Special Senses

Olfaction

Taste

Vision

Hearing

Balance & Equilibrium

Olfaction: Sense of Smell

• Smell and taste are chemical senses. The human nose contains 10 million to 100 million receptors for smell (olfaction) in the olfactory epithelium of the superior part of the nasal cavity.
• Can detect at least one trillion different odors
• The olfactory epithelium covers the inferior surface of the cribriform plate (of the ethmoid bone of the skull) and extends along the superior nasal concha.

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Olfaction: Sense of Smell

Olfaction: Sense of Smell

There are 3 types of cells:

• Olfactory receptor cells
• Supporting cells
1. Located in the mucous membrane lining the nose
2. Physical support, nourishment and electrical insulation for olfactory receptor cells.
• Basal cells
• Undergo mitosis to replace olfactory receptor cells

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Mucous glands also help to transduce the signal by providing a layer of mucous and water.

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Olfaction: Sense of Smell

Olfaction: Sense of Smell

• Olfactory transduction: binding of an odorant molecule to an olfactory receptor protein.
• Chemical reactions involving cyclic AMP (cAMP) cause depolarization
• Action potential travels to the primary olfactory area.
• Impulse travels to the frontal lobe (orbitofrontal area) for odor identification.

Olfaction: Sense of Smell

Gustation: Sense of Taste

• Taste is a chemical sense, but it is much simpler than olfaction.
• 5 primary tastes: sour, sweet, bitter, salt and umami (meaty, savory).

Gustation: Sense of Taste

• Taste buds contain receptors for the sensation of taste. Approximately 10,000 taste buds are found on the tongue of a young adult and on the soft palate, pharynx, and epiglottis.
• Taste buds contain 3 kinds of epithelial cells: supporting cells, gustatory receptor cells and basal stem cells.

Gustation: Sense of Taste

Gustation: Sense of Taste

• Taste buds are located in elevations on the tongue called papillae.
• 3 types of papillae that contain taste buds:
• Vallate papillae (about 12 that contain 100–300 taste buds)
• Fungiform papillae (scattered over the tongue with about 5 taste buds each)
• Foliate papillae (located in lateral trenches of the tongue—most of their taste buds degenerate in early childhood).
• Filiform papillae cover the entire surface of the tongue.
• Contain tactile receptors but no taste buds.
• Increase friction to make it easier for the tongue to move food within the mouth.

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Gustation: Sense of Taste

Gustation: Sense of Taste

• Three cranial nerves are involved the sense of taste.
• Facial (VII) nerve carries taste information from the anterior 2/3 of the tongue.
• Glossopharyngeal (IX) nerve carries taste information from the posterior 1/3 of the tongue.
• Vagus (X) nerve carries taste information from taste buds on the epiglottis and in the throat.

Gustation: Sense of Taste

Vision T Th

Vision uses visible light which is part of the electromagnetic spectrum with wavelengths from about 400 to 700 nm.

Vision

Wavelength is defined as the distance between two consecutive peaks of an electromagnetic wave.

Vision

Accessory structures of the eyes include the eyelids, eyelashes, eyebrows, lacrimal (tear-producing) apparatus and extrinsic eye muscles.

Copyright © 2014 John Wiley & Sons, Inc. All rights reserved.

Vision

• Palpebral muscles control eyelid movement and extrinsic eye muscles are responsible for moving the eyeball itself in all directions.
• The conjunctiva is a thin, protective mucous membrane that lines the eyelids and covers the sclera.
• The tarsal plate: a fold of connective tissue that gives form to the eyelids. Contains a row of sebaceous glands (tarsal glands/Meibomian glands) that keeps the eyelids from sticking to each other.

Vision

Vision – Pathway of tears

Vision

• Six extrinsic eye muscles move the eyes in almost any direction. These muscles include the superior rectus, inferior rectus, lateral rectus, medial rectus, superior oblique and inferior oblique.
• The eyeball contains two tunics (coats): the fibrous tunic (the cornea and sclera) and the vascular tunic (the choroid, ciliary body and iris).

Vision

Vision

Vision

The iris (colored portion of the eyeball) controls the size of the pupil based on autonomic reflexes.

Vision

• The retina lines the posterior three-quarters of the inner layer of the eyeball. It may be viewed using an ophthalmoscope.
• The optic (II) nerve is also visible.
• The point at which the optic nerve exits the eye is the optic disc (blind spot).
• The exact center of the retina is the macula lutea. In its center is the fovea centralis (area of highest visual acuity).

Vision

Vision

• The retina contains sensors (photoreceptors) known as rods and cones.
• Rods to see in dim light
• Cones produce color vision
• From these sensors, information flows through the outer synaptic layer to bipolar cells through the inner synaptic layer to ganglion cells. Axons of these exit as the optic (II) nerve.

Vision

Vision

Vision

• The eye is divided into an anterior chamber and a posterior chamber by the iris (colored portion of the eyeball).
• The anterior chamber (between the iris and cornea) is filled with aqueous humor (a clear, watery liquid).
• The posterior chamber lies behind the iris and in front of the lens and is also filled with aqueous humor.
• Behind this is the posterior cavity (vitreous chamber) filled with a transparent, gelatinous substance, the vitreous humor.

Vision

Light passes through the cornea, the anterior chamber, the pupil, the posterior chamber, the lens, the vitreous humor, and is projected onto the retina.

