Unit 7.9

• Sensory receptors


  Specialized cells capable of detecting changes in internal or external conditions, and of communicating that information to the central nervous system
• Sensory transduction


  Conversion of an event (stimulus) occurring in the environment into a nerve impulse
• Perceptions


  Any sensory stimuli of which humans, and perhaps other animals, become conscious
• Types of sensory receptors


  • Chemoreceptors
  • Photoreceptors
  • Mechanoreceptors
  • Thermoreceptors
• Chemoreception


  • Found almost universally in all animals
  • Thought to be the most primitive sense
  • Allows organisms to locate food, find a mate, detect dangerous environmental chemicals, etc.
• Taste buds


  • Located primarily on the tongue
  • Open at a taste pore
  • Have supporting cells and elongated taste cells that end in microvilli

  
• Primary tastes


  • Sweet
  • Sour
  • Salty
  • Bitter
  • Umami (savory)

  
• Olfactory cells


  • Located within olfactory epithelium in the roof of the nasal cavity
  • Nerve fibers from olfactory cells lead to the same neuron in the olfactory bulb

  
• Sense of taste and smell


  • Work together to create a combined effect
  • Interpreted by the cerebral cortex

  
• Photoreceptors


  Sensory receptors that are sensitive to light

  
• Arthropod eyes


  • Contain compound eyes composed of many independent ommatidia
  • Photoreceptors generate nerve impulses which pass to the brain by way of optic nerve fibers
  • Insects have limited color vision, some able to see UV

  
• Human eye


  • Three layers: sclera, choroid, retina
  • Cornea, lens, iris, pupil
  • Retina contains rod cells and cone cells
  • Fovea centralis is region of densely packed cone cells where light is focused
  • Optic nerve connects to eye, creating a blind spot

  
• Focusing of the eye


  1. Light rays pass through pupil and are focused on retina

  2. Focusing starts at cornea and continues as rays pass through lens, which provides visual accommodation

  3. Distant object - ciliary muscle is relaxed

  4. Near object - ciliary muscle is contracted

  
• Rods and cones


  • Outer segment joined to inner segment by a stalk
  • Pigment molecules embedded in membrane of disks in outer segment
  • Rhodopsin - pigment composed of opsin and retinal, a derivative of vitamin A
  • Rods permit vision in low light, peripheral vision and motion
  • Cones permit vision in bright light, fine detail and color
  • Three different types of cones, each containing a different type of photopigment: B (blue), G (green), R (red)

  
• Retina


  • Three layers of neurons: rod and cone cells, bipolar cells, ganglion cells
  • Rod and cone cells synapse with bipolar cells, which synapse with ganglion cells responsible for initiating nerve impulses
  • Human retina has approximately 150 million rod cells, 6 million cone cells, 1 million ganglion cells
  • More rod cells synapse on a ganglion cell than cone cells, so cone cells provide sharper more detailed images

  
• Tympanum


  A membrane stimulated to vibrate by sound waves that directly activates nerve impulses in attached receptor cells
• Lateral line system


  Detects water currents and pressure waves from nearby objects, helping the organism guide their movement and locate other fish
• Human ear


  • Outer ear: pinna and auditory canal
  • Middle ear: tympanic membrane, ossicles (malleus, incus, stapes), auditory tube
  • Inner ear: semicircular canals, vestibule, cochlea

  
• Hearing in the human ear


  1. Sound waves enter auditory canal

  2. Strike tympanic membrane, causing it to vibrate

  3. Malleus takes pressure from inner surface of tympanic membrane

  4. Passes it via incus to stapes, multiplying the pressure

  5. Stapes strikes membrane of oval window, passing pressure to fluid within cochlea

  6. Basilar membrane in cochlea vibrates, bending stereocilia of hair cells

  7. Nerve impulses begin in cochlear nerve and travel to brain stem, then auditory cortex

  
• Mechanoreceptors for equilibrium


  • Ampullae of semicircular canals contain hairs that shift when rotational force occurs, sending nerve impulse to vestibular nerve
  • Utricle and saccule contain hair cells that shift in membrane fluid when head bends, sending nerve impulse to vestibular nerve
• Somatic senses


  • Proprioceptors (mechanoreceptors involved in reflex actions that maintain muscle tone)
  • Cutaneous receptors (make skin sensitive to touch, pressure, pain, temperature)
  • Pain receptors (also termed free nerve endings or nociceptors)