Sensory Receptors and Pain Perception Quiz

Pain and Temperature Transmission

• Pain is a subjective experience, influenced by individual variation, genetics, past experiences, and current situation.
• Physiological (nociceptive) pain is crucial for survival, while chronic/neuropathic pain arises from nerve damage.
• Pain transmission involves the ascending pathways transmitting pain signals to the brain.
• The concept of a "pain matrix" highlights the interconnected brain areas involved in processing pain.
• Descending pain modulatory pathways play a role in regulating pain signals.
• Neuropathic pain has two distinct mechanisms.

Session Objectives

• Understand the significance of physiological/nociceptive pain and its distinction from chronic/neuropathic pain.
• Identify and pinpoint the ascending pathway responsible for mediating pain, temperature, and touch sensations.
• Grasp the concept of the "pain matrix."
• Analyze the importance of descending pain modulatory pathways.
• Describe the two underlying mechanisms of neuropathic pain.

Outline

• Sensory receptors and transduction
• Somatosensory receptor types
  • Mechanoreceptors
  • Thermoreceptors
  • Nociceptors
• Somatosensory pathways
  • Dorsal column system
  • Anterolateral System/Spinothalamic tract
• Pain matrix and descending pathway
• Neuropathic pain

Somatosensation

• Different somatosensory receptors respond to various external stimuli.
• Information about these stimuli travels to the brain through two pathways.
  • Touch and position pathway (mechanoreceptors)
  • Pain and temperature pathway

Types of Somatosensory Receptors: Nociceptors

• Nociceptors are free nerve endings located in superficial skin, bones, joints, and blood vessels.
• Mechanical, thermal, or chemical tissue damage activates nociceptors.

Thermoreceptors

• Thermoreceptors are free nerve endings sensitive to temperature changes (warmth or coolness) in the skin.
• Several receptor subtypes detect a range of temperatures.
• These receptors detect noxious cold and heat.
• Transduction involves Transient Receptor Potential (TRP) channels.
• Opening of these channels depolarizes or hyperpolarizes sensory neurons.

Receptors on the Receptors

• Various receptor types (TRPV1, TRPV2, TRPV3, TRPV4, TRPM8, TRPA1) respond to distinct temperature ranges.
  • TRPV1: Responds to heat and capsaicin.
  • TRPV2: Responds to noxious heat.
  • TRPV3: Responds to warmth
  • TRPV4: Responds to warmth and body temperature.
  • TRPM8: Responds to coolness.
  • TRPA1: Responds to noxious cold.

PAIN

• Pain is an unpleasant sensory and emotional experience.
• Perception is subjective, influenced by individual factors.
• Pain is critical for survival, and its absence can cause serious harm.
• Pain (e.g., burns, steps on nails) is sometimes relevant from a protective or survival function.
• Clinical pain states include physiological (nociceptive) and neuropathic.
  • Physiological (nociceptive): the neural processing of encoding/processing noxious stimuli.
  • Neuropathic: result from injury to a peripheral or central nervous system that consequently affects circuit sensitivity & connections, creating permanent changes.

Pathological Sensitivity to Temperature

• CNS injury can heighten sensitivity to warm and cool temperatures.
• Chemotherapy-induced peripheral neuropathy is one example of this.
• Certain chemotherapies (cisplatin, oxaliplatin) can affect up to 75% of patients.
• Toxicity to peripheral sensory nerves may cause paresthesia/cold-induced paresthesia of the mouth and throat.
• Treatment may include selective norepinephrine reuptake inhibitors (SNRIs).

The Somatosensory Pathways

• Pathways transmit sensory information.
  • Posterior column/medial lemniscus pathway: Transmits fine touch, vibration, pressure, and proprioception.
  • Anterolateral (spinothalamic) pathway: Transmits crude touch, pressure, pain, and temperature.

Anterolateral/Spinothalamic Pathway

• First-order neurons in the periphery have modified dendrites.
• The entire dendrite/axon extension is either a myelinated A fiber or unmyelinated C fiber.
• Axons extend from the dorsal root ganglia into the dorsal horn, travelling approximately two vertebral levels. These axons then decussate (cross to the opposite side) to the anterolateral corner of the spinal cord.
• Second-order neurons synapse in the ventral posterior lateral thalamus.
• Third-order neurons transmit signals to the primary somatosensory cortex, cingulate cortex, and insular cortex.

