Lecture on Pain as a Defense Mechanism

Overview

• Pain serves as an important defense mechanism.
• It alerts the body to potential or actual injuries/diseases, prompting protective actions.

Pain Pathway

• Noxious Signals:
  • Transmitted to the spinal cord; relayed to the brain.
  • Brain interprets these signals as pain and instructs the body to react.

Pain Receptors (Nociceptors)

• Located in the skin, superficial tissues, and most organs, except the brain.
• First-Order Neurons:
  • Myelinated A-Type Fibers: Fast conduction, responsible for initial sharp pain.
  • Unmyelinated C-Type Fibers: Slower conduction, responsible for prolonged, dull pain.

Signal Processing

• First-Order Neurons:
  • Travel via spinal nerves to the spinal cord.
  • Synapse with second-order neurons in the dorsal horn.
• Second-Order Neurons:
  • Cross to the opposite side of the spinal cord.
  • Ascend to the brain.

Major Pain Pathways

• Spinothalamic Tract:
  • Second-order neurons travel to the thalamus and synapse with third-order neurons.
  • Third-order neurons project to somatosensory cortex.
  • Involved in pain localization.
• Spinoreticular Tract:
  • Second-order neurons ascend to the brainstem's reticular formation.
  • Continue to the thalamus, hypothalamus, and cortex.
  • Responsible for the emotional aspect of pain.

Facial Pain Pathway

• Pain signals travel via the trigeminal nerve to the brainstem.
• Synapse with second-order neurons ascending to the thalamus.

Types of Pain

• Somatic Pain:
  • Originates from skin, muscles, and joints.
• Visceral Pain:
  • Originates from internal organs.
  • May be perceived in different locations (referred pain).

Referred Pain Example

• Heart Attack Pain:
  • Can be felt in the left shoulder, arm, or back instead of the chest.
  • Due to convergence of pain pathways at the spinal cord level.
  • Spinal segments T1 to T5 receive signals from both the heart and shoulders/arms.
  • The brain assumes the pain is from the more superficial areas (shoulder/arm).