1. CHAPTER 29 THE SENSES

2. PROCESSING SENSORY PERCEPTION The sensory nervous system tells the CNS what is happening. Sensory neurons carry impulses to the CNS from special cells, called sensory receptors, that detect changes outside and inside the body. Particularly complex sensory receptors, made up of many cells and tissue types, are called sensory organs.

3. PROCESSING SENSORY PERCEPTION The path of sensory information to the CNS is a simple one composed of three stages: 1.Stimulation is when a stimulus impinges on a sensory receptor. 2.Transduction is the conversion of the stimulus to an electrical potential, or nerve impulse. 3.Transmission by a sensory neuron conducts the impulse along an afferent pathway to the CNS.

4. PROCESSING SENSORY PERCEPTION All sensory receptors are able to initiate nerve impulses by opening or closing stimulus-gated channels. Except for visual photoreceptors, these channels are sodium ion channels. Exteroreceptors sense stimuli that arise in the external environment. Interoreceptors sense stimuli that arise from within the body.

5. PROCESSING SENSORY PERCEPTION The body uses a variety of interoreceptors to respond to different aspects of its internal environment. temperature change blood chemistry pain muscle contraction blood pressure touch Skeletal muscle fiber Specialized muscle fibers Connective tissue sheath Nerve endings Sensory nerve fiber (stretch receptor)

6. SENSING GRAVITY AND MOTION Sensory receptors that detect gravity are hair cells in the inner ear The tips of the hair cells project into a gelatinous matrix with embedded particles called otoliths. The otoliths shift in the matrix in response to the pull of gravity, stimulating hair cells. Sensory receptors in the semicircular canals of the inner ear detect changes in motion.

7. HOW THE INNER EAR SENSES GRAVITY AND MOTION 1 3 Flow of fluid Direction of body movement Stimulation Cilia of hair cells Hair cells Supporting cell Sensory nerve fibers Saccule Utricle 2 Otoliths Gelatinous matrix Hair cells Supporting cells Nerve Fluid Cupula Semicircular canals

8. SENSING CHEMICALS: TASTE AND SMELL Embedded within the surface of the tongue are taste buds that contain many chemical receptors. Chemicals from food dissolve in saliva and contact the taste cells The tastes perceived are salty, sour, sweet, bitter, and umami (a “meaty taste”). The “hot” sensation of foods, such as chili peppers, is detected by pain receptors, not chemical receptors. Taste papillae Taste bud Support cell Taste pore Receptor cell with microvilli Nerve fiber

9. SENSING CHEMICALS: TASTE AND SMELL In the nose, chemically sensitive neurons are embedded within the epithelium of the nasal passage. Basal cell Support cell Nasal passage Axon Receptor cell Cilia Olfactory nerveOlfactory mucosa

10. SENSING SOUNDS: HEARING When you hear a sound, you are detecting the air vibrating, as waves of pressure push the eardrum membrane in and out. On the other side of the eardrum are three small bones, called ossicles, that transfer the vibration to the inner ear fluid. Sound receptors within the cochlea of the inner ear interpret sound as changes in fluid that move a sensitive membrane. The membrane vibrates differently according to sounds of different frequencies. Membrane supporting hair cells Hair cells Branch of auditory nerve Membrane covering hair cells Fluid-filled canals Ear canal Eustachian tube Eardrum Auditory nerve Semicircular canals Ossicles Cochlea

11. SENSING SOUNDS: HEARING A lateral line system provides a sense of “distant touch”. Vibrations carried through the fish’s environment travel down a longitudinal canal and other canals in the fish’s skin. These vibrations produce movements of cupula that contain hair cells, causing the hair cells to bend. This stimulates sensory neurons. Lateral line Opening Canal Cilia Cupula Sensory nerves Afferent axons Nerve Lateral line organ Lateral line scales Hair cell

12. SENSING SOUNDS: HEARING Some mammals perceive distance by means of sonar. Using echolocation, they emit sounds and then determine the time it takes these sounds to reach an object and return. Examples of mammals that use sonar are bats, shrews, whales, and dolphins.

13. SENSING LIGHT: VISION The perception of light is carried out by a special sensory apparatus called an eye. Eyes contain sensory receptors, called photoreceptors, that capture light energy. Many invertebrates have simple visual systems with photoreceptors clustered in an eyespot that can perceive the direction of light but cannot form an image. Light Photoreceptors Eyespot Pigment layer Flatworm will turn away from light

14. SENSING LIGHT: VISION Well-developed, image-forming eyes have evolved in four animal phyla: annelids mollusks arthropods vertebrates These are examples of convergent evolution although all use the same type of light- capturing molecule.

15. EYES IN THREE PHYLA OF ANIMALS Lenses Retinular cell InsectVertebrateMollusk Optic nerve Retina Lens Optic nerve Eye muscles Optic nerve (left): © David M. Dennis; (middle, right): © Corbis RF

16. SENSING LIGHT: VISION The vertebrate eye works like a lens-focused camera. Light first passes through a transparent covering called the cornea. a lens helps to focus the light from the cornea to the rear of the eye. The shape of the lens can be adjusted by ciliary muscles. The iris, located between the cornea and the lens, acts as a shutter to control the amount of light that enters the eye. The pupil is the transparent zone in the middle of the iris. The retina is an array of photoreceptors in the back of the eye.

17. THE STRUCTURE OF THE HUMAN EYE Sclera Retina Suspensory ligament Iris Ciliary muscle Lens Cornea Pupil Artery Fovea Optic nerve Choroid Vein

18. SENSING LIGHT: VISION The retina is the light-sensing portion of the eye and contains two kinds of photoreceptors: Rods are very sensitive to light intensity but do not detect color or produce sharp images. Cones can detect color and produce sharp images. The center of the vertebrate retina contains a tiny pit, called the fovea, that is densely packed with cones and produces the sharpest image. Pigment discs Outer segment Connecting cilium Inner segment Mitochondria Nucleus Synaptic terminal RodCone

19. SENSING LIGHT: VISION Three kinds of cone cells provide us with color vision. Each possesses a different version of the opsin protein, which affects the wavelength of light absorbed by retinal. There are three colors absorbed by these different cone cells: blue, green, and red. Light absorption (percent of maximum) 400 Wavelength (nm) 700 600 500 20 100 80 60 40 Blue cones 420 nm Red cones 560 nm Green cones 530 nm Rods 500 nm

20. SENSING LIGHT: VISION Color blindness occurs when individuals are not able to perceive all three colors. It typically occurs due to an inherited lack of one or more types of cones. It is a sex-linked trait, so men are more likely to be colorblind than women.

21. SENSING LIGHT: VISION In primates and most predators the image each eye sees is slightly different because each eye views the object from a different angle. This slight displacement permits binocular vision, the ability to perceive 3-D images and to sense depth or the distance to an object. Other animals, such as prey animals, have eyes located on the sides of the head, enlarging the overall visual field but preventing binocular vision.

22. OTHER VERTEBRATE SENSES Vertebrates can sense their environment via parts of the electromagnetic spectrum other than visible light. Heat Pit vipers possess a pair of heat-detecting pit organs located on either side of the head between the eye and the nostril. Inner chamber Pit Outer chamber Membrane

23. OTHER VERTEBRATE SENSES Electricity Elasmobranches (sharks, rays, and skates) have electroreceptors. Magnetism Eels, sharks, bees, and many birds can navigate along the magnetic field lines of the earth.