Touch & pain

CHAPTER 12: The Somatic Sensory System

Feeling and Treating Pain

  • Pain is a fact of life:

    • 30% of visits to physicians are for pain symptoms, as are 50% of emergency room visits.

    • Pain has many causes.

  • Pain is necessary:

    • The occasional person born without pain receptors experiences body deformities through failure to adjust posture, and acute injuries through failure to avoid harmful situations.

PAIN AND NOCICEPTION

  • Pain:

    • The feeling (perception) of sore, aching, throbbing, etc.

    • Result of cortical processing of sensory input.

  • Nociception:

    • A sensory process that provides signals that trigger the feeling of pain.

  • Nociceptors:

    • Receptors responsible for transduction of sensory signals for pain.

    • Types include:

    • Free nerve endings

    • Ion channels in free nerve endings that open in response to noxious stimuli

      • Strong mechanical stimulation (pressure), temperature extremes, oxygen deprivation, chemicals.

Pain Isn't Simple…

  • THE MYSTERIOUS SCIENCE OF PAIN

Examples of Human Circadian Rhythms

  • Various functions vary based on time of day:

    • Peak mental alertness and memory functions: around 9:00 A.M. and 9:00 P.M.

    • Lowest body temperature: about 97°F at 4:00 A.M.

    • Highest body temperature: about 99°F at 4:00 P.M.

    • Peak physical strength: around 11:00 A.M. and 7:00 P.M.

    • Peak hearing, visual, taste, and smell sensitivity: around 3:00 A.M. and 6:00 P.M.

    • Lowest sensitivity to pain: around 4:00 P.M.

    • Peak sensitivity to pain: around 4:00 A.M.

    • Peak degree of sleepiness: between 1:00 A.M. and 3:00 A.M.

    • Peak allergic sensitivity to pollen and dust: between 11:00 P.M. and 1:00 A.M.

Nociception Is a Separate Sensation

  • Nociceptive and nonnociceptive thermoreception share similar stimuli but respond differently.

    • Magnitude of afferent response:

    • Example of threshold:

      • Thermoreceptor turns on at 45° Celsius (~113°F).

TRANSDUCTION OF PAINFUL STIMULI

  • Types of Nociceptors:

    • Mechanical: respond to force.

    • Thermal: activated above 43 °C (~110 °F).

    • Chemical: respond to substances like bradykinin (a peptide produced by proteases), ATP, and K+.

    • Polymodal: respond to all forms of painful stimuli.

  • Hyperalgesia: phenomenon where tissue that has already been damaged may become supersensitive to further stimulation, impacting both primary (damaged area) and secondary (surrounding tissues) responses due to chemical activation of nociceptors.

CHEMICAL MEDIATORS OF HYPERALGESIA

  • Damaged cells release substances that open ion channels (termed inflammatory soup), which enhances the sensitivity of nociceptors.

    • Key mediators include:

    • Bradykinin: depolarizes nociceptors.

    • Prostaglandins: increase sensitivity of nociceptors, produced via breakdown of lipid membranes.

    • K+ (Potassium)

    • Histamine: can produce itch.

    • Substance P: released after activation of nociceptors, causes vasodilation (swelling) and activation of mast cells, leading to the release of histamine.

TOUCH (AND PAIN)

  • Primary Afferent Axons:

    • Enter via the vertebral column through the dorsal root.

    • Four types of axons:

    • Aa: large diameter, fast conduction.

    • Ab: medium diameter, touch and pressure.

    • Ad: small diameter, fast pain.

    • C: small diameter, slow pain.

FIRST AND SECOND PAIN

  • Primary Afferents and Spinal Mechanisms:

    • First pain:

    • Fast and sharp, mediated by faster Aδ fibers.

    • Second pain:

    • Slow and dull, mediated by slower C fibers.

THE SPINOTHALAMIC PATHWAY

  • Diagram:

    • Dorsal column, small dorsal root axons, thalamus, primary somatosensory cortex (S1), medulla.

    • Note: The pathway has immediate decussation (crossing over).

REFERRED PAIN

  • Pain in the heart (associated with heart attack) is often perceived as pain in the left shoulder and upper arm due to “cross talk” of neurons in the spinal cord, which relay pain and temperature messages from both the body’s surface and internal organs.

  • Other examples of referred pain:

    • Kidneys can refer pain to lower back.

    • Blood vessels can cause headache (the brain itself has no pain receptors).

SPINOTHALAMIC VERSUS DORSAL COLUMN-MEDIAL LEMNISCAL PATHWAYS

  • While similar pathways exist, they are not identical.

  • Pain and temperature signals from the face/head follow a similar pathway as touch, known as the Trigeminal Pain Pathway.

