Pain and thermal sensations

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The detection of potentially harmful stimuli and changes in environmental temperature is fundamental to our survival. In this study unit, you will learn about the receptors and neural pathways that our nervous system uses to detect and process harmful and thermal stimuli.

When we experience potentially harmful stimuli, we feel pain. These stimuli activate specialized neurons called nociceptors. The activation of nociceptors located in tissues on the surface of the body results in somatic pain, whereas visceral pain refers to the sensations perceived when the active nociceptors are in the internal organs. Pain sensations may also be perceived in a location different from where the active nociceptors are, and this is called referred pain.

Two main pathways transmit nociceptive information to the brain:

• The neospinothalamic tract carries fast, localized pain sensations via myelinated Aδ fibers to the somatosensory cortex in the parietal lobe.
• The paleospinothalamic tract carries slow, widespread pain sensations via unmyelinated C fibers to various subcortical structures.

Our central nervous system also has mechanisms to avoid harmful stimuli and to reduce the intensity of perceived pain:

• Activation of nociceptors results in a withdrawal reflex, a spinal reflex that produces a motor response to ‘pull away’ from the harmful stimulus to limit potential tissue damage.
• Gate control theory is a mechanism where the activation of fast, non-painful afferents limits the amount of information transmitted by nociceptors in the same body region.
• Descending pain modulation occurs when the periaqueductal gray substance is activated, which sends signals to the spinal cord to suppress incoming nociceptive signals.
  

The brain integrates nociceptive information with memories and emotions to create subjective perceptions of pain and appropriate responses. Some of the nociceptive pathways described in this study unit are also important for the development of chronic pain.

Warmth and cold thermal receptors are activated by temperatures warmer or cooler than our body temperature. When temperatures become extreme, the activation of hot-pain and cold-pain receptors warns our brain of the potential danger. Like nociceptive input, thermal information ascends to the brain via the lateral spinothalamic tract, reaching the somatosensory cortex and subcortical structures.

Watch the video below to understand more about how we perceive pain and temperature.

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Summary