Cutaneous receptors

• What is sensation?

  subconscious or conscious awareness of changes in the external or internal environment
• somatic sensory modalities

  • tactile (touch, pressure, vibration)
  • pain
  • proprioception (position of limbs, stationary or moving)
  • thermal (warm, cold)
• Process of sensation

  • stimulation of sensory receptor
  • transduction of stimulus (graded potential)
  • generation of nerve impulse (action potential)
  • integration of sensory input
• Sensory neurons only carry one modality
• Mechanoreceptors (deformation, stretching, bending of cells) respond to touch, pressure, vibration, proprioception
• Thermoreceptors respond to absolute and relative changes in temperature
• Nociceptors respond to pain resulting from physical or chemical damage
• Types of cutaneous receptors

  • free nerve endings
  • pain, thermal, tickle, itch and some touch sensation
  • encapsulated nerve endings
  • touch, pressure, vibration
  • type of capsule enhances sensitivity
• Distribution of somatic sensation is uneven.
• What areas of the body have the highest density of somatic receptors?

  lips, fingertips, tongue
• Accuracy of somatic sensation depends on density of receptors and receptor fields
• For good sensory discrimination

  • sensory receptor density must be high
  • receptor fields must be small
• Generator potential decreases in amplitude with a sustained stimulus - perception of sensation decreases even though the stimulus persists
• Touch, pressure and vibration sensation: sensory neurons with

  • encapsulated nerve endings (dendrites)
  • large diameter axon
  • myelinated A fibres
• Vibration, itch and tickle sensation: sensory neurons with

  • free nerve endings (dendrites)
  • small diameter axon
  • unmyelinated C fibres
• Rapidly adapting receptors

  • Meissner's and Pacinian corpuscles
  • hairless skin (hands, lips, tongue etc.)
  • hair root plexus
  • around hair follicles
• Slowly adapting receptors

  • merkel disc
  • free nerve endings
  • fine touch
  • ruffini corpuscle
  • encapsulated
  • deep in dermis
  • sensitive to stretching during movement
• Pain receptors (nociceptors)

  • found everywhere except the brain
  • free nerve endings
  • very little adaptation
• Nociceptors are activated by intense mechanical, thermal or chemical stimuli that are capable of causing tissue damage
• Fast pain

  • acute, sharp
  • carried by myelinated A(delta) fibres
  • precisely located
• Slow pain

  • chronic, burning
  • carried by unmyelinated C fibres
  • diffusely located
• Proprioception information

  • body position
  • muscle contraction
  • tension in tendons
  • position of joints
  • perception of movement - kinesthesia
• Proprioceptors

  • muscle spindles
  • golgi tendon organs
  • joint kinesthetic receptors
• Muscle spindles detect changes in muscle stretch (length)
• Golgi tendon organs detect changes in tendon tension
• Joint kinesthetic receptors monitor stretch in synovial joints and provide information about joint position and movement
• Thermoreceptors

  • free nerve endings
  • basal firing at low frequencies, rapidly adapting
• Cold receptors

  • activated between 10 - 40 oC (painful if < 10 oC)
  • myelinated A(delta) fibres or unmyelinated C fibres
• Warm receptors

  • activated between 32 - 48 oC (painful if > 48 oC)
  • unmyelinated C fibres
• Dorsal column-medial lemniscal

  • discriminative touch
  • crossover in medulla
• spinocerebllar tract

  • proprioception
  • no functional crossover
• spinothalamic tract

  • pain and temperature
  • crossover in spinal cord
• Spinothalamic tract decussates close to their entry to the spinal cord
• Dorsal columns tract remains uncrossed until the medulla