Pain (2)
- Pain is the most common symptom that brings a patient to a physician’s attention.
- Pain is defined as an unpleasant sensation localized to a part of the body.
* It is often described in terms of a penetrating or tissue-destructive process (e.g., stabbing, burning, twisting, tearing, squeezing) and/or of a bodily or emotional reaction (e.g., terrifying, nauseating, sickening). - Pain of moderate or higher intensity is accompanied by anxiety and the urge to escape or terminate the feeling; pain is both a physical sensation and an emotion.
* Has a strong neurological basis
Function of Pain
The function of the pain sensory system is to protect the body and maintain homeostasis by detecting, localizing, and identifying potential or actual tissue-damaging processes.
Acute Pain
- associated with behavioral arousal and a stress response consisting of increased blood pressure, heart rate, pupil diameter, and plasma cortisol levels.
- In addition, local muscle contraction (e.g., limb flexion, abdominal wall rigidity) is often present.
* Limb flexion: to withdraw limb from painful stimulation
* Abdominal wall rigidity: bear down when we feel pain
How is Pain sensed?
- The Primary Afferent Nociceptor
- Nociception: physical sensing of pain
- Nociceptor: neuronal cell that senses painful stimulation
* They are afferent nerves, which are types of peripheral nerves (peripheral nerves start off as spinal nerves)
Classifications of Primary Afferent Nerves (3)
- Diameter,
- Degree of myelination
- Conduction velocity
A-beta fibers (4)
- Have the largest diameter
- Respond maximally to light touch and/or moving stimuli
* Somatosenosray fibers that convey touch and movement across skin - They are present primarily in nerves that innervate the skin
- The activity of these fibers DOES NOT PRODUCE PAIN
myelinated A-delta fibers (2)
- Smallest diameter
2. Respond to low intensity painful stimulation
Unmyelinated C-fibers (3)
High intensity painful stimulation
Painful Stimuli (4)
- Includes noxious stimuli such as: intense heat and cold, intense mechanical
- Stimuli like a pinch, changes in pH (especially acidity), and application of chemical irritants (both endogenous and exogenous.)
- Exogenous irritants: things from the outside world
- Endogenous: inflammatory mediators and some components of intracellular fluid, like potassium is irritating to nerves
C-fibers that project into dorsal horn of spinal cord (2)
- These are somatosensory
2. Convey painful stimulation from the surface of the skin that we are conscious of
C-fibers that project into the sympathetic ganglia (2)
- These are visceral afferent, and conveys visceral pain/stimulation
* Ex: if there is death of myocardial tissue that triggers inflammation/pain, the pain will be conveyed through the c-fiber that goes into the sympathetic chain - Project into the sympathetic ganglia where the autonomic nerves project into
Sensitization (4)
- When we injure our body, the area of the wound becomes extremely sensitive and even an area surround the laceration becomes tender
* The lightest tough that would normally not be painful all of a sudden becomes painful
* This process is called sensitization - When intense, repeated, or prolonged stimuli are applied to damaged or inflamed tissues.
- The threshold for activating primary afferent nociceptors is lowered, and the frequency of firing is higher for all stimulus intensities.
* This is process is called sensitization and a variety of inflammatory mediators contribute the process. - Designed to stimulate you to protect the wounded area; cover it, shield it, etc. from further injury
Primary Activation Stage of Sensitization (3 steps)
Happens at the site of injury
1st: Cell damage and death at site of injury
* When cells die, their intracellular material becomes acidic, and the acidic material gets released into the extracellular fluid
* Also have release of intracellular potassium
2nd: As a result of inflammation and cell damage → get production of prostaglandins (PG) and bradykinin (BK)
3rd: All of these mediators are increasing the excitability of the nocicpetor nerve ending
* This increased excitability is sensitizing the nerve ending, meaning that even the lightest touch on the area of injury will trigger pain because the pain receptor is already highly excited by the mediators
Secondary Activation Stage of Sensitization (6 steps)
Nerve endings connected to the site of injury convey pain from the surrounding areas of the skin
1st: The pain message comes down from primary location to spinal cord and secondary branches
2nd: It causes a release of substance P (SP) at secondary sites
3rd: Substance P triggers mast cells in the area, and stimulates platelets
* Platelets and mast cells = players in inflammatory response that will release histamine, serotonin, etc.
4th: Histamine and serotonin will increase excitability of the nerve ending, just like acidity, potassium, prostaglandins, and bradykinin did in primary pathway
5th: Result is increased excitability on these nerve endings
6th: Now, not only is the area of the injury sensitive to pain, but now the surrounding area is highly sensitive
Direct Activation
Occurs by intense pressure and consequent cell damage. Cell damage induces lower pH (H+) and leads to release of potassium (K+) and to synthesis of prostaglandins (PG) and bradykinin (BK). Prostaglandins increase the sensitivity of the terminal to bradykinin and other pain-producing substances.
Secondary Activation
Impulses generated in the stimulated terminal propagate not only to the spinal cord but also into other terminal branches where they induce the release of peptides, including substance P (SP). Substance P causes vasodilation and neurogenic edema with further accumulation of bradykinin (BK). Substance P also causes the release of histamine (H) from mast cells and serotonin (5HT) from platelets
Referred Pain
The spatial displacement of pain sensation from the site of the injury that produces it.
- We can think of referred pain as being primary visceral pain from a visceral organ
- Instead of us feeling the pain where the organ is, the pain is felt on the surface of the body
Referred Liver Pain
Right shoulder and upper abdominal quadrant
Referred Cardiac Pain
left chest, shoulder, left arm, can be up into left side of jaw
Referred Colon Pain
middle of abdomen
Referred Small intestine pain
epigastric region of stomach
Mechanism of Referred Pain (3)
CONVERGENCE
- When we feel pain in a visceral organ, that pain is being conveyed by the c-fibers that ultimately are going to converge onto a spinal nerve
- The convergence onto the spinal nerve is converging with a somatosensory afferent nerve
* This is including somatosensory nociceptors (a-delta and somatosensory c-fibers) - When the visceral pain comes in and moves up to the brain, it is perceived by the brain as coming from the corresponding dermatome for that spinal nerve due to this convergence
Ascending Pain Pathways
Pain that is felt at the surface of the skin is sensed by peripheral nociceptors, and it is conveyed into the spinal cord
6 Steps of Ascending Pain Pathway
1st: Noxious stimuli activate the peripheral ending of the primary afferent nociceptor.
2nd: The message is then transmitted to the spinal cord, where it synapses with cells of origin of the major ascending pain pathway that ultimately go to the brain: the spinothalamic tract.
3rd: It goes from the dorsal horn of the spinal cord then the to the contralateral side and enters the white matter of the spinal cord
4th: Ascends through the spinalthalamic tract, which is the white matter tract that carries affernt fibers up to brain centers
5th: At the midbrain, it terminates at the thalamus, which is a major relay station for a lot of ascending projections
6th: The message is relayed to the contralateral thalamus to the somatosensory cortex and/or broad areas of cerebral cortex such as the anterior cingulate and the frontal insular
