A real head-scratcher! Expert reveals the science behind what makes us itch and how it is similar to the reasons why we feel pain

Have you ever wondered why the inside of your mouth can get itchy, but your stomach can't? You can feel pain inside your stomach, but it never itches.

If this confuses you, you are not alone. 

It is one of the differences between pain and itching that scientists are still trying to understand and have been discussed in the book Touch: The Science of the Sense that Makes Us Human.

You can feel pain inside your stomach, but it never itches and yet your skin can both itch and feel pain. If this confuses you, you're not alone. It is one of the differences between pain and itching that scientists are trying to understand and have been discussed in the book Touch: The Science of the Sense that Makes Us Human

Although still not fully understood, the two sensations are more similar than you might think.

'Some who believe that an itch is a pattern rather than a unique type of touch contend that it is merely a particular type of pain,' said author Professor David Linden, professor of neuroscience at Johns Hopkins University in Baltimore.

Itching and pain have some similarities, he explains. 

Both can be triggered by force, chemicals and heat, and both can be sometimes relieved by anti-inflammatory drugs.

Pain and itching are both related to attention, anxiety and expectation, and they are signals of things on our bodies that should be avoided.

Itch sensations trigger reflexes that protect us from disease-carrying insects. In a similar way, pain acts as a warning signal that alerts us to noxious mechanical and chemical stimuli that are potentially tissue damaging

'For example, acute pain acts as a warning signal that alerts us to noxious mechanical, chemical and thermal stimuli, which are potentially tissue damaging,' said Professor Linden.

'Likewise, itch sensations trigger reflexes that may protect us from disease-carrying insects.'

They share a common pathway in the spinal cord, called the spinothalamic tract. 

Patients who are insensitive to pain are also insensitive to itch and they both activate similar sensory areas in the brain. 

If itching is a special form of touch, then scientists say they expect to find sensory neurons that can be electrically stimulated to give rise to an itch

But there are some crucial differences between the two sensations, for example they produce very different responses.

When we feel light touch, we feel an itch and respond by scratching to get the bug or other irritant off of the skin.

But pain brings about a very different behaviour, the urge is to pull the injured body part away and to keep it away.

'Behaviourally, pain initiates a withdrawal response but itching induces attention towards the area of stimulation,' Dr Hongzhen Hu from the Centre for the Study of Itch at the Washington University School of Medicine in St. Louis told MailOnline.

Drugs such as opiates may suppress pain, but they make itch worse.

Scratching, which can induce pain, relieves itch, at least momentarily. 

If itching is a special form of touch, then scientists say they expect to find sensory neurons that can be electrically stimulated to give rise to an itch.

'Elevated pain signalling can inhibit itch sensation so there is some level of communication between the two pathways,' Dr Hu said.

'Although the cellular and molecular mechanisms underlying pain and itch are not fully understood, there are cells in the dorsal root ganglia (DRG) and spinal cord that are selectively involved in itching without affecting the pain responses.' 

A study conducted by a separate team at Dr Hu's university in 2014 found serotonin lies at the intersection of pain and itch.

Zong-Qiu Zhao and colleagues were interested in the role of the chemical messenger serotonin in itch and pain. 

They found that when the mouse scratched an itch, the pain caused the release of serotonin. The serotonin caused pain relief, but then activated a type of serotonin receptor which made the mice itch.

SEROTONIN AT THE INTERSECTION OF ITCH AND PAIN 

Zong-Qiu Zhao and colleagues were interested in the role of the chemical messenger serotonin in itch and pain.

Serotonin is often referred to in relation to psychiatric conditions and mood. But chemical messengers, or neurotransmitters, have different effects depending on where they are released.

In the brain, serotonin plays roles in mood, circadian rhythms and hallucinations. In the tummy, serotonin helps control whether or not you end up with diarrhoea. And in the skin, serotonin can play a role inhibiting pain and exacerbating itch.

Zhao and colleagues used mice to study serotonin's role in the skin by watching to see how much the mice scratched.

They found that when the mouse scratched an itch, the pain caused the release of serotonin. The serotonin caused pain relief, but then activated a type of serotonin receptor which made the mice itch.

Serotonin is often referred to in relation to psychiatric conditions and mood. But chemical messengers, or neurotransmitters, have different effects depending on where they are released

But there are still a lot of unanswered questions regarding the links between pain and itching, including why painful stimuli can suppress itching and how some people experience chronic itching but not pain.

'We are interested in understanding how specific molecules and cells in the skin initiate itch, especially chronic itch, but not pain,' Dr Hu told MailOnline.

'We investigate how skin cells communicate with pruriceptors, itch mediating receptors, to generate a specific itch response.'

'Other labs in our centre for the study of itch investigate itch mechanisms at the levels of primary sensory neurons, spinal cord, and brain,' he said.

'By identifying unique itch signaling pathways at different stations we will be able to distinguish pain and itching at different levels.'