Chapter 44: Sensory Systems (Part 2, Week 10)

Question 1

What is a visual organ in flatworms that detects light and its direction but does not form an image?

Answer 1

Eye cups

Question 2

What is a type of image-forming organ in arthropods and some annelids consisting of several hundred to several thousand light detectors called ommatidia?

Answer 2

Compund eyes

Question 3

What is a visual unit in the compound eye of arthropods and some annelids that functions as a separate photoreceptor capable of forming an independent image?

Answer 3

Ommatidium

Question 4

For each ommatidium of the compound eye, what consists of it?

Answer 4

• a two-part lens

- Rhabdom

Question 5

How is the two-part lens composed?

Answer 5

An outer region called the cornea and an inner crystalline cone

Question 6

What does the two-part lens focus?

Answer 6

Focuses light on a long central structure called a rhabdom.

Question 7

Since rhabdom is a column of light-sensitive microvilli that recieves input from the two-part lens, what does this column of microvilli belong to?

Answer 7

They are projections from the cell membrane of the photoreceptor cells of the ommatidium.

Question 8

What imparts the eye with increased sensitivity to light?

Answer 8

The extensive surface area of the microvilli.

Question 9

What do the pigmented cells surrounding photoreceptor cells do which isolates each ommatidium from its neighbors and allows each commatidium to be pointed at one narrow area in space?

Answer 9

They absorb excess light

Question 10

Facts about the Ommatidium

Each ommatidium senses the intensity and color of light. Although each ommatidium receives light from only a very narrow field, collectively they provide animals with a wide viewing area.

Combining the different inputs from neighboring ommatidia, the compound eye is believed to form a mosaic-type image that the brain interprets. Animals such as bees and fruit flies, with large numbers of ommatidia, presumably have sharper vision and a wider field of vision than do those with fewer sensory cells, such as grasshoppers.

As anyone who has tried to swat a fly knows, the compound eye is extremely sensitive to movement as an object moves across successive ommatidia. This helps flying insects evade birds and other predators.

Behavioral studies have shown, however, that the resolving power of even the best compound eye is considerably less than that of the single-lens eye.

Answer 10

N/A

Question 11

What is a type of eye found in vertebrates and some invertebrates that has only one lens, as opposed to compound eyes with many lenses?

Answer 11

Single-lens eye

Question 12

What is a small opening in the eye of a vertebrate that transmits different patterns of light emitted from or reflected off objects in the animal’s field of view?

Answer 12

Pupil

Question 13

What is a sheetlike layer of photoreceptors at the back of the vertebrate eye?

Answer 13

Retina

The light inputs form a visual image of the environment on the retina.

The activation of these photoreceptors triggers electrical changes in neurons that pass out of the eye through the optic nerves, which carry the signals to the brain. The brain then interprets the visual image that was transmitted.

Question 14

What is the tough outer sheath of the vertebrate eye called?

Answer 14

Sclera (whites of the eyes)

Question 15

What are the layer of blood vessels that exist between the sclera and the retina?

Answer 15

The choroid

Question 16

What becomes a clear layer, which is continous of the sclera, and thinner as it passes the front of the eyes?

Answer 16

The cornea

So the cornea is actually a part of the sclera but becomes transparent to not block visual stimuli.

Question 17

What is the role of the cornea? (2)

Answer 17

Focuses light and protection.

In single-lens eyes, however, the lens plays the major role in focusing light onto photoreceptors.

Question 18

What are the two cavitites of the vertebrate eye and what do they consist of?

Answer 18

• The anterior cavity is the part of the eye between the lens and the cornea where the iris is the circle of pigmented smooth muscle responsible for eye color.

The anterior cavity is largely filled witha thin liquid called the aqueous humor that helps maintain eye pressureand shape, and may serve a nutritive function.

• The larger posterior cavity between the lens and the retina contains the thicker vitreous humor, which further helps maintain the shape of the eye.

Question 19

Why do pupils widen and contract?

Answer 19

Because the smooth muscles of the iris, contract and release to let in or reduce the amout of light coming in.

