Mechanoreceptors

By: Jeffrey Connell


Overview
     Mechanoreceptors allow an organism to recognize and respond to physical pressure that are being acted upon the organism. Mechanoreceptors relay information back to the central nervous system, where it is then decoded and the body responds accordingly. The human hand has a very complex set of mechanoreceptors that allows it to distinguish many different types of stimuli. However, mechanoreceptors are found throughout the body, and differ between organisms (Purves, 2001).
   
     Different types of mechanoreceptors help an organism sense all different types of physical pressures, which can range from a very sensitive light touch, to a very hard applied pressure (Purves, 2001).
    
    The four main receptors for light touches (low threshold/high sensitivity) are: Meissner's corpuscles, Pacinian corpuscles, Merkel’s disks, and Ruffini’s corpuscles (Purves, 2001).

http://www.ncbi.nlm.nih.gov/books/NBK10895/figure/A616/?report=objectonly

http://www.ncbi.nlm.nih.gov/books/NBK10895/figure/A616/?report=objectonly
   
  • Meissner’s corpuscles lie between the dermal papillae and the epidermis of the inside of the hand. This type of receptor is composed of Schwann Cells. Meissner’s corpuscle is the most common type of mechanoreceptor found on hairless skin. They account for 40% of the sensory information the hand collects and processes. Very fine tuned to recognize vibrations and textures that come in contact with the skin.
  • Pacinian corpuscles are found in the subcutaneous tissue. Pacinian corpuscles are bigger in size when compared to Meissner's. This type of mechanoreceptor has a fluid filled sac that separates the two layers of lamella which helps filter out different levels of touch. Pacinian corpuscle's nerve endings are stimulated within the range of 250-350 hertz. These corpuscles are more sensitive than the meissner’s corpuscle. This allows an organism to determine the feel of an object  in greater detail. They account for 10-15% of the sensory information the hand collects and processes. Similar structures are found within other organisms such as, duck’s beaks, legs of cranes, and wings of birds.
  • Merke’s disk are located in the epidermis. They lie beneath the dermal ridges of the papillae and account for around 25% of the sensory information the hand collects. This type of mechanoreceptors is most congregated in the finger tips and external areas of genitalia. When stimulated, the nerve fiber enlarges at the ending that excites another specialized cell to release peptides that move into the nerve terminal. This type of corpuscle is intended to help feel a general light pressure applied to an area. It is not as exact as the Meissner’s or Pacinian Corpuscle.
  • Ruffini’s corpuscles are very similar in structure to the previous three mechanoreceptors mentioned, however, they are not understood as well. Ruffini’s corpuscles are elongated, spindle capsular receptors located deep within the skin, as well as in ligaments and tendons. This type of corpuscle is usually aligned along the stretch lines in the skin which leads researchers to believe it is specialized to recognize movements in the body, especially within the limbs. Ruffini's corpuscules account for around 20% of the sensory information the hand collects. Believed to be more of a general stimulus recognition, not as specific as Meissner’s and Pacinian corpuscles.
(Purves, 2001)

Development of Mechanoreceptors

As previously noted, Mechanoreceptors allow us to perceive touch, body position and pain. The most common are the low threshold mechanoreceptors, which are located in the skin or deep tissues. Mechanoreceptors can be divided into groups based of how they adapt to response in sustained stimuli. Rapid adapting mechanoreceptors are the Meissner's corpuscles located in the dermal papilla, and the Pacinian corpuscle, which are located in the joints of bones. Slow adapting mechanoreceptors are Merkels discs and Ruffini's corpuscles. Slow adapting mechanoreceptors respond to skin movement or stretching of the body. Both rapid adapting, and slow adapting mechanoreceptors work together to allow us to perceive touch, texture, shapes, and vibrations.

