Pain signal travels from injury site to the brain.
This pathway is referred to as the ascending pathway.
Pain perception occurs in the somatosensory cortex.
Key areas of the brain involved:
Cerebrum
Brainstem (includes midbrain, pons, medulla)
Cerebellum
Somatosensory Cortex:
Located posterior to the central sulcus.
Areas correlate to different body parts (left side of cortex represents the right side of body).
Spinal Cord and Nerve Pathways
Cross-section of the spinal cord segment (Cervical spine region).
Nerves originate from:
Anterior root (motor)
Posterior root (sensory)
Spinal Thalamic Tract:
Contains lateral and anterior spinal thalamic tracts.
Pain Mechanism from Injury
Injury (e.g., right hand injury) triggers immune response:
Damaged cells release cytokines and prostaglandins (PG).
Sensory nerve fibers respond to prostaglandins, sending signals to the dorsal horn of the spinal cord.
Neuron Pathways in the Dorsal Horn
First Order Neuron:
Located in the dorsal horn.
Synapses with the second order neuron.
Second Order Neuron:
Crosses to the opposite side, enters spinal thalamic tract.
Ascends to the thalamus (relay station).
Third Order Neuron:
Relays signal to the somatosensory cortex.
Responsible for pain perception.
Important Chemicals in Pain Transmission
Substance P:
Released by first order neurons to transmit signals.
Plays a crucial role in the ascending pathway.
Descending Pain Pathway
Responsible for controlling and inhibiting the ascending pathway.
Important areas include:
Peri-aqueductal gray matter (midbrain)
Nucleus raphe magnus (medulla)
Descending neurons synapse with second neuron (serotonergic noradrenergic neuron) which:
Inhibits communication between first and second order neurons.
Reduces pain signals.
Role of Opioid Neurons
Substantia Gelatinosa:
Area in the dorsal horn critical for pain modulation (often referred to as a gate).
Contains interneurons that release endogenous opioids (e.g., encephalins).
Effects of Encephalins:
Inhibit presynaptic neuron from releasing substance P.
Inhibit postsynaptic neuron from depolarizing, halting pain signal transmission.
Summary
Ascending pathway transmits pain signals to the brain.
Descending pathway modulates and inhibits pain responses.
Interplay between these pathways and neurotransmitters (substance P, serotonin, norepinephrine, and encephalins) determines pain perception and control.