Itch
Encyclopedia
Itch is a sensation that causes the desire or reflex
to scratch. Itch has resisted many attempts to classify it as any one type of sensory experience. Modern science has shown that itch has many similarities to pain
, and while both are unpleasant sensory
experiences, their behavioral response patterns are different. Pain creates a withdrawal reflex while itch leads to a scratch reflex
. Unmyelinated nerve fibers for itch and pain both originate in the skin
; however, information for them is conveyed centrally in two distinct systems that both use the same peripheral nerve bundle and spinothalamic tract
.
(dermal or neuropathic) or in the central nervous system
(neuropathic, neurogenic, or psychogenic).
Itch originating in the skin is known as pruritoceptive, and can be induced by a variety of stimuli, including mechanical, chemical, thermal, and electrical stimulation. The primary afferent neurons responsible for histamine
-induced itch are unmyelinated C-fibres
. Two major classes of human C-fibre nociceptors exist: mechano-responsive nociceptors and mechano-insensitive nociceptors. Mechano-responsive nociceptors have been shown in studies to respond to mostly pain and mechano-insensitive receptors respond mostly to itch induced by histamine. However it does not explain mechanically induced itch or when itch is produced without a flare reaction which involves no histamine
. Therefore it is possible that pruritoceptive nerve fibres have different classes of fibres, which is unclear in current research.
Studies have been done to show that itch receptors are found only on the top two skin
layers, the epidermis and the epidermal/dermal transition layers. Shelley and Arthur had verified the depth by injecting individual itch powder spicules (Mucuna pruriens
) and found that maximal sensitivity was found at the basal cell layer or the innermost layer of the epidermis. Surgical removal of those skin
layers removed the ability for a patient to perceive itch. Itch is never felt in muscle
, joints, or inner organs, which strongly suggests that deep tissue probably does not contain itch signaling apparatuses.
Sensitivity to pruritic stimuli is evenly distributed across the skin and has a clear spot distribution with similar density to that of pain. The different substances that elicit itch upon intracutaneous injection (injection within the skin) elicit only pain when injected subcutaneously (beneath the skin). Itch is readily abolished in skin areas treated with nociceptor excitotoxin
capsaicin
but remains unchanged in skin areas which were rendered touch-insensitive by pretreatment with saponin
s, an anti-inflammatory agent. Although experimentally induced itch can still be perceived under a complete A-fiber conduction block, it is significantly diminished. Overall, itch sensation is mediated by A-delta and C nociceptors located in the uppermost layer of the skin.
. They could include diseases or disorders in the central nervous system
or peripheral nervous system
. Examples of neuropathic itch in origin are notalgia paresthetica, brachioradial pruritis
, brain tumors, multiple sclerosis
, peripheral neuropathy
, and nerve irritation
.
s (as in OCD-related neurotic scratching).
.
The inhibition of itch by painful stimuli, including heat
, physical stimulus, and chemical stimulus, has been shown experimentally. In an article written by Louise Ward and others, the effects of noxious and non-noxious counterstimuli, such as heat, physical vibration, or chemical stimulation on skin, were studied in healthy adults after they had experimentally induced itch (transdermal iontophoresis
of histamine) and pain (with topical mustard oil) in their skin. They found that when they induced non-noxious counterstimuli, the reduction of pain and itch only lasted for up to 20 seconds. However when they induced noxious counterstimuli, there was a significant inhibition of itch for an extended period of time but no inhibition of pain. In addition, it was found that brief noxious stimuli created an anti-itch state for more than 30 minutes. These findings show that itch is not a subliminal form of pain and that noxious counterstimulus is likely to act through a central mechanism, instead of a peripheral one.
Chloroquine has been used in animal studies and in humans as a tool to study the itch mechanisms. In studies in rats, an inverted U-shaped dose response curve to chloroquine pruritogenicity was first described by Onigbogi et al., and has been subsequently confirmed by others. This may suggest the involvement of several receptor types centrally. Indeed, nalrexone and Mu opiate down-regulation with chronic morphine attenuated the frequency of chloroquine
pruritus in rats and in mice. Further, a kappa-opioid antagonist, nalfurafine
further attenuated the chloroquine-induced itching frequency in mice. These results further suggested that multiple opiate receptors (mu and kappa) interact to modulate itching behavior to chloroquine. Chloroquine exhibits a pharmacogenetic variation in its itch sensitivity, being common in Africans but rare in other races. It also shows an age-related difference in prevalence. Chloroquine itching is uncommon in children but increases in age with a peak in the fourth decade of life and with increased familial concurrence in families and concordance among twins. Whether the pain sensation exhibits such distribution in Africans is yet to be studied.
Painful electrical stimulation reduced histamine
-induced itch for several hours at a maximum distance of 10 cm from the stimulated site, which suggests a central mode of action. A new method has been recently found, by Hans-Jorgen Nilsson and others, that is able to relieve itch without damaging the skin: cutaneous field stimulation (CFS). CFS consists of a flexible rubber plate with 16 needle-like electrodes placed regularly at 2-centimeter intervals in a 4 by 4 matrix used to electrically stimulate nerve
fibers in the surface of the skin. The electrodes were stimulated continuously at 4 hertz
per electrode, with a pulse duration of 1 millisecond, and an intensity of 0.4–0.8 milliampere lasting for 25 minutes. CFS resulted in a pricking and burning sensation that usually faded away very quickly. The burning sensation was still present during a selective block of impulse conduction of A-fibres in myelinated fibers, which indicates that nociceptive C-fibres are activated by CFS. In addition, a flare reaction was noted to develop around the CFS electrodes, suggesting activation of axon reflexes in nociceptive C-fibres. Itch, which was induced by transdermal iontophoresis of histamine, was inhibited within the skin area treated with CFS and was also significantly reduced at 10 cm from the treatment area. CFS proves to offer a new method of combating itch by using painful electrical stimulation as it creates a long-lasting inhibitory effect, does not create any significant skin injuries, and is simple to apply. It is able to activate powerful itch-inhibitory mechanisms possibly routed through central mechanisms, which could normally be activated by scratching the skin.
