Chapter 9 The Senses

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Chapter 9The Senses
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CLASSIFICATION OF SENSE ORGANS

• General sense organs (Table 9-1)
• Often exist as individual cells or receptor units
• Widely distributed throughout the body
• Special sense organs (Table 9-2)
• Large and complex organs
• Localized grouping of specialized receptors

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CLASSIFICATION OF SENSE ORGANS

• Classification by presence or absence of covering
  capsule
• Encapsulated
• Unencapsulated (free or naked)
• Classification by type of stimuli required to
  activate receptors
• Photoreceptors (light)
• Chemoreceptors (chemicals)
• Pain receptors (injury)
• Thermoreceptors (temperature change)
• Mechanoreceptors (movement or deforming of
  capsule)
• Proprioceptors (position of body parts or changes
  in muscle length or tension)

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CONVERTING A STIMULUS INTO A SENSATION

• All sense organs have common functional
  characteristics
• All 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

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GENERAL SENSE ORGANS (Table 9-1)

• Distribution is widespread single-cell receptors
  are common
• Examples (Figure 9-1, Table 9-1)
• Free nerve endingspain, temperature, and crude
  touch
• Meissners corpusclesfine touch and vibration
• Ruffinis corpusclestouch and pressure
• Pacinian corpusclespressure and vibration
• Krauses end-bulbstouch
• Golgi tendon receptorsproprioception
• Muscle spindlesproprioception

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SPECIAL SENSE ORGANS

• The eye (Figure 9-2)
• Layers of eyeball
• Scleratough outer coat white of eye cornea
  is transparent part of sclera over iris
• Choroidpigmented 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
• Retina (Figure 9-4)innermost layer of the eye
  contains rods (receptors for night vision) and
  cones (receptors for day vision and color vision)
• Conjunctivamucous membrane covering the front
  surface of the sclera and also lines the eyelid
  kept moist by tears found in the lacrimal gland
• Lenstransparent body behind the pupil focuses
  light rays on the retina

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SPECIAL SENSE ORGANS

• The eye (cont.)
• Eye fluids
• Aqueous humorin the anterior chamber in front of
  the lens
• Vitreous humorin the posterior chamber behind
  the lens
• Visual pathway
• Innermost layer of retina contains rods and cones
• Impulse travels from the rods and cones through
  the bipolar and ganglionic layers of retina
  (Figure 9-4)
• Nerve impulse leaves the eye through the optic
  nerve the point of exit is free of receptors and
  is therefore called a blind spot
• Visual interpretation occurs in the visual cortex
  of the cerebrum

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SPECIAL SENSE ORGANS

• The ear
• The ear functions in hearing and in equilibrium
  and balancereceptors called mechanoreceptors
• Divisions of the ear (Figure 9-5)
• External ear
• Auricle (pinna)
• External auditory canal
• Curving canal 2.5 cm (1 inch) in length
• Contains ceruminous glands
• Ends at the tympanic membrane