The skin detects touch, pressure,
cold, warmth, and pain.
Vertebrate eyes have a lens focusing
light on a sensitive retina.
The outer ear focuses sound onto the tympanic membrane.
The middle ear has 1 to 3 vibrating bones (ossicles).
The inner ear is
filled with fluid and aids in both hearing and balance.
Interoceptors: nerves that detect the status of the body's internal organs.
Proprioceptors: detect the position of the body's muscles and joints.
- Neuromuscular spindles detect the state of contraction of muscles.
- Stretch receptors detect tension in ligaments and tendons.
- Position receptors (in arthropods) detect the angular position of joints.
Exteroceptors: all other receptors, responsive to external conditions.
- Thermoreceptors are sensitive to temperature.
- Mechanoreceptors detect mechanical stimuli (movement,
vibration, sound).
- Photoreceptors are sensitive to light.
- Chemoreceptors are sensitive to chemical stimuli (taste, smell).
Cutaneous sensations: Nerve endings in skin detect 5 different sensations.
- Naked (unencapsulated) nerve endings perceive pain.
- Meissner's corpuscles are sensitive to light touch.
- Pacini corpuscles respond to deep pressure.
- Ruffini corpuscles are sensitive to warmth.
- Krause end bulbs are sensitive to cold.
Smell: The nasal epithelium (olfactory mucosa) is chemically
sensitive to substances in very low concentrations. The sensory cells are
unusual because they grow inward toward the olfactory lobe of the brain.
Taste: The taste buds are sensitive to moderate concentrations
of chemicals of 4-5 kinds: sweet, sour, bitter, salty, and "umami".
Photoreception (vision): Many simple organisms detect light and dark without
any special organs. Some protists, like Euglena, have photosensitive
"pigment spots." Flatworms and many other simple animals have light-sensitive
optic nerves beneath an overlying transparent skin.
Image-forming eyes use various optical principles:
- Compound eyes, as in insects, use an array of receptors to
form an image, like a TV picture, out of many colored pixels.
- Pinhole eyes focus light through a very small hole onto a
sensitive retina.
- Lens-based eyes, as in mollusks and vertebrates, use a
transparent lens to focus an image upon a retina.
Vertebrate eyes include:
- Eyelids: protective folds of skin
- Cornea: transparent layer protecting front of eye
- Lens: focuses light upon retina
- Ciliary body: holds lens in place and controls focus of image
- Iris diaphragm: controls amount of light entering eye
- Aqueous humor: watery fluid in front of lens
- Vitreous humor: glassy fluid behind lens
- Retina: light-sensitive surface, containing rods and cones
- Choroid and scleroid coats: protective layers around eye
- Optic nerve: carries impulses to the brain
Stato-acoustic senses: special forms of mechanoreception
- Lateral line in fishes: sensitive to swimming movements of
other fishes or to waves reflected from obstacles
- Outer ear in mammals: focuses sound waves on tympanic membrane
- Middle ear: contains tiny bones (ossicles) that transmit vibrations.
Mammals have 3 ossicles: hammer (malleus), anvil (incus), and stirrup
(stapes); other land vertebrates have stapes only.
- Inner ear:
- Cochlear portion contains cochlea (coiled in
mammals), including vibration-sensitive organ of Corti.
- Vestibular portion, sensitive to balance and acceleration,
contains semicircular canals.
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