Early development of the mechanoreceptors happens to be fairly unknown. Current research has shown that neurotrophic factors play a vital role in the early development of the rapid adapting and slow adapting mechanoreceptors. Studies have shown that certain neurotrophins are required to maintain functioning Merkel cells, and also are required for the correct development of Meissner's corpuscles (Ma, 2009).

Further studies have shown the importance of the Ret signaling pathway in the development of these mechanoreceptors. Researchers have been able to uncover the role of the Ret neurons at the beginning of the pathway except for 5% of them. Researchers have since identified that the missing piece of the Ret neurons in the Ret pathway develop into the rapid adapting mechanoreceptors. Without the Ret pathway, these mechanoreceptors would not develop (Ma, 2009).

The researchers solidified their study by marking the early Ret neurons, believed to develop into the mechanoreceptors, using green fluorescent protein. This allowed them to trace the Ret neurons through development. The results showed the green fluorescent protein in the Meissner's corpuscles within the dermal papillae, and also in the Pacinian corpuscles in the periosteum of the fibula. The green fluorescent protein did not show up in any of the slow adapting mechanoreceptors. Their development origin is still controversial. The lack of green fluorescent protein leads many to believe a different pathway could be responsible for the slow adapting mechanoreceptors. The researchers believe that neurons found later in the Ret signaling pathway may have a role in the development of slow adapting mechanoreceptors (Ma, 2009).


Other Types of Mechanoreceptors
Lateral-Line Organs

http://www.britannica.com/EBchecked/topic/371976/mechanoreception/64718/Lateral-line-organs
A. Shows Location of Lateral Lines on Body; B. Diagram of Lateral Canals; C. Diagram of a Neuromast

  

          Specialized mechanoreceptors are found in other species as well. Aquatic vertebrates have evolved mechanoreceptors that have allowed them to thrive in an aqueous environment. These mechanoreceptors are called Lateral-Line organs.

            Most aquatic vertebrates have lateral line organs in their epidermis. Lateral line organs are sensitive to water displacements, but not just any water displacement. The aquatic vertebrate’s mechanoreceptors have evolved to know the difference between water displacement caused by their own movement compared to the movement of other organisms, which could be potential predators, mates, or members of their own species. This type of mechanoreceptor allows the organism to sense other organisms before there is any physical contact taking place (Dijkgraaf, 2012).

            Lateral-Line mechanoreceptors are made up of neuromast, which are are hair-like clusters on the epidermal surface which are projected into a cupula. The cupula bends in response to water displacement (Photo above, Figure C.). Very similar to how the inner ear of humans functions (Dijkgraaf, 2012).

            The name, lateral-line, comes from the distribution of the sense organs on the head and body of the organism (See above photo, section A). The actual sensory cell of a single neuromast is made up of one longer hair, and around fifty shorter hairs. The smaller hairs are arranged in parallel rows. This adds to the organisms ability to sense even the slightest changes in their environment,  less than one thousandth of a millimeter of displacement can alter the impulses.

            Further evolution has led to some aquatic organisms acquiring the ability to use these lateral line organs angled out on the head and tail axis which allows them the ability to hone in on exactly where the displacement is coming from. Similar to a radar (Dijkgraaf, 2012).

            Some fish have very organized lateral-line canals (Photo above, Figure B).  The canals are formed from a groove that develops in the epidermis along the fish' main lateral lines. Some of the neuromast move down into the bottom of the groove. The canal then closes over the neuromast, but leaves small openings called canal pores. Simulation is still the same, except it is more localized to a single canal pore which then displaces the neuromast within the fluid filled canal. This type of specialization is most commonly found in fish that are usually continuously swimming, and in bottom dwellers that are in running/tidal waters. This adaptation helps organisms that are commonly exposed to constant water displacement a more localized way of detecting water displacement caused by other proximate organisms. For lateral-line organs to be effective, the organism producing the stimulus (water displacement) must be within one body length to the organisms receiving the stimulus (Dijkgraaf, 2012).