Question 20

What, in the vertebrate eye, is the process in which contraction and relaxation of the ciliary muscles adjust the lens according to the angle at which light enters the eye?

Answer 20

Accommodation

Question 21

What do you call the region of the retina that is directly in line with the pupil?

What do you call the center of this region above that contains the highest density of photoreceptors for color and is responsible for the sharpness with what we see in daylight?

Answer 21

The macula.

The fovea.

Question 22

What do you call the point in the retina where the optic nerve leaves the eye and it does not have photoreceptors which forms a blind spot where light does not activate a response?

Answer 22

Optic disc

Invertebrates with single-lens eyes do not have a blind spot, because the photoreceptors in their eyes are at the front of the retina. Therefore, the optic nerve does not pass through the layer of photoreceptors before leaving the eye.

Question 23

What are the two photoreceptors with names that are derived from their shapes within vertebrates?

Answer 23

Rods and cones

Question 24

What photoreceptor is sensitive to low-intensity light and can respond to as little as one photon, but they do not discriminate different colors?

Answer 24

Rods

Hence why at night it is hard to see color. Rods are useful mostly at night, and they send signals to the brain that generate a black-and-white visual image.

Question 25

What photoreceptor are LESS sensitive to low levels of light, but are sensitive to wavelengths of light that allow animals to perceive color?

Answer 25

Cones

Question 26

What are the three functional parts of rod and cone photoreceptors?

Answer 26

Outer segment, inner segment, and synaptic terminal.

Question 27

What is the highly convoluted plasma membrane found in the rods and cones of the eye?

Answer 27

Outer segment - forms stacks like discs

These discs contain the pigment molecules that absorb light.

Question 28

What does the inner segment of rods and cones contain?

Answer 28

Cell nucleus and organelles

Question 29

What do the photoreceptors, cones and rods, not contain but have something else which has neurotransmitter-containing vesicles, which synapse with neurons within the retina?

Answer 29

They don’t have axons but do have synaptic terminals.

Question 30

Why are humans able to see low-intensity light easier if the light comes in at an angle of the lens?

Answer 30

Because a majority of the low-light sensitive photoreceptors, rods, are located around the periphery of the retina away from the fovea. (just the majority are on the periphery not just only there)

You can easily verify this. In early evening, before many stars are visible, look at the sky until you notice a star out of the corner of your eye. Now shift your gaze to where you thought you saw the star. You will probably not be able to locate it anymore.

When you look away again so that light from the dim star enters your eye at an angle, it will reappear. This demonstrates that under low-light conditions, your vision is better when the light is directed to the part of the retina that contains only rods.

Question 31

Why do cones provide sharp images?

Answer 31

Because of their density at the fovea.

Question 32

What are the two components that are bonded together within the visual pigments that are embedded in the photoreceptors of rods in cones in the outer segments?

Answer 32

Retinal, a derivative of vitamin A capable of absorbing light.

And

A protein called opsin, of which there are several types.

Question 33

What is the important of opsin?

Answer 33

Opsins are examples of G-protein-coupled receptors, which trigger a signal transduction pathway that changes the permeability of membrane channels to ions.

Question 34

T/F Rods and Cones contain different types of opsin proteins in their visual pigments.

Answer 34

True.

These pigments are named according to the type of opsin they contain. In rods, the visual pigment is named rhodopsins.

Cones contain any one of several types of visual pigments called cone pigments, or photopsins.

Question 35

Since photopsins are composed of retinal, and one of three types of opsin proteins, what does each type of opsin protein determine?

Answer 35

The wavelength of light that the retinal in a cone can absorb.

For example, each cone pigment in humans responds best to red, green, or blue light. Any given cone cell makes only one type of cone pigment.

Many different shades of these colors can be perceived, however, because the brain uses information about the proportion of each type of cone that was stimulated to generate perceptions of all other colors.

Question 36

How do photoreceptors differ from other sensory receptor cells?

Answer 36

Because at rest in the dark their membrane is slightly depolarized, whereas in response to a light stimulus, it becomes hyperpolarized.

