PAIN AND ANALGESIA
I. PAIN:
- Defined as an “Unpleasant sensory & emotional experience associated with actual or potential tissue damage”.
TYPES OF PAIN:
1. Nociceptive pain:
- Musculoskeletal pain.
- Due to injury/inflammatory/degenerative disorders.
- E.g. Bone fracture/Arthritis/Burn.
2. Neuropathic pain:
- Due to dysfunction/nerve damage.
- Eg. Intervertebral disc prolapse.
3. Mixed pain:
- Includes components of both nociceptive & neuropathic pain.
4. Idiopathic/unspecified pain:
- Purely psychological in nature; Hence, “Psychogenic pain”.
RECEPTORS & INNERVATION:
1. Receptors:
- Referred as “Nociceptors”.
- Are mostly “Free nerve endings”.
– Found maximum in skin.
– Also in periosteum, arterial walls, joint surface, falx cerebri & tentorium.
– Pain receptors absent in Brain.
2. Innervation:
- Impulse from nociceptors transmitted via two fiber types –
– Aδ & C fibers.
- Based on type of fibers, pain classified into,
– Fast & slow pain.
2a. Fast pain/Epicritic pain/First pain:
- Carried by thin myelinated Aδ (Type-III) fibers.
- Conduction velocity – 12-30 m/s.
Neurotransmitter:
- Glutamate.
- Secreted by Aδ fibers.
Receptor:
- Sensitive to mechanical/thermal stimuli of noxious strength.
Pain type:
- Produce sharp & well-localized pain.
2b. Slow pain/Protopathic pain/Second pain:
- Carried by unmyelinated C (Type IV) fibers.
- Conduction velocity – 0.5 m/s.
Neurotransmitter:
- Mainly substance P & some glutamate.
- Released by C fibers.
Receptor:
- Sensitive to noxious mechanical or thermal stimuli.
- Also sensitive to chemicals.
- Histamine, serotonin, bradykinin, K+ ions, H+ ions, prostaglandins, substance P & acetylcholine.
- Serotonin – Most potent allogenic substance.
Pain type:
- Produce dull-aching & poorly localized pain.
HYPERALGESIA & ALLODYNIA:
- Pain accomponied by hyperalgesia & allodynia.
- Hyperalgesia – Exaggerated response to noxious (painful) stimulus.
- Allodynia – Pain sensation in response to non-noxious (non-painful) stimuli.
Mechanism:
Noxious stimulus.
↓
Stimulates nociceptors.
↓
Produces tissue injury.
↓
Releases pain-producing substances.
- Eg: K+ ions, Histamine, Serotonin, Prostaglandins, Bradykinin, Acetylcholine & Substance P.
↓
Increases sensitivity of silent nociceptors.
↓
Hence, increased nociceptors response to noxious stimulus, during tissue injury.
PRIMARY & SECONDARY HYPERALGESIA:
Primary hyperalgesia –
- Due to nociceptors sensitization.
- Repetitive stimuli –> Cause tissue damage.
- This increases nociceptors sensitivity (hypersensitivity) –> Producing hyperalgesia.
Secondary hyperalgesia –
- Due to spinothalamic tract sensitization.
- Caused by long-term potentiation.
- Mainly on continuous primary nociceptor C fibers discharge.
– Hence, “Central sensitization/Winding up”.
Summary:
- Primary hyperalgesia – Exaggerated response in nociceptors itself.
- Secondary hyperalgesia – Exaggerated response in spinothalamic tract (nociceptor neurons).
VISCERAL PAIN:
- A viscus does not have any other sensation (e.g. touch, temperature etc) except pain.
- Poorly localized – Due to sparse distribution of nociceptors.
Stimulus:
- Any stimulus exciting pain nerve endings in diffuse visceral areas.
- Ischemia of visceral tissue
– Eg. MI.
- Smooth muscle spasm of hollow viscus.
– Eg. Gallbladder/bile duct/ureter.
- Excessive distension of a hollow viscus.
- Stretching of connective tissue surrounding viscus.
– Eg. Mesentery stretching in violent intestinal contraction.
- Inflammation.
– Eg. appendicitis.
- Chemical stimuli.
– Eg. gastric acid leak through perforated ulcer-causing chemical peritonitis).
Nerve involved:
- Visceral pain carried by – Type C afferents.
– In sympathetic system – From thoracic & abdominal viscera.
– In parasympathetic system – From pelvic viscera.
VARIATIONS IN PAIN SENSITIVITY AMONG STRUCTURES:
Important pain producing viscera:
- Heart.
- Esophagus.
- Gallbladder.
- Stomach.
- Intestine.
- Appendix
- Ureter.
- Kidney.
- Urinary bladder.
Completely insensitive viscera:
- Liver parenchyma.
- Lung alveoli.
- Yet, Liver capsule, bronchi & parietal lung pleura are pain sensitive.
Intracranial structures sensitive to pain:
- Scalp.
- Aponeurotica.
- Duramater around venous sinuses & vessels.
- Falx & Tentorium cerebri.
- Cranial venous sinuses & some arteries.
Intracranial structures insensitive to pain:
- Brain parenchyma.
- Ventricular ependyma.
- Dura materr over convexity of skull.
- Pia mater.
- Pial veins.
- Arachnoid mater.
- Choroid plexus.
Exam Important
- Idiopathic/unspecified pain – Purely psychological in nature; Hence, “Psychogenic pain”.
- Brain is the most conspicuous part of body which has no pain receptors at all.
- Pain receptors (Nociceptors) – Free nerve endings.
- Pain receptors absent in Brain.
- Fast pain/Epicritic pain/First pain – Carried by thin myelinated Aδ (Type-III) fibers with conduction velocity – 12-30 m/s.
- Slow pain/Protopathic pain/Second pain – Carried by unmyelinated C (Type IV) fibers.
– Neurotransmitter released by C fibers – Mainly substance P.
- Slow pain receptor sensitive to chemicals – Histamine, serotonin, substance P & acetylcholine.
– Serotonin – Most potent allogenic substance.
- Hyperalgesia – Exaggerated response to noxious (painful) stimulus.
- Allodynia – Pain sensation in response to non-noxious (non-painful) stimuli.
– Repetitive stimuli → Cause tissue damage.
– Thus increasing nociceptors sensitivity → Producing primary hyperalgesia.
- A viscus does not have any other sensation (e.g. touch, temperature etc) except pain.
- Visceral pain is poorly localized due to sparse distribution of nociceptors.
- Stimuli for visceral pain –
– Excessive distension of a hollow viscus.
– Stretching of connective tissue surrounding viscus.
– Eg. Mesentery stretching in violent intestinal contraction.
- Visceral pain sensation carried by – Type C afferents.
– In sympathetic system – From thoracic & abdominal viscera.
– In parasympathetic system – From pelvic viscera.
– Important pain producing viscera – Heart & Stomach
– Completely insensitive viscera – Liver parenchyma.
– Intracranial structures sensitive to pain – Duramater around venous sinuses & vessels,
– Intracranial structures insensitive to pain – Brain parenchyma, pia meter & pial veins.
Click Here to Start Quiz
