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22 Cards in this Set
- Front
- Back
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Special Senses
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All sense organs, regardless of size, type or location, have in common some important functional characteristics. First, they must be able to sense or detect a stimulus in their environment. Of course, different sense organs detect different types of stimuli. Whether its is light, sound, temperature change, mechanical pressure, or the presence of chemicals ultimately identified as taste or smell, the stimulus must be changed into an electrical signal or nerve impulse. This signal is then transmitted over a nervous system "pathway" to the brain, where the sensation is actually perceived.
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The Eye
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The eyes are complex organs that are supported externally by several structures including orbital cavities, eyebrows, eyelids, eyelashes, conjunctiva, lacrimal apparatus and the extrinsic apparatus of the eye.
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Eye Physiology
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The eyeball has three layers (tunics); an outer fibrous tunic (Sclera), a middle vascular (choroid) tunic and retina.
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The lens of the eye
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lies directly behind the pupil. It is held in place by a ligament attached to the ciliary muscle. In most young people the lens is transparent and somewhat elastic so that it is capable of changing shape. Aqueous humour is the name of the water fluid in front of the lens (in the anterior cavity of the eye) and vitreous humour is the name of the jelly-like fluid behind the lens (in the posterior cavity). Aqueous humour is constantly being formed, drained and replaced in the anterior cavity. It supplies nutrients and oxygen to the lens and cornea which do not have blood supplies. By its constant production, it also maintains an intra-ocular pressure of about 20mm Hg within the anterior cavity. This pressure is important in maintaining the shape of the eyeball.
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Vision
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Vision is the culmination of several processes involving the focusing and reception of light rays and the transmission of sensory information to the occipital lobe for integration and interpretation.
Light is the stimulus that results in vision (that is, ability to see objects as they exist in our environment). Light enters the eye through the pupil and is refracted or bent so that it is focused on the retina. Refraction occurs as light passes through the cornea, the aqueous humor, the lens and the vitreous humor on its way to the retina. The innermost layer of the retina contains the rods, and cones, which are the photoreceptor cells of the eye. They respond to a light stimulus by producing nervous impulse. The rod and cone photoreceptor cells synapse with neurons in the bipolar and gang ionic layers of the retina. |
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OPTIC NERVE
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Nervous signals eventually leave the retina and exit the eye through the optic nerve on the posterior surface of the eyeball. No rods or cones are present in the area of the retina where the optic nerve fibres exist. The result is a blind spot known as the optic disc.
After leaving the eye, the optic nerves enter the brain and travel to the visual cortex of the occipital lobe. In this area the visual interpretation of the nervous impulses generated by light stimuli in the rods and cones of the retina, results in “seeing.” |
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The Ear
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In addition to its role in hearing, the ear also functions as the sense organ of equilibrium and balance. The stimulation or “trigger” that activates receptors involved in hearing and equilibrium is mechanical, and the receptors themselves are called mechanoreceptors. Physical forces that involve sound vibrations and fluid movements are responsible for initiating nervous impulses perceived as sound and balance.The ear is more than an appendage on the side of the head. A large part of the ear- and its most important part – lies hidden from view inside the temporal bone. It is divided into the following anatomical areas:
External ear Middle ear Inner ear |
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The External Ear
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The external ear is where sound waves are collected and directed inward toward the eardrum and onto the middle ear.
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Middle Ear
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The tympanic membrane separates the outer ear from the middle ear. This is an air filled cavity that contains the three smallest bones in the body that transmit sound waves from the tympanic membrane across to the inner ear. There are two passages that connect with the middle ear; the eustachian tube and mastoid air cells. If unequal pressure occurs between the middle ear and the atmosphere the this can cause distortion to sound and also pain.
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The Inner Ear
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The activation of specialized mechanoreceptors in the inner ear generates nervous impulses that result in hearing and equilibrium. Anatomically the inner ear consists of three spaces in the temporal bone, assembled in a complex maze called the bony labyrinth. Certain structures within the inner ear are specifically concerned with hearing and balance of the body.
In addition to its role in hearing, the ear also functions as the sense organ of equilibrium and balance. The stimulation or “trigger” that activates receptors involved in hearing and equilibrium is mechanical, and the receptors themselves are called mechanoreceptors. Physical forces that involve sound vibrations and fluid movements are responsible for initiating nervous impulses perceived as sound and balance |
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Taste: Gustation
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Taste buds are found on mainly on the tongue. They are also found on the palate, lips and throat. They are sensory structures for taste. The perception of taste is affected by whether the food or fluid that is placed in the mouth is hot or cold.
