3 main types of receptors:
Mechanical Receptors I. Chemical Receptors
A. Taste Receptors, e.g., Sweet, Sour, Salt, Bitter, Umami
Umami – a taste of glutamic acid, a natural amino acid associated with protein rich foods.
Monosodium glutamate (MSG) is a seasoning that gives the same umami taste. Because MSG has no smell or specific texture of its own, it can be used in many different dishes where it enhances the original flavor of the food.
Reminder: General principle: Sensations are not an inherent property of the stimulus, but are present exclusively in the nervous system itself.
B. Olfactory Receptors – Sense of Smell
Note that our sense of taste and sense of smell are controlled by chemical receptors and in both cases the substance must be dissolved in a fluid before the receptors can respond. II. Visual Receptors
Two types of animal visual systems.
A. Compound Eyes are found in arthropods (e.g., insects, crustaceans). Each compound eye consists of Ommatidia, the subunits, where each subunit consists of its own lens, receptors, and sensory neurons. Compound eyes are very sensitive to movements.
B. Camera Type Eyes are found in vertebrates, such as Humans. Each camera type eye consists of a single lens that focuses a single image on an array of receptor cells. Each receptor cell records only a small part of the total image.
The Structure of the Eye
Sclera - white outer tough layer, surrounds entire eyeball for protection.
Cornea – transparent part of the sclera – allows light to pass through.
Choroid – contains blood vessels to supply eye cells with oxygen and nutrients.
Pupil – opening or aperture of the eye. The size of the aperture is regulated by the iris.
Iris – pigmented part of eye that gives people their eye color. The iris is composed of
smooth muscle and controlled by reflex.
Lens – adjusts the focus for far and near objects and forms a single image on the inside
back of the eye. The lens consists of living cells.
Ciliary Muscles – control the thickness of the lens to alter its focus.
The lens is thin for normal, distance focusing, while the lens is allowed to become rounder and thicker for near focusing. The process of the lens becoming rounder is called accommodation.
Image is actually projected upside down on the retina. The brain then gives you the
perception of right side up.
Vitreous Humor – transparent gel that fills the eyeball.
Retina – light image is focused on the retina and the retina contains the photosensitive
Note that the receptors are located behind the sensory neurons, so that only about 10% of the light that enters the eye actually gets through to hit the receptors.
Optic Nerve – contains the axons of the sensory neurons relaying visual information to the brain.
Blind Spot (Optic Disc) – the location where there are no receptors, since the sensory neuron axons come together to form the optic nerve at that point.
Macula – The macula is a small area at the back of the eye that contains the fovea.
Fovea – the center of the macula and the area of most acute vision.
Cones are located primarily in the fovea and function in bright light.
There are about 6.5 – 7 million cones in an eye and each one synapses with its own sensory neuron.
Types of Cones
1. Sensitive to red light (64%)
2. Sensitive to green light (32%)
3. Sensitive to blue light (2%)
Rods are located outside the fovea, are especially abundant around the edges, number about 125 million in each eye, and are exceedingly sensitive to light. Rods can not detect color. The images rods produce are coarse and poorly defined. Each rod synapses with many different sensory neurons. New Photoreceptor discovered: intrinsically photosensitive retinal ganglion cells
Whereas rods and cones rapidly communicate changes in brightness and are responsible for coloring our world, the new class of cells send signals about overall brightness, somewhat like the light meter of a camera, telling the brain when it is night and when it is day. They number about 2,000 in an eye and function relative to our circadian rhythm.
Common Disorders and Abnormalities of the Eye
Near-sighted Eye = Myopia
Seeing is clearer at near distances. Eye ball is longer than normal. Concave lens to correct.
Far-sighted Eye = Hyperopia
Seeing is clearer at farther distances. Eye ball is shorter than normal. Convex lens to correct. May cause headaches because of more ciliary muscle contraction.
Astigmatism Caused by an uneven cornea or lens that prevents a single focal point, so that indistinct images are formed, i.e., a double, fuzzy image. Appropriate lens to correct.
Glaucoma Condition where there is an increase in pressure inside the eye to a point that kills eye cells. Glaucoma is the leading cause of blindness in the USA.
Cataract A cataract is not a growth. It is a cloudiness of the lens of the eye – like seeing through a dirty window.(The best foods to prevent cataracts and macular degeneration are fresh, uncooked fruits and vegetables. Clinical studies have demonstrated that vitamin C can actually halt cataract progression.)
Harold Ridley: Physician to Royal Air Force pilots in the Second World War.
Perspex – a tough plastic also called Plexiglas, Acrylic, and Lucite still used today. Silicone
Color Deficiency – also known as “color blindness.” Trouble discriminating colors.
Affects about eight to ten percent of all males. Red-green deficiency is most common. Hereditary color deficiencies cannot be cured and will remain unchanged.
III. Mechanical Receptors
A. The nature of sound: Sound is transmitted through the air as a disturbance of molecules (regions of compression and regions of rarefaction). No sound in a vacuum.
B. Hearing: The Ear
Pinna (height location of sound determined by time delays of reflections from the ridges of the pinna.), Auditory Canal, Tympanic Membrane = eardrum
Middle Ear: Hammer, Anvil, Stirrup, Oval Window
There are only 32,000 hearing receptors (16,000 in each ear) – compared to 131 million receptors in each eye.
A low frequency stimulates a sensation of low pitch or a low note, while a high frequency stimulates a sensation of high pitch or a high note.
Low frequency sounds stimulate receptors near the apex of the cochlea.
High frequency sounds stimulate receptors near the base of the cochlea.
Intermediate sounds stimulate intermediate areas – like a piano keyboard.
2. Volume is a function of the intensity or amplitude of vibrations. The decibel is a measure of sound volume. (You know that you are listening to a sound that is at least 85-dB if you must raise your voice to be heard by somebody else. Any sound above 85 dB can cause permanent hearing loss.) Permanent hearing loss is related both to the power of the sound as well as the length of exposure. For example, permanent hearing loss occurs after only 15 minutes at 115 decibels. Some rock bands have in their contracts that they will be playing at least 120 decibels. General Principle: Most loss of hearing in the USA is due to noise pollution, not due to aging. Older people living in a quiet environment have hearing ability just as good as younger people. 3. Tone is a result of a different pattern of stimulated receptors along the cochlea. C. Balance
Dynamic Equilibrium detects movement and orientation of the body – while the body is moving, e.g., rotation. Receptors are found in the semicircular canals. The ampulla at the base of the semicircular canal is the chamber where the hair receptor cells are located.
2. Static Equilibrium
Static Equilibrium deals with the stationary status of orientation – when the body is not moving. The receptors are located in chambers at the base of the semicircular canals.
Static Equilibrium Chambers: Sacculus, Utriculus Contain otoliths (“ear stones”) that respond to gravity and activate the hair cells.
D. Skin Receptors
Skin receptors are concentrated in various parts of the skin on the body, e.g., there are more receptors for touch on the finger tips than on your back.
Types of Skin Receptors:
1. Free Nerve Endings – detect pain.
2. Meissner’s Corpuscle – sensitive to very light touch (and vibration).