Sensory Systems: Pain and Temperature Sensation

Modalities and Nerve Endings

Deep (wide spread) pain (nociceptors): free nerve endings, group IV unmyelinated

Prinking (localized) pain (nociceptors): free nerve endings, group III myelinated

Temperature (thermoreceptors): groups III and IV, distinct cool (III) and warm (IV) endings

Light ““crude” touch and pressure (tickle, itch): groups III and IV

*free nerve endings: free from any other structures/ bare nerve ending entering skin to pick up stimuli

Experiment: Pain and Temperature

• Area of overlap between cold and warm receptors allows for sensitivity up to 1° temperature changes

• As the temperature increases, the response of the nociceptor increases. The nociceptor stops responding as soon as the stimulus is removed

• As the temperature increases, the subject gives a higher pain rating, but there is a slight delay in response

• Even after the stimulus has been removed, the subject still notes pain, due to slow conduction velocity (slow conducting and slow adapting)

Transduction mechanism

tissue damage releases intracellular materials (K+) which depolarizes nociceptor

thermal or mechanical alteration of ion channels also depolarizes

• Cold: depolarized when the membrane becomes more rigid

• Hot: depolarized when the membrane becomes more fluid

Thresholds vary

low for touch, temperature (tell difference of 1 C)

high for pain (variable/depends on person)

Slow to very slow adaptation

Single stimulus may lead to very long increase in action potential frequency

Very slow: No longer responding to stimuli

Slow: will always response to stimulus because adaptation is low

Central Connections -- Pain & Temperature (body)

Cell bodies of primary afferents in dorsal root ganglia

First synapses in spinal cord dorsal horn (posterior)

• Local reflex circuits (immediate to protect from pain/heat/cold - recoil back)

  •  DRG synapses at the posterior horn on substantia gelatinosa (posterior) or dorsal funicular gray/nucleus proprius (more anterior) → anterior horn motor neuron → muscle for withdrawal reflex/flexor reflex

    • Rapid because no communication with the brain

• Interaction with other pathways (dim/reduce painful stimulus)

  •  controls the magnitude of the perception of pain (increase or decrease)

Second order fibers cross midline to form spinothalamic tract (body)

Thalamic relay in ventral posterior lateral (VPL, body) nucleus

Central Connections -- Pain & Temperature (body) Endings

Somatotopic destination in postcentral gyrus, parietal lobe (Brodmann areas 3,1,2)

• leg on medial surface; upper limb dorsal & lateral

Additional endings in brainstem reticular formation, hypothalamus

• input to pleasure/pain/emotion pathways, etc.

Second order fibers cross midline to form trigeminothalamic tract (face)

Thalamic relay in ventral posterior medial (VPM, face) nucleus

Central Connections -- Pain & Temperature (head)

Cell bodies of primary afferents in cranial nerve (mostly V) ganglia

First synapses in spinal trigeminal nucleus of brainstem

• Local reflex circuits (poked → rub → reduce sensation)

• Interaction with other pathways

Second order fibers cross midline to form trigeminothalamic tract (face)

Thalamic relay in ventral posterior medial (VPM, face) nucleus

Central Connections -- Pain & Temperature (head) Endings

Somatotopic destination in postcentral gyrus, parietal lobe (Brodmann areas 3,1,2)

• face ventral & lateral

Additional endings in brainstem reticular formation, hypothalamus

• input to pleasure/pain/emotion pathways, etc