Understanding Pain biology

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The International Association for Study of Pain defines “pain as an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.”

It is important to note here that pain is not only a physical sensation but may also be an emotional experience.  Pain need not always be associated with tissue damage; it may also occur when there is potential tissue damage or even when there is no actual tissue damage (referred to as neuropathic pain).

However, pain is an important mechanism that tells that something is wrong with our body.

TYPES OF PAIN

Broadly speaking pain is of two types—Acute pain and chronic pain.

Acute pain: This type of pain is sharp, sudden and caused by something specific and usually does not last longer than 6 months.  It goes away when there is no underlying cause for pain.  Some causes of acute pain are:

  • Burns and cuts.
  • Labor and childbirth.
  • Fractured bones.

Chronic pain:  This type of pain lasts longer than 6 months and can be caused by a variety of conditions and reasons and knowing the exact cause can sometimes be tricky and complex.  However, chronic pain is linked to certain conditions like:

  • Nerve pain.
  • Back pain.
  • Fibromyalgia pain.

Chronic pain may also result from damage to sensory nerves or painful reflex muscle contraction or psychological conditions like depression or stress.

TYPES OF PAIN

Depending on the source, pain is of two types:

Nociceptive pain

Nociceptive pain is the pain associated with tissue damage.  Nociceptors are nerves which sense and respond when parts of the body suffer damage.  They signal tissue damage or actual injury and when activated they send signals via peripheral nerves and spinal cord to the brain.  Nociceptive pain is time limited and it resolves when tissue damage resolves.  This type of pain responds well to opioids. An example for nociceptive pain is pain that a person feels when a pin has pierced his trunk.

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Nociceptive pain has two subtypes— somatic pain (associated with bone, muscles and joints etc.,) and visceral pain (associated with organs like heart, liver, gall bladder).  Pain associated with somatic parts is well localized, constant and aching or throbbing in quality whereas the pain associated with the visceral parts is episodic and poorly localized.

Neuropathic pain

It is a special type of pain due to damage to part of the ‘pain pathway’. It is not associated with true tissue damage; instead, the pain pathway is ‘misfiring’, causing the experience of pain without actual injury.

This type of pain is chronic in nature, severe and described as tingling, burning and having electric-shock like quality.  This type of pain is associated with allodynia, which is severe pain on light touch.  Neuropathic pain may be due to a variety of conditions like:

  • Amputation (phantom limb pain).
  • Back, leg and hip problems.
  • Facial nerve problems.
  • Multiple sclerosis.
  • Spine surgery.

Neuropathic pain is resistant to standard pain medications like opiates, antidepressants etc.,

HOW WE FEEL PAIN (MECHANISM OF PAIN)

Here we shall specifically learn about nociception.  Nociceptors are special nerve cell endings that initiate the mechanism of pain.  These are present all over the body.  In addition to nociceptors brain, brainstem and spinal cord together make us feel pain.  Let’s learn briefly about the structure of spinal cord before proceeding further.

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As we can see in this picture the spinal cord is surrounded by three membranes called meninges(piamater, arachnoid, duramater).  We also see a butterfly shaped grey matter with white matter surrounding it.  The white matter contains axons of the nerve cells whereas the grey matter contains cell bodies of the nerve cells.  The motor nerve fibres carry information out of the spinal cord and the sensory nerve fibres carry information into the spinal cord which is then transmitted to the brain to process pain.

Let’s learn the actual mechanism with the help of an example.  Let’s consider that a pin has pierced a person’s trunk.  When this happens our body releases chemicals which stimulate nociceptors (as we have discussed above nociceptors are special nerve cells present all over our body).  When they are stimulated they bring information to the spinal cord through the sensory nerve root which is at the back of the spinal cord.  In the spinal cord the nociceptors release special chemicals called neurotransmitters which are received by a second order neuron present there.  From there the information will travel towards the brain through spinothalamic tract (called so as it passes through thalamus).  In the thalamus the neurotransmitters are received by third order neurons present there.  In this stage location and discrimination of pain occurs (at this step our body discriminates that pin has pierced the trunk).

PAIN MANAGEMENT

There are two basic models for pain management; one is the biomedical model of pain and the other is the holistic model.

Biomedical model—this model treats body and mind as two separate entities and aim to treat the pain without taking emotional and psychological aspects into consideration.

Holistic model—this model acknowledges that pain involves complex interactions, some of which are listed below:

  • Physical: In addition to medication, physical factors like nutrition and exercise are given importance.
  • Emotional: Emotional factors like anxiety, guilt, blame, and depression affect the way we experience pain.
  • Thoughts: The way we think affect messages travelling from brain to physical parts like nerves, hormones, immune system.
  • Social and environmental: Relationships affect the way we perceive pain.
  • Spiritual: Addressing such as meaning, acceptance, hope and acceptance have a positive influence on pain.

This model aims to empower a person to manage his pain better.

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