A study done by Gil Yosipovitch, Katharine Fast, and Jeffrey Bernhard showed that noxious heat and scratching were able to inhibit or decrease itch induced by transdermal iontophoresis of histamine and, most interestingly, decrease skin blood flow. Twenty-one healthy volunteers participated in the study. Baseline measurements of skin blood flow were obtained on the flexor part of the forearm
and then compared with skin blood flow after various stimuli. Then transdermal iontophoresis of histamine was performed and tested with various stimuli. It is well known that skin blood flow is significantly increased during mechanical scratching, warming, and noxious heat. This study is the first to examine the changes of blood flow by stimuli during iontophoresis
of histamine and how itch is perceived in those conditions. Its examination provided an unexpected result that noxious heat and scratching have an inhibitory effect.
A negative correlation
was found between pain sensitivity and itch sensitivity. In a study done by Amanda Green and others, they aimed to determine itch-related genetic
factors and establish a more useful animal model for itch. They looked at 11 inbred mouse strains and compared their scratching behavior in response to two itch-inducing agents, histamine
and chloroquine
. Every strain revealed an inverted-U-shaped dose response relationship from chloroquine, indicating that moderate dosages produced more scratching than at higher dosages. An explanation is that higher dosage produces more pain and the presence of pain inhibits itch thereby lowering the amount of overall scratching. Another notable result was that histamine-induced scratching occurred in female mice
on average 23% more than in males. Finally, it was found that mice having strains sensitive to pain were resistant to itch and vice versa.
mediators—such as bradykinin
, serotonin
(5-HT) and prostaglandins—released during a painful or pruritic inflammatory condition not only activate pruriceptors but also cause acute
sensitization of the nociceptors. In addition, expression of neuro growth factors (NGF) can cause structural changes in nociceptors, such as sprouting. NGF is high in injured or inflamed tissue. Increased NGF is also found in atopic dermatitis
, a hereditary and non-contagious skin disease with chronic
inflammation
. NGF is known to up-regulate neuropeptides, especially substance P. Substance P has been found to have an important role in inducing pain; however, there is no confirmation that substance P directly causes acute sensitization. Instead, substance P may contribute to itch by increasing neuronal sensitization and may affect release of mast cells, which contain many granules rich in histamine, during long-term interaction.
is known to produce central sensitization, which consists of allodynia
, exaggeration of pain, and punctuate hyperalgesia
, extreme sensitivity to pain. Two types of mechanical hyperalgesia can occur: 1) touch that is normally painless in the uninjured surroundings of a cut or tear can trigger painful sensations (touch-evoked hyperalgesia), and 2) a slightly painful pin prick stimulation is perceived as more painful around a focused area of inflammation (punctuate hyperalgesia). Touch-evoked hyperalgesia requires continuous firing of primary afferent nociceptors, and punctuate hyperalgesia
does not require continuous firing which means it can persist for hours after a trauma
and can be stronger than normally experienced. In addition, it was found that patients with neuropathic pain, histamine ionophoresis resulted in a sensation of burning pain rather than itch, which would be induced in normal healthy patients. This shows that there is spinal hypersensitivity to C-fiber input in chronic
pain.
Infections and infestations:
Environmental and allergic:
Dermatological disorders:
Medical disorders:
Medication:
Related to pregnancy:
Topical
antipruritics in the form of cream
s and spray
s are often available over-the-counter. Oral
anti-itch drugs also exist and are usually prescription drug
s. The active ingredient
s usually belong to the following classes:
Phototherapy is helpful for severe itching, especially if caused by renal failure. The common type of light used is UVB.
Sometimes scratching relieves isolated itches, hence the existence of devices such as the back scratcher. Often, however, scratching can intensify itching and even cause further damage to the skin, dubbed the "itch-scratch-itch cycle".
The mainstay of therapy for dry skin is maintaining adequate skin moisture and topical emollients.
which leads to retraction and therefore a reaction trying to protect an endangered part of the body. Itch creates a scratch reflex
which draws one to the affected skin site. For example, responding to a local itch sensation is an effective way to remove insects on the skin. Scratching has traditionally been regarded as a way to relieve oneself by reducing the annoying itch sensation. However there are hedonic aspects of scratching as one would find noxious scratching highly pleasurable. This can be problematic with chronic
itch patients, such as ones with atopic dermatitis
, who may scratch affected spots until it no longer produces a pleasant or painful sensation instead of when the itch sensation disappears. It has been hypothesized that motivational aspects of scratching include the frontal brain areas of reward and decision making. These aspects might therefore contribute to the compulsive nature of itch and scratching.
itch" are very common occurrences. Even a discussion on the topic of itch can give one the desire to scratch. Itch is likely to be more than a localized phenomenon in the place we scratch. Results from a recent study showed that itching and scratching were induced purely by visual stimuli
in a public lecture on itching. There is currently little detailed data on central activation for contagious itching but it is hypothesized that a human mirror neuron
system exists in which we imitate certain motor actions when we view others performing the same action. A similar phenomenon in which mirror neurons are used to explain the cause is contagious yawning.
Reflex
A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus. A true reflex is a behavior which is mediated via the reflex arc; this does not apply to casual uses of the term 'reflex'.-See also:...
to scratch. Itch has resisted many attempts to classify it as any one type of sensory experience. Modern science has shown that itch has many similarities to pain
Pain
Pain is an unpleasant sensation often caused by intense or damaging stimuli such as stubbing a toe, burning a finger, putting iodine on a cut, and bumping the "funny bone."...
, and while both are unpleasant sensory
Sensory system
A sensory system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory receptors, neural pathways, and parts of the brain involved in sensory perception. Commonly recognized sensory systems are those for vision, hearing, somatic...
experiences, their behavioral response patterns are different. Pain creates a withdrawal reflex while itch leads to a scratch reflex
Scratch reflex
The scratch reflex is a response to activation of sensory neurons whose peripheral terminals are located on the surface of the body. Some sensory neurons can be activated by stimulation with an external object such as a parasite on the body surface. Alternately, some sensory neurons can respond to...
