Sensory Pathways

• Receptors


  • Can be classified structurally on the basis of cell type and their position in relation to stimuli they sense
  • Can also be classified functionally on the basis of the transduction of stimuli, or how the mechanical stimulus, light, or chemical changed the cell membrane potential
• Cells that interpret information about the environment

  • Neuron with free nerve ending
  • Neuron with encapsulated ending
  • Specialized receptor cell
• Chemoreceptor


  Interprets chemical stimuli, such as an object's taste or smell
• Osmoreceptor


  Responds to solute concentrations of body fluids
• Nociceptor


  Interprets the presence of chemicals from tissue damage, or similar intense stimuli
• Mechanoreceptor


  Interprets physical stimuli, such as pressure and vibration, as well as the sensation of sound and body position (balance)
• Thermoreceptor


  Sensitive to temperatures above (heat) or below (cold) normal body temperature
• General sense (somatic sensory division)


  Distributed throughout the body and has receptor cells within the structures of other organs, often contribute to the sense of touch or to proprioception and kinesthesia
• Special sense


  Receives sensory information on sight, sound, balance, pressure, and taste and transmits them to the CNS for interpretation
• Visceral sense


  Receives sensory information from viscera (thoracic and abdominal organs) and transmits to CNS for interpretation
• Mechanoreceptors


  • The closer the receptors are together, the greater the tactile acuity of that region
• Two-point discrimination test


  Ability of a person to perceive two fine points pressed against the skin, used to determine tactile acuity
• Warm receptors


  Activated at ~40 C
• Cold receptors


  • Activated at temperatures below 25 C
  • Also activated at very hot temperatures (>45 C), known as paradoxical cold
• Both warm and cold receptors display a rapid adaptive response
• Gustation


  • Lined by a stratified squamous epithelium
  • Raised bumps called papillae contain the structures for gustatory transduction
  • Within the papillae are taste buds that contain specialized gustatory receptor cells for the transduction of taste stimuli

  
• Salty taste


  Perception of Na+ ions in the saliva
• Sour taste


  Perception of H+ concentration
• Bitter taste


  Result from food molecules binding to a G protein–coupled receptor, one major group are alkaloids
• Sweet taste


  Sensitivity of gustatory cells to the presence of glucose dissolved in the saliva
• Umami taste


  Molecule that activates this receptor is the amino acid L-glutamate, perceived while eating protein-rich foods
• Olfactory neurons


  • Transmit impulses along their axons whose ends become enlarged olfactory bulbs
  • Connect with association neurons to the olfactory cortex in the temporal and frontal lobes

  
• Organs of Corti


  • Contain hair cells with hair-like stereocilia extending from the cell's apical surfaces
  • When stereocilia bend toward the tallest member, tension in the protein tethers opens ion channels, depolarizing the hair cell and triggering nerve impulses
  • When stereocilia bend toward the shortest member, the tension slackens and the ion channels close
  • When no sound is present, a small amount of tension still exists on the tethers, keeping the membrane potential of the hair cell slightly depolarized
• Vision


  • Based on the transduction of light stimuli received through the eyes
  • Movement of the eye within the orbit is accomplished by the contraction of six extraocular muscles
  • Light and color perception are detected by the retina within the eye
• Four of the extraocular muscles are arranged at the cardinal points around the eye and are named for those locations
• The tendon of the oblique muscles threads through a pulley-like piece of cartilage known as the trochlea
• The levator palpebrae superioris is responsible for elevating and retracting the upper eyelid, a movement that usually occurs in concert with elevation of the eye by the superior rectus