The Skeletal System: bone, bone formation, bone diseases ...



Chapter 8: The Special Senses

Lecture Notes taken predominantly from:

Marieb,E.N. 2009. Essentials of Human Anatomy and Physiology. PBC

The special senses include smell, taste, sight, hearing, and equilibrium. They are known as ‘special senses’ because, in contrast to the generalized sense of touch (pressure, pain, and temperature) sensors found throughout the body, the special senses are made up of specialized sensory neurons found only in certain parts of the body. In the case of hearing and sight, these special senses are also aided by relatively complex organs, i.e. the ear and the eye, in order to function properly.

I. Vision: The eye is arguably the most sensitive organ of the special senses. Together, both eyes contain about 70% of all the sensory neurons found throughout the human body (more than any other type of sensory system combined). Each eye is well protected by a bony orbit of the skull and a padding of adipose tissue that lines each bony orbit.

Accessory organs of the eye include the eyelids and eyelashes (protect the exposed portion of the eye and secrete modified sebum that lubricates the eye from tarsal glands located between and just posterior to the eyelashes); the conjunctiva (a mucus membrane that lines the eyelids and covers the exposed outer surface of the eye for additional lubrication); the lacrimal apparatus (composed of the lacrimal gland, lacrimal canals, lacrimal sac, and nasolacrimal duct that produce and manage lacrimal fluid, i.e. tears which help to cleanse, lubricate, and protect the eye from bacterial infection); and the extrinsic eye muscles (6 separate muscles that allow for conscious manipulation of the eye within the socket).

The conjunctiva are prone to inflammation known as conjunctivitis (a common infectious form = pink eye). An extraordinary production of lacrimal fluid during emotional stress or irritation often overwhelms the drainage capabilities of the lacrimal apparatus and results in tears; because drainage is into the nasal cavity, the ‘sniffles’ often coincide with tears. [pic]

The eye itself is composed of 3 major tissue layers: 1) the fibrous layer – outermost layer of fibrous connective tissue includes the sclera (white part of the eye) and the cornea (a clear window in front of the iris and pupil, subject to damage but heals quite readily, can be transplanted without fear of rejection due to a lack of direct blood supply); 2) the vascular layer – middle layer that includes the highly vascular choroid (delivers nutrients and removes waste from eye cells as well as contains pigments that help absorb excess light), the ciliary body (smooth muscle and tendon-like structures which control the shape of the lens for focus), and the iris (colored part of the eye responsible for changing the size of the pupil – opening through which light passes, the size of the pupil is controlled by reflex pathways); and 3) the sensory layer – innermost layer that contains the retina (the area lining the back of the eye that maintains the photoreceptors = rods and cones which actually detect the incoming light and relay nerve impulses to the brain). The sensory neurons send messages to the brain via the optic nerve which exits the back of the eye in an area known as the optic disc. The arrangement of photoreceptors in the retina is such that no photoreceptors are present within the optic disc. As a result, light focused on the optic disc (a.k.a. blind spot) cannot be detected by the eye. Be able to identify these structures on a diagram like the one seen above.

More on rods and cones: Rods are rod-shaped photoreceptors that sense dim light in gray tones; they are important in peripheral vision and night vision. You may notice that you see a world of gray tones by moonlight as a result. When rods are not functioning properly, we may suffer from ‘night blindness’ and reduced peripheral vision. Cones, on the other hand, sense detail and color. Three different types of cones detect slightly different shades of color. The fovea centralis, located near the center of the retina, is an area with nothing but cone receptors that can detect the highest detail. We automatically focus light when viewing objects for fine detail on the fovea centralis. If 1 or more of the 3 types of cones are not functioning properly, we suffer from various degrees and forms of ‘color blindness’.

More on the lens: The lens is a clear, mainly protein structure that is bowed outwards on both sides (=biconvex). As light passes through the lens, it is bent so that a real (inverted) image is projected onto the retina. For objects greater than 20 ft away, the lens remains ‘relaxed’ and a person with 20/20 vision can see clearly. For objects closer than 20 ft, the ciliary body muscles contract and cause the lens to bulge further outwards. The greater bulge bends light more dramatically and allows us to see close objects clearly – this is known as accommodation. To avoid excess eye strain while reading, one can simply focus on far away objects from time to time so that the ciliary body muscles have a chance to relax. As we age, the proteins of the lens can bind together and change shape enough to cause a clouding of the lens known as cataracts.

Internal fluid of the eye: The aqueous humor (water-like body fluid) of the eye is found in the most anterior chamber inside the eye between the cornea and the lens. This fluid is derived from blood plasma and helps maintain the lens and cornea; it is also continuously replenished and cycled back to the blood stream. If drainage of the aqueous humor back to the blood stream is somehow blocked, pressure inside the eye can build to a damaging level. Damage of this sort is known as glaucoma and is common in older people. The vitreous humor fills the rest of the eye (posterior chamber) between the lens and the retina. The vitreous humor is more of a gel and does not circulate like the aqueous humor. Debris formed in the vitreous humor, therefore, remains unless surgically removed.

