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