The Special Senses Chapter 7 (Pages 237-265)

The 6 special senses Sight Hearing Equilibrium Touch Smell Taste

How do we sense? TRANSDUCTION Sensory cells translate chemical, electromagnetic, and mechanical stimuli into action potentials that our nervous system can make sense of There are three basic categories of senses, depending on the types of receptors Chemical Light Mechanical

Light senses SIGHT Type of receptor: Photoreceptors Receptors that detect light waves Rods: Only register black  white (Grayscale) and general shapes Rods rule your peripheral vision Cones: Color (red, green, blue) and fine details Cones only work in bright conditions

Basic external anatomy of the eye

Extrinsic muscles Attach to the outer surface of the eye and are responsible for moving the eye within the orbital socket

Basic internal Anatomy of the eye

How do you see? Light is reflected from an object that you see The light passes through the cornea  pupil  lens  retina The rods and cones in the retina are stimulated to send impulses to the optic nerve The optic nerve transmits sensory signals to the occipital lobe of the brain

The image of an object travels as sensory information through the optic nerves The optic nerves cross at the optic chiasma The optic tracts then carry the visual stimuli to the occipital lobe of the brain Images seen on the left side of the body are interpreted in the right visual cortex Images seen on the right side of the body are interpreted in the left visual cortex

Myopia (Nearsightedness) the lens focuses objects in front of the retina due to an elongated eye shape, nearby objects are seen clearly, but distant objects appear blurry

Hyperopia (farsightedness) The distance from the lens to the retina is shortened because the eye is flatter, objects at a distance can be seen clearly, but objects close up appear blurry

Presbyopia (age related farsightedness) the lens ages and stiffens, images are formed behind the retina causing blurry close up vision

Astigmatism irregular curvature of the cornea or lens results in blurred vision

Color blindness the cone cells on the retina are affected and certain colors cannot be distinguished Remember cones sense Red, Green, and Blue light Color blindness is an inherited disorder (recessive) Much more common in males Son  only needs to inherit it from his mother Daughter  needs to inherit it from both mother and father A common form of color blindness is red-green color blindness Inability to distinguish red from green

Stare at this flag for 30 seconds without moving your eyes

What did you see on the white screen? You should have seen a full American flag (Red, White, and Blue) This is a common optical illusion called an “after image” When we stare at a bright object for some time, we can experience retinal fatigue - when the image disappears, we see the complimentary color. Or in other words, your cones get tired!

Mechanical senses HEARING, TOUCH & EQUILIBRIUM Type of receptor: Mechanoreceptors Receptors that detect sound waves and pressure on the skin and in the inner ear

anatomy of the outer ear Auricle (Pinna) Channels sound waves into the ear Auditory Canal Tube that carries sound waves to the middle ear Tympanic Membrane the eardrum Sound waves cause this to vibrate

Anatomy of the middle ear Ossicles: 3 smallest bones in the body, they transmit and amplify sound waves Hammer (malleus) Anvil (Incus) Stirrup (stapes) Oval Window The stapes attaches to the oval window and transmits sound waves into the inner ear Eustachian Tube Connects the middle ear to the pharynx to equalize pressure on each side of the tympanic membrane

Anatomy of the inner ear Cochlea Snail shaped structure, enables one to hear Vestibule Chamber that connects the three semicircular canals Semicircular Canals Channels containing receptor hair cells that play an important role in balance

How do you hear? Sound waves enter the outer ear (auricle/pinna) and move through the auditory canal towards the tympanic membrane The sound waves vibrate the tympanic membrane which transfers to the ossicles (hammer, anvil, stirrup) The vibrations are then amplified by the ossicles and transferred to the cochlea The cochlea is attached to nerve fibers which join to form the cochlear nerve The vibrations are translated into action potentials which are sent via the cochlear nerve to the brain where they are interpreted as sound in the auditory cortex (in the temporal lobe)

How do you balance? The vestibule of the inner ear contains three semicircular canals filled with fluid and hair cells Each semicircular canal is on a different axis As your head moves, the fluid moves accordingly, stimulating the hair cells The hair cells send action potentials to the vestibular nerve which communicates with the cerebellum Your cerebellum then interprets these signals about the orientation of the body and motion of the head

Motion sickness Usually a result of contradictory messages being sent to the brain from receptors Example: You spin quickly in a chair The hair cells in your semicircular canals are being stimulated due to rotation You suddenly stop spinning and sit still The fluid in your semicircular canals is still moving (aka the hair cells are telling your brain you’re still spinning) But your eyes and the receptors in your spine tell your brain that you are sitting still (NOT spinning) These contradictory messages confuse the brain and can cause motion sickness

Chemical Senses SMELL & TASTE Type of receptor: Chemoreceptors Receptors that only respond to chemicals, detect molecules from the environment Excited by chemicals dissolved in saliva and airborne chemicals dissolved in nasal membranes

Anatomy of the olfactory region

How do you smell? When you smell something, odor molecules dissolve in the mucous layer surrounding the olfactory hairs Olfactory receptor cells send action potentials through to the olfactory nerve The olfactory nerve sends the impulses through the ethmoid bone to the olfactory bulb which then sends impulses to the olfactory cortex in the temporal lobe

Smelly facts Humans have about 40 million olfactory receptors that allow us to identify about 10,000 different smells Dogs have a better sense of smell because they have 20 times more olfactory receptor cells than humans Their olfactory cortex is 40 times larger than ours It only takes a few odor molecules to trigger an action potential, which is why people can become used to smells easily Example: Someone who wears the same perfume everyday tends to not smell it on themselves The nerve pathway between the nose and the brain travels through the limbic system (responsible for emotions) This is why smells can trigger positive or negative emotions

rhinitis Rhinitis: an inflammation of the mucous membranes that line the nasal passage Usually caused by the common cold (the rhinovirus) Causes the release of histamines Molecules that trigger a reaction causing nasal congestion and drainage Congestion = thicker mucous layer = odor molecules can’t reach olfactory hairs Why you can’t smell well when you’re sick Treated with antihistimines

anosmia Loss of functioning olfaction or inability to smell Unfortunate because smell is responsible for 80% of taste! Can be temporary or permanent Possible causes: Nasal polyps Cocaine abuse Head trauma Toxic chemical exposure Radiation (cancer treatment) Certain medications

Septum problems Deviated Septum Perforated Septum A large shift of the septum away from the center Usually caused by injury Can be surgically repaired Perforated Septum Development of one or more holes in the septum Can be caused by injury, ulcer, long-term exposure to toxic fumes, or illegal drug abuse

Basic tongue & taste bud anatomy

Specific gustatory cells Basal epithelial cells Stem cells that replace gustatory cells every week or so Why burned tongues heal so quickly Gustatory epithelial cells (gustatory receptor cells in picture) Do the actual tasting using gustatory hairs

How do you taste? When you eat something, chemical molecules from the food called tastants are dissolved in saliva Tastants diffuse through the taste pore, hitting the gustatory hairs This activates the gustatory cells which triggers action potentials through three cranial nerves to the gustatory cortex of the brain

5 basic tastes Sweet Salty Sour Bitter Umami Taste maps are WRONG! The “savory” taste of beef or MSG Taste maps are WRONG! Each gustatory cell can only respond to one type of taste, BUT there are 50-100 gustatory cells per taste bud In other words, each taste bud can sense all 5 tastes