SOMATOSENSORY NEUROTRANSMISSON

• cutaneous mechanoreceptors


  mediate responses to touch and pressure
• proprioceptors


  relay information about muscle length and tension
• thermoreceptors


  sensations of warmth and cold
• nociceptors


  respond to potentially harmful stimuli such as pain, extreme heat, and extreme cold
• chemoreceptors


  stimulated by change in the chemical composition of the local environment
• receptors for taste and smell , as well as visceral receptors

  chemoreceptors
• sensitive to changes in the plasma level of oxygen, ph, and osmolality

  chemoreceptors
• in the rods and cones in the retina respond to light

  photoreceptors
• dendrites encapsulated in connective tissue beneath the epidermis of glabrous skin

  meissner corpuscles
• responds to slow vibration

  meissner corpuscles
• are expanded on dendritic endings in epidermis of glabrous skin

  merkel cells
• responds to sustained pressure and touch

  merkel cells
• enlarged dendritic endings with elongated capsules in the dermis of glabrous and hairy skin

  ruffini corpuscles
• responds to stretch and fluttering vibration

  ruffini corpuscles
• largest cutaneous mechanoreceptor

  pacinian corpuscles
• pacinian corpuscles are 2 mm long and about 1 mm in diameter; found in the dermis of glabrous and hairy skin
• responds to fast vibration and deep pressure

  pacinian corpuscles
• PAIN AND TEMPERATURE arise from receptors located on unmyelinated dendrites of sensory neurons located throughout the glabrous and hairy skin as well as deep tissue
• mechanical nociceptors


  responds to strong pressure
• thermal nociceptors


  responds to skin temperatures above 45 degrees celsius or by severe cold (less than 20)
• chemically sensitive nociceptors


  responds to chemicals such as bradykinin, histamine, high acidity, and environmental irritants
• polymodal nociceptors

  combinations of these stimuli
• nociceptors

  transmitted via thinly myelinated Aδ fibers that conduct at rates of ∼12–35 m/s and unmyelinated C fibers that conduct at low rates of ∼0.5–2 m/s
• Activation of Aδ fibers -> release glutamate

  • is responsible for first pain (fast pain), rapid response and mediates the discriminative aspect of pain or the ability to localize the site and intensity of the noxious stimulus.
• Activation of C fibers, -> release a combination of glutamate and substance P

  • is responsible for the delayed second pain (slow pain), which is the dull, intense, diffuse, and unpleasant feeling associated with a noxious stimulus.
  • Itch is elated to pain sensation
• Innocuous cold receptors - on dendritic endings of Aδ and C fibers
• Innocuous warm receptors - are on C fibers
• TRPM8 receptors

  activated by moderate cold.
• Cold receptors are inactive at temperatures of 40°C, but then steadily increase their firing rate as skin temperature falls to about 24°C.
• As skin temperature further decreases, the firing rate of cold receptors decreases until the temperature reaches 10°C.
• TRPV3 and TRPV4 receptors

  on sensory nerve endings
• TRPV3 and TRPV4 receptors

  • activated when skin temperatures reach 33–39°C and 25–34°C, respectively.
• The firing rate of warm receptors can further increase as the skin temperature reaches about 45°C
• sensory coding


  Converting a receptor stimulus to a recognizable sensation
• Modality - type of energy transmitted by the stimulus.
• Intensity - is signaled by the response amplitude or frequency of action potential generation.
• Location - site on the body or space where the stimulus originated.
• Duration - time from start to end of a response in the receptor.
• receptor specificity

  A sensory receptor is specialized to respond to either mechanical, chemical, thermal, or electromagnetic stimuli.
• adequate stimulus

  • The form of energy to which a receptor is most sensitive