. Unmyelinated nerve fibers for itch and pain both originate in the skin
Skin
-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...
; however, information for them is conveyed centrally in two distinct systems that both use the same peripheral nerve bundle and spinothalamic tract
Spinothalamic tract
The spinothalamic tract is a sensory pathway originating in the spinal cord. It transmits information to the thalamus about pain, temperature, itch and crude touch...
.
Mechanism
Itch can originate in the peripheral nervous systemPeripheral nervous system
The peripheral nervous system consists of the nerves and ganglia outside of the brain and spinal cord. The main function of the PNS is to connect the central nervous system to the limbs and organs. Unlike the CNS, the PNS is not protected by the bone of spine and skull, or by the blood–brain...
(dermal or neuropathic) or in the central nervous system
Central nervous system
The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...
(neuropathic, neurogenic, or psychogenic).
Dermal/pruritoceptive
Histamine
Histamine is an organic nitrogen compound involved in local immune responses as well as regulating physiological function in the gut and acting as a neurotransmitter. Histamine triggers the inflammatory response. As part of an immune response to foreign pathogens, histamine is produced by...
-induced itch are unmyelinated C-fibres
Group C nerve fiber
-Location:C fibers are found in the peripheral nerves of the somatic sensory system. They are afferent fibers, conveying input signals from the periphery to the central nervous system.-Structure:...
. Two major classes of human C-fibre nociceptors exist: mechano-responsive nociceptors and mechano-insensitive nociceptors. Mechano-responsive nociceptors have been shown in studies to respond to mostly pain and mechano-insensitive receptors respond mostly to itch induced by histamine. However it does not explain mechanically induced itch or when itch is produced without a flare reaction which involves no histamine
Histamine
Histamine is an organic nitrogen compound involved in local immune responses as well as regulating physiological function in the gut and acting as a neurotransmitter. Histamine triggers the inflammatory response. As part of an immune response to foreign pathogens, histamine is produced by...
. Therefore it is possible that pruritoceptive nerve fibres have different classes of fibres, which is unclear in current research.
Studies have been done to show that itch receptors are found only on the top two skin
Skin
-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...
layers, the epidermis and the epidermal/dermal transition layers. Shelley and Arthur had verified the depth by injecting individual itch powder spicules (Mucuna pruriens
Mucuna pruriens
Mucuna pruriens is a tropical legume known as velvet bean or cowitch and by other common names , found in Africa, India and the Caribbean. The plant is infamous for its extreme itchiness produced on contact, particularly with the young foliage and the seed pods...
) and found that maximal sensitivity was found at the basal cell layer or the innermost layer of the epidermis. Surgical removal of those skin
Skin
-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...
layers removed the ability for a patient to perceive itch. Itch is never felt in muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
, joints, or inner organs, which strongly suggests that deep tissue probably does not contain itch signaling apparatuses.
Sensitivity to pruritic stimuli is evenly distributed across the skin and has a clear spot distribution with similar density to that of pain. The different substances that elicit itch upon intracutaneous injection (injection within the skin) elicit only pain when injected subcutaneously (beneath the skin). Itch is readily abolished in skin areas treated with nociceptor excitotoxin
Excitotoxicity
Excitotoxicity is the pathological process by which nerve cells are damaged and killed by excessive stimulation by neurotransmitters such as glutamate and similar substances. This occurs when receptors for the excitatory neurotransmitter glutamate such as the NMDA receptor and AMPA receptor are...
capsaicin
Capsaicin
Capsaicin 2CHCH=CH4CONHCH2C6H3-4--3- ) is the active component of chili peppers, which are plants belonging to the genus Capsicum. It is an irritant for mammals, including humans, and produces a sensation of burning in any tissue with which it comes into contact...
but remains unchanged in skin areas which were rendered touch-insensitive by pretreatment with saponin
Saponin
Saponins are a class of chemical compounds, one of many secondary metabolites found in natural sources, with saponins found in particular abundance in various plant species...
s, an anti-inflammatory agent. Although experimentally induced itch can still be perceived under a complete A-fiber conduction block, it is significantly diminished. Overall, itch sensation is mediated by A-delta and C nociceptors located in the uppermost layer of the skin.
Neuropathic
Neuropathic itch can originate at any point along the afferent pathway as a result of damage of the nervous systemNervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...
. They could include diseases or disorders in the central nervous system
Central nervous system
The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...
or peripheral nervous system
Peripheral nervous system
The peripheral nervous system consists of the nerves and ganglia outside of the brain and spinal cord. The main function of the PNS is to connect the central nervous system to the limbs and organs. Unlike the CNS, the PNS is not protected by the bone of spine and skull, or by the blood–brain...
. Examples of neuropathic itch in origin are notalgia paresthetica, brachioradial pruritis
Brachioradial pruritis
Brachioradial pruritus is an intense itching sensation of the arm usually between the shoulder and elbow of either or both arms. The itch can be so intense that sufferers will scratch their own skin to a bleeding condition. The cause is not known, although there are a few lines of thought on...
, brain tumors, multiple sclerosis
Multiple sclerosis
Multiple sclerosis is an inflammatory disease in which the fatty myelin sheaths around the axons of the brain and spinal cord are damaged, leading to demyelination and scarring as well as a broad spectrum of signs and symptoms...
, peripheral neuropathy
Peripheral neuropathy
Peripheral neuropathy is the term for damage to nerves of the peripheral nervous system, which may be caused either by diseases of or trauma to the nerve or the side-effects of systemic illness....
, and nerve irritation
Nerve injury
Nerve injury is injury to nervous tissue. There is no single classification system that can describe all the many variations of nerve injury. Most systems attempt to correlate the degree of injury with symptoms, pathology and prognosis...
.
Neurogenic
Neurogenic itch, which is itch induced centrally but with maximum neural damage, is mostly associated with increased accumulation of exogenous opioids and possibly synthetic opioids.Psychogenic
Itch is also associated with some symptoms of psychiatric disorders such as tactile hallucinations, delusions of parasitosis or obsessive-compulsive disorderObsessive-compulsive disorder
Obsessive–compulsive disorder is an anxiety disorder characterized by intrusive thoughts that produce uneasiness, apprehension, fear, or worry, by repetitive behaviors aimed at reducing the associated anxiety, or by a combination of such obsessions and compulsions...
s (as in OCD-related neurotic scratching).
