Copyright © 2013 by John Wiley & Sons, Inc. All rights reserved. Chapter 8 The Special Senses Copyright © 2013 by John Wiley & Sons, Inc. All rights reserved.

© 2013 by John Wiley & Sons, Inc. All rights reserved. Senses Somatic senses Involve receptors from more than one place in the body Help coordinate muscle movement Maintain body temperature Special senses Extremely sensitive receptors Supply us with detailed information about the world around us Sight Sound Smell Taste © 2013 by John Wiley & Sons, Inc. All rights reserved.

© 2013 by John Wiley & Sons, Inc. All rights reserved. Special Senses Special senses include Photoreceptors for vision Mechanoreceptors for hearing and balance Chemoreceptors for smell and taste Sensory adaptation The perception that sense simply decreases to the point where we are not aware of it any longer Special senses evolved to protect organisms from danger as they move through their environment so they can reach reproductive age Anything that affects reproduction can have species-wide effects For example, color vision evolved to help animals find ripe fruit © 2013 by John Wiley & Sons, Inc. All rights reserved.

© 2013 by John Wiley & Sons, Inc. All rights reserved. Olfaction Smelling Occurs in the upper chambers of the nasal passages Sensory receptors respond to chemicals dissolved in the mucus lying over them When a receptor binds its specific odor molecule A sensory impulse is sent to the olfactory bulb and on to the brain Messages (signals) travel along nerve fibers Neural connections between the olfactory bulb and the limbic system explain why smells trigger memories and emotions © 2013 by John Wiley & Sons, Inc. All rights reserved.

© 2013 by John Wiley & Sons, Inc. All rights reserved. Olfaction © 2013 by John Wiley & Sons, Inc. All rights reserved.

© 2013 by John Wiley & Sons, Inc. All rights reserved. Gustation Taste The sense of taste is linked with the sense of smell Receptors for taste are in roughly 10,000 taste buds Most of which are on the tongue In small bumps called papillae Taste buds can distinguish five categories of taste Sweet, sour, salty, bitter, and umami (savory) When stimulated, taste bud receptor cells send information on to the brain © 2013 by John Wiley & Sons, Inc. All rights reserved.

© 2013 by John Wiley & Sons, Inc. All rights reserved. Gustation © 2013 by John Wiley & Sons, Inc. All rights reserved.

© 2013 by John Wiley & Sons, Inc. All rights reserved. Hearing The ear has three functional parts The outer ear Composed of the pinna and external auditory canal Both capture sound waves and funnel them to the middle ear The middle ear Ear drum (tympanic membrane) marks the beginning of the middle ear Compression waves in the air (sound) cause the membrane to vibrate Converting sound into mechanical motion Attached to the inside of the tympanic membrane is the malleus The vibrating tympanic membrane moves the malleus, which in turn moves the incus through a synovial joint The stapes (ossicle) is the final small bone of the middle ear © 2013 by John Wiley & Sons, Inc. All rights reserved.

© 2013 by John Wiley & Sons, Inc. All rights reserved. Hearing The inner ear Beyond the stapes lies the oval window A membrane that functions like the tympanic membrane The oval window bounces in response to movement of the stapes Creating “fluid waves” in the inner ear © 2013 by John Wiley & Sons, Inc. All rights reserved.

The Middle and Inner Ear The entire middle and inner ear are within a hollow portion of the temporal bone This area is filled with air and communicates with the external environment through the eustachian tube (auditory tube) The cochlea of the fluid-filled inner ear has three compartments Vestibular canal, organ of Corti, and tympanic canal The tectorial membrane lies on top of the organ of Corti and rests on hair cells, which are the sensory receptors of the auditory system Hair cells do not have hairs - rather, they have organelles called stereocilia which look like hairs - stereocilia are sound-sensing organelles As the stapes pushes air against the membrane, the stereocilia move and a nerve impulse is created in the neuron of that particular hair cell Each part of the tectorial membrane is sensitive to a different pitch © 2013 by John Wiley & Sons, Inc. All rights reserved.

© 2013 by John Wiley & Sons, Inc. All rights reserved. Hearing © 2013 by John Wiley & Sons, Inc. All rights reserved.

© 2013 by John Wiley & Sons, Inc. All rights reserved. Hearing © 2013 by John Wiley & Sons, Inc. All rights reserved.

Equilibrium is Housed in the Inner Ear Equilibrium is the sense of balance The vestibule and semicircular canals of the inner ear are responsible for the two types of equilibrium Static equilibrium (also called gravitational equilibrium) The physical response to gravity that tells us which direction is down Dynamic equilibrium (also called rotational equilibrium) Detects acceleration or deceleration of your head © 2013 by John Wiley & Sons, Inc. All rights reserved.

