A&PI Chapter 17
Chapter 17
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The Special Senses
Lecture Outline
Chapter 17• Smell, taste, vision, hearing and equilibrium
• Housed in complex sensory organs
• Ophthalmology is science of the eye
• Otolaryngology is science of the ear
• Interaction of molecules with receptor cells
• Olfaction (smell) and gustation (taste)
• Both project to cerebral cortex & limbic system
Anatomy of olfactory receptors
• The receptors for olfaction, which are bipolar neurons, are in the nasal epithelium in the superior portion of the nasal cavity (Figure 17.1).
• They are first-order neurons of the olfactory pathway.
• Supporting cells are epithelial cells of the mucous membrane lining the nose.
• Basal stem cells produce new olfactory receptors.
• 1 square inch of membrane holding 10-100 million receptors
• Covers superior nasal cavity and cribriform plate
• 3 types of receptor cells
• Olfactory receptors
– bipolar neurons with cilia or olfactory hairs
• Supporting cells
– columnar epithelium
• Basal cells = stem cells
– replace receptors monthly
• Olfactory glands
– produce mucus
• Both epithelium & glands innervated cranial nerve VII.
• Genetic evidence suggests there are hundreds of primary scents.
• In olfactory reception, a generator potential develops and triggers one or more nerve impulses.
• Adaptation to odors occurs quickly, and the threshold of smell is low: only a few molecules of certain substances need be present in air to be smelled.
• Olfactory receptors convey nerve impulses to olfactory nerves, olfactory bulbs, olfactory tracts, and the cerebral cortex and limbic system.
• Hyposmia, a reduced ability to smell, affects half of those over age 65 and 75% of those over 80. It can be caused by neurological changes, drugs, or the effects of smoking .
• Odorants bind to receptors
• Na+ channels open
• Depolarization occurs
• Nerve impulse is triggered
• Adaptation = decreasing sensitivity
• Olfactory adaptation is rapid
• Low threshold
• Axons from olfactory receptors form the olfactory nerves (Cranial nerve I) that synapse in the olfactory bulb
• Second-order neurons within the olfactory bulb form the olfactory tract that synapses on primary olfactory area of temporal lobe
• Other pathways lead to the frontal lobe (Brodmann area 11) where identification of the odor occurs
• Taste is a chemical sense.
– Taste stimuli classes include sour, sweet, bitter, and salty.
• Taste requires dissolving of substances
• Four classes of stimuli--sour, bitter, sweet, and salty
– Other “tastes” are a combination of the four taste sensations plus olfaction.
• 10,000 taste buds found on tongue, soft palate & larynx
• Found on sides of circumvallate & fungiform papillae
• 3 cell types: supporting, receptor & basal cells
• An oval body consisting of 50 receptor cells surrounded by supporting cells
• A single gustatory hair projects upward through the taste pore
• Basal cells develop into new receptor cells every 10 days.
• Receptor potentials developed in gustatory hairs cause the release of neurotransmitter that gives rise to nerve impulses.
• Complete adaptation in 1 to 5 minutes
• Thresholds for tastes vary among the 4 primary tastes
• Mechanism
• First-order gustatory fibers found in cranial nerves
• Signals travel to thalamus or limbic system & hypothalamus
• Taste fibers extend from the thalamus to the primary gustatory area on parietal lobe of the cerebral cortex
• More than half the sensory receptors in the human body are located in the eyes.
• A large part of the cerebral cortex is devoted to processing visual information.
• Eyelids or palpebrae
– protect & lubricate
– epidermis, dermis, CT, orbicularis oculi m., tarsal plate, tarsal glands & conjunctiva
• Tarsal glands
– oily secretions
• Conjunctiva
– palpebral & bulbar
– stops at corneal edge
• The eyelids shade the eyes during sleep, protect the eyes From superficial to deep, each eyelid consists of epidermis, dermis, subcutaneous tissue, fibers of the orbicularis oculi muscle, a tarsal plate, tarsal glands, and conjunctiva (Figure 17.4a).
– The tarsal glands secrete a fluid to keep the eye lids from adhering to each other.
– The conjunctiva is a thin mucous membrane that lines the inner aspect of the eyelids and is reflected onto the anterior surface of the eyeball.
• Eyelashes & eyebrows help protect from foreign objects, perspiration & sunlight
• Sebaceous glands are found at base of eyelashes (sty)
• Palpebral fissure is gap between the eyelids
• About 1 ml of tears produced per day. Spread over eye by blinking. Contains bactericidal enzyme called lysozyme.
• Six muscles that insert on the exterior surface of the eyeball
• Innervated by CN III, IV or VI.
• 4 rectus muscles -- superior, inferior, lateral and medial
• 2 oblique muscles -- inferior and superior
• The eye is constructed of three layers (Figure 17.5).