There are five different primary tastes. However a combination of these five tastes allows a variety of different tastes to be experienced. |
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The Nose
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The nose is part of the upper respiratory tract and is discussed in detail when we cover the respiratory system. It is part of the conduction zone. It is divided into internal & external portions. The external nose provides the supporting framework of bone & hyaline cartilage. The openings below external nose are called the external nares (nostrils). The internal nose is referred to as the nasal cavity, it is divided by the septum. The internal nose consists of Olfactory epithelium and is lined by pseudostratified ciliated columnar cells.
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Smell: Olfaction
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Odours stimulate sensory olfactory epithelial receptors in the nose. Olfactory bulbs are contained in the roof of the nose in the cribriform plate. Approx 10 million neurons project from these bulbs into the olfactory epithelium which bulge out to form vesicles. These vesicles contain cilia on their surface called olfactory hairs.
On the above diagram, can you identify the olfactory bulb, olfactory epithelium, olfactory cilia and olfactory neuron? When air airborne molecules enter the nasal cavity the olfactory hairs are stimulated and they contain odorant receptors which are chemoreceptors and detect the sense of smell. The stimulus is sent to the limbic system in the brain for interpretation. Like taste there are primary odours but researchers are not sure of the exact number. There is thought to be around seven. |
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When people are asked to list the senses, they usually come up with five:
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vision, taste,
smell, hearing, and touch. Actually, touch reflects the combined activity of the general senses. |
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special senses
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The remaining senses—vision, taste, smell,
hearing, and equilibrium—are called special senses. |
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Most of us tend to forget the sense
of equilibrium, |
whose receptors are housed in the ear along with the organ of hearing.
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In contrast to the widely distributed general receptors (most of which are modified
nerve endings of sensory neurons), |
the special sensory receptors are distinct receptor
cells. These receptor cells are confined to the head region and are highly localized, either housed within complex sensory organs (eyes and ears) or in distinct epithelial structures (taste buds and olfactory epithelium). |
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Eyebrows
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The eyebrows are short, coarse hairs that overlie the supraorbital margins of the skull. They help shade the eyes from sunlight and prevent perspiration trickling down the forehead from reaching the eyes.
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Eyelids
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Anteriorly, the eyes are protected by the mobile eyelids or palpebrae (pal'pĕ-bre). The eyelids are separated by the palpebral fissure (“eyelid slit”) and meet at the medial and lateral angles
of the eye—the medial and lateral commissures (canthi), respectively. The medial commissure sports a fleshy elevation called the lacrimal caruncle (kar'ung-kl; “a bit of flesh”). The caruncle contains sebaceous and sweat glands and produces the whitish, oily secretion (fancifully called the Sandman’s eye-sand) that sometimes collects at the medial commissure, especially during sleep. In most Asian peoples, a vertical fold of skin called the epicanthic fold commonly appears on both sides of the nose and sometimes covers the medial commissure. The eyelids are thin, skin-covered folds supported internally by connective tissue sheets called tarsal plates. |
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eyelashes
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Projecting from the free margin of each eyelid are the eyelashes.
The follicles of the eyelash hairs are richly innervated by nerve endings (hair follicle receptors), and anything that touches the eyelashes (even a puff of air) triggers reflex blinking. Several types of glands are associated with the eyelids. The tarsal glands are embedded in the tarsal plates, and their ducts open at the eyelid edge just posterior to the eyelashes. These modified sebaceous glands produce an oily secretion that lubricates the eyelid and the eye and prevents the eyelids from sticking together. Associated with the eyelash follicles are a number of smaller, more typical sebaceous glands, and modified sweat glands called ciliary glands lie between the hair follicles (cilium = eyelash). |
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chalazion & sty
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An infected tarsal gland results in an unsightly cyst called a
chalazion (kah-la9ze-on; “swelling”). Inflammation of any of the smaller glands is called a sty. |
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eye upward muscle
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The tarsal plates also anchor the orbicularis oculi and levator
palpebrae superioris muscles that run within the eyelid. The orbicularis muscle encircles the eye, and the eye closes when it contracts. Of the two eyelids, the larger, upper one is much more mobile, mainly because of the levator palpebrae superioris muscle, which raises that eyelid to open the eye. The eyelid muscles are activated reflexively to cause blinking every 3–7 seconds and to protect the eye when it is threatened by foreign objects. Each time we blink, accessory structure secretions (oil, mucus, and saline solution) spread across the eyeball surface, keeping the eyes moist. |
