Sensory Organs

Pain:

Definition

  • discomfort induced by tissue injury or noxious stimulation

Receptors

  • nociceptors = respond to chemicals released by injured tissues
  • location = skin, mucous membranes and nearly all organs (except brain)

General types

  • somatic pain = skin, muscles, joints
  • visceral = internal organs of body cavity

Specific types

  • Referred pain = where viscera pain is mistaken to come from skin
  • Phantom pain = pain coming from a limb that has been amputated
  • Spinal gating = absence or minimal sense of pain

Gustation:

Definition:

  • Begins with chemical stimulation of sensory cells

Location:

  • Tongue, inside the cheeks, on the palate, pharynx and epiglottis

Structure:

  • ovoid clusters, banana-shaped taste cells, nonsensory supporting cells and unspecialised basal cells

Taste cells:

  • microvilli (taste hairs)
  • taste hairs project into taste pore (apex of taste bud)
  • epithelial cells

Primary taste sensations:

1) Salty

  • produced by metal ions (Na & K)

2) Sweet

  • produced by sugars and organic compounds

3) Sour

  • produced by acids

4) Bitter

  • produced by organic alkaloid compounds (i.e. nicotine or caffeine)

5) Umami

  • meaty taste, produced by amino acids such as aspartic

Olfaction:

Definition:

  • the sense of smell, in response to airborne chemicals

Structure:

  • patch of sensory epithelium (olfactory mucosa)

Olfactory cells:

  • true neurons
  • swollen tip bearing 10-20 cilia (olfactory hairs)
  • bulbous body containing the nucleus
  • basal end tapers to thin axon leading to the brain

Steps:

1) Odor molecule diffuses through mucus on surface of olfactory epithelium

2) odor binds to one of the hairs on a receptor cell

3) Depolarization of the cell = generating action potential in axon

Further structures:

  • olfactory nerve = multiple olfactory cells converged to form little bundles
  • cranial nerve penetrate through pores in cribriform plate in ethmoid plate
  • nerve fibers end in a pair of olfactory bulbs of the brain
  • olfactory tracts = neurons form bundles of fibers. relay signals to multiple locations in the cerebrum and brainstem, especially primary olfactory cortex