– Fibrous Tunic– Vascular Tunic– Nervous Tunic• Transparent
• Helps focus light(refraction)
– astigmatism
• 3 layers
– nonkeratinized stratified squamous
– collagen fibers & fibroblasts
– simple squamous epithelium
• Transplants
– common & successful
– no blood vessels so no antibodies to cause rejection
• Nourished by tears & aqueous humor
• “White” of the eye
• Dense irregular connective tissue layer -- collagen & fibroblasts
• Provides shape & support
• At the junction of the sclera and cornea is an opening (scleral venous sinus)
• Posteriorly pierced by Optic Nerve (CNII)
• Choroid
– pigmented epithilial cells (melanocytes) & blood vessels
– provides nutrients to retina
– black pigment in melanocytes absorb scattered light
• Ciliary body
– ciliary processes
• folds on ciliary body
• secrete aqueous humor
– ciliary muscle
• smooth muscle that alters shape of lens
• Colored portion of eye
• Shape of flat donut suspended between cornea & lens
• Hole in center is pupil
• Function is to regulate amount of light entering eye
• Autonomic reflexes
– circular muscle fibers contract in bright light to shrink pupil
– radial muscle fibers contract in dim light to enlarge pupil
• Constrictor pupillae (circular) are innervated by parasympathetic fibers while Dilator pupillae (radial) are innervated by sympathetic fibers.
• Response varies with different levels of light
• Avascular
• Crystallin proteins arranged like layers in onion
• Clear capsule & perfectly transparent
• Lens held in place by suspensory ligaments
• Focuses light on fovea
• Suspensory ligaments attach lens to ciliary process
• Ciliary muscle controls tension on ligaments & lens
• Posterior 3/4 of eyeball
• Optic disc
– optic nerve exiting back of eyeball
• Central retina BV
– fan out to supply nourishment to retina
– visible for inspection
• hypertension & diabetes
• Detached retina
– trauma (boxing)
• fluid between layers
• distortion or blindness
• shapes of their outer segments differ
• Rods
– permit us to see shapes and movement.
• Cones
– most densely concentrated in the central fovea, a small depression in the center of the macula lutea.
• The macula lutea is in the exact center of the posterior portion of the retina, corresponding to the visual axis of the eye.
– The fovea is the area of sharpest vision because of the high concentration of cones.
– Rods are absent from the fovea and macula and increase in density toward the periphery of the retina.
• Pigmented epithelium
– nonvisual portion
– absorbs stray light & helps keep image clear
• 3 layers of neurons (outgrowth of brain)
– photoreceptor layer
– bipolar neuron layer
– ganglion neuron layer
• 2 other cell types (modify the signal)
– horizontal cells
– amacrine cells
• Rods----rod shaped
– shades of gray in dim light
– 120 million rod cells
– shapes & movements
– distributed along periphery
• Cones----cone shaped
– sharp, color vision
– 6 million
– fovea of macula lutea
• densely packed region
• at exact visual axis of eye
• 2nd cells do not cover cones
• sharpest resolution (acuity)
• Light penetrates retina
• Rods & cones transduce light into action potentials
• Rods & cones excite bipolar cells
• Bipolars excite ganglion cells
• Axons of ganglion cells form optic nerve leaving the eyeball (blind spot)
• To thalamus & then the primary visual cortex
• The eyeball contains the nonvascular lens, just behind the pupil and iris.
• The lens fine tunes the focusing of light rays for clear vision.
• Anterior cavity (anterior to lens)
• Posterior cavity (posterior to lens)
• The pressure in the eye, called intraocular pressure, is produced mainly by the aqueous humor.
• The intraocular pressure, along with the vitreous body, maintains the shape of the eyeball and keeps the retina smoothly applied to the choroid so the retina will form clear images.
• Glaucoma
• Continuously produced
• Flows from posterior chamber
• Scleral venous sinus
– canal of Schlemm
– opening in white of eye• Refraction of light
• Accommodation of the lens
• Constriction of the pupil
• Bending of light as it passes from one substance (air) into a 2nd substance with a different density(cornea)
• In the eye, light is refracted by the anterior & posterior surfaces of the cornea and the lens
• Image focused on retina is inverted & reversed from left to right
• Brain learns to work with that information
• 75% of Refraction is done by
• Light rays from > 20’ are nearly parallel and only need to be bent enough to focus on retina
• Light rays from < 6’ are more divergent & need more refraction
– extra process needed to get additional bending of light is called accommodation
• Accommodation is an increase in the curvature of the lens, initiated by ciliary muscle contraction, which allows the lens to focus on near objects (figure 17.10c).
• Convex lens refract light rays towards each other
– Lens of eye is convex on both surfaces
• Viewing a distant object
– lens is nearly flat by pulling of suspensory ligaments
• View a close object
– ciliary muscle is contracted & decreases the pull of the suspensory ligaments on the lens
– elastic lens thickens as the tension is removed from it
– increase in curvature of lens is called accommodation
– The near point of vision is the minimum distance from the eye that an object can be clearly focused with maximum effort.
• Near point is the closest distance from the eye an object can be & still be in clear focus
• Reading glasses may be needed by age 40
• Myopia is nearsightedness (Figure 17.11).
• Hyperopia is farsightedness (Figure 17.11).
• Astigmatism is a refraction abnormality due to an irregular curvature of either the cornea or lens.
• Emmetropic eye (normal)
– can refract light from 20 ft away
• Myopia (nearsighted)
– eyeball is too long from front to back
– glasses concave |