Lecture 21: Sensory Systems

• Sensory Systems

  • Taste and Smell (chemosensory systems)
  • Touch, Hearing, and Balance (mechanoreceptor systems)
  • Sight (electromagnetic receptor systems)
• Sensory Receptor Cells

  • Rely on membrane receptors
  • Imbedded in cells that communicate with neurons (taste and sight)
  • Or membrane receptors are embedded directly in neuron membranes (smell)

  
• In most multicellular animals, the receptor cells are organized into sensory organs
• Sensory Transduction

  • Conversion of physical or chemical stimuli into nerve impulse
  • Initial transformation at the sensory receptor
• Sensory Transduction

  1. Stimulated membrane sensory receptor causes ion channels in the plasma membrane to open

  2. Sensory receptors then either fire action potentials themselves or synapse with neurons that do

  3. Signals interpreted in the CNS (Perception)
• Chemoreceptors

  • Provide the sense of smell and taste
  • Respond to molecules in the environment (or our mouth) that bind to receptors in the cell membrane
• Chemoreceptors - Smell

  • Chemosensitive projections of neurons extend into the mucus that line the upper part of our nasal passage

  
• Chemoreceptors - Taste

  • Taste buds on our tongue are specialized cells that synapse with sensory neurons
• Mechanoreceptors

  • Respond to physical deformation of the plasma membrane
  • Opens a Na+ ion channel, causing an action potential to fire
  • Responsible for our sense of touch
• Mechanoreceptors

  • Merkel's disks respond to light touch
  • Meissner's corpuscles respond to touch and low-frequency vibration
  • Ruffini endings detect stretch, deformation within joints, and warmth
  • Pacinian corpuscles detect transient pressure and high-frequency vibration

  
• Hair Cells

  • Specialized mechanoreceptors that sense mechanical vibration using Stereocilia on their surface
  • Do not fire action potentials, but do synapse with neurons
  • Stimulation of hail cells alters the rate at which neurons fire
• Hearing

  1. Sound vibrations enter outer ear and vibrate the tympanic membrane

  2. The tympanic membrane then moves the bones in the middle ear (incus, malleus, and stapes)

  3. Middle ear bones then vibrate the oval window in the cochlea

  
• Hair Cells - Balance

  • The vestibular system in the inner ear detects gravity, acceleration, and deceleration using 3 semicircular canals
  • Each semicircular canal detects angular momentum in one plane that the head can turn (nodding up-down, turning left or right, or moving side to side)
  • When the body moves, fluid in the canals move hair cells, and the hair cell movement is converted into nerve impulses

  
• Electromagnetic Receptors

  • Respond to electrical, magnetic, and light stimuli
  • Photoreceptors are the most common
• Opsin

  Light-sensitive protein that converts light energy into electrical signals
• Opsin - Photoreceptors

  1. Opsin protein contains a pigment called Retinal

  2. When light energy strike retinal, it shifts from a cis- to trans- isomer configuration

  3. This shift in configuration opens Na+ channels to depolarize the cell
• Animal Eyes

  • Eyecups - only detect light intensity and direction
  • Compound Eyes - each lens is called an ommatidium. The more ommatidia, the greater the visual resolution. Predatory insects have far more than herbivores.
  • Single-lens Eyes - focus an inverted image on the retina, a thin, photoreactive tissue in the back of the eye

  
• Rod cells

  Blue-green sensitive; the brain interprets this as black-and-white vision
• Cone cells

  Provide color vision