© 2013 by John Wiley & Sons, Inc. All rights reserved. Static Equilibrium © 2013 by John Wiley & Sons, Inc. All rights reserved.

© 2013 by John Wiley & Sons, Inc. All rights reserved. Dynamic Equilibrium © 2013 by John Wiley & Sons, Inc. All rights reserved.

© 2013 by John Wiley & Sons, Inc. All rights reserved. Vision The eye has three layers Sclera (fibrous layer) Choroid (vascular layer) Retina (nervous layer) © 2013 by John Wiley & Sons, Inc. All rights reserved.

Eye Structures and Functions © 2013 by John Wiley & Sons, Inc. All rights reserved.

Visual Acuity and the Lens Visual acuity requires the eye to focus entering light onto the retina at the back of the eyeball The changing of lens shape to view nearby objects is called accommodation Accommodation gets more difficult with age Because the lens continues to add layers over time Making the lens thicker and stiffer © 2013 by John Wiley & Sons, Inc. All rights reserved.

© 2013 by John Wiley & Sons, Inc. All rights reserved. Visual Impairments Nearsightedness and farsightedness are both caused by the lens' inability to accommodate light properly Nearsightedness The eye is too long for the lens to focus the light rays on the retina Results in forming an image that is spread out and fuzzy when it hits the retina Farsightedness is the opposite of nearsightedness Astigmatism The cornea is imperfectly shaped, resulting in an uneven pattern of light hitting the retina Some areas of the image are in focus, but not others © 2013 by John Wiley & Sons, Inc. All rights reserved.

© 2013 by John Wiley & Sons, Inc. All rights reserved. Visual Impairments © 2013 by John Wiley & Sons, Inc. All rights reserved.

© 2013 by John Wiley & Sons, Inc. All rights reserved. The Retina Behind the lens lies a large chamber filled with vitreous humor, a gel-like fluid that holds the retina in place © 2013 by John Wiley & Sons, Inc. All rights reserved.

The Retina is Sensitive to Light Composed entirely of layers of neurons Rods and cones are neurons that detect light - the photoreceptors Photoreceptors detect light in the retina The cones respond to bright light, providing color vision and resolution Cones are concentrated near the center of the retina, where incoming light is strongest The rods function in low levels of light, providing only vague images Rods are spread across the periphery of the retina The bipolar cells and ganglionic cells are interneurons that carry the action potential generated by the photoreceptors to the brain © 2013 by John Wiley & Sons, Inc. All rights reserved.

© 2013 by John Wiley & Sons, Inc. All rights reserved. Anatomy of the Retina © 2013 by John Wiley & Sons, Inc. All rights reserved.

Rods and Cones Use Different Chemical Mechanisms When a photon of light hits a rod, a neural response is initiated via the chemical rhodopsin Rhodopsin is a visual pigment that responds to low levels of white light Light splits it into retinal and opsin, which causes a series of events that generate an action potential Cones Use the visual pigments retinal and opsin There are three types of cones, each of which is sensitive to different wavelengths of light Red Green Blue © 2013 by John Wiley & Sons, Inc. All rights reserved.

© 2013 by John Wiley & Sons, Inc. All rights reserved. Rods and Cones © 2013 by John Wiley & Sons, Inc. All rights reserved.

© 2013 by John Wiley & Sons, Inc. All rights reserved. The Path of Vision Visual nerve impulses first pass toward the front of the eye From the photoreceptors to the bipolar neurons The bipolar neurons transmit the impulse to the ganglionic cells In the anterior of the retina Ganglionic cells collect impulses from a small cluster of bipolar cells And pass them to the brain via the optic nerve © 2013 by John Wiley & Sons, Inc. All rights reserved.

© 2013 by John Wiley & Sons, Inc. All rights reserved. The Path of Vision Vision occupies more space in the brain than any other special sense Visual impulses travel along the optic nerve, through the thalamus to the occipital lobe of the brain Some impulses cross to the opposite side of the brain at the optic chiasma The view from the right eye is partially projected on the left side of the visual cortex of the cerebrum, and that from the left is partially projected on the right side The image reaching the occipital lobe is upside down and inverted The brain must flip and invert the image before it makes sense to us © 2013 by John Wiley & Sons, Inc. All rights reserved.

© 2013 by John Wiley & Sons, Inc. All rights reserved. Sensory Loss Vision and hearing can diminish with age Some hearing loss is due to mechanical malfunctions Conduction deafness Sound is poorly conducted from the outer ear to the inner ear Hearing aids can help those suffering from conduction deafness by increasing the amplitude of sound that enters the ear Nerve deafness Cannot be resolved by a hearing aid Sound is either not detected by the cochlear nerves or the nerve impulse is not transmitted to the brain Cochlear implants convert sound vibrations into electrical impulses and have shown some promise in treating nerve deafness © 2013 by John Wiley & Sons, Inc. All rights reserved.