46.1 Animals employ a wide variety of sensory receptors.
Categories of Sensory Receptors and Their Actions
• Sensory information is conveyed and perceived in a four-step process: stimulation, transduction, transmission, and interpretation. (p. 970)
• Exteroceptors sense stimuli that arise in the external environment, and interoceptors sense stimuli that arise from within the body. (pp. 970-971)
• The greater the sensory stimulus, the greater the depolarization of the receptor potential and the higher the frequency of action potentials. (p. 972)
46.2 Mechanical and chemical receptors sense the body's condition.
Detecting Temperature and Pressure
• Cutaneous receptors respond to stimuli at the border between the external and internal environments. (p. 973)
• Thermoreceptors are naked dendritic endings sensitive to changes in temperature. (p. 973)
• Nociceptors are free nerve endings that perceive tissue damage as pain. (p. 973)
• Mechanoreceptors contain sensory cells with ion channels sensitive to a mechanical force. (p. 973)
• Pacinian corpuscles are phasic, pressure-sensitive receptors in the subcutaneous tissue. (p. 973)
Sensing Muscle Contraction and Blood Pressure
• Proprioceptors are sensory receptors that provide information about the relative position or movements of body parts. (p. 974)
• Distortion of mechanoreceptors produces nerve impulses that monitor muscle length (p. 974)
• Baroreceptors monitor blood pressure by detecting tension in blood vessel walls. (p. 974)
Sensing Taste, Smell, and Body Position
• Chemoreceptors contain membrane proteins that can bind to particular chemicals or ligands in the extracellular fluid. (p. 975)
• The taste buds of terrestrial vertebrates are located within the epithelium of the tongue within papillae, while the sense of smell involves chemoreceptors in the upper portions of the nasal passage. (pp. 975)
• Internal chemoreceptros monitor blood and blood fluids. (p. 976)
• The lateral line system in fish consists of sensory structures within a longitudinal canal filled with hair cells that detect water movements. (p. 977)
• Hair cells in the inner ear of terrestrial vertebrates detect gravity, and hair cells in the vestibular apparatus sense angular acceleration and balance. (pp. 977-979)
46.3 Auditory receptors detect pressure waves in the air.
The Ears and Hearing
• Auditory stimuli travel farther and more quickly than chemical stimuli, and auditory receptors provide better directional information than chemoreceptors. (p. 980)
• Vibrations of the tympanic membrane cause movement of ossicles in the middle ear, creating pressure waves in the fluid-filled chambers. (p. 980)
• Cilia from hair cells bend in response to movement, depolarizing hair cells and stimulating the production of action potentials in sensory neurons projecting to the brain where they are interpreted as sound. (pp. 980-982)
• Many mammals use echolocation as a means of perceiving distance. (p. 982)
46.4 Optic receptors detect light over a broad range of wavelengths.
Evolution of the Eye
• Four phyla (annelids, mollusks, arthropods, and chordates) have evolved image-forming eyes. (p. 983)
• The eyes of vertebrates admit light through the pupil and focus the light with an adjustable lens onto the retina. (pp. 983-984)
Vertebrate Photoreceptors
• The vertebrate retina contains two types of photoreceptors: rods, which are responsible for black and white vision, and cones, which are responsible for sharpness and color vision. (p. 985)
• Light inhibits photoreceptors from releasing an inhibitory neurotransmitter, thus stimulating bipolar cells and ganglion cells that transmit action potentials to the brain. (p. 986)
Visual Processing in the Vertebrate Retina
• Action potentials propagated along ganglion cells are relayed through the thalamus and projected to the occipital lobe of the brain. (p. 987)
• The frequency of impulses in each ganglion cell provides information about light intensity, while the relative activity of ganglion cells connected with the three types of cones provides color information. (p. 987)
• Color blindness is due to an inherited lack of at least one type of cone. (p. 987)
• Binocular vision occurs when a displacement of images (parallax) allows the perception of three-dimensional images and depth. (p. 987)
46.5 Some vertebrates use heat, electricity, or magnetism for orientation.
Diversity of Sensory Experiences
• Pit vipers sense heat with a pair of pit organs located on either side of the head. (p. 988)
• All aquatic animals generate electrical currents in their muscles, and some groups can detect electrical currents. (p. 988)
• Many animals, including eels, sharks, bees, and many birds, use the earth's magnetic fields for navigation. (p. 988)