Pain inhibits itch
The sensation of itch can be reduced by many painful sensations. Numerous studies done in the last decade have shown that itch can be inhibited by many other forms of painful stimuli, such as noxious heat, physical rubbing/scratching, noxious chemicals, and electric shockElectric shock
Electric Shock of a body with any source of electricity that causes a sufficient current through the skin, muscles or hair. Typically, the expression is used to denote an unwanted exposure to electricity, hence the effects are considered undesirable....
.
The inhibition of itch by painful stimuli, including heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
, physical stimulus, and chemical stimulus, has been shown experimentally. In an article written by Louise Ward and others, the effects of noxious and non-noxious counterstimuli, such as heat, physical vibration, or chemical stimulation on skin, were studied in healthy adults after they had experimentally induced itch (transdermal iontophoresis
Iontophoresis
Iontophoresis is a technique using a small electric charge to deliver a medicine or other chemical through the skin. It is basically an injection without the needle...
of histamine) and pain (with topical mustard oil) in their skin. They found that when they induced non-noxious counterstimuli, the reduction of pain and itch only lasted for up to 20 seconds. However when they induced noxious counterstimuli, there was a significant inhibition of itch for an extended period of time but no inhibition of pain. In addition, it was found that brief noxious stimuli created an anti-itch state for more than 30 minutes. These findings show that itch is not a subliminal form of pain and that noxious counterstimulus is likely to act through a central mechanism, instead of a peripheral one.
Chloroquine has been used in animal studies and in humans as a tool to study the itch mechanisms. In studies in rats, an inverted U-shaped dose response curve to chloroquine pruritogenicity was first described by Onigbogi et al., and has been subsequently confirmed by others. This may suggest the involvement of several receptor types centrally. Indeed, nalrexone and Mu opiate down-regulation with chronic morphine attenuated the frequency of chloroquine
Chloroquine
Chloroquine is a 4-aminoquinoline drug used in the treatment or prevention of malaria.-History:Chloroquine , N'--N,N-diethyl-pentane-1,4-diamine, was discovered in 1934 by Hans Andersag and co-workers at the Bayer laboratories who named it "Resochin". It was ignored for a decade because it was...
pruritus in rats and in mice. Further, a kappa-opioid antagonist, nalfurafine
Nalfurafine
Nalfurafine is a κ-opioid receptor agonist marketed as a treatment for uremic pruritus in people undergoing hemodialysis....
further attenuated the chloroquine-induced itching frequency in mice. These results further suggested that multiple opiate receptors (mu and kappa) interact to modulate itching behavior to chloroquine. Chloroquine exhibits a pharmacogenetic variation in its itch sensitivity, being common in Africans but rare in other races. It also shows an age-related difference in prevalence. Chloroquine itching is uncommon in children but increases in age with a peak in the fourth decade of life and with increased familial concurrence in families and concordance among twins. Whether the pain sensation exhibits such distribution in Africans is yet to be studied.
Painful electrical stimulation reduced histamine
Histamine
Histamine is an organic nitrogen compound involved in local immune responses as well as regulating physiological function in the gut and acting as a neurotransmitter. Histamine triggers the inflammatory response. As part of an immune response to foreign pathogens, histamine is produced by...
-induced itch for several hours at a maximum distance of 10 cm from the stimulated site, which suggests a central mode of action. A new method has been recently found, by Hans-Jorgen Nilsson and others, that is able to relieve itch without damaging the skin: cutaneous field stimulation (CFS). CFS consists of a flexible rubber plate with 16 needle-like electrodes placed regularly at 2-centimeter intervals in a 4 by 4 matrix used to electrically stimulate nerve
Nerve
A peripheral nerve, or simply nerve, is an enclosed, cable-like bundle of peripheral axons . A nerve provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons. Nerves are found only in the peripheral nervous system...
fibers in the surface of the skin. The electrodes were stimulated continuously at 4 hertz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
per electrode, with a pulse duration of 1 millisecond, and an intensity of 0.4–0.8 milliampere lasting for 25 minutes. CFS resulted in a pricking and burning sensation that usually faded away very quickly. The burning sensation was still present during a selective block of impulse conduction of A-fibres in myelinated fibers, which indicates that nociceptive C-fibres are activated by CFS. In addition, a flare reaction was noted to develop around the CFS electrodes, suggesting activation of axon reflexes in nociceptive C-fibres. Itch, which was induced by transdermal iontophoresis of histamine, was inhibited within the skin area treated with CFS and was also significantly reduced at 10 cm from the treatment area. CFS proves to offer a new method of combating itch by using painful electrical stimulation as it creates a long-lasting inhibitory effect, does not create any significant skin injuries, and is simple to apply. It is able to activate powerful itch-inhibitory mechanisms possibly routed through central mechanisms, which could normally be activated by scratching the skin.
A study done by Gil Yosipovitch, Katharine Fast, and Jeffrey Bernhard showed that noxious heat and scratching were able to inhibit or decrease itch induced by transdermal iontophoresis of histamine and, most interestingly, decrease skin blood flow. Twenty-one healthy volunteers participated in the study. Baseline measurements of skin blood flow were obtained on the flexor part of the forearm
Forearm
-See also:*Forearm flexors*Forearm muscles...
and then compared with skin blood flow after various stimuli. Then transdermal iontophoresis of histamine was performed and tested with various stimuli. It is well known that skin blood flow is significantly increased during mechanical scratching, warming, and noxious heat. This study is the first to examine the changes of blood flow by stimuli during iontophoresis
Iontophoresis
Iontophoresis is a technique using a small electric charge to deliver a medicine or other chemical through the skin. It is basically an injection without the needle...
of histamine and how itch is perceived in those conditions. Its examination provided an unexpected result that noxious heat and scratching have an inhibitory effect.
