Physiology of pain

🖇 Pain Sensation (Nociception)

📌 Basic Concepts

• Nociception = Pain Sensation (من مستقبل خاص اسمه Nociceptor).
• Specificity:
• ال Pain له مستقبل خاص
• • (مش استجابة قوية لمستقبلات أخرى).
  • إثبات: Analgesics ↓ pain فقط بدون تأثير على heat/cold/touch.
• Dissociation: ممكن فصل الإحساس بالألم عن باقي الأحاسيس.

📌 Characteristics of Pain

• High Intensity: لازم Stimulus قوي يسبب الألم.
• Non-Adaptation: لا يحدث تعود على الألم.
• Withdrawal Reflex: الألم يسبب سحب الطرف.
• Prepotent Reflex: Reflex الألم يتغلب على أي Reflex آخر يحدث في نفس الوقت.

📌 Location of Pain Receptors

• High Density (أكثر مناطق حساسية): Skin – Periosteum – Arterial walls – Joint surfaces – Dura & cranial sinuses.
• Low Density: Muscles – Joints – Intestine.
• Absent: Liver parenchyma – Lung alveoli – Brain parenchyma – Tooth enamel & Cornea.

📌 Mechanism of Pain

• Trigger = Tissue Damage → release of Pain Factors (P-factors):
• • Bradykinin (B factor) – Lactic acid – Histamine – K+ – Prostaglandins – Protoenzymes.
  • Substance p and prostaglandins facilitate pain not directly excite it (خلي بالك من نقطة دي mcq مهمة )
• هذه المواد تفتح Channels → Depolarization → AP في Nociceptors.

Channels

1- degenerin channel : for mechanosnsetive pain

2- vanilloid channel: for extreme heat or caspacin

3- acid sensing channels: for chemosensitive pain.

خلي بالك بيجوا mcq

📌 Types of Pain & Pathways

Feature

🟠 Fast Pain (Acute/Sharp)

Fibers

Type A (myelinated)

Pathway

Neo-spinothalamic

NT

Glutamate

Localization

Accurate, well localized

Center

Cortex (interpretation)

Function

Withdrawal reflex (avoidance)

🟣 Slow Pain (Chronic/Dull)

Fibers

Type C (unmyelinated)

Pathway

Paleo-spinothalamic

NT

Substance P

Localization

Diffuse, poorly localized

Center

​Thalamus

Function

Activates RAS → arousal, insomnia

Deep Pain: muscles/joints (due to spasm → ischemia).

• Cautenous pain : from skin fast pain not referred well localized sharp pain

Related to autonomic reflex as in acute pain causing tachycardia and hypertension while in chronic pain cause bradycardia and hypotension

somatic reflexes as prepotent protective reflex

Emotional reflex as crying and depression

And skin hyperalgesia

• Visceral Pain: من الأعضاء الداخلية، غالبًا مع Referred Pain (ألم يظهر في منطقة سطحية لها نفس Segment في SC).

📌 Pain Modification

• ↓ Pain by:
• • Pressure / Hypoxia.
  • Local Anesthesia (especially deep pain).
• Autonomic Effects: Severe pain → sweating, HR changes, even cardiac arrest.

▶️ Referred pain: is pain occur in another site from original site of pain which from the same embryonic segment

🔰Mechanism :

1- convergence projection mechanism

The afferent pain nerve from the site of pain in visceral structure converge with the afferent neurons from skin then brain receive pain sensation from somatic structure not from visceral structure and project pain sensation to somatic structure who have the same embryonic origin with the visceral structure

2- facilitation theory

The afferent neurons of visceral structure facilitate spinothalamic tract which carry sensations from somatic structure to minimize threshold of pain sensation so by little stimulus from somatic sensation will produce pain because it is facilitated by afferent visceral structure

ال referred pain مهم ريان مهم جدا و خاصة convergence projection mechanism

🚨 Key Questions (Focus Points)

1. Specificity of pain receptors (Nociceptors ≠ other receptors).
2. Absence of pain receptors: Liver, brain tissue, lung alveoli, enamel, cornea.
3. Fast vs Slow pain:
4. • Fibers (A vs C)
   • Pathway (Neo vs Paleo spinothalamic)
   • NT (Glutamate vs Substance P).
4. Mechanism of pain: Release of P factors (Bradykinin, K+, etc).
5. Referred Pain: تعريف + مثال (heart → arm, gallbladder → shoulder).

؟