Overview
This lecture explains the mechanisms of pain perception, including the pathways, types of nerves involved, and phenomena like referred pain.
Pain as a Defense Mechanism
- Pain alerts the body to potential or actual injury or disease.
- The brain interprets pain signals and helps localize and respond to danger.
Pain Receptors and Nerve Types
- Nociceptors are pain receptors located in skin, tissues, and organs (but not in the brain).
- Nociceptors are nerve endings of first-order neurons in the pain pathway.
- Myelinated A fibers conduct fast, sharp pain signals.
- Unmyelinated C fibers conduct slow, dull, and diffuse pain signals.
The Pain Pathway
- First-order neurons carry pain signals through spinal nerves to the spinal cord and synapse in the dorsal horn.
- Second-order neurons cross to the opposite side of the spinal cord and ascend toward the brain.
Major Pain Signal Pathways
- The spinothalamic tract transmits pain to the thalamus, then to the somatosensory cortex for localization.
- The spinoreticular tract transmits pain to the reticular formation, thalamus, hypothalamus, and cortex, influencing emotion.
Facial Pain Pathway
- Facial pain signals travel via the trigeminal nerve to the brainstem, then to the thalamus.
Types of Pain and Referred Pain
- Somatic pain arises from skin, muscles, and joints.
- Visceral pain comes from internal organs.
- Referred pain is when pain is felt in a different area than its source due to convergent pathways in the spinal cord (e.g., heart pain felt in the left arm).
Key Terms & Definitions
- Nociceptor — a pain receptor located in tissues and organs.
- First-order neuron — the initial neuron carrying pain signals from the body.
- Myelinated A fiber — a fast-conducting nerve fiber responsible for sharp pain.
- Unmyelinated C fiber — a slow-conducting nerve fiber responsible for dull pain.
- Spinothalamic tract — pathway for localizing pain to the cortex.
- Spinoreticular tract — pathway for the emotional aspect of pain.
- Somatic pain — pain from skin, muscles, or joints.
- Visceral pain — pain from internal organs.
- Referred pain — pain perceived at a site other than its origin due to shared nerve pathways.
Action Items / Next Steps
- Review the structures and functions of nociceptors and pain pathways.
- Prepare examples of referred pain for discussion.