Vision pathways to the brain: The optic nerve is a bundle of several nerve pathways from the eye; and, the medial nerves of each eye actually branch from the optic nerve and travel to the opposite side of the brain at the optic chiasma. The more lateral nerves of each eye actually remain on the same side of the brain. The nerves from the lateral side of 1 eye join and travel with the nerves from the medial side of the other eye in pathways known as optic tracts. A stroke (i.e. cerebrovascular accident = blood clot in the brain) can damage the optic tracts and cause hemianopia = loss of vision to the right or left side.

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II. Hearing and Equilibrium: The ear houses mechanoreceptors – specialized sensors that respond to changes in pressure by bending hairs. Both hearing and balance are detected by mechanoreceptors. The ear itself is divided into 3 sections: 1) the external ear (hearing only) – is composed of the auricle, or pinna (the cup-like projection we commonly recognize as the ear) and the external acoustic meatus (a skin-lined canal that leads to chambers inside the temporal bone and produces cerumen = earwax from modified sebaceous glands); 2) the middle ear (tympanic cavity, also hearing only) – begins at the tympanic membrane (eardrum) and also includes the 3 ossicles (malleus, incus, and stapes) as well as the pharyngotympanic tube (a tube that connects the air space of the middle ear to the throat in order to equalize pressure around the tympanic membrane, common pathway for infections from the throat to the middle ear causing otis media = middle ear infections); and 3) the inner ear (bony labyrinth: hearing and equilibrium) – houses the cochlea (hearing organ), vestibule (static equilibrium organ), and the semicircular canals (dynamic equilibrium).

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Static equilibrium is detected by tiny organs known as maculae found in chambers inside the vestibule – these sensory organs help us know which way is up and detects the angle of tilt of the head; also detect acceleration. The mechanoreceptors of the maculae are stimulated when the jellylike otolithic membrane responds to gravity or acceleration. Tiny stones of calcium salts called otoliths embedded in the otolithic membrane fall forward, for example, when we tilt our head forward with the pull of gravity, this action bends the hairs of the mechanoreceptors that send the signals to the brain.

Dynamic equilibrium sensors are found in the semicircular canals and dynamic equilibrium itself is detected by tiny organs known as crista ampullaris. As fluid inside the semicircular canals flow in response to angular motion (spinning motion) they bend the gel-like cupula of the crista ampullaris that in turn bends embedded hairs of mechanoreceptors. The brain then interprets these signals to determine dynamic equilibrium.

Hearing is sensed by a series of coordinated events that begins at the tympanic membrane. The tympanic membrane responds to tiny pressure differentials created by sound waves and vibrates at the same frequency as the incoming sound waves. This in turn causes the malleus, incus, and stapes bones to vibrate as well. The lever-like arrangement of these bones actually amplifies the incoming sound waves. The stapes transfers the amplified pressure waves to the oval window of the inner ear. This action creates hydrostatic (fluid) pressure waves to radiate through the fluid of the cochlea. These hydrostatic pressure waves force membranes of the cochlea to vibrate against the Spiral Organ of Corti that houses the mechanoreceptors of hearing. As the hairs bend from the pressure waves, signals are sent to the brain for interpretation. Volume is sensed by the number of hairs that are stimulated (more=louder) and pitch is sensed by where in the cochlea the hairs are being stimulates.

III. Taste and Smell: Both rely on chemoreceptors – sensory nerves stimulated by chemicals dissolved in saliva (taste) or mucus (smell) that originate from whatever is being tasted or smelled. Smell (a.k.a. olfaction) receptors are found on the roof of the nasal cavity. Long cilia of the olfactory receptors maintain chemical binding sites that bind to many different types of chemicals and allow us to recognize over 10,000 different smells. Smell signals are sent to the brain via the olfactory nerve. Taste = gustation is sensed by similar chemoreceptors found mainly on the tongue with a few also found on the soft palate and cheeks. Gustatory cells = taste receptors are found embedded in tiny tasting organs known as taste buds. Taste buds are in turn found in association with different types of small projections known as papillae. Tiny gustatory hairs (microvilli) of the gustatory cells have chemical binding sites that respond to chemicals dissolved in saliva from food. 5 different types of taste have been identified: sweet (sugars), sour (acids), bitter (alkaloids), salty (metal ions), and umami (glutamate of proteins). Overall sense of taste comes from taste receptors and the complimentary smells of food. Up to 80% of our sense of taste is actually derived from smell.

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