Topic 8: Modulation of Nociception

What are the two primary types of modulation of nociception in the CNS?

Segmental modulation (spinal gating) and suprasegmental modulation (descending).

Where does segmental modulation of nociception occur?

In the dorsal horn of the spinal cord.

Where does suprasegmental modulation of nociception originate?

From brain structures (cortex, brainstem) descending to the spinal cord dorsal horn.

What is meant by "spinal gating" in nociceptive modulation?

Local interneurons in the dorsal horn inhibit or amplify pain signals before they reach the brain.

What is the main principle of suprasegmental (descending) pain modulation?

Descending fibers from the brainstem or cortex can inhibit or facilitate nociceptive transmission in the spinal cord.

Is there always a direct relationship between tissue injury and pain perception?

No—there is no simple relationship; pain perception can be influenced by CNS modulation and psychological factors.

Give an example of how pain and tissue injury do not always correlate.

Traumatic injuries in athletes or combat may be described as relatively painless at first.

Name three psychological factors that can alter pain perception.

Arousal, attention, and expectation.

What determines whether pain is transmitted or inhibited in the CNS?

The balance of inhibitory and facilitatory influences on somatosensory neural circuits.

At what CNS levels does integration of pain modulation occur?

At the spinal cord, brainstem, and multiple cortical regions.

Why are derangements in central pain modulation systems important clinically?

They are often critical in the generation and maintenance of chronic pain.

What is the first site of central modulation of nociception?

The dorsal horn of the spinal cord.

According to the Gate Control Theory, what determines whether pain signals are transmitted?

The balance of activity between nociceptive and non-nociceptive afferent fibers.

Which fibers "open the gate" to pain transmission?

Small-diameter nociceptive fibers (A-delta and C fibers).

Which fibers "close the gate" to pain transmission?

Large-diameter non-nociceptive fibers (A-β mechanoreceptors).

What common everyday action demonstrates the gate control theory?

Rubbing a painful area reduces pain by activating A-β mechanoreceptors.

Which laminae of the dorsal horn contain interneurons involved in nociceptive modulation?

Laminae I, II, and V.

What neurotransmitters do inhibitory interneurons in the dorsal horn release?

Glycine and dynorphin (an opioid peptide).

Which second-order neurons in the dorsal horn receive modulatory input from interneurons?

Wide Dynamic Range (WDR) and Nociceptive-Specific (NS) neurons.

How do A-β mechanoreceptors affect WDR and NS neurons of the spinothalamic tract (STT)?

They exert an inhibitory influence, reducing nociceptive activity.

Which clinical therapy is based on the principle of activating A-β mechanoreceptors to inhibit pain?

Transcutaneous Electrical Nerve Stimulation (TENS).

What is the role of low-threshold mechanical stimulation in pain modulation?

It can override or desensitize nociceptive signals within the STT.

In a normal joint, which type of afferent input dominates?

Large-diameter mechanoreceptive afferents (proprioception).

In a normal joint, what is the level of nociceptive input?

Low nociceptive input.

What happens to mechanoreceptive input in a subluxated joint?

Mechanoreceptive input decreases.

What happens to nociceptive input in a subluxated joint?

Nociceptive input increases.

After a chiropractic adjustment, what happens to mechanoreceptive input?

Mechanoreceptive input increases again.

After a chiropractic adjustment, what happens to nociceptive input compared to the subluxated state?

Nociceptive input decreases.

What overall effect does a chiropractic adjustment have on afferent input balance?

It restores mechanoreceptive dominance and reduces nociceptive drive.

Which neurophysiological principle helps explain how increased mechanoreceptor activity reduces pain perception?

The Gate Control Theory of pain modulation.

Why does rubbing or massaging a painful area often reduce pain?

It activates mechanoreceptors (A-β fibers), which inhibit nociceptive signaling via Gate Control Theory.

How does a chiropractic adjustment influence afferent input to the spinal cord?

It increases mechanoreceptive input and decreases nociceptive input from the joint.

What is the clinical significance of increased mechanoreceptive afferentation after adjustment?

It can improve proprioception, normalize joint function, and reduce pain perception.

What happens when mechanoreceptive input decreases and nociceptive input dominates?

The CNS receives more pain-related signals, which can contribute to chronic pain and dysfunctional motor control.

Which type of afferent fibers are associated with nociception?

Small-diameter fibers (A-delta and C fibers).

Which type of afferent fibers are associated with mechanoreception?

Large-diameter A-β fibers.

Why might a subluxated joint lead to altered motor patterns or reflexes?

Increased nociceptive input and decreased mechanoreceptive input disrupt CNS integration of sensory information.

What neurophysiological model explains how chiropractic adjustments reduce pain?

The Gate Control Theory combined with restoration of normal joint afferentation.

What midbrain structure is the principal site for endogenous pain control?

The periaqueductal gray (PAG).

Which inputs can activate PAG opioid neurons?

Mesolimbic structures, the hypothalamus, and the paleospinothalamic tract (PSTT).

How does the PAG exert descending pain modulation?

By disinhibition of bulbospinal projections from the raphe nuclei and reticular nuclei.

Which brainstem nuclei relay descending signals from the PAG?

The nucleus raphe magnus (NRM) and the reticular magnocellular nuclei (RMC: NRGC & NpGL).

What nuclei form the reticular magnocellular group (RMC)?

Nucleus reticularis gigantocellularis (NRGC) and nucleus paragigantocellularis lateralis (NpGL).

What nuclei form the nucleus raphe magnus (NRM)?

Nucleus raphe alatus and nucleus raphe lateralis.

Which tract carries descending projections from NRM and RMC to the spinal cord?

The dorsolateral funiculus (DLF).

Which spinal cord laminae receive input from descending projections?

Laminae I, II, and V of the dorsal horn.

What neurotransmitters are released by inhibitory interneurons in the dorsal horn?

Glycine, enkephalin, and dynorphin.

What is the net effect of suprasegmental modulation from PAG → NRM/RMC → spinal cord?

Inhibition of nociceptive transmission and reduced pain perception.

What type of afferent input do chiropractic adjustments increase?

Mechanoreceptive input from joint and muscle receptors.

How does increased mechanoreceptive input affect nociception?

It activates descending inhibitory pathways and reduces nociceptive signaling.

Which neurotransmitters are released by descending brainstem pathways to modulate pain in the dorsal horn?

Norepinephrine (NE) and serotonin (5-HT).

Which neurotransmitters are released by inhibitory interneurons in the dorsal horn to suppress pain transmission?

Enkephalin (ENK) and dynorphin.

What excitatory neurotransmitters are released by primary nociceptive afferents in the dorsal horn?

Glutamate and substance P.

Where do descending fibers from the brainstem travel in the spinal cord?

The dorsolateral funiculus (DLF).

Which spinal cord laminae are the main targets of descending inhibitory fibers?

Laminae I, II, and V of the dorsal horn.

How do enkephalin-containing interneurons inhibit pain transmission?

By presynaptic inhibition of neurotransmitter release and postsynaptic inhibition of projection neurons.

Which midbrain structure is the principal site of endogenous pain control that initiates descending modulation?

The periaqueductal gray (PAG).

Overall, how do chiropractic adjustments reduce pain perception?

By increasing mechanoreceptor input, activating descending anti-nociceptive pathways, and suppressing nociceptive transmission in the dorsal horn.