Transcript Chapter 29
THE SENSES CHAPTER 29 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. 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. 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. PROCESSING SENSORY PERCEPTION • The body uses a variety of interoreceptors to respond to different aspects of its internal environment. Sensory • • • • • • temperature change blood chemistry pain muscle contraction blood pressure touch nerve fiber (stretch receptor) Nerve endings Skeletal muscle fiber Specialized muscle fibers Connective tissue sheath 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. Otoliths Gelatinous matrix Hair cells Supporting cells Utricle 1 2 Semicircular canals Saccule Fluid Flow of fluid Nerve Cupula Cilia of hair cells Hair cells HOW THE INNER EAR SENSES GRAVITY AND MOTION Supporting cell Stimulation Sensory nerve fibers 3 Direction of body movement SENSING CHEMICALS: TASTE AND SMELL • Embedded within the surface of the tongue are taste buds that contain many chemical Taste papillae 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 bud Support cell Taste pore Receptor cell with microvilli SENSING CHEMICALS: TASTE AND SMELL • In the nose, chemically sensitive neurons are embedded within the epithelium of the nasal passage. Olfactory nerve Nasal passage Olfactory mucosa Axon Basal cell Support cell Receptor cell Cilia 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. Ossicles Semicircular canals Auditory nerve Cochlea Ear canal Eardrum Eustachian tube Fluid-filled canals Branch of Membrane covering auditory hair cells nerve Hair cells Membrane supporting hair cells 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 Lateral line scales Opening Canal Lateral line organ Nerve Cupula Hair cell Afferent axons Sensory nerves Cilia 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. SENSING LIGHT: VISION • The perception of light Photoreceptors Eyespot is carried out by a Light special sensory apparatus called an eye. Pigment layer • Eyes contain sensory receptors, called photoreceptors, that Flatworm will turn capture light energy. away from light • Many invertebrates have simple visual systems with photoreceptors clustered in an eyespot that can perceive the direction of light but cannot form an image. 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 lightcapturing molecule. EYES IN THREE PHYLA OF ANIMALS Lenses Optic nerve Retina Optic nerve Lens Optic nerve Eye muscles Retinular cell Retina Insect Mollusk (left): © David M. Dennis; (middle, right): © Corbis RF Vertebrate Lens 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. THE STRUCTURE OF THE HUMAN EYE Suspensory ligament Sclera Retina Choroid Lens Optic nerve Cornea Vein Pupil Artery Iris Ciliary muscle Fovea 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 Rod Cone • 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. Light absorption (percent of maximum) SENSING LIGHT: VISION Blue cones 420 nm Green Red Rods cones cones 500 nm 530 nm 560 nm 100 80 60 40 20 400 600 500 Wavelength (nm) • There are three colors absorbed by these different cone cells: blue, green, and red. 700 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. 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. 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. Pit Outer Membrane chamber Inner chamber 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.