Chapter 8 the Special Senses

Chapter 8 The Special Senses Sensory receptors – dendrites specialized to detect certain types of stimuli  Exteroceptors detect stimuli from outside the body (e.g. taste, hearing, vision)  Interoceptors receive stimuli from inside the body (e.g. change in blood pressure) Types of sensory receptors  Chemoreceptors – respond to the binding of certain chemicals (smell and taste)  Photoreceptors – respond to light energy (vision)  Mechanoreceptors – respond to mechanical forces such as pressure (hearing and balance) The special senses  Sensory adaptation can occur when perception of a particular sense decreases to the point where we are no longer aware of it o This is usually due to repeated or constant stimulation  The special senses likely evolved to protect organisms as they moved through their environment Olfaction  The chemoreceptors responsible for sensing smell are located in the upper chambers of the nasal passage  The chemicals will dissolve in the mucus coating the receptors  There are specific receptors for specific molecules  When a particular receptor binds its specific odor molecule, a sensory impulse is sent to the olfactory bulb and then to the brain  The olfactory bulb also connects with the limbic system of the brain o This explains why smells can trigger such deep memories and emotions Taste  The chemoreceptors responsible for sensing taste are located in the approximately 10,000 buds located in small bumps (papillae) on the tongue  They can distinguish 5 categories of flavor - sweet, sour, salty, bitter, and umami (savory)  Binding of the flavor molecules initiates signal transmission to the brain Anatomy of the ear The ear functions in hearing and balance  3 divisions o Outer ear functions in hearing; filled with air o Middle ear functions in hearing; filled with air o Inner ear functions in hearing and balance; filled with fluid A. The ear: Outer ear  Includes o Pinna - the external ear flap that catches sound waves o Auditory canal - directs sound waves to the tympanic membrane . Lined with fine hairs and modified sweat glands that secrete earwax B. The ear: Middle ear  Includes o Tympanic membrane (eardrum) - membrane that vibrates to carry the wave to the bones o 3 small bones (malleus, incus, stapes) - amplify the sound waves o Eustachian tube - a tube that connects from the throat to the middle ear used to equalize pressure so the eardrum does not burst C. The ear: Inner ear  Important for both hearing and balance  3 areas - cochlea, semicircular canals, vestibule o Stapes (middle ear bone) vibrates and strikes the membrane of the oval window causing fluid waves in the cochlea o Vestibule – gravitational equilibrium o Semicircular canals – rotational equilibrium Cochlea  Converts vibrations into nerve impulses  Contains the organ of Corti (spiral organ), a sense organ containing hairs for hearing  Bending of embedded hairs cause vibrations that send nerve impulses to the cochlear nerve and then to the brain  Pitch is determined by varying wave frequencies that are detected by different parts of the organ of Corti  Volume is determined by the amplitude of sound waves Semicircular canals and vestibule  Detects movement of the head in the vertical and horizontal planes (gravitational equilibrium) o Depends on hair cells in the utricle and saccule  Detects angular movement (rotational equilibrium) o Depends on hair cells at the base of each semicircular canal (ampulla) Anatomy of the eye  Made of 3 layers/coats – o Sclera - mostly white and fibrous except the cornea o Choroid - darkly, pigmented vascular layer o Retina - inner layer containing photoreceptors A. The eye: Sclera  Sclera – the white of the eye that maintains eye shape  Cornea - transparent portion of the sclera that is important in refracting light  Pupil - a hole that allows light into the eyeball B. The eye: Choroid  Choroid – middle layer that absorbs light rays that are not absorbed by the retina  Iris - donut-shaped, colored structure that regulates the size of the pupil  Ciliary process - a structure behind the iris that contains a muscle that controls the shape of the lens  Lens – attached to the ciliary body and functions to refract and focus light rays o The lens is a flexible, transparent, biconvex structure o The lens changes shape (accommodates) to focus light on the retina in order to form an image o As we age the lens loses elasticity and we use glasses to correct for this C. The eye: Retina  Rods are sensitive to light  Cones require bright light and see wavelengths of light (color)  The fovea centralis is an area of the retina densely packed with cones where images are focused  Sensory receptors from the retina form the optic nerve that takes impulses to the brain o The blindspot is where the optic nerve attaches C. The eye: Photoreceptors of the retina  Rods o Contain a visual pigment called rhodopsin o Important for peripheral and night vision o Vitamin A is important for proper functioning  Cones o Located mostly in the fovea o Allow us to detect fine detail and color o 3 different kinds of cones containing red, green, and blue pigments Abnormalities of the eye o Cataracts – lens of the eye is cloudy o Glaucoma – fluid pressure builds up in the eye o Astigmatism – condition in which the cornea or lens is uneven leading to a fuzzy image o Nearsightedness – eyeball is too long making it hard to see distant objects o Farsightedness – eyeball is too short making it hard to see close objects o Color blindness – genetic disease more common in males in which they cannot distinguish red and green o Scientists have used gene therapy to cure red-green colorblindness in squirrel monkeys