Chapter Summary

• Sensory receptor cells, also known as sensors or receptors, transduce information about an animal’s external and internal environment into action potentials that the brain perceives as different forms of sensory information.

• Receptor potentials can spread to regions of the cell membrane that generate action potentials. Some sensors do not fire action potentials but release neurotransmitter onto sensory neurons that do fire action potentials. Review Figure 45.1

• Sensors have receptor proteins that cause ion channels to open or close, affecting the receptor cell’s membrane potential. Some receptors open ion channels physically through forces such as pressure or stretch. Other receptors act through signal transduction pathways to generate receptor potentials. A large family of genes called transient receptor potential (TRP) genes is responsible for many modalities of sensory transduction. Review Figure 45.2, Investigating Life: How Do Pit Vipers “See” in the Dark?

• The interpretation of action potentials as particular sensations depends on which neurons in the central nervous system receive them.

• Adaptation enables the nervous system to ignore irrelevant or continuous stimuli while remaining responsive to relevant or new stimuli.

• Chemoreceptors are responsible for olfaction, gustation, and the sensing of pheromones.

• Mammalian olfactory receptor neurons (ORNS) project directly to the olfactory bulb of the brain. ORNs for the same odorant project to the same area of the olfactory bulb.

• Each ORN expresses one receptor protein that can bind a specific type of molecule or ion. Binding causes a second messenger to open ion channels, which creates an action potential. Review Figure 45.3

• Pheromones are chemicals that communicate information among individuals of the same species.

• In vertebrates, taste buds in the mouth cavity are responsible for gustation. The five basic tastes are sweet, salty, sour, bitter, and umami. Review Figure 45.5

• The skin contains a variety of mechanoreceptors that respond to touch and pressure. The density of mechanoreceptors in any skin area determines the sensitivity of that area. Review Figure 45.6

• Stretch receptors in muscle spindles and in Golgi tendon organs inform the CNS of the positions of and loads on parts of the body. Review Figure 45.7

• Hair cells are mechanoreceptors of the auditory and vestibular systems. Physical bending of the hair cells’ stereocilia alters their receptor proteins and therefore their membrane potentials. Review Figure 45.8

• In mammalian auditory systems, ear pinnae collect and direct sound waves to the tympanic membrane, which vibrates in response to sound waves. The movements of the tympanic membrane are amplified through a chain of ossicles that conduct the vibrations to the oval window. Movements of the oval window create pressure waves in the fluid-filled cochlea. Review Focus: Key Figure 45.9, Activity 45.1

• The basilar membrane running down the center of the cochlea is distorted by pressure waves at specific locations that depend on the frequency of the wave. These distortions cause hair cells in the organ of Corti to bend and to release neurotransmitter, generating action potentials in the cochlear nerve that are transmitted to the auditory cortex of the brain. Review Figure 45.10, Animation 45.1

• Hair cells are also the mechanoreceptors of the organs of equilibrium in the mammalian vestibular system, which include the semicircular canals and the saccule and utricle. Review Figure 45.11, Activity 45.2

• Visual systems range from the simple eye cups of flatworms, which sense the direction of a light source, to the compound eyes of arthropods, which detect shapes and patterns, to the image-forming eyes of vertebrates and cephalopods. Review Figures 45.12, 45.13

• Vertebrate and cephalopod eyes focus detailed images of the visual field onto dense arrays of photoreceptors that transduce the visual image into neural signals. Review Figures 45.13, 45.14, Activity 45.3

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• Photosensitivity in animals depends on the absorption of photons of light by the visual pigment opsin, which contains a light absorbing group called 11-cis-retinal. Absorption of light is the first step in a cascade of intracellular events leading to a change in the membrane potential of the photoreceptor cell. Review Figures 45.15, 45.18, Animation 45.2

• Vertebrates have two types of photoreceptors, rod cells and cone cells. Rod cells are more sensitive to light and are responsible for dim light vision. Cone cells are less sensitive to light but are responsible for high-acuity and color vision.

• Photoreceptors do not fire action potentials. Light hyperpolarizes rod cells, and their release of neurotransmitter decreases. Review Figure 45.17

• Rhodopsin is the visual pigment of rod cells. The visual pigments of cone cells have three different opsin components, which gives them different spectral sensitivities. Review Figure 45.19

• The vertebrate retina consists of layers of neurons lining the back of the eye. The light-absorbing photoreceptor cells are at the rear of the retina. The axons of the ganglion cells are bundled together in the optic nerves. Between the photoreceptors and the ganglion cells are neurons that process information from the photoreceptors. Review Figure 45.20, Activity 45.4