Chapter 9
The General and Special Senses
Sensory System
• Sensory system allows us to experience the world
– External information– Internal information
Receptors and Sensation
• Receptor: specialized area of a sensory neuron that detects a specific stimulus
• Five types of sensory receptors:
– Chemoreceptors– Pain receptors (nociceptors)– Thermoreceptors– Mechanoreceptors– Photoreceptors
• Sensation: conscious awareness ofincoming sensory information
• Four components of sensation perception:– Stimulus–Receptor– Sensory nerve– Special area of the brain
• Characteristics of sensation:
– Projection: process by which the brain, afterreceiving a sensation, refers that sensation backto its source
– Adaptation: when sensory receptors arecontinuously stimulated, the receptors sendfewer signals to the area of the brain thatinterprets that particular sensory information
The General Senses• Five general senses:
– Pain– Touch– Pressure– Temperature– Proprioception
• Pain receptors (nociceptors):
– Consist of free nerve endings stimulated bytissue damage
– Do not adapt; may continue to send signalsafter stimulus is removed
– Widely distributed throughout the skin, visceralorgans, and other internal tissues
– Not present in nervous tissue of the brain
Figure 9.2
• Touch and pressure receptors:
– Mechanoreceptors; respond to forces that press,move, or deform tissue
– Touch receptors are found mostly in the skin;also called tactile receptors
– Pressure receptors are located in the skin,subcutaneous tissue, and deep tissue
Figure 9.3
• Thermoreceptors (receptors of temperature):
– Two types of thermoreceptors:• Cold receptors• Heat receptors
– Found in free nerve endings and other specializedsensory cells beneath the skin
– Scattered widely throughout the body
– Both types display adaptation
• Proprioception: sense of orientation or position
• Proprioreceptors:
– Located in muscles, tendons, joints, and inner ear
– Sensory information about movement and position issent to the parietal lobe
– Sensory information pertaining to coordination ofskeletal muscle activity is sent to the cerebellum
The Special Senses
• Five special senses:– Smell– Taste– Sight– Hearing– Balance
Sense of Smell
• Olfaction: sense of smell
• Olfactory receptors:
– Chemoreceptors; stimulated by chemicals thatdissolve in the moisture of the nasal tissue
– Sensory information interpreted withinolfactory area of the temporal lobe
– Quick adaptation
Figure 9.6a
Sense of Taste
• Gustation: sense of taste
• Taste buds: special organs of taste. Modified epithelial cells
• Taste receptors:– Chemoreceptors; sensitive to the chemicals in food
– Four basic taste sensations: salty, sweet, sour, and bitter
Figure 9.7
Sense of Sight• Vision: sense of sight• Eyes: organs of vision– Visual receptors– Visual accessory organs– Eyebrows– Eyelids– Conjunctiva– Eyelashes– Lacrimal apparatus– Extrinsic eye muscles
Figure 9.8
• Eyeball:
– Spherical shape approximately 2 to 3 cmDiameter
– Composed of three layers: sclera, choroid,and retina
Eye Features
• Blind Spot• The area where your optic nerve
attaches to your eye.
• Fovea• The highest concentration of cones• Center of color vision and sharpest
vision
Figure 9.9
• Layers of the eyeball:
– Sclera
• Outermost layer
• Made of tough fibrous connective tissue
• Cornea is a forward extension of the sclera
Eye Cavities
• Posterior Cavity• Also called the vitreous chamber• Contains vitreous body
• Anterior Cavity• Contains the anterior and posterior
chamber
Anterior Cavity
• Anterior Chamber• The space between the iris and the
cornea
• Posterior Chamber• The space between the suspensory
ligament and the iris
Figure 9.10c
• Layers of the eyeball (cont’d.):
– Choroid• Area between the Sclera and the retina
• Highly vascular
Lens of the Eye
• Held in place by suspensory ligaments
• Focuses visual images• Suspensory ligaments control the
shape of the eye• The closer the object, the suspensory
ligaments relax and the lens appears more round
Lens Continued
• Cataract: Loss of transparency in the lens. (looks cloudy)
• Layers of the eyeball (cont’d.):
– Retina• Innermost layer that lines posterior two-thirdsof eyeball
• Contains photoreceptors: rods most abundant inperiphery, cones most abundant in center
• Optic disk: blind spot because no rods or cones
• How seeing occurs:
– Light enters the cornea through the pupil
– Lens bends (refracts) the light wavesto focus them
– Photoreceptors in retina transmit nervousImpulses to optic nerve
– Optic nerve sends signal to occipital lobe
Cones
• Allow color vision to occur• Are Red, Blue, Green• Higest concetration in the fovea
Figure 9.17
Factors affecting Blindness
• Glaucoma: Interference with the circulation of the aqueous humor. Increases pressure inside the eye.
• Diabetes• Heredity• Retinal Detachment
Hearing
• Provided by receptors in the semicircular canals
• Located in the Organ of Corti
• Hearing range: 20-2000 Hertz• Measured in decibels
Sense of Hearing
• Structure of the ear:– Three parts:
• External ear: composed of auricle and externalauditory canal; extends to eardrum
• Middle ear: contains eardrum, three tiny bones, andeustachian tube
• Inner ear: three parts include vestibule, semicircularcanals, and cochlea
Figure 9.22
Anatomy: Middle Ear
• Tympanic membrane (Eardrum)• Collects vibrations from external
environment
• Ear Bones (Ossicles)• Connect Tympanic membrane to oval
window• Incus • Stapes (smallest)• malleus
How Hearing Works
• Soundwaves collect in the tympanic membrane
• Membrane vibrates and activates the ossicles
• Ossicles vibrate on the oval window
• Vibration sent to the Vestibule and through the Cochlea
Hearing Continued
• Once the vibration reaches the Cranial nerves from the Cochlea, the soundwave is converted to chemical energy and interpreted by the brain.
• Hearing range: 20-2000 Hertz
Sense of Balance• Receptors for balance:
– Located within the vestibule and thesemicircular canals of the inner ear
– Mechanoreceptors; hair like projectionsimmersed in fluid of the inner ear
Figure 9.25e
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