A negative correlation
Correlation
In statistics, dependence refers to any statistical relationship between two random variables or two sets of data. Correlation refers to any of a broad class of statistical relationships involving dependence....
was found between pain sensitivity and itch sensitivity. In a study done by Amanda Green and others, they aimed to determine itch-related genetic
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....
factors and establish a more useful animal model for itch. They looked at 11 inbred mouse strains and compared their scratching behavior in response to two itch-inducing agents, histamine
Histamine
Histamine is an organic nitrogen compound involved in local immune responses as well as regulating physiological function in the gut and acting as a neurotransmitter. Histamine triggers the inflammatory response. As part of an immune response to foreign pathogens, histamine is produced by...
and chloroquine
Chloroquine
Chloroquine is a 4-aminoquinoline drug used in the treatment or prevention of malaria.-History:Chloroquine , N'--N,N-diethyl-pentane-1,4-diamine, was discovered in 1934 by Hans Andersag and co-workers at the Bayer laboratories who named it "Resochin". It was ignored for a decade because it was...
. Every strain revealed an inverted-U-shaped dose response relationship from chloroquine, indicating that moderate dosages produced more scratching than at higher dosages. An explanation is that higher dosage produces more pain and the presence of pain inhibits itch thereby lowering the amount of overall scratching. Another notable result was that histamine-induced scratching occurred in female mice
Mouse
A mouse is a small mammal belonging to the order of rodents. The best known mouse species is the common house mouse . It is also a popular pet. In some places, certain kinds of field mice are also common. This rodent is eaten by large birds such as hawks and eagles...
on average 23% more than in males. Finally, it was found that mice having strains sensitive to pain were resistant to itch and vice versa.
Peripheral sensitization
InflammatoryInflammation
Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...
mediators—such as bradykinin
Bradykinin
Bradykinin is a peptide that causes blood vessels to dilate , and therefore causes blood pressure to lower. A class of drugs called ACE inhibitors, which are used to lower blood pressure, increase bradykinin further lowering blood pressure...
, serotonin
Serotonin
Serotonin or 5-hydroxytryptamine is a monoamine neurotransmitter. Biochemically derived from tryptophan, serotonin is primarily found in the gastrointestinal tract, platelets, and in the central nervous system of animals including humans...
(5-HT) and prostaglandins—released during a painful or pruritic inflammatory condition not only activate pruriceptors but also cause acute
Acute (medicine)
In medicine, an acute disease is a disease with either or both of:# a rapid onset, as in acute infection# a short course ....
sensitization of the nociceptors. In addition, expression of neuro growth factors (NGF) can cause structural changes in nociceptors, such as sprouting. NGF is high in injured or inflamed tissue. Increased NGF is also found in atopic dermatitis
Atopic dermatitis
Atopic dermatitis is an inflammatory, chronically relapsing, non-contagious and pruritic skin disorder...
, a hereditary and non-contagious skin disease with chronic
Chronic (medicine)
A chronic disease is a disease or other human health condition that is persistent or long-lasting in nature. The term chronic is usually applied when the course of the disease lasts for more than three months. Common chronic diseases include asthma, cancer, diabetes and HIV/AIDS.In medicine, the...
inflammation
Inflammation
Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...
. NGF is known to up-regulate neuropeptides, especially substance P. Substance P has been found to have an important role in inducing pain; however, there is no confirmation that substance P directly causes acute sensitization. Instead, substance P may contribute to itch by increasing neuronal sensitization and may affect release of mast cells, which contain many granules rich in histamine, during long-term interaction.
Central sensitization
Noxious input to the spinal cordSpinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...
is known to produce central sensitization, which consists of allodynia
Allodynia
Allodynia is a pain due to a stimulus which does not normally provoke pain. Temperature or physical stimuli can provoke allodynia, and it often occurs after injury to a site...
, exaggeration of pain, and punctuate hyperalgesia
Hyperalgesia
Hyperalgesia is an increased sensitivity to pain, which may be caused by damage to nociceptors or peripheral nerves. Temporary increased sensitivity to pain also occurs as part of sickness behavior, the evolved response to infection.-Types:...
, extreme sensitivity to pain. Two types of mechanical hyperalgesia can occur: 1) touch that is normally painless in the uninjured surroundings of a cut or tear can trigger painful sensations (touch-evoked hyperalgesia), and 2) a slightly painful pin prick stimulation is perceived as more painful around a focused area of inflammation (punctuate hyperalgesia). Touch-evoked hyperalgesia requires continuous firing of primary afferent nociceptors, and punctuate hyperalgesia
Hyperalgesia
Hyperalgesia is an increased sensitivity to pain, which may be caused by damage to nociceptors or peripheral nerves. Temporary increased sensitivity to pain also occurs as part of sickness behavior, the evolved response to infection.-Types:...
does not require continuous firing which means it can persist for hours after a trauma
Physical trauma
Trauma refers to "a body wound or shock produced by sudden physical injury, as from violence or accident." It can also be described as "a physical wound or injury, such as a fracture or blow." Major trauma can result in secondary complications such as circulatory shock, respiratory failure and death...
and can be stronger than normally experienced. In addition, it was found that patients with neuropathic pain, histamine ionophoresis resulted in a sensation of burning pain rather than itch, which would be induced in normal healthy patients. This shows that there is spinal hypersensitivity to C-fiber input in chronic
Chronic (medicine)
A chronic disease is a disease or other human health condition that is persistent or long-lasting in nature. The term chronic is usually applied when the course of the disease lasts for more than three months. Common chronic diseases include asthma, cancer, diabetes and HIV/AIDS.In medicine, the...
pain.
Causes
- Allergic reaction to contact with specific chemicals, such as UrushiolUrushiolUrushiol is an oily organic allergen found in plants of the family Anacardiaceae, especially Toxicodendron spp. . It causes an allergic skin rash on contact, known as urushiol-induced contact dermatitis...
