somatic sensory II

The Somatic Sensory System - Pain

Overview of Pain and Nociception

  • Nociception:

    • Definition: Activation of detectors of tissue injury.

  • Pain:

    • Definition: An unpleasant sensory and emotional experience associated with tissue injury or potential tissue injury, requiring higher brain processing.

Distinction Between Nociception and Pain

  • Separable Concepts:

    • Nociception can occur without pain, exemplified by wartime valor where individuals may not feel pain despite injuries.

    • Pain can exist without nociception, demonstrated through phantom limb sensations where amputees perceive pain in the absent limb.

Nociceptors

  • Characteristics:

    • Type: Free, unmyelinated nerve endings.

    • Function: Detects strong mechanical stimulation, temperature extremes, oxygen deprivation, and various chemicals.

Activation of Nociceptors

  • Mechanisms of Activation:

    • Mechanically gated ion channels activated by physical forces.

    • Chemicals released by cellular damage include:

    • Proteases

    • ATP

    • K+ ions

    • Histamine

    • Prostaglandins

    • Low pH

Hyperalgesia

  • Definition: Increased sensitivity to painful stimuli.

  • Mechanism: Involves interaction between the immune system and the peripheral nervous system, where inflammation sensitizes nociceptors.

  • Substance P:

    • Definition: A neuropeptide released by nociceptors and present in both the central nervous system (CNS) and periphery.

    • Role: Peripheral release contributes to hyperalgesia and acts as a potent vasodilator causing redness and swelling.

Thermoreceptors and Pain

  • Temperature Detection:

    • Utilizes the TRP (Transient Receptor Potential) channel family.

    • Specific channels for hot/cold detection:

    • Menthol activates ‘cold’ TRP channels.

    • Capsaicin activates ‘hot’ TRP channels.

Pain Afferents and Spinal Mechanisms

  • Types of Pain:

    • First pain: Sharp, stabbing sensation.

    • Second pain: Dull, aching sensation.

  • Neurotransmitter Release:

    • Primary afferents release glutamate and substance P (as a cotransmitter) when high-frequency action potentials occur.

  • Substance P's Role:

    • Encodes moderate to intense pain and is released from nociceptor terminals in the spinal cord.

Differential Speed of Nerve Fibers

  • Types of Fibers:

    • Ad Fibers: Fast transmission at approximately 10 m/s.

    • C Fibers: Slow transmission at approximately 1 m/s.

  • Function of Slow C-Fibers: Serve as a teaching signal to prevent future injuries.

Referred Pain

  • Definition: Pain perceived at a location other than the site of origin.

    • Example: Heart attacks are often felt in the neck and shoulders rather than the chest.

  • Cause: Arises from the mixing of visceral and cutaneous nociceptor signals within the spinal cord. This mixing occurs in the spinal cord itself.

Ascending Pain Pathways

  • Anatomy of Pathways:

    • Right Side of the Body:

    • Thalamus, first neuron, receptors for pain, cold, warmth, tickle, and itch.

    • Connections to midbrain and medulla oblongata.

    • Left Side of the Body:

    • Primary somatosensory area of the cerebral cortex.

    • Pathways:

    • Lateral Spinothalamic Tract: Carries pain and temperature signals.

    • Anterior Spinothalamic Tract: Additionally involved in the sensory transmission.

Comparison of Ascending Pathways

  • Separation of Touch and Pain Systems:

    • Touch: Dorsal-column medial lemniscal pathway ascends ipsilaterally and crosses at the medulla.

    • Pain and Temperature: Spinothalamic pathway crosses immediately at the entry level into the spinal cord and ascends contralaterally.

Brown-Sequard Syndrome Overview

  • Neurological Implications:

    • Ipsilateral loss of all sensory modalities and paralysis at the lesion level.

    • Ipsilateral spastic paraparesis occurs below the lesion.

    • Contralateral loss of pain and temperature sensation below the lesion.

  • Affected Areas:

    • Posterior columns (Cuneate fasciculus, Gracilis fasciculus).

    • Lateral corticospinal tract.

Modulation of Pain

  • Adaptive Nature of Pain:

    • Pain is protective and adaptive, preventing harm and encouraging healing.

    • Absence of pain increases the risk of injury through lack of feedback.

    • Pain can interfere with normal functioning, and hence mechanisms to modulate pain exist.

  • Endogenous Mechanisms:

    • Involve multiple descending systems that utilize:

    • Endogenous opioids

    • Various neurotransmitters

Gate Theory of Pain

  • Basic Concept:

    • Pain relief post-injury can be achieved through physical manipulation (e.g., pressing, rubbing, or moving the affected area).

  • Mechanism: C-fibers carry slow pain, while faster non-nociceptive fibers (Aa and Ab) can override some pain signals through inhibition by interneurons in the dorsal horn of the spinal cord.

Descending Control of Pain

  • Components of Descending Control:

    • Tissue Damage Pathway: Involves DRG neurons and the spinothalamic tract, influenced by enkephalin-secreting neurons.

    • Clinical Correlates:

    • The periaqueductal gray can activate during stress to limit pain signals.

    • Drugs that elevate norepinephrine (NE), serotonin (5HT), and opioids engage this descending control system to reduce pain.

    • Opioids bind to mu-opioid receptors, affecting both pre- and post-synaptic transmission to limit ascending pain transmission.

Pathological Pain Syndromes

  • General Overview: Pain may exist without nociceptive stimuli.

  • Nociceptive Pain vs. Neuropathic Pain:

    • Types of neuropathic pain include:

    • Peripheral neuropathic pain: Examples include Varicella Zoster (“Shingles”) and diabetic polyneuropathy.

    • Treatments: Diminish signaling along peripheral axons using Na-channel blockers (e.g., carbamazepine) and Ca-channel modulators (e.g., gabapentin).

  • Central Neuropathic Pain:

    • Characterized by its aversive quality, beyond expected levels for simple nociceptive pain.

    • Conditions include strokes (commonly affecting the thalamus), spinal cord injuries, multiple sclerosis, and phantom limb pain.

    • Syndromes associated with central neuropathic pain include deafferentation syndromes (e.g., tinnitus after hearing loss).

Cognitive Aspects of Limbs

  • Cognitive Phenomena:

    • Concepts such as hemispatial neglect and the rubber hand illusion demonstrate how the brain's perception of limbs can be manipulated.

Treatment Approaches to Phantom Pain

  • Mirror Box Therapy: Utilizes the principle of visual feedback to alleviate phantom limb pain, allowing patients to visualize the missing limb in a way that reduces pain sensations.