Sensory receptors

• Sensory receptors allow the body to respond to stimuli caused by changes in our internal or external environment
• The body responds to stimuli by converting them into nerve impulses. Different types of receptors respond to different stimuli
• Receptors for special senses of smell, taste, vision, hearing and equilibrium are grouped into localized areas or complex organs
• General sense organs of somatic senses are microscopic receptors widely distributed throughout the body in the skin, mucosa, connective tissue, muscles, tendons, joints and viscera
• Receptor potential is the potential that develops when an adequate stimulus acts on a receptor. When a threshold is reached, an AP in the sensory neuron’s axon is triggered
• Impulses travel over sensory pathways to the brain and spinal cord, where they are interpreted as a particular sensation or initiate a reflex action
• Structural characteristics of sensory receptors can be used to group them into different classes

  • free nerve endings
  • encapsulated nerve endings
  • separate specialized cells
• Free nerve endings are the simplest structure. They consist of bare dendrites that lack any specialization at their ends. They are used for pain, thermal, tickle and itch receptors
• Encapsulate nerve endings are dendrites enclosed in connective tissue capsule. They are used for touch, pressure and vibration
• Separate specialized cells are specific cells that detect sensation like the hair cells in the inner ear or photoreceptors in the retina of the eye. They synapse with sensory neurons.
• Sensory neurons can be grouped according to the type of stimulus they detect

  • mechanoreceptors: touch, pressure, vibration
  • thermoreceptors: temperature
  • photoreceptors: vision
  • chemoreceptor: taste and smell
  • osmoreceptor: osmotic pressure
• Mechanoreceptors are sensitive to mechanical stimuli such as deformation, stretching and bending of cells
• Photoreceptors detect light that strikes the retina of the eye. They are only found in the eyes
• Chemoreceptors detect chemicals in the mouth (taste), nose (smell), and body fluids
• Osmoreceptors detect the osmotic pressure (amount of water) of body fluids