, derived from Poison IvyPoison ivyToxicodendron radicans, better known as poison ivy , is a poisonous North American plant that is well known for its production of urushiol, a clear liquid compound found within the sap of the plant that causes an itching rash in most people who touch it...
or Poison OakPoison oakPoison oak may refer to* Toxicodendron diversilobum, grows on West Coast of North America* Toxicodendron pubescens, grows in the Eastern United Statesdamnnnnn tissss is terribleee...
. - Body louseBody louseThe body louse is a louse which infests humans. The condition of being infested with head lice, body lice, or pubic lice is known as pediculosis.-Origins:...
. BodyBodyWith regard to living things, a body is the physical body of an individual. "Body" often is used in connection with appearance, health issues and death...
lice can be seen in substandard living conditions. - Cutaneous larva migransCutaneous larva migransCutaneous larva migrans is a skin disease in humans, caused by the larvae of various nematode parasites of the hookworm family...
. - Head lice, if limited to the neck and scalp.
- Herpes.
- Insect biteInsect biteInsect bites and stings occur when an insect is agitated and seeks to defend itself through its natural defense mechanisms, or when an insect seeks to feed off the bitten person. Some insects inject formic acid, which can cause an immediate skin reaction often resulting in redness and swelling in...
s, such as those from mosquitos or chiggers. - PhotodermatitisPhotodermatitisPhotodermatitis, sometimes referred to as "sun poisoning" or photoallergy is a form of allergic contact dermatitis in which the allergen must be activated by light to sensitize the allergic response, and to cause a rash or other systemic effects on subsequent exposure...
– (sun)light reacts with chemicals in the skin, leading to the formation of irritant metabolites. - Pubic lice, if limited to the genital area.
- ScabiesScabiesScabies , known colloquially as the seven-year itch, is a contagious skin infection that occurs among humans and other animals. It is caused by a tiny and usually not directly visible parasite, the mite Sarcoptes scabiei, which burrows under the host's skin, causing intense allergic itching...
especially when several other persons in close contact also itch. - ShavingShavingShaving is the removal of hair, by using a razor or any other kind of bladed implement, to slice it down to the level of the skin. Shaving is most commonly practiced by men to remove their facial hair and by women to remove their leg and underarm hair...
, which may irritate the skin. - Swimmer's itchSwimmer's itchSwimmer’s itch, also known as lake itch, duck itch, cercarial dermatitis, and Schistosome cercarial dermatitis, is a short-term, immune reaction occurring in the skin of humans that have been infected by water-borne schistosomatidae...
- Varicella (Chickenpox) Most common in young children. Highly contagious.
Environmental and allergic:
- UrticariaUrticariaUrticaria is a kind of skin rash notable for pale red, raised, itchy bumps. Hives is frequently caused by allergic reactions; however, there are many non-allergic causes...
(also called hives) usually causes itching.
Dermatological disorders:
- DandruffDandruffDandruff is the shedding of dead skin cells from the scalp . Dandruff is sometimes caused by frequent exposure to extreme heat and cold. As it is normal for skin cells to die and flake off, a small amount of flaking is normal and common; about 487,000 cells/cm2 get released normally after...
. (An unusually large amount of flaking is associated with this sensation.) - ScabWound healingWound healing, or cicatrisation, is an intricate process in which the skin repairs itself after injury. In normal skin, the epidermis and dermis exists in a steady-state equilibrium, forming a protective barrier against the external environment...
healing; scarScarScars are areas of fibrous tissue that replace normal skin after injury. A scar results from the biological process of wound repair in the skin and other tissues of the body. Thus, scarring is a natural part of the healing process. With the exception of very minor lesions, every wound results in...
growth; and the development or emergence of molesMelanocytic nevusA melanocytic nevus is a type of lesion that contains nevus cells .Some sources equate the term mole with "melanocytic nevus". Other sources reserve the term "mole" for other purposes....
, pimplePimpleA pimple, zit or spot is a kind of acne, and one of the many results of excess oil getting trapped in the pores. Some of the varieties are pustules or papules....
s and ingrown hairIngrown hairIngrown hair is a condition where the hair curls back or grows sideways into the skin. The ingrown hair condition is seen primarily among people having curly hair. It may or may not be accompanied by an infection of the hair follicle or "razor bumps" , which vary in size...
s from below the epidermis. - Skin conditionsDermatologyDermatology is the branch of medicine dealing with the skin and its diseases, a unique specialty with both medical and surgical aspects. A dermatologist takes care of diseases, in the widest sense, and some cosmetic problems of the skin, scalp, hair, and nails....
(such as psoriasisPsoriasisPsoriasis is an autoimmune disease that appears on the skin. It occurs when the immune system mistakes the skin cells as a pathogen, and sends out faulty signals that speed up the growth cycle of skin cells. Psoriasis is not contagious. However, psoriasis has been linked to an increased risk of...
, eczemaEczemaEczema is a form of dermatitis, or inflammation of the epidermis . In England, an estimated 5.7 million or about one in every nine people have been diagnosed with the disease by a clinician at some point in their lives.The term eczema is broadly applied to a range of persistent skin conditions...
, sunburnSunburnA sunburn is a burn to living tissue, such as skin, which is produced by overexposure to ultraviolet radiation, commonly from the sun's rays. Usual mild symptoms in humans and other animals include red or reddish skin that is hot to the touch, general fatigue, and mild dizziness. An excess of UV...
, athlete's footAthlete's footAthlete's foot is a fungal infection of the skin that causes scaling, flaking, and itch of affected areas. It is caused by fungi in the genus Trichophyton and is typically transmitted in moist areas where people walk barefoot, such as showers or bathhouses...
, hidradenitis suppurativaHidradenitis suppurativaHidradenitis suppurativa is a skin disease that most commonly affects areas bearing apocrine sweat glands or sebaceous glands, such as the underarms, breasts, inner thighs, groin and buttocks.-Overview:...
and many others). Most are of an inflammatory nature. - Punctate palmoplantar keratodermaPunctate palmoplantar keratodermaPunctate palmoplantar keratoderma is a form of palmoplantar keratoderma in which many tiny "raindrop" keratoses involve the palmoplantar surface, skin lesions which may involve the whole of the palmoplantar surface, or may be more restricted in their distribution.* Type 1: Keratosis punctata...
