Nociception
Fall 2002

Nociception is the sensory system designed to detect the presence of noxious stimuli. That is, stimuli that produce pain.
There are two types of axons for nociception:

1) Unmyelinated axons called C-fibers are responsible for slow throbbing pain (conduction velocity of 1 m/s).

2) Thin myelinated axons called A-delta fibers are responsible for fast sharp pain (conduction velocity of 5-30 m/s).


Three neurons are necessary to carry pain messages from the skin to the cortex (Figure 12.16).

1. Nociceptors carry messages from both the skin and inside the body (e.g., muscles, bones, guts) to the dorsal horn of the spinal cord. Nociceptors have their soma in the dorsal root ganglion just like mechanoreceptors.

2. Dorsal horn neurons send axons to the contralateral spinal cord to ascend in the spinothalamic tract. As the name indicates, these axons synapse in the thalamus.

3. Thalamic neurons relay the message to the cortex (somatosensory and other regions).
 

Noxious stimuli also trigger reflexes. In addition to sending messages to the brain, dorsal horn neurons receiving input from nociceptors also send messages to motoneurons in the ventral horn of the spinal cord. These motoneurons activate muscles to remove your skin from the noxious stimulus. This reflex response occurs without conscious perception. The reflex also occurs before the brain receives the message. That is, you move away from the stimulus before you perceive it.
 

A comparison of nociception and mechanoreception reveals several important differences:

  • Nociception is carried by A-delta and C fibers (slow), whereas mechanoreception is carried by A-beta fibers (fast).
  • Nociceptors terminate in the spinal cord whereas mechanoreceptors terminate in the medulla.
  • Nociception crosses to the contralateral side of the nervous system in the spinal cord, whereas mechanoreception crosses to the contralateral side in the medulla.


    • Because of the different pathways for nociception and mechanoreception, damage to the right half of the spinal cord will disrupt mechanoreception from the right side of the body and nociception from the opposite side of the body.
     

    The following pain conditions show the importance of a well regulated pain system:

    Congenital Insensitivity to pain is a condition in which a person does not feel pain because of a lack of C-fibers. These people do not suffer despite  severe damage to their bodies.

    Chronic pain can be caused by a wide range of conditions (e.g., arthritis; sciatica; neuropathic pain). This pain can be constant and severe to the point of disrupting all aspects of life (e.g., sleep, work, movement). There is often no treatment.


    Nociception is especially fascinating because it is subjected to strong modulation. Although there are many ways to reduce pain (e.g., acupuncture, hypnosis, ice), the most effective treatments are drugs. These include:

    Aspirin (salicylic acid)--blocks chemicals in periphery that enhance pain by promoting inflammation.

    Opioids (morphine, codeine, fentynal, etc.) bind to the body's natural receptors for endorphins (endogenous opioids such as enkephalin). Neurons with opioid receptors are located in the spinal cord and brain. In particular, opioids activate neurons in the periaqueductal gray (PAG) in the midbrain which send messages to the spinal cord (via the medulla) to inhibit nociception (Figure 12.26). Natural stimuli like fear can also activate PAG neurons and block pain.
     

     Return to Psych 372 Syllabus