Somatosensation (Chapter 5a)

History:

• Solomon’s group and inherent need for stimulation
  • Individuals become distressed w/in 4-8 hrs, few lasted more than 24 hours

General Sensory Processing:

• Sensory transduction::when a receptor cells converts the energy of a stimulus into action potentials
• Each sense has it own specific set of receptor cells
• Hierarchical organization
  • Stimuli → receptor cells → thalamic relay nuclei → primary sensory cortex → secondary sensory cortex → association cortex
  • Functional segregation and parallel processing
  • Multiple specialized areas at multiple levels - interconnected by multiple parallel pathways
• Think on a comparative level - how is there variation across species?
  • Black fly, panther chameleon, American bald eagle - examples of ocular differences

Labeled Lines:

• Each nerve input to the brain reports only a particular type of info
• Distinguishing b/w different types of touch
• Travel thru the spinal cord and then are fully processed by the brain

Somatosensation:

• Types of skin -
  • Hairy skin
  • Glabrous::non-hairy skin; high sensitivity (palms of hands, lips, tongues, soles of feet)
• 2-point sensitivity test
  • Glabrous skin - 3mm
  • Hairy skin - 2+cm

Somatosensory Receptors:

• Nociception::perception of pain and temp; free nerve endings
• Haptic sensation::perception of fine touch and pressure

Dorsal Column System:

• Sense of touch neurons are longest in the body - going from longest toe upward thru the spinal cord into the medulla at the brain stem and then to the thalamus
• Toe → medulla; medulla → thalamus (crisscrosses - if on right side, will move to left); thalamus → somatosensory cortex
  • Area of the medulla = decassation::criss-crosses
• Dermatomes::a strip of skin innervated by a particular spinal nerve

Pain:

• Unpleasant sensory and emotional experience associated w/actual or potential tissue damage

• 3 dimensions of pain -

  • Sensory-Discriminative::good pain vs. bad pain; where is the pain?
  • Motivational-Affective::situational motivations and emotions
  • Cognitive-Evaluation::thoughts on the pain, address it as decisions

• Pain Paradox

  1. Adaptiveness of pain - pain is bad, but important for survival
  2. Lack of clear cortical representation - remove SI and SII and we still perceive pain
  3. Pain can be suppressed - sports injuries, religious rituals, life-threatening situations

Congenital Insensitivity to Pain:

• Born w/o perception of pain; no increase in HR, blood pressure, or respiration due to pain
  • Weak gag reflex, complete lack of corneal reflex
• The “Human Pincushion”
• Small cluster in Sweden w/this specific gene

Pain Pathways - Anterolateral System:

• A Delta fibers::large in diameter, myelinated axons, fast travel = sharp pain
• C fibers::thin, unmyelinated axons, slow travel = dull or throbbing pain
• Withdrawal reflex to get away from the pain; vocal reflex at the midbrain (exclamation or curse)

Dual Pathways:

• Haptic and Proprioceptic Perception
  • Dorsal Column System
  • Enters spinal cord and ascends to the medulla prior to crossing
• Nociception
  • Anterolateral system
  • Synapse in the spinal cord and cross over

Peripheral Mediation of Pain:

• Inflammation and potential activation of the immune system
• Neurons are activated - collaterals that come off and release Substance P
  • Substance P::further stimulates cells to promote action at the skin level
  • Pain fibers release glutamate which goes up the spinal cord and signals the brain

Pain Perception/Long Term Pain:

• Glutamate::mild pain
• Substance P::intense pain
• Endorphins::if enough endorphins are produced, Substance P turns down = less pain
• Neuropathic pain::complicated gray area about why pain exists - neurons coded wrong? treat w/opiods?
• Phantom limb pain::some patients can be treated w/mirror image boxes and are more receptive to prosthesis b/c brain is still maintaining cortical space for the missing limb

Drugs of Interest:

• Naloxone::opioid antagonist
• Methadone::opioid agonist
  • Long acting, decreased euphoria, remove withdrawal effects

Cortical Pathways and Somatosensation:

• Cingulate cortex; somatosensory agnosias
• Astereognosia::inability to recognize objects by touch - focal cortical lesion at area SII, which integrates inputs from finger joints w/tactile info
• Asomatognosia::failure to recognize parts of one’s own body - damage in the right posterior parietal cortex or premotor cortex
  • Almost always in a left leg of left arm
  • Almost always in stroke victims
  • Recovery is weak

\