Anatomy and Histology

• Receptor specificity: receptor is sensitive to a particular type of stimulus (structure-function)

•  Receptive field: area monitored by a single receptor cell (size of field and localization are inversely related)

•  Output via the axon is electrochemical

• Tonic receptors: always active, signal frequency varies

  according to stimulus, e.g. eye photoreceptors

• Phasic receptors: inactive, signal propagated only when stimulated, e.g. skin touch and pressure receptors

• Combined tonic and phasic receptors: e.g. proprioceptors Adaptation: reduced sensitivity due to constant stimulation 

Types of Receptors

Classification by location

• Exteroceptors: located on or near body surface. Respond to stimuli that arise external to the body. Detect pressure, touch, pain and temperature

• Internoceptors or Visceroceptors: located internally. Receive stimuli from the internal body environment. Activated by pressure, stretching and chemical changes. Also mediate hunger and thirst.

• Proprioceptors: specialized type of visceroceptors. Located in skeletal muscle, joint capsules and tendons. Information of body movement, spatial orientation and muscle stretch 

1. Classification by structure

   • Free nerve endings

     • Simplest, most common

     • Terminates in dendritic knobs

   • Encapsulated nerve endings

     • Connective tissue capsule surrounding their terminal or dendritic end

     • Primarily mechanoreceptors 

Classification by stimulus detected

• Nociceptors: Pain

  • Location: skin, joints and bone periostea, blood vessels, some viscera

  • Free nerve endings, large receptive field (making localizing the source difficult)

  • Thermo, mechano and chemoreceptors

  • Fast pain: “stabing” pain perceived in primary sensory cortex, triggers somatic reflexes

  • Slow pain: “aching” pain activates the thalamus and reticular formation

• Thermoreceptors: Temperature

  • Location: skin dermis, skeletal muscle, liver, hypothalamus

  • Cold and warm receptors, phasic, i.e. adaptation

  • Same pathways as nociception to the primary sensory cortex, thalamus and reticular formation

• Chemoreceptors: Chemical compounds

  • Location: respiratory centres (pH and Pco2), carotid bodies (pH, Pco2, Po2),

    aortic bodies (pH, Pco2, Po2)

  • Water and lipid soluble compounds

• Mechanoreceptors: Touch

• Photoreceptors: Light - 

  • Only in the eye 

Hornhaut (Augen)

• Two components: sclera and cornea

• Provides: structure, protection, points of attachment

• Tough fibro-elastic capsule

• Sclera = opaque, dense

  fibrous connective tissue,

  the “whites” of the eye

• Provides attachment for 6

  extra-ocular muscles

• Nerves and vessels

  penetrate the anterior surface (i.e. blood-shot eyes)

• Corneal limbus: separates sclera and cornea 

Three components: choroid, ciliary body, iris