Senses

• Sense organs


  • General
  • Special
• General sense organs


  • Often exist as individual cells or receptor units
  • Widely distributed throughout the body
• Special sense organs


  • Large and complex organs
  • Localized grouping of specialized receptors
• Sensory receptor types


  • Encapsulated
  • Unencapsulated ("free" or "naked")
• Classification of senses by type of stimuli required to activate receptors

  • Photoreceptors (light)
  • Chemoreceptors (chemicals)
  • Pain receptors (injury) (nociceptors)
  • Thermoreceptors (temperature change)
  • Mechanoreceptors (movement or deforming of capsule)
  • Proprioceptors (position of body parts or changes in muscle length or tension)
• Sensory pathways


  • All sense organs have common functional characteristics:
  • They are able to detect a particular stimulus
  • A stimulus is converted into a nerve impulse
  • A nerve impulse is perceived as a sensation in the central nervous system (CNS)
• General senses


  • Distribution is widespread; single-cell receptors are common
• General sense receptors


  • Free nerve endings (pain, temperature, crude touch)
  • Tactile (Meissner) corpuscles (fine touch)
  • Bulbous (Ruffini) corpuscles (touch, pressure)
  • Lamellar (Pacini) corpuscles (pressure, vibration)
  • Bulboid corpuscles (Krause end bulbs) (touch)
  • Golgi tendon receptors (important proprioceptors)
  • Muscle spindles (important proprioceptors)
• Merkel's disc


  Receptors for light, sustained touch
• Hair receptor


  Receptors for hair movement and very gentle touch
• Pacinian corpuscle


  Receptors for vibrations and deep pressure
• Ruffini endings


  Receptors for deep pressure
• Meissner's corpuscle


  Receptors for light, fluttering touch
• Special senses: Vision


  • Eyes capture patterns of illumination in the environment as an "optical picture" on a layer of light-sensitive cells, the retina
  • Coded image on the retina is transmitted through the steps of visual processing until it is finally consciously perceived
• Eye


  Composed of an optical component, which focuses the visual image on the receptor cells, and a neural component, which transforms the visual image into a pattern of graded and action potentials
• Components of the eye

  • Sclera
  • Cornea
  • Conjunctiva
  • Choroid
  • Lens
  • Retina
  • Aqueous humor
  • Vitreous humor
  • Iris
  • Pupil
  • Ciliary body
  • Optic nerve
• Sclera


  Tough outer coat; "white" of eye; cornea is transparent part of sclera over iris
• Cornea


  Transparent circle on the anterior of the fibrous layer; "window of the eye"
• Conjunctiva


  Mucous membrane covering the front surface of the sclera and also lines the eyelid; kept moist by tears found in the lacrimal gland
• Choroid


  Pigmented vascular layer prevents scattering of light; front part of this layer made of ciliary muscle and iris, the colored part of the eye; the pupil is the hole in the center of the iris; contraction of iris muscle dilates or constricts pupil
• Lens


  Transparent body behind the pupil; focuses light rays on the retina
• Retina


  Innermost layer of the eye; contains rods (receptors for night vision) and cones (receptors for day vision and color vision)
• Aqueous humor


  In anterior chamber in front of the lens
• Vitreous humor


  In posterior chamber behind the lens
• Iris


  Thin, pigmented, smooth muscle; forms a visible ringlike structure within the aqueous humor; controls the amount of light entering the eye
• Pupil

  The hole in the center of the iris
• Ciliary body


  Contains the ciliary muscle that regulates the shape of the lens for accommodation
• Optic nerve


  Transmits visual information from the retina to the brain
• Photoreceptors

  Rods (receptors for night vision) and cones (receptors for day vision and color vision)
• Phototransduction

  Process by which retinal cells convert light stimuli into neural signals
• Rods


  Provide indistinct gray vision at night; have high sensitivity; provide vision in shades of gray
• Cones

  Provide sharp color vision during the day; have lower sensitivity; provide color vision
• Color vision


  Depends on the ratios of stimulation of the three cone types, each of which is most effectively activated by a particular wavelength of light
• Visual information processing


  Visual information is modified and separated before reaching the visual cortex; the thalamus and visual cortex elaborate the visual message
• Color vision


  Depends on the ratios of stimulation of the three cone types
• Cone types


  • Each cone type is most effectively activated by a particular wavelength of light in the range of color indicated by its name
• Eye sensitivity


  The sensitivity of the eyes can vary markedly through dark and light adaptation
• Visual information processing


  1. Visual information is modified and separated before reaching the visual cortex

  2. The thalamus and visual cortex elaborate the visual message
• Parts of the eye

  • Right eye
  • Left eye
  • Optic nerve
  • Optic chiasm
  • Lateral geniculate nucleus
  • Optic tract
  • Visual cortex
  • Occipital lobe
• Visual field


  • Monocular zone is the portion of the visual field associated with only one eye
  • Binocular zone is where left and right visual fields overlap