Concept 35.1 Sensory Systems Convert Stimuli into Action Potentials
- Sensory receptor cells, also known as sensors or receptors, transduce information about an animal's external and internal environment into action potentials.
- Receptor potentials can spread to regions of the cell's plasma 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 35.1
- Sensory receptor cells have receptor proteins that cause ion channels to open or close, affecting the cells' membrane potential. This causes a graded membrane potential called a receptor potential. Review Figure 35.2
- The interpretation of action potentials as particular sensations depends on which neurons in the CNS receive them.
- Adaptation enables the nervous system to ignore irrelevant or continuous stimuli while remaining responsive to relevant or new stimuli.
Concept 35.2 Chemoreceptors Detect Specific Molecules or Ions
- Chemoreceptors are responsible for olfaction, gustation, and the sensing of pheromones.
- Mammalian olfactory sensors project directly to the olfactory bulb of the brain. Sensors for the same odorant project to the same area of the olfactory bulb.
- Each olfactory receptor cell expresses one receptor protein that can bind to a set of similar odorant molecules. Review Figure 35.3
- Pheromones are detected with chemoreceptors that are usually separate from the main olfactory system. In vertebrates, pheromones are detected with the vomeronasal organ. Review Figure 35.4
- 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 35.5
Concept 35.3 Mechanoreceptors Detect Physical Forces
- The skin contains a variety of mechanoreceptors that respond to touch and pressure. Review Figure 35.6
- Stretch receptors in muscle spindles and Golgi tendon organs inform the CNS of the loads on muscles and tendons. Review Figure 35.7 and INTERACTIVE TUTORIAL 35.1
- Hair cells are mechanoreceptors in the mammalian inner ear. Physical bending of their stereocilia alters their membrane potentials. Hair cells are found in the cochlea and semicircular canals and function in the auditory and vestibular systems. Review Figures 35.8 and 35.9
- In mammalian auditory systems, ear pinnae collect and direct sound waves into the auditory canal, where the tympanic membrane 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 Figure 35.9 and WEB ACTIVITY 35.1
- 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 waves, generating information we interpret as the pitch of sound. These distortions cause the bending of hair cells in the organ of Corti. Review Figures 35.9 and 35.10 and ANIMATED TUTORIAL 35.1
Concept 35.4 Photoreceptors Detect Light
- Photosensitivity depends on the absorption of photons of light by rhodopsin, which consists of an opsin protein and a nonprotein light-absorbing group called 11-cis-retinal. Absorption of light by 11-cis-retinal leads to a change in the membrane potential of the photoreceptor cell. Review Figure 35.12 and ANIMATED TUTORIAL 35.2
- When excited by light, vertebrate photoreceptor cells hyperpolarize and release less neurotransmitter. They do not fire action potentials. Review Figures 35.13 and 35.14 and WORKING WITH DATA 35.1
- 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, which focus detailed images onto dense arrays of photoreceptors. Review Figure 35.15
- The vertebrate retina consists of five layers of neurons lining the back of the eye, including the light-absorbing photoreceptor cells at the back. Extensive processing of visual information occurs in the retina and also in the brain. Review Figure 35.16, WEB ACTIVITIES 35.2 and 35.3, and INTERACTIVE TUTORIAL 35.2
- Vertebrates have two types of photoreceptors, rod cells and cone cells. In humans, the fovea contains almost exclusively cone cells, which are responsible for color vision but are not very sensitive in dim light. Color vision in humans arises from three types of cone cells with different spectral absorption properties. Review Figure 35.17