Itching

Discomfort can arise from a number of neural signals. The sensation that we feel as "the itch" is, while not completely understood, often traceable back to several different triggers. In 1660, Samuel Hafenreffer wrote of the first widely-recognized definition for itching as a symptom, describing it as "an unpleasant desire to scratch." (MSNBC). In the 350+ years since this recognition, much has been discovered and hypothesized about the sometimes mysterious sensation.
Figure 1.2. Primary Afferent Axons. Type C has the smalest diameter, and is unmyelinated.
Figure 1.2. Primary Afferent Axons. Type C has the smalest diameter, and is unmyelinated.




The Ventral Spinothalamic Tract
Itching is commonly resultant of a free nerve ending ascending the nocioceptive pathway toward the CNS. Conscious sensation of the itch typically follows the following pathway: Free nerve endings (Figure 1.1) comprised of unmyelinated, thin, group IV fibers with “C” type axons (Figure 1.2) synapse with dorsal root ganglion cells. This 2nd order neuron then decussates at the spinal level, travels up the ventrolateral spinothalamic tract (Figure 1.3) in the ventral lateral horn of the spinal column, then through the brainstem to the VPL of the thalamus. Here the 2nd order neuron synapses with the third order neuron, transmitting information to the somatosensory cortex, specifically the postcentral gyrus (Byrne).
Figure 1.1. Two major categories of skin and their afferent receptors
Figure 1.1. Two major categories of skin and their afferent receptors



Possible physiology
The recognition of the itch elicits an innate motor program response – the scratch. The action of scratching is a spinal reflex that doesn't reach higher centers (Moses). The action of scratching is often counterproductive, and it’s because scratching will trigger skin inflammation, temperature increase, vasodilation, and not least of all, stimulation of various cutaneous and nocioceptors. In addition, excessive itching can break open the skin, leaving it vulnerable to a litany of various infections, and deeper problems (Moses).

Histamine, associated with mast cells, is one of the most common neurotransmitters with respect to pain mediation and allergic reactions. It has been shown that subcutaneous injections of histamine has itself brought on sensations of pain. (Byrne) . Tissue damage in healthy individuals stimulates mast cells which release histamine, which in turn stimulate nocioceptors. Histamine cells are located in the basal posterior hypothalamus (Byrne), and anti-histamines, which are drugs that block histamine receptors, are often used to treat pain symptoms and allergic reactions. Anti-histamines are found in common over-the-counter cold medications such as NyQuil and allergy-relief medications like Claritin.

Seratonin has been linked to itching in diseases including, but not limited to, polycythemia vera, uremia, cholestasis and lymphoma, and of morphine-associated pruritus. (Moses). In addition, serotonin has been linked to the excitation of nocioceptors.
Opioids have been found to trigger pruritus intravenously, and significantly more often when injected into the spinal column. (Moses). Opiate receptors have been found in Rexed laminae I, II and IV of the brainstem. Intravenous morphine will bind to opiate receptors to produce analgesia. It has been shown that intravenous morphine releases histamine (Twycross et. al.).

“Delusions of parasitosis” is the label for a hallucinatory phenomenon in which an individual falsely perceives the presence of parasitic organisms underneath the skin, and can lead to harsh self-inflicted skin damage. This is a commonly found side effect in crystal methamphetamine users – narcotics investigators colloquially term these fictional parasites “Crank Bugs.” Additionally, this can be a side-effect of other serious psychological disorders (Litt).

Peripheral neuropathies can also cause an array of sensations, including the itch. Damage, especially injury and inflammation, can cause polarization and
Figure 1.4. A map of dermatomes in the body.
Figure 1.4. A map of dermatomes in the body.
thus an itching sensation to travel across a variable range of area, traveling down a single nerve, a group of nerves, or even throughout the span of an entire dermatome (Figure 1.4).

external image a_03_cl_dou_1c.jpg
Atopic dermatitis is an excessive dryness of the skin, and can be linked genetically, but has a variety of origins. This is another frequent cause of itching.

Impaired immune response, such as t-cell lymphoma and HIV infection can also cause itch.




BUT WHY DOES SCRATCHING FEEL SO GOOD?

Itching sensations are elicited by the same free nerve endings that sense pain, and therefore elicit a noxious stimulus to our higher cortical structures. Scratching is thus a mechanism in humans and animals that is designed to rid the body of irritants. Still, without removing the irritant:

-Scratching diverts the attention of the affected nerve endings by bringing minor pain sensations (noxious stimuli) to the somatosensory cortex using the same pathway, allowing the body to ignore the itch for the duration of the scratch. Once scratching ceases, itch returns, often worse. Scratching is, essentially, a distraction (McLeod).

-Touching and rubbing the affected area stimulates different cutaneous receptors within the dermis, resulting in a confusion of signals and a net masking of the uncomfortable itchy sensation (Andrews).


-One study suggests that scratching in response to an itch stimulus lowers activation in the cingulate cortex, which is linked to memory and emotional response to pain (Raymond).






