. * K C Catania, J H Kaas: 'Organization of the somatosensory cortex of the star-nosed mole.' J Comp Neurol 351 (1995): 549–567. * T Eimer: 'Die Schnauze des Maulwurfs als Tastwerkzeug.' Arch Micr Anat 7, (1873): 181–201. * Z Halata: 'The mechanoreceptors of the mammalian skin ultrastructure and morphological classification.' Adv Anat Embryol Cell Biol 50, (1975): 3-77. * P D Marasco, P R Tsuruda, D M Bautista, D Julius, K C Catania: 'Neuroanatomical evidence for segregation of nerve fibers conveying light touch and pain sensation in Eimer's organ of the mole.' Proc Natl Acad Sci USA 103, (2006): 9339–9344. * U Proske, J E Gregory, A Iggo: 'Sensory receptors in monotremes.' Philos Trans R Soc Lond B Biol Sci 353, (1998): 1187–1198. Mammal anatomy Olfactory system