In the dark, the cell membranes of the outer segments of resting cells are highly permeable to sodium ions. Na+ diffuses into the cytosol of the cell through open Na+channels in the outer segment membrane.

The Na+ channels are gated by intracellular cyclic guanosine monophosphate (cGMP). In the dark, cytosolic concentrations of cGMP are high, keeping Na+ channels open and depolarizing the cell.

This depolarization results in a continuous release of the neurotransmitter glutamate from the synaptic terminal of the photoreceptor.

The photoreceptor synapses with a postsynaptic cell that is the next neuron in the visual pathway. This initiates a series of events within the retina that is interpreted by the brain as an absence of light.

In contrast, when exposed to light, the Na+
channels in the outersegment membranes of the photoreceptor close. The resulting decrease in Na+ concentration leads to a hyperpolarization of the cell.

In response, the release of glutamate is stopped. This results in a series of cellular activations within the retina and brain that is interpreted as a visual image.

Question 37

Explain in steps how transduction happens in a rod cell in response to light (a photon).

Answer 37

1. In the disc membrane, the cis-retinal in the visual pigment absorbs a photon of light, isomerizing it to trans-retinal (conformational change in a double bond)
2. The change to trans-retinal causes a conformational change in opsin that activates transducin, a G protein in the disc membrane.
3. Transducin activates a phosphodiesterase, which converts cGMP into GMP. cGMP is the gating mechanism of Na+ ion channels.
4. Since the cGMP concentrations in the cytosol decreases, it dissociates with Na+ channels in the outer segment of the membrane and the Na+ channels are closed which hyperpolarizes the cell.
5. Lastly, since the hyperpolarization of the cell occurs, glutamate release is greatly reduced ultimately leading to a visual image.

Question 38

When it comes to the layers of the retina, which part do the cones and rods lie?

What is behind these photoreceptors?

Answer 38

The deepest part closest to the sclera.

Pigmented epithelium that absorbs light that missed the photoreceptors; this prevents scattering of light within the retina, which would degrade the sharpness of vision.

Question 39

What are the two transparent layers of cells that light must pass through to reach the photoreceptors?

Answer 39

Middle layer called bipolar cells and the top layer called ganglion cells.

Question 40

What are the cells in the vertebrate eye that make synapses with photoreceptors and relay responses to the ganglion cells?

Answer 40

Bipolar cells

Question 41

What are the cells in the vertebrate eye whose axons extend into the optic nerve?

Answer 41

Ganglion cells

Question 42

What are two other types of cells, other than bipolar and ganglion cells, that are interspersed across the retina?

Answer 42

Horizontal and amacrine cells

Question 43

Light Reception Begins at the Rods and Cones (Facts)

• These photoreceptor cells release neurotransmitter molecules that affect the membrane potential of bipolar cells. The membrane potential of the bipolar cells determines the amount of neurotransmitter that they release, which, in turn, controls the membrane potential of ganglion cells.
• When a threshold potential is reached in ganglion cells, action potentials are sent out of the eye via the optic nerve to the brain. These signals travel along pathways that include the thalamus, brainstem, cerebellum, and the cerebral cortex.
• Visual information is further refined and interpreted within the vision centers of the cortex. The cortex responds to such characteristics of the visual scene as whether something is moving, how far away it is, how one color compares with another, and the nature of the image (for example, a face).
• The cortex does not form a picture in the brain, but forms a spatial and temporal pattern of electrical activity that is perceived as an image.

Answer 43

N/A

Question 44

What cells modify electrical signals as they pass from the photoreceptors to the ganglion cells and adjust the signals significantly, enhancing an animal’s ability to visualize a scene by emphasizing the differences between images?

Answer 44

Horizontal and amacrine cells

Animals, like birds and reptiles, have extremely specialized and complex retinas that process the image before the brain interprets the signal.

Question 45

What cells make connections between photoreceptors and help to define the BOUNDARIES of an image?

Answer 45

Horizontal cells

Question 46

What cells are important in adjusting the eye to different light intensities and increasing the sensitiivity of the eye to moving images?

Answer 46

Amacrine cells

Question 47

What is a type of vision in animals having two