Mechanoreceptors in the dermis can also divert attention from an itch via the Dorsal Column Medial Lemniscal pathway.

Pacinian Corpuscle: Sensitive to Vibration


Meissner's Corpuscle: Sensitive to buzzing or fluttering sensations (slower than vibration)


Ruffini's ending: Sensitive to skin stretching


Merkel's Disc: Sensitive to skin indentation







Related Terms:

Pruritus: Itch

Paresthesia: Prickling, tingling, itching, or any sort of sense of skin abnormality that has no apparent cause. Not necessarily malicious.

Urticaria: Hives


Quiz:
1. Which Structure in the dermis is responsible for skin stretching?
A) Ruffini's ending
B) Pacinian corpuscle
C) Meissner's corpuscle
D) Merkel's Disc
E) Free nerve ending

2. What type of afferent axonal fiber is responsible for relaying itch information?
A) A alpha
B) A beta
C) A gamma
D) C

3. What pathway is responsible for the sending itch information afferently?
A) Spinothalamic
B) Dorsal Column Medial Lemniscus
C) Spinocerebellar
D) Vestibulocerebellar
E) Trigeminal

4) TRUE/FALSE Scratching feels good because it releases histamines, which are the neurotransmitters responsible for feelings of well-being and satisfaction.

5) TRUE/FALSE Scratching often makes the skin feel worse afterward because of vasodilation, nearby nocioceptive stimulation, and small abrasions to the epidermis.

6) TRUE/FALSE Methamphetamine is a pheromone for spiders, inviting them to crawl all over the bodies of chronic users.

7) TRUE/FALSE Scratching can feel good because it distracts from an uncomfortable, "itchy" stimulus.

8) What are two neurotransmitters that have been linked to nocioceptor stimulation?

9) Where does the ascending pathway for nocioceptive information project to in the parietal lobe of the brain?

10) What nocioceptive stimulus might make someone itchy and, physiologically, why?

Works Cited / Further Readings


Andrews, Mark A.W. (2007). Why and How do Body Parts Itch? [online]. Scientific American. http://www.scientificamerican.com/article.cfm?id=experts-why-we-itch-and-scratch [15 December 2011].

Byrne, J. H. (ed.) (1997). Neuroscience Online: An Electronic Textbook for the Neurosciences [online]. Department of Neurobiology and Anatomy, The University of Texas Medical School at Houston (UTHealth)
http://nba.uth.tmc.edu/neuroscience/ [14 December 2011].

Kandel, Eric R. and Schwartz, James H. Principles of Neural Science. London: Elsevier North Holland, 1981.

Litt, Jerome (2009). Formication [online]. Psychology Today. http://www.psychologytoday.com/blog/odd-curious-and-rare/200911/formication [12 December 2011].

McLeod, Jamie (2009). Healthy Living [online]. Farmers' Almanac. http://www.farmersalmanac.com/health/2009/03/30/why-does-scratching-an-itch-feel-good/ [15 December 2011].

Moses, Scott (2003). Pruritus [online]. American Academy of Family Physicians. http://www.aafp.org/afp/2003/0915/p1135.html [12 December 2011].

Nicholls, John G., Martin, Robet A. and Wallace, Bruce G. From Neuron to Brain: A Cellular and Molecular Approach to the Function of the Nervous System [Third Edition]. Massachusetts: Sinauer Associates, 1992.

(2011) Peripheral Neuropathy Factsheet [online]. National Institute of Neurological Disorders and Stroke. http://www.ninds.nih.gov/disorders/peripheralneuropathy/detail_peripheralneuropathy.htm [15 December 2011].

Perkins, Sid (1997). Scientists Finally Find Where to Scratch [online]. Science News.http://www.nasw.org/users/sperkins/itch.html [6 December 2011].

Raymond, Joan (2008). The Science of Scratching [online]. Newsweek. http://www.thedailybeast.com/newsweek/2008/01/30/it-feels-good-and-everybody-does-it.html [15 December 2011].

Twycross, R, M.W. Greaves, H. Handwerker, E.A. Jones, S.E. Libretto, J.C. Szepietowski,Z. Zylicz. Itches Scratching More than the Surface [online]. Oxford Journals: Medicine: QJM: An International Journal of Medicine, Volume 96 Issue 1, pp. 7-26. http://qjmed.oxfordjournals.org/content/96/1/7.full [14 December 2011].

(2006) Why do we itch? Blame your brain [online]. Health on MSNBC.com. http://www.msnbc.msn.com/id/14910412/ns/health-livescience/t/why-do-we-itch-blame-your-brain/#.TugSfvLO3YQ [30 November 2011].



Quiz answers
1. A 2. D 3. A 4. F 5. T 6. F 7. T 8. Histamine, Seratonin 9. Postcentral gyrus 10. A mosquito bite, a hive, chicken pox, etc. can manifest itself in the skin and stimulate free nerve endings, traveling up the spinothalamic pathway and eventually causing histamine release to help localize the itch.