. - XerosisXerosisXerosis cutis is the medical term for dry skin.It can have many different causes, including general dehydration, atopic dermatitis, Vitamin A deficiency, and maybe diabetes. Treatment is primarily symptomatic. "Xero", meaning dry or dehydrated, "osis" usually referring to a medical disease or...
(dry skin). This is the most common cause,, frequently seen in the winter and also associated with older age, frequent bathing in hot showers or baths, and high-temperature and low-humidity environments.
Medical disorders:
- Diabetes MellitusDiabetes mellitusDiabetes mellitus, often simply referred to as diabetes, is a group of metabolic diseases in which a person has high blood sugar, either because the body does not produce enough insulin, or because cells do not respond to the insulin that is produced...
. - HyperparathyroidismHyperparathyroidismHyperparathyroidism is overactivity of the parathyroid glands resulting in excess production of parathyroid hormone . The parathyroid hormone regulates calcium and phosphate levels and helps to maintain these levels...
. - Iron deficiency anemiaIron deficiency anemiaIron-deficiency anemia is a common anemia that occurs when iron loss occurs, and/or the dietary intake or absorption of iron is insufficient...
. - JaundiceJaundiceJaundice is a yellowish pigmentation of the skin, the conjunctival membranes over the sclerae , and other mucous membranes caused by hyperbilirubinemia . This hyperbilirubinemia subsequently causes increased levels of bilirubin in the extracellular fluid...
and CholestasisCholestasisIn medicine, cholestasis is a condition where bile cannot flow from the liver to the duodenum. The two basic distinctions are an obstructive type of cholestasis where there is a mechanical blockage in the duct system such as can occur from a gallstone or malignancy and metabolic types of...
. (BilirubinBilirubinBilirubin is the yellow breakdown product of normal heme catabolism. Heme is found in hemoglobin, a principal component of red blood cells. Bilirubin is excreted in bile and urine, and elevated levels may indicate certain diseases...
is a skin irritant at high concentrations.) - Malignancy or internal cancer such as lymphomaLymphomaLymphoma is a cancer in the lymphatic cells of the immune system. Typically, lymphomas present as a solid tumor of lymphoid cells. Treatment might involve chemotherapy and in some cases radiotherapy and/or bone marrow transplantation, and can be curable depending on the histology, type, and stage...
, Hodgkin's disease. - MenopauseMenopauseMenopause is a term used to describe the permanent cessation of the primary functions of the human ovaries: the ripening and release of ova and the release of hormones that cause both the creation of the uterine lining and the subsequent shedding of the uterine lining...
or changes in hormonal balances associated with aging. - PolycythemiaPolycythemiaPolycythemia is a disease state in which the proportion of blood volume that is occupied by red blood cells increases...
, which can cause generalized itching due to increased histamine. - ThyroidThyroidThe thyroid gland or simply, the thyroid , in vertebrate anatomy, is one of the largest endocrine glands. The thyroid gland is found in the neck, below the thyroid cartilage...
illness. - Uraemia.
Medication:
- Directly (e.g. morphineMorphineMorphine is a potent opiate analgesic medication and is considered to be the prototypical opioid. It was first isolated in 1804 by Friedrich Sertürner, first distributed by same in 1817, and first commercially sold by Merck in 1827, which at the time was a single small chemists' shop. It was more...
and other opiates) - ChloroquineChloroquineChloroquine is a 4-aminoquinoline drug used in the treatment or prevention of malaria.-History:Chloroquine , N'--N,N-diethyl-pentane-1,4-diamine, was discovered in 1934 by Hans Andersag and co-workers at the Bayer laboratories who named it "Resochin". It was ignored for a decade because it was...
.
Related to pregnancy:
- Gestational pemphigoidGestational pemphigoidGestational Pemphigoid or Pemphigoid Gestationis is a dermatosis of pregnancy, being an autoimmune blistering skin disease that occurs during pregnancy, typically in the second or third trimester, and/or immediately following pregnancy...
- Intrahepatic cholestasis of pregnancyIntrahepatic Cholestasis of PregnancyIntrahepatic cholestasis of pregnancy in the United Kingdom, is a medical condition in which cholestasis occurs during pregnancy...
- Pruritic urticarial papules and plaques of pregnancy (PUPPP)
- Psychiatric disease.
Treatment
A variety of over-the-counter and prescription anti-itch drugs are available. Some plant products have been found to be effective anti-pruritics, others not. Non-chemical remedies include cooling, warming, soft stimulation.Topical
Topical
In medicine, a topical medication is applied to body surfaces such as the skin or mucous membranes such as the vagina, anus, throat, eyes and ears.Many topical medications are epicutaneous, meaning that they are applied directly to the skin...
antipruritics in the form of cream
Cream (pharmaceutical)
A cream is a topical preparation usually for application to the skin. Creams for application to mucus membranes such as those of the rectum or vagina are also used. Creams may be considered pharmaceutical products as even cosmetic creams are based on techniques developed by pharmacy and...
s and spray
Spray
Spray can refer to:* Spray ** Aerosol spray** Blood spray** Road/tire spray, kicked up from a vehicle tire** Snow spray, a spray that creates artificial snow fall...
s are often available over-the-counter. Oral
Mouth
The mouth is the first portion of the alimentary canal that receives food andsaliva. The oral mucosa is the mucous membrane epithelium lining the inside of the mouth....
anti-itch drugs also exist and are usually prescription drug
Prescription drug
A prescription medication is a licensed medicine that is regulated by legislation to require a medical prescription before it can be obtained. The term is used to distinguish it from over-the-counter drugs which can be obtained without a prescription...
s. The active ingredient
Active ingredient
An active ingredient is the substance of a pharmaceutical drug or a pharmaceutical ingredient and bulk active in medicine; in pesticide formulations active substance may be used. Some medications and pesticide products may contain more than one active ingredient...
s usually belong to the following classes:
- AntihistamineAntihistamineAn H1 antagonist is a histamine antagonist of the H1 receptor that serves to reduce or eliminate effects mediated by histamine, an endogenous chemical mediator released during allergic reactions...
s such as diphenhydramineDiphenhydramineDiphenhydramine hydrochloride is a first-generation antihistamine possessing anticholinergic, antitussive, antiemetic, and sedative properties which is mainly used to treat allergies. Like most other first-generation antihistamines, the drug also has a powerful hypnotic effect, and for this reason...