  • Fibers make initial synapses in the trigeminal nucleus of the brainstem, cross and ascend to S1 (primary somatosensory cortex).

ASCENDING PAIN PATHWAY

  • Similarities and differences in touch and pain pathways to the brain:

    • Free nerve endings participate in nociception in the skin, while mechanoreceptors for touch are also found in the skin.

    • Axon diameter:

    • Large for touch (Aβ fibers), smaller for pain (C and Aδ fibers).

    • Pathway relay:

    • No relay for touch in the spinal cord, one relay for pain.

    • Pain travels slightly more laterally.

    • Within the medulla:

    • No relay for pain, but one relay for touch.

    • In the Thalamus:

    • Both touch and pain relay occurs.

    • General fiber path to brain:

    • Touch ascends ipsilaterally; pain ascends contralaterally.

UNILATERAL DAMAGE TO SPINAL CORD

  • If an individual sustains unilateral damage to the left spinal cord (at T10), the effects are:

    • Loss of touch sensation below T10 on the left side of the body.

    • Loss of pain and temperature sensation below T10 on the right side of the body.

PAIN REGULATION: AFFERENT REGULATION

  • Gate Theory of Pain (Melzack and Wall, 1960s):

    • Aβ fibers and C fibers both make synapses on an inhibitory interneuron, prompting discussion on which fiber is faster and how this affects the perception of pain.

“TREATING” PAIN AFFERENTLY

  • The Gate Theory in action:

    • When stubbing a toe, pain is perceived because the pain pathway to the brain is open.

    • Rubbing the toe activates the haptic-proprioceptive pathway (Ab fibers), reducing pain sensation as the “pain gate” partially closes.

  • Methods such as massage, acupuncture, and immersion in warm water may relieve pain by selectively activating haptic and proprioceptive fibers to close the pain gate.

  • This differs from hyperalgesia, which refers to oversensitivity to touch following tissue damage, due to varying timing and severity of the injury.

DESCENDING REGULATION OF PAIN

  • Descending Regulation:

    • The Periaqueductal Gray (PAG): receives input from brain regions involved in emotion; stimulation acts as a powerful analgesic.

    • The Raphe Nuclei: stimulated by PAG, sends serotonin projections to the dorsal horn, which reduces pain sensation.

    • Endogenous opiates: opiates and endorphins activate PAG and Raphe neurons, thus suppressing pain signals from the spinal cord.

    • Naloxone: an opioid antagonist that inhibits the actions of these endogenous opiates.

PERCEIVING PAIN

  • The spinothalamic tract is recognized as the main pain pathway to the brain.

  • There are additional pathways that carry pain information from the spinal cord to the brain, including:

    • Reticular formation: associated with arousal.

    • Amygdala: associated with emotional responses.

    • Hypothalamus: associated with hormonal and cardiovascular responses.

  • It is critical to note that simply severing the spinothalamic pathway will not necessarily result in pain relief.

ITCH… Poorly Understood

  • QUESTION: Why do we itch?

ITCH… Poorly Understood

  • Defined as a disagreeable sensation inducing a desire or reflex to scratch.

  • It serves as a natural defense against parasites and toxins on the skin.

  • Similar to pain, itch can be triggered by chemicals and touch, mediated by distinct thin sensory axons, separate from nociceptive axons.

  • Importantly, pain can suppress itch, hence the act of scratching may relieve itch.

  • Some forms of itch are mediated by histamines; therefore, antihistamines are effective in relieving this kind of itch.

TEMPERATURE

  • Thermoreceptors: present in the skin for temperature perception.

    • Distinct receptors for “cold” and “hot” sensations.

  • Trp channels: located in thermoreceptors that are also activated by substances like menthol (Trpm8) and capsaicin (Trpv1).

  • There are varying temperature sensitivities; solid lines represent temperature-only responses, while dotted lines indicate independent nociceptive responses.

ADAPTATION OF THERMORECEPTORS

  • Hot and cold receptors show heightened responses to sudden changes in temperature (exaggerated response at the onset of change).

  • Changes in firing patterns are predictable based on the direction of temperature shifts.

TEMPERATURE PATHWAY

  • The organization of the temperature pathway mirrors that of the pain pathway.

    • Cold receptors are coupled to Aδ and C fibers.

    • Hot receptors are coupled only to C fibers.

SUMMARY

  • Sensory systems (visual, auditory, somatosensory) exhibit similar organization and general function.

  • Somatic sensory information is segregated within the spinal cord and cerebral cortex.

  • Parallel processing of information takes place, with some integration at the thalamus and at primary and secondary cortical areas.

  • Accurate identification and processing of stimuli rely on the seamless coordination of somatic sensory information, thus working alongside other senses to complete our sensory perception of the world.