Skin Sense

• Sensory input


  • Sensory impulses reach the CNS and become part of a large pool of sensory input
  • Each piece of incoming information is combined with other arriving and previously stored information
• Sensory modality


  The property by which one sensation is distinguished from another

  
• Different types of sensations

  • Touch
  • Pain
  • Temperature
  • Vibration
  • Hearing
  • Vision

  
• Generally, each type of sensory neuron can respond to only one type of stimulus
• Classes of sensory modalities

  • General senses
  • Special senses
• Somatic senses


  • Tactile sensations (touch, pressure, vibration, itch, and tickle)
  • Thermal sensations (warm and cold)
  • Pain sensations
  • Proprioception (awareness of limb and joint position in space)
• Visceral senses


  Provide information about conditions within internal organs (pressure, chemicals, stretch, nausea, hunger, temperature)
• Sensory receptors


  • Demonstrate selectivity - respond to only one type of stimulus

  
• Process of sensation


  1. Stimulation of the receptor

  2. Transduction (conversion) of stimulus into a graded potential

  3. Generation of impulses when graded potentials reach threshold

  4. Integration of sensory input by the CNS

  
• General sensory receptors


  May have simple or complex structure
• General sensory receptors (somatic receptors)

  • Free nerve endings that provide pain, tickle, itch, temperature
  • Receptors with structural specializations for touch, pressure & vibration

  
• Structural classification of sensory receptors


  • Free nerve endings
  • Encapsulated nerve endings

  
• Types of sensory receptors by response to stimulus

  • Mechanoreceptors
  • Thermoreceptors
  • Nociceptors

  
• Generator potential


  • Produced by free nerve endings, encapsulated nerve endings & olfactory receptors
  • When large enough, it generates a nerve impulse in a first order neuron
• Receptor potential


  • Receptor cells release neurotransmitter molecules on first-order neurons producing postsynaptic potentials
  • PSP may trigger a nerve impulse
  • Amplitude of potentials vary with stimulus intensity
• Location of receptors & origin of stimuli


  • Exteroceptors
  • Interoceptors
  • Proprioceptors
• Meissner's corpuscle


  • Dendrites enclosed in CT in dermal papillae of hairless skin
  • Discriminative touch & vibration-- rapidly adapting
  • Generate impulses mainly at onset of a touch
• Crude touch


  Ability to perceive that something has simply touched the skin
• Discriminative touch (fine touch)

  Provides specific information about a touch sensation such as location, shape, size, and texture of the source of stimulation
• Receptors for touch


  • Corpuscles of touch (Meissner's corpuscles)
  • Hair root plexuses
• Slowly adapting receptors for touch

  • Type I Cutaneous Mechanoreceptors (tactile or Merkel discs)
  • Type II Cutaneous Mechanoreceptors (end organs of Ruffini)
• Merkel's disc


  • Flattened dendrites touching cells of stratum basale
  • Used in discriminative touch (25% of receptors in hands)
• Ruffini corpuscle


  • Found deep in dermis of skin
  • Detect heavy touch, continuous touch, & pressure
• Pressure


  • A sustained sensation that is felt over a larger area than touch
  • Pressure sensations generally result from stimulation of tactile receptors in deeper tissues and are longer lasting and have less variation in intensity than touch sensations
• Receptors for pressure


  • Type II cutaneous mechanoreceptors
  • Lamellated (Pacinian) corpuscles
• Vibration


  Sensations result from rapidly repetitive sensory signals from tactile receptors
• Receptors for vibration


  • Corpuscles of touch
  • Lamellated corpuscles
• Pacinian corpuscle


  • Onion-like connective tissue capsule enclosing a dendrite
  • Found in subcutaneous tissues & certain viscera
  • Sensations of pressure or high-frequency vibration
• Itch and tickle


  • Itch and tickle receptors are free nerve endings
  • Tickle is the only sensation that you may not elicit on yourself
• Itch and tickle sensations

  • Experienced when mild stimulation of the pain nerve endings occurs
  • There are also specific free nerve endings for itch sensation
  • Transmitted by group C unmyelinated nerve fibres
• Histamine


  Produces itch while pain signals suppress it
• Tickle


  Itch produced by light external moving stimuli and is a pleasurable sensation
• Itch


  • An annoying sensation while pain is unpleasant
  • Itch sensation excites the scratch reflex
• Hair root plexus


  Free nerve endings found around follicles, detects movement of hair
• Thermal sensations


  • Free nerve endings with 1mm diameter receptive fields on the skin surface
  • Cold receptors in the stratum basale respond to temperatures between 10- 40.5 degrees C
  • Warm receptors in the dermis respond to temperatures between 32.2 -47.8 degrees C
  • Both adapt rapidly at first but continue to generate impulses at a low frequency
  • Pain is produced below 10 and over 40.5 degrees C
• Proprioceptive sensations


  • Receptors located in skeletal muscles, in tendons, in and around joints, and in the internal ear convey nerve impulses related to muscle tone, movement of body parts, and body position
  • This awareness of the activities of muscles, tendons, and joints and of balance or equilibrium is provided by the proprioceptive or kinaesthetic sense
• Proprioceptive or kinaesthetic sense


  • Awareness of body position & movement
  • Proprioceptors adapt only slightly
  • Sensory information is sent to cerebellum & cerebral cortex

  
• Muscle spindles


  • Specialized intrafusal muscle fibres enclosed in a CT capsule and innervated by gamma motor neurons
  • Stretching of the muscle stretches the muscle spindles sending sensory information back to the CNS
  • Spindle sensory fibres monitor changes in muscle length
  • Brain regulates muscle tone by controlling gamma fibres
• Golgi tendon organs


  • Found at junction of tendon & muscle
  • Consists of an encapsulated bundle of collagen fibres laced with sensory fibres
  • When the tendon is overly stretched, sensory signals head for the CNS resulting in the muscle's relaxation

  
• Joint receptors


  • Ruffini corpuscles
  • Pacinian corpuscles