Stimulus: sensory input that causes some change within or outside the body. Heat, pressure, sound waves, light, chemical

Transcription

1 Chapter 12 Sensory Mechanisms Receptors Receive and Convert Stimuli Stimulus: sensory input that causes some change within or outside the body Heat, pressure, sound waves, light, chemical Receptor: structure that detects stimuli and converts its energy into another form Mechanoreceptors Respond to mechanical energy Thermoreceptors Respond to heat or cold Receptors Receive and Convert Stimuli Pain receptors Respond to tissue damage or excessive heat or pressure Chemoreceptors Respond to presence of chemicals Photoreceptors Respond to light CNS Interprets Nerve Impulses Based on Origin and Frequency Nerve impulses are transmitted from receptors to specific portions of brain Stronger stimuli Activate a greater number of receptors Trigger a greater frequency of action potentials in sensory neurons

2 Some Receptors Adapt to Continuing Stimuli Sensory adaptation Allows the CNS to concentrate on important stimuli and ignore noncritical ones to maintain homeostasis Receptors that adapt Light touch, pressure, and smell Receptors that do not adapt Pain, joint, and muscle monitoring receptors Somatic Sensations and Special Senses Provide Sensory Information Somatic sensations Arise from receptors located throughout the body Temperature, pressure, touch, vibration, pain, awareness of body positions Special senses Arise from receptors restricted to specific areas of the body Taste, smell, hearing, balance, vision Mechanoreceptors Detect Limb Position, Muscle Length, and Tension Mechanoreceptors In joints: detect joint position In skeletal muscles: muscle spindles Specialized mechanoreceptors for monitoring muscle length, which relay information about limb position In tendons: detect tension

3 Thermoreceptors Detect Temperature Thermoreceptors near skin surface provide information about external environment Surface thermoreceptors adapt quickly Thermoreceptors in thoracic and abdominal organs monitor core temperature Core temperature receptors do not adapt quickly Taste: Chemoreceptors Bind with Dissolved Substances Taste buds Chemoreceptors bind with dissolved substances Most are distributed around edge, front, back of tongue Taste categories Sweet Salty Sour Bitter Umami Smell: Chemoreceptors Bind with Odorants Olfactory receptor cells Chemoreceptors that bind with odorants Detect >1,000 different odorants Correlation between taste and smell Chewed food releases chemicals that come in contact with olfactory receptors

4 Hearing: Mechanoreceptors Detect Sound Waves Properties of sound and sound waves Loudness Related to amplitude of sound waves Measured in decibels Pitch (tone) Related to frequency (number of wave cycles/sec) higher frequency: higher pitch lower frequency: lower pitch Outer Ear Channels Sound Waves Pinna: outer visible portion of ear Auditory canal: directs sound waves Tympanic membrane (ear drum): separates outer ear from middle ear Middle Ear Amplifies Sound Air-filled space with three small bones Malleus (hammer), Incus (anvil), Stapes (stirrup) Sound converted to vibrations of tympanic membrane, which then pass through these three bones in sequence Several-fold amplification of sound Stapes passes vibrations along to oval window from which they will enter the inner ear (cochlea) Auditory tube (eustachian tube) Connects to throat

5 Equalizes pressure The Inner Ear Sorts and Converts Sounds Cochlea converts sound to action potential Structure of cochlea Looks like a coiled snail Uncoiled, it is a tapered tube with two outer canals (vestibular and tympanic canals) and inner fluid-filled duct (cochlear duct) Contains hair cells (mechanoreceptors), with hair-like extensions embedded in tectorial membrane Organ of Corti Hair cells (mechanoreceptors) and tectorial membrane The Inner Ear Sorts and Converts Sounds Vibrations of oval window pass through cochlear fluid as pressure waves Pressure waves result in physical bending of hair cells in organ of Corti Different pitched sounds result in stimulation of hair cells (mechanoreceptors) in different regions of the cochlea Pitch is distinguished by the region of cochlea stimulated Action potentials carried by auditory nerve to vestibulocochlear nerve to brain Inner Ear Plays an Essential Role in Balance Vestibular apparatus Three semicircular canals and vestibule Contain hair cells (mechanoreceptors) embedded in gel-like material Hair cells bend in response to movement Sensing rotational movement

6 Movement of fluid in semicircular canals bends hair cells (mechanoreceptors) Head position and linear acceleration Movement of otoliths bends hair cells in vestibule (utricle and saccule) Vision: Structure of the Eye Vision involves detecting and interpreting visual stimuli by converting light energy to nerve impulses and transmitting them to the brain Structures Sclera (whites of the eye) and cornea Aqueous humor, iris, lens Vitreous humor, retina, optic nerve Macula, optic disc Regulating the Amount of Light and Focusing the Image Regulating light Iris opens or closes to control amount of light entering the eye Focusing Includes bending of light by cornea and lens Accommodation Adjustment of lens curvature to enable focusing on near and far objects Made possible by ciliary muscle Eyeball Shape Affects Focus Normal shape allows focusing on the retina Myopia: nearsighted

7 Eye longer than normal Distant objects focus in front of retina Corrected with concave lenses Hyperopia: farsighted Eye shorter than normal Near objects focus behind the retina Corrected with convex lenses Astigmatism: Irregularities in cornea or lens Corrected with specially ground lenses that compensate for lens irregularities Light Converted to Action Potentials Retina Allows us to see in color, adapt to varying light intensities, and perceive images Layers of retina Outermost: pigmented cells, absorb excess light Photoreceptors: rods and cones Bipolar cells: synapse with rods and cones Innermost: ganglion cells, axons become the optic nerve Rods and Cones Respond to Light Photoreceptors Rods and cones Response to light: photopigment (protein) changes shape Rods: approximately 120 million

8 Cones: approximately 6 million 1 million ganglion cells Significant amount of convergence Rods Provide Vision in Dim Light Rhodopsin: photopigment within rods More sensitive to light than the photopigment in cones In dim light, vision primarily dependent on rods Rods do not enable color vision The farther from the fovea, the greater the density of rods Cones Provide Color Vision and Accurate Images Three types of cones enable color vision Red, green, blue Brain interprets ratios of impulses coming from ganglion cells connected to the three kinds of cones Cones require stronger light to be activated Cones responsible for visual acuity Disorders of the Ear Deafness: loss of hearing Nerve deafness: damage to hair cells Conduction deafness: damage to tympanic membrane or bones of middle ear

9 In some cases, can be corrected with a cochlear implant Otitis media Inflammation of the middle ear Ménière s syndrome Inner ear condition impairs hearing and balance Disorders of the Eye Retinal detachment Retina separates from choroid Cataracts Lens becomes opaque Glaucoma Pressure inside the eye rises due to improper draining of aqueous humor If untreated, may lead to blindness