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 of incoming 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, after receiving a sensation, refers that sensation back to its source – Adaptation: when sensory receptors are continuously stimulated, the receptors send fewer signals to the area of the brain that interprets that particular sensory information

The General Senses Five general senses: – Pain – Touch – Pressure – Temperature – Proprioception

Pain receptors (nociceptors): – Consist of free nerve endings stimulated by tissue damage – Do not adapt; may continue to send signals after stimulus is removed – Widely distributed throughout the skin, visceral organs, 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 specialized sensory 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 is sent to the parietal lobe – Sensory information pertaining to coordination of skeletal 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 that dissolve in the moisture of the nasal tissue – Sensory information interpreted within olfactory 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 cm Diameter – 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-thirds of eyeball Contains photoreceptors: rods most abundant in periphery, 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 waves to focus them – Photoreceptors in retina transmit nervous Impulses 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: Hertz Measured in decibels

Sense of Hearing Structure of the ear: – Three parts: External ear: composed of auricle and external auditory canal; extends to eardrum Middle ear: contains eardrum, three tiny bones, and eustachian tube Inner ear: three parts include vestibule, semicircular canals, 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: Hertz

Sense of Balance Receptors for balance: – Located within the vestibule and the semicircular canals of the inner ear – Mechanoreceptors; hair like projections immersed in fluid of the inner ear

Figure 9.25e