Pain Processing and Sensitization Review

Pain Processing Mechanisms: The Spinal Cord and Dorsal Horn

• Pain processing involves three primary components: Input, Processing, and Output. These mechanisms interact with tissues and the environment.

• Input mechanisms relate to the incoming signals from the environment and tissues into the system.

• Processing mechanisms occur primarily within the central nervous system (CNS), specifically involving the spinal cord, the dorsal horn, and second-order neurons.

• Output mechanisms involve the response generated by the system back toward the tissues and environment.

• The dorsal horn of the spinal cord is a critical site for the processing of nociceptive information. It contains interneurons and second-order neurons that manage incoming signals.

• Nociceptor input from the periphery enters the dorsal horn and is immediately met by interneurons.

• Interneurons act as filters or modulators; they can either block the information or allow it to be passed on to the second-order neuron.

• Once the second-order neurons receive the passed information, they transmit these signals to the brain.

Structural Organization and the Rexed Laminae

• Processing of nociception specifically occurs in Rexed Laminae (RL) I, II, and IV of the dorsal horn.

• Sensory (Dorsal Horn) Laminae include:

  • Lamina I: Marginal layer.

  • Lamina II: Substantia gelatinosa.

  • Lamina III.

  • Lamina IV: Nucleus proprius.

• Intermediate and Autonomic regions include:

  • Lamina V.

  • Lamina VI: Nucleus dorsalis.

  • Lamina VII: Intermediolateral cell column and Intermediomedial cell column.

• Motor (Ventral Horn) Laminae include:

  • Lamina VIII.

  • Lamina IX: Medial and lateral motor nuclei.

• The primary "players" in the dorsal horn processing nociceptive information include:

  • C Fibers: Responsible for transmitting "danger" signals.

  • A-Beta Fibers: Responsible for transmitting light touch or innocuous sensations.

  • Interneurons: Specifically endogenous interneurons which provide modulation.

  • Wide Dynamic Ranging (WDR) Neurons: Handle various types of input.

  • Nociceptive Specific Neurons: Respond only to painful stimuli.

  • Inputs from other dermatomes, the other side of the body, and descending signals from the brain.

The Loss of Inhibitory Interneurons and Central Sensitization

• The loss of inhibitory interneurons (INs) in the dorsal horn leads to a critical loss of modulation for incoming action potentials (APs).

• This loss results in "zillions" of nociceptive-specific second-order neurons firing constantly.

• Input amplification occurs: a single impulse can become two impulses. This amplification initiates central sensitization.

• Structural and functional changes in second-order neurons:

  • Standard receptors are replaced with receptors that specifically facilitate nociceptive messages.

  • Receptors remain open longer, a state referred to as an "open gate."

• A bombardment of the CNS occurs, driven not just by nociceptive fibers but also by other neurons, including those from the opposite side of the body.

Clinical Consequences of C-Fiber Barrage into the CNS

• Table 2.3 outlines the specific processes and their clinical consequences:

  • Death of inhibitory neurons: Results in a decreased ability to inhibit peripheral nociception.

  • C-fibers pull back and A-beta fibers grow into their place: Results in allodynia (pain from non-painful stimuli).

  • Upregulation of second-order neurons: Results in increased firing toward the brain.

  • Further upregulation of second-order neurons: Results in functional shifts in the brain's pain neuromatrix.

  • Inappropriate synapsing at other spinal levels: Results in spreading pain.

  • Inappropriate synapsing with other fibers: Leads to sympathetic, immune, and motor system contributions.

  • Inappropriate synapsing from the other side of the body: Results in bilateral "mirror" pains.

  • Decreased endogenous mechanisms: Results in both allodynia and hyperalgesia.

  • Altered information from the periphery: Leads to structural shifts in the brain, documented as "homuncular smudging."

  • Immune function alterations: Invovles glial cells and the opening of the spinal-cord-blood-barrier.

Peripheral and Central Sensitization Definitions

• Peripheral Sensitization: Defined as the increased responsiveness of nociceptors in the peripheral nervous system due to acute tissue damage.

• Central Sensitization: Defined as the increased responsiveness of nociceptive neurons in the central nervous system to their normal or subthreshold afferent input.

• Early stages of central sensitization are reversible.

• Later stages are considered pathologic and may become irreversible.

• Sensitization is clinically identified through two primary assessments:

  • Hyperalgesia: An increased pain response from a stimulus that normally provokes pain.

  • Allodynia: A pain response from a stimulus that normally does NOT provoke pain.

• Both can occur in both peripheral and central sensitization conditions.

Messengers of Peripheral Sensitization

• Tissue damage and inflammation lead to the release of various chemicals that lower the nociceptor threshold (increasing sensitivity).

• Chemicals and ions involved include:

  • Hydrogen ions and Potassium ions.

  • Histamine, Purines, Leucotrienes, and Bradykinin.