Current Studies Involving Mechanoreceptors

A recent and growing field of interest is the complex role ligaments play within the joints of the body. Ligaments are not solely stabilizers of the joints. Many studies have looked into the role that mechanoreceptors play within a ligament. A lot of the research is centered on the knee since it is injured so often. It is believed that mechanoreceptors in the ligaments of the knee influence motor function. If a person tears their anterior cruciate ligament (ACL) and has it replaced, then they will lose all the benefits from the mechanoreceptors that was present in the original ACL (Hogervost, 1998).

Studies have shown that each ligament in a cat contains 6-20 mechanoreceptor on average. This statistic is similar in humans as well. The number of mechanoreceptors decrease with age and disease. Since each ligament has a such a small number of mechanoreceptors, many argue that losing just one ligament will have very little lasting damage. Further studies show that not only does the ACL have a higher than average number of mechanoreceptors, but many mechanoreceptors have been located to be around the attachment sites of the ACL. There is some controversy to the importance of the mechanoreceptors in the ACL. The receptors in the ACL are mostly Ruffini receptors (stretch receptors). These help protect the knee by sending signals to the brain when the knee is being hyper extended. Mechanoreceptors are only activated when the knee is flexed at the limit of, or beyond a normal range (Hogervost, 1998).

Clinical Studies have shown a difference in personal perception of the stability of the knee in normal use and also while hyperextension is taking place in subjects with torn ACL's. This can be accredited to the lack of properly functioning mechanoreceptors in a very important ligament for a person’s perception of knee stability.

Mechanoreceptors in the ligaments provide input that influence how stiff the muscles around the joint are. When a ligament is injured, two things happen: The mechanoreceptors signal output is reduced, and the signaling from tissues and other ligaments around the injured ligament produce more signal output and are put under more stress (Hogervost, 1998).


Conclusion

    Mechanoreceptors are a type of sensory receptor that respond to physical pressure or distortion, which have allowed organisms to adapt and respond to their environment quicker and more efficiently. They are found throughout the body, not just on the surface. Mechanoreceptors are located in ligaments, tendons, organs, and many other internal parts of the body. These type of mechanoreceptors focus more on recognizing and responding to unnatural stretch/stress. 

    Mechanoreceptors in areas such as the human hand have become very complex over time. The complexity allows recognition of a wide arrange of mechanical pressures or stimulus. It allows humans the ability to discriminate between textures and hardness at a very fine detail.

    Mechanoreceptors play different roles in different organisms depending on their environment. The lateral line organs of a fish are adapted to their aquatic environment. Human's mechanoreceptors are very similar in theory and function, yet are implicated throughout the body very differently.

    There is still a lot to know about the development of mechanoreceptors and also exactly how vital their role is to normal function. As more research is completed, scientist are discovering that mechanoreceptors have many roles within the body's normal function other than just recognizing physical pressure and distortion.


Resources

Dijkgraaf, Sven. Lateral Line Organs: Mechanoreceptor Function. Encyclopedia Britannica. 2012. Retrieved Online. http://www.britannica.com/EBchecked/topic/371976/mechanoreception/64718/Lateral-line-organs

Hogervost, Tom. M. D.; Brand, Richard A. M.D. Current Concepts Review - Mechanoreceptors in Joint Function. The journal of Bone and Joint Surgery 80:9, 1998. Retrieved Online. http://jbjs.org.prox.lib.ncsu.edu/article.aspx?articleid=24135#Overview

Ma, Quifa. REToucing upon Mechanoreceptors. Neuron 64:6. 2009. Cancer Institute and Department of Neurobiology, Harvard Medical School. 2009. Retrieved Online. http://www.sciencedirect.com.prox.lib.ncsu.edu/science/article/pii/S0896627309010046

Purves, D; Augustine, GJ, Fitzpatrick, D. Mechanoreceptors Specialized to Receive Tactile Information. Neuroscience. Sinaure Associates, 2001. Retrieved Online. http://www.ncbi.nlm.nih.gov/books/NBK10895/