CHAPTER 45 Sensory Systems

Chapter 45: Sensory Systems Sensory Cells, Sensory Organs, and Transduction Chemoreceptors: Responding to Specific Molecules Mechanoreceptors: Detecting Stimuli that Distort Membranes

Chapter 45: Sensory Systems Photoreceptors and Visual Systems: Responding to Light Sensory Worlds Beyond Our Experience

Sensory Cells, Sensory Organs, and Transduction Sensory cells transduce information about an animal’s external and internal environment into action potentials. Review Figures 45.1, 45.2 4

figure 45-01a.jpg Figure 45.1 – Part 1 Figure 45.1 – Part 1

figure 45-01b.jpg Figure 45.1 – Part 2 Figure 45.1 – Part 2

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Sensory Cells, Sensory Organs, and Transduction The interpretation of action potentials as particular sensations depends on which neurons in the CNS receive them. 8

Sensory Cells, Sensory Organs, and Transduction Membrane receptor proteins of sensory cells cause ion channels to open or close, generating receptor potentials. Receptor potentials can spread to regions of the sensory cell plasma membrane that generate action potentials, or influence release of neurotransmitter from the sensory cell. Review Figure 45.3 9

figure 45-03.jpg Figure 45.3 Figure 45.3

Sensory Cells, Sensory Organs, and Transduction Adaptation enables the nervous system to ignore irrelevant stimuli while remaining responsive to relevant or to new stimuli. 11

Chemoreceptors: Responding to Specific Molecules Smell, taste, and the sensing of pheromones are examples of chemosensation. Chemoreceptor cells have receptor proteins that can bind to specific molecules that come into contact with the sensory cell membrane. Review Figures 45.5, 45.6 12

figure 45-05.jpg Figure 45.5 Figure 45.5

Chemoreceptors: Responding to Specific Molecules Binding of an odorant molecule to a receptor protein causes production of a second messenger in the chemoreceptor cell. The second messenger alters ion channels and creates a receptor potential. 15

Chemoreceptors: Responding to Specific Molecules Chemoreceptors in the mouth cavities of vertebrates are responsible for the sense of taste. Review Figure 45.6 16

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Mechanoreceptors: Detecting Stimuli that Distort Membranes The skin has a diversity of mechanoreceptors that respond to touch and pressure. The density of mechanoreceptors in any skin area determines the sensitivity of that area. Review Figure 45.7 18

figure 45-07.jpg Figure 45.7 Figure 45.7

Mechanoreceptors: Detecting Stimuli that Distort Membranes Stretch receptors in muscles, tendons, and ligaments inform the CNS of the positions of and the loads on parts of the body. Review Figure 45.8 20

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Mechanoreceptors: Detecting Stimuli that Distort Membranes Hair cells are mechanoreceptors that are not neurons. Bending of their stereocilia alters their membrane proteins and therefore their receptor potentials. Hair cells are found in organs of equilibrium and orientation. Review Figures 45.9, 45.10, 45.11 22

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figure 45-11a.jpg Figure 45.11 – Part 1 Figure 45.11 – Part 1

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Mechanoreceptors: Detecting Stimuli that Distort Membranes Hair cells are responsible for mammalian auditory sensitivity. Ear pinnae collect and direct sound waves to the tympanic membrane, which vibrates in response to sound waves. The vibrations are amplified through a chain of ossicles that conduct them to the oval window. Movements of the oval window create pressure waves in the fluid-filled cochlea. Review Figure 45.12, 45.13 29

figure 45-12a.jpg Figure 45.12 – Part 1 Figure 45.12 – Part 1

figure 45-12b.jpg Figure 45.12 – Part 2 Figure 45.12 – Part 2

figure 45-12c.jpg Figure 45.12 – Part 3 Figure 45.12 – Part 3

Mechanoreceptors: Detecting Stimuli that Distort Membranes The basilar membrane running down the center of the cochlea is distorted at specific locations depending on the frequency of the pressure wave. These distortions cause bending of hair cells in the organ of Corti, which rests on the basilar membrane. Changes in hair cell receptor potentials create action potentials in the auditory nerve, which conducts the information to the CNS. Review Figure 45.13 31

figure 45-13.jpg Figure 45.13 Figure 45.13

Photoreceptors and Visual Systems: Responding to Light Photosensitivity depends on the capture of photons of light by rhodopsin. Rhodopsin is a photoreceptor molecule consisting of opsin and retinal. Absorption of light by retinal is the first step in a cascade of intracellular events leading to a change in the receptor potential of the photoreceptor cell. Review Figure 45.15 33

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Photoreceptors and Visual Systems: Responding to Light When excited by light, vertebrate photoreceptor cells hyperpolarize and release less neurotransmitter onto neurons with which they form synapses. They do not fire action potentials. Review Figures 45.16, 45.17, 45.18 35

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figure 45-17a.jpg Figure 45.17 – Part 1 Figure 45.17 – Part 1

figure 45-17b.jpg Figure 45.17 – Part 2 Figure 45.17 – Part 2

Photoreceptors and Visual Systems: Responding to Light Vision results when eyes focus patterns of light onto layers of photoreceptors. The simple eye cups of flatworms can sense the direction of a light source The compound eyes of arthropods can detect shapes and patterns The eyes of cephalopods and vertebrates have lenses. Review Figures 45.18, 45.19, 45.20 40

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Photoreceptors and Visual Systems: Responding to Light The eyes of vertebrates and cephalopods focus detailed images of the visual field onto dense arrays of photoreceptors that transduce the visual image into neural signals. Review Figure 45.21 43

figure 45-21.jpg Figure 45.21 Figure 45.21

Photoreceptors and Visual Systems: Responding to Light The vertebrate photoreceptors are rod and cone cells. Rod cells are responsible for dim light and black-and-white vision Cone cells are responsible for color vision by virtue of their spectral sensitivities. Review Figure 45.23 45

Photoreceptors and Visual Systems: Responding to Light The vertebrate retina is a dense array of neurons lining the back of the eyeball. It consists of five layers of cells. The outermost layer consists of rods and cones. The innermost layer consists of ganglion cells, which send their axons in the optic nerve to the brain. Between the photoreceptors and ganglion cells are neurons that process information from the photoreceptors. Review Figure 45.24 47

figure 45-24.jpg Figure 45.24 Figure 45.24

Photoreceptors and Visual Systems: Responding to Light The area of the retina that receives light from the center of the visual field, the fovea, has the greatest density of photoreceptors. In humans it contains almost exclusively cone cells, which are responsible for color vision but not very sensitive in dim light. 50

Photoreceptors and Visual Systems: Responding to Light Each ganglion cell is stimulated by light falling on a small circular patch of photoreceptors called a receptive field. Receptive fields have a center and a surround, which have opposing effects on the ganglion cell. If the center is excitatory, the surround is inhibitory, and vice versa. See fig.45.25 51

figure 45-25a.jpg Figure 45.25 – Part 1 Figure 45.25 – Part 1

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Sensory Worlds Beyond Our Experience Many animals have sensory abilities that we do not share. Bats echolocate Insects see ultraviolet radiation Pit vipers “see” infrared radiation Fish sense electric fields. 52