Trigeminothalamic Tract

• Primary axons from pain and temperature receptors for the face enter through the trigeminal nerve.
• Cell bodies of these neurons reside in the trigeminal ganglia.
• Axons descend as the spinal trigeminal tract before synapsing in the spinal trigeminal nucleus or the spinal nucleus of V.
• Second-order neurons synapse, and then most decussate and travel in the ventral trigeminal thalamic tract to the ventral posterior medial nucleus in the thalamus.

Modulation of Pain

• Nociceptors' transmission is modulated through adaptive processes involving excitatory and inhibitory mechanisms.
• In normal functioning, responses from second-order neurons are suppressed or facilitated based on other bodily events.
• Many of these modulations occur subconsciously.

Central Pain Processing: Pain Matrix

• Pain perception is multidimensional and produced by distributed neural patterns.
• Sensory input triggers activity in the pain matrix, an interconnected network of brain areas.
• The thalamus and somatosensory cortex process pain signals and receive input from other brain regions.
• Pain experience is affected by cognition, emotion, motivation, and sensation, consistent with the pain matrix concept.
• Brain regions in the pain matrix can enhance or reduce pain perception.

Central Pain Processing: Specific Brain Regions

• Anterior cingulate cortex: Part of the limbic system, involved in emotional learning and sensory aspects of pain.
• Insula: Part of the limbic system, involved in self-awareness and the sense of self in pain, including visceral pain (e.g., gastric distension).
• Prefrontal cortex: Anticipates pain and is involved in the placebo effect.
• Periaqueductal gray (PAG): Important area for endogenous pain inhibition.

Descending Pathways

• Areas of the pain matrix project to the PAG (periaqueductal grey), a crucial hub of pain inhibition.
• The PAG projects to brainstem regions (locus coeruleus, rostral ventromedial medulla, and raphe nucleus).
• Descending inhibitory projections influence the dorsal horn of the spinal cord.

Descending Pathways (Potential Mechanism)

• PAG stimulates noradrenergic, serotonergic, and opioidergic pathways.
• Potential mechanisms of SNRIs and SSRIs for neuropathic pain involve enhancing inhibitory actions within the descending pain pathway.
• This action diminishes ascending pain signals.

Role of opioid receptors in the spinal cord

• Opioids act as "gates" by exerting analgesic effects in the dorsal horn of the spinal cord and other CNS sites.
• Interneurons release neuromodulators like endorphins and enkephalins influencing pain signalling.
• Opioid receptors are inhibitory, present presynaptically on sensory afferents. Activation decreases the release of neurotransmitters like glutamate/substance P.
• Morphine and other opiates exert pain-inhibiting effects here.

Neuropathic Pain

• Neuropathic pain arises from nerve damage, altering touch and pain processing.
• Neuropathic pain persists or worsens after initial injury.
• Neuropathic pain is associated with spontaneous pain, abnormal sensations, hypersensitivity, and heightened responses to stimuli (allodynia & hyperalgesia).

Mechanisms of Neuropathic Pain: Peripheral Sensitization

• Peripheral sensitization reduces thresholds for nociceptors and increases their responsiveness.
• Occurs due to chemical release and injury in disease states (e.g., diabetes).
• Chemicals (histamine, bradykinin, serotonin) bind to receptors, depolarizing them.
• Persistent stimulation can increase Na+ channel number and subtype, and TRP channels, resulting in stronger depolarization.

Mechanisms of Neuropathic Pain: Central Sensitization

• Central sensitization involves increased excitability of neurons in the CNS.
• This is due to peripheral/central nerve damage/inflammation.
• Normal input may now evoke abnormal responses.
• Nociceptor activity strengthens synaptic connections to spinal cord neurons (hyperalgesia).
• Low-threshold sensory fibers (normally not pain-producing) might now activate pain neurons (allodynia).

Cellular and Molecular Mechanisms of Central Sensitization

• Multiple mechanisms underlie central sensitization, including alterations in NMDA-mediated neuronal signaling.
• Changes in inhibitory pathways (e.g., GABA/Glycine) and glial-neuronal interactions add to the complex nature of pain processing.

Test your knowledge on sensory receptors, pain types, and the mechanisms of pain perception with this quiz. Delve into the specifics of thermoreceptors, nociceptors, and the processes involved in pain transmission. Understand how different stimuli can affect pain sensation in various ways.