Eimer's Organ - Function

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 cells 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 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.