Vision

Vision

Vision

Light refracts (bends) when it passes through a transparent substance with one density into a second transparent substance with a different density. This bending occurs at the junction of the two substances.

Vision

• Images focused on the retina are inverted and right-to-left reversed due to refraction. The brain corrects the image.
• The lens must accommodate to properly focus the object.
• The image is projected onto the central fovea, the site where vision is the sharpest.

Vision

Vision

The normal (emmetropic) eye will refract light correctly and focus a clear image on the retina.

Vision

• Myopia (nearsightedness)
• The eyeball is longer than it should be and the image converges in front of the retina.
• These people see close objects sharply, but perceive distant objects as blurry.
• A concave lens is used to correct the vision.

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Vision

• Hyperopia (farsightedness)
• The eyeball is shorter than it should be and the image converges behind the retina.
• These individuals can see distant objects clearly, but have difficulty with close objects.
• A convex lens is used to correct this abnormality.

Vision

Astigmatism is a condition where either the cornea or the lens (or both) has an irregular curve. This causes blurred or distorted vision.

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Vision

Rods and cones, the photoreceptors in the retina that convert light energy into neural impulses, were named for the appearance of their outer segments.

Vision

• Rods and cones contain photopigments necessary for the absorption of light that will initiate the events that lead to production of a receptor potential.
• Rods contain only rhodopsin.
• Cones contain three different photopigments, one for each of the three types of cones (red, green, blue).
• Photopigments respond to light in a cyclical process.

Test for Red-Green Color Blindness

Vision

Vision

• Part of this difference is related to the rates of bleaching and regeneration of photopigments in rods and cones.

Vision

Light adaptation

Occurs when an individual moves from dark surroundings to light ones.

It occurs in seconds.

Vision

• Dark adaptation
• Takes place when one moves from a lighted area into a dark one.
• This takes minutes to complete.

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• In darkness, rod photoreceptors release the inhibitory neurotransmitter glutamate.
• This inhibits bipolar cells from transmitting signals to ganglion cells which provide output from the retina to the brain.

Vision

Vision

The anterior location of our eyes leads to visual field overlap. This gives us binocular vision.

Visual information from the right half of each visual field travels to the left side of the brain.

Visual information from the left half of each visual field travels to the right side of the brain.

Hearing and Equilibrium

• The transduction of sound vibrations by the ear’s sensory receptors into electrical signals is 1000 times faster than the response to light by the eye’s photoreceptors.
• The ear also contains receptors for equilibrium.
• The ear is divided into 3 regions: the external ear, middle ear and internal ear.

Hearing and Equilibrium

Hearing and Equilibrium

Hearing and Equilibrium

Hearing and Equilibrium

Hearing and Equilibrium

Hearing and Equilibrium

Hearing and Equilibrium

Hearing and Equilibrium

Hearing and Equilibrium

Hearing and Equilibrium

• Equilibrium (balance) exists in two forms:
• Static equilibrium: maintenance of the body’s position relative to the force of gravity
• Dynamic equilibrium: the maintenance of the body’s position in response to sudden movements.
• Vestibular apparatus: The organs that maintain equilibrium. Includes saccule, utricle (both otolithic organs) and semicircular canals.
• Otoliths are calcium carbonate crystals. The walls of the utricle and saccule contain a macula. The two maculae are receptors for static equilibrium.

Hearing and Equilibrium

The otolithic membrane sits on top of the macula. Movement of the head causes gravity to move it down over hair cells. The hair cells synapse with neurons in the vestibular branch of the vestibulocochlear (VIII) nerve.

Hearing and Equilibrium

Hearing and Equilibrium

Otolith

membrane

Kinocilium

Stereocilia

Receptor potential

Depolarization

Hyperpolarization

Nerve impulses generated

in vestibular fiber

When hairs bend toward

the kinocilium, the hair cell

depolarizes, exciting the

nerve fiber, which generates

more frequent action potentials.

When hairs bend away

from the kinocilium, the hair cell

hyperpolarizes, inhibiting the nerve

fiber, and decreasing the action

potential frequency.

Hearing and Equilibrium

• Three semicircular canals are responsible for dynamic equilibrium. The ducts lie at right angles to each other which allows for rotational acceleration or deceleration.
• An ampulla in each canal contains the crista with a group of hair cells. Movement of the head affects the endolymph and hair cells.
• This generates a potential leading to nerve impulses that travel along the vestibular branch of the vestibulocochlear (VIII) nerve.

Hearing and Equilibrium

Hearing and Equilibrium

Hearing and Equilibrium

Aging and the Special Senses

• Smell and taste are not affected by aging until around age 50 when the gradual loss of receptors and the slower rate of regeneration have an affect.
• The lens begins to lose elasticity and has difficulty focusing on close objects (presbyopia). This begins around age 40.
• Muscles of the iris weaken and react more slowly to light and dark causing elderly people to have difficulty adjusting to changes in lighting.

Aging and the Special Senses

• Retinal diseases such as macular disease, detached retina and glaucoma (damage to the retina due to increased intraocular pressure) occur more frequently in the elderly.
• By about age 60, approximately 25% of individuals experience a noticeable hearing loss. Age associated loss is called presbycusis.
• Tinnitus (ringing in the ears) and vestibular imbalance also occur more frequently in the elderly.