  • Cytokines: Specifically $\text{IL-1}$, $\text{IL-6}$, and $\text{TNF alpha}$.

  • Norepinephrine and Neuropeptides.

  • Nerve Growth Factor (NGF).

  • Prostaglandins.

  • Substance P.

Mechanisms of Central Sensitization: From Early to Chronic

• Early Central Sensitization:

  • The spinal cord neurons become sensitive to "day to day danger," "remembered danger" (memory), and immune/inflammatory activation.

  • The spinal cord acts as a sorter, deciding what information ascends to the brain.

  • Under threat, the spinal cord increases sensitivity to create a stronger danger signal.

• Normal vs. Abnormal Sensitization to Input:

  • Normal response: Brief, mild pain (e.g., sharp/dull test).

  • Normal response to injury: Peripheral sensitization leading to early central sensitization (hypersensitivity).

  • Abnormal response: Occurs with no peripheral input or a severely heightened response when input is present.

• Stages of Central Sensitization:

  • Early Stage: Adaptive response to protect the individual, characterized by allodynia and hypersensitivity.

  • Late Stage: Still reversible, but abnormal. More receptors are laid down in the second-order neuron, leading to higher responsiveness and diffuse pain sensitivity.

  • Chronic Pain Syndrome (Disinhibition Phase): Not reversible. Characterized by a loss of inhibitory interneurons (no "braking" of input) and altered descending modulation pathways. This becomes "hardwired" or maladaptive neuroplasticity.

Anti-nociception and Pro-nociception

• Pro-nociception: A bottom-up amplification of pain signals. It involving the local release of glutamate, aspartate, Substance P, and Prostaglandins that amplify the signal traveling to the brain.

• Anti-nociception: A top-down inhibition of pain signals. This utilizes descending pathways and endogenous opioids like endorphins and enkephalins.

• The Brain's Decision Matrix: The brain weighs whether the body is in real danger. If danger is perceived, it results in amplification. If the brain decides "you're okay," it results in inhibition.

The Brain and the Pain Neuromatrix

• The Pain Neuromatrix refers to brain structures that process and regulate pain information. It is capable of creating pain perception even in the absence of nociceptive input (e.g., phantom limb pain).

• Anatomy of the Pain Matrix: includes the brainstem, limbic system, hypothalamus, thalamus, areas of the cerebral cortex, and ascending/descending pathways.

• Common areas activated during pain:

  • Anterior cingulate gyrus.

  • Thalamus.

  • Prefrontal cortex.

  • Posterior parietal cortices.

• Ascending Nociceptive Tracts:

  • Lateral Tract (Spinothalamic): Processes location and intensity.

  • Medial Tract (Divergent pathways): Processes affective and cognitive aspects.

• According to Butler & Moseley, the brain's challenge is to construct a sensible story by "weighing the world."

Clinical Impacts of Pain Matrix Changes

• Persistent pain often correlates with problems in attention, focus, concentration, reasoning, short-term memory, body temperature regulation, and sleep.

• Structural Changes: Chronic pain patients show different representation in the primary sensory cortex ($S1$ or the sensory homunculus).

• Body maps may expand or contract, and brain volume changes occur in the pain matrix areas.

• Cortical Smudging: Altered cortical body representations with increased activation, leading to pain perception without peripheral stimulation.

• Laterality Issues: Difficulty with left-right discrimination.

• Body perception changes: Experiencing the painful body part as larger or smaller than it is.

Pain as a Stressor: The Output Mechanisms

• Adrian Louw describes pain using the metaphor of a "roaring African lion."

• Long-term pain triggers the fight-or-flight response, releasing ACTH from the pituitary gland to stimulate adrenaline production in the adrenal glands.

• Cortisol: Acts longer than adrenaline. It makes tissues sore, tired, sensitive, and fatigued. It impacts the immune system via cytokine signaling ($\text{IL-1}$, $\text{IL-6}$, $\text{TNF alpha}$) and leads to persistent inflammation and brain changes.

• Effects of chronic stress on systems:

  • Brain: Short-term memory loss, sleep disturbance, decreased focus, low libido, appetite changes.

  • Tissues: Increased sensitivity, increased inflammation, decreased blood flow.

  • Immune System: Immuno-deficiency, potential failure, fatigue.

  • General: Sensitized gut, muscle hyperactivity, and atrophy.

• Development of disease from overworked "supersystems":

  • Immune System Stress $\rightarrow$ Fibromyalgia.

  • Sympathetic Nervous System Stress $\rightarrow$ Adrenal Fatigue.

  • Gastrointestinal System Stress $\rightarrow$ Irritable Bowel Syndrome (IBS) or Non-Celiac Gluten Sensitivity.

  • Endocrine System Stress $\rightarrow$ Chronic Fatigue Syndrome or Chronic Lyme Disease.