(Benadryl) - CorticosteroidCorticosteroidCorticosteroids are a class of steroid hormones that are produced in the adrenal cortex. Corticosteroids are involved in a wide range of physiologic systems such as stress response, immune response and regulation of inflammation, carbohydrate metabolism, protein catabolism, blood electrolyte...
s such as hydrocortisone topical cream, see topical steroidTopical steroidTopical steroids are the topical forms of corticosteroids. Topical steroids are the most commonly prescribed topical medications for the treatment of rash, eczema, and dermatitis. Topical steroids have anti-inflammatory properties, and are classified based on their vasoconstriction abilities. There... - CounterirritantCounterirritantA counterirritant is a substance which creates inflammation in one location with the goal of lessening the inflammation in another location. They can be used as antipruritics. This strategy falls into the more general category of counterstimulation....
s, such as mint oilMenthaMentha is a genus of flowering plants in the family Lamiaceae . The species are not clearly distinct and estimates of the number of species varies from 13 to 18. Hybridization between some of the species occurs naturally...
, mentholMentholMenthol is an organic compound made synthetically or obtained from peppermint or other mint oils. It is a waxy, crystalline substance, clear or white in color, which is solid at room temperature and melts slightly above. The main form of menthol occurring in nature is -menthol, which is assigned...
, or camphorCamphorCamphor is a waxy, white or transparent solid with a strong, aromatic odor. It is a terpenoid with the chemical formula C10H16O. It is found in wood of the camphor laurel , a large evergreen tree found in Asia and also of Dryobalanops aromatica, a giant of the Bornean forests... - CrotamitonCrotamitonCrotamiton is a drug that is used both as a scabicidal and as a general antipruritic . It is a prescription and OTC lotion based medicine that is applied to the whole body to get rid of the scabies parasite that burrows under the skin and causes itching.-Use:For treating scabies, crotamiton should...
(trade name Eurax) is an antipruritic agent available as a cream or lotion often used to treat scabiesScabiesScabies , known colloquially as the seven-year itch, is a contagious skin infection that occurs among humans and other animals. It is caused by a tiny and usually not directly visible parasite, the mite Sarcoptes scabiei, which burrows under the host's skin, causing intense allergic itching...
. Its mechanism of action remains unknown. - Local anestheticLocal anestheticA local anesthetic is a drug that causes reversible local anesthesia, generally for the aim of having local analgesic effect, that is, inducing absence of pain sensation, although other local senses are often affected as well...
s such as benzocaineBenzocaineBenzocaine is a local anesthetic commonly used as a topical pain reliever, or in cough drops. It is the active ingredient in many over-the-counter anesthetic ointments...
topical cream (Lanacane)
Phototherapy is helpful for severe itching, especially if caused by renal failure. The common type of light used is UVB.
Sometimes scratching relieves isolated itches, hence the existence of devices such as the back scratcher. Often, however, scratching can intensify itching and even cause further damage to the skin, dubbed the "itch-scratch-itch cycle".
The mainstay of therapy for dry skin is maintaining adequate skin moisture and topical emollients.
Sensations associated with scratching
Pain and itch have very different behavioral response patterns. Pain evokes a withdrawal reflexWithdrawal reflex
The withdrawal reflex is a spinal reflex intended to protect the body from damaging stimuli...
which leads to retraction and therefore a reaction trying to protect an endangered part of the body. Itch creates a scratch reflex
Scratch reflex
The scratch reflex is a response to activation of sensory neurons whose peripheral terminals are located on the surface of the body. Some sensory neurons can be activated by stimulation with an external object such as a parasite on the body surface. Alternately, some sensory neurons can respond to...
which draws one to the affected skin site. For example, responding to a local itch sensation is an effective way to remove insects on the skin. Scratching has traditionally been regarded as a way to relieve oneself by reducing the annoying itch sensation. However there are hedonic aspects of scratching as one would find noxious scratching highly pleasurable. This can be problematic with chronic
Chronic (medicine)
A chronic disease is a disease or other human health condition that is persistent or long-lasting in nature. The term chronic is usually applied when the course of the disease lasts for more than three months. Common chronic diseases include asthma, cancer, diabetes and HIV/AIDS.In medicine, the...
itch patients, such as ones with atopic dermatitis
Dermatitis
-Etymology:Dermatitis derives from Greek derma "skin" + -itis "inflammation" and genetic disorder.-Terminology:There are several different types of dermatitis. The different kinds usually have in common an allergic reaction to specific allergens. The term may describe eczema, which is also called...
, who may scratch affected spots until it no longer produces a pleasant or painful sensation instead of when the itch sensation disappears. It has been hypothesized that motivational aspects of scratching include the frontal brain areas of reward and decision making. These aspects might therefore contribute to the compulsive nature of itch and scratching.
Contagious itch
Events of "contagiousContagious disease
A contagious disease is a subset category of infectious diseases , which are easily transmitted by physical contact with the person suffering the disease, or by their secretions or objects touched by them....
itch" are very common occurrences. Even a discussion on the topic of itch can give one the desire to scratch. Itch is likely to be more than a localized phenomenon in the place we scratch. Results from a recent study showed that itching and scratching were induced purely by visual stimuli
Stimulus (physiology)
In physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity....
in a public lecture on itching. There is currently little detailed data on central activation for contagious itching but it is hypothesized that a human mirror neuron
Mirror neuron
A mirror neuron is a neuron that fires both when an animal acts and when the animal observes the same action performed by another. Thus, the neuron "mirrors" the behaviour of the other, as though the observer were itself acting. Such neurons have been directly observed in primate and other...
system exists in which we imitate certain motor actions when we view others performing the same action. A similar phenomenon in which mirror neurons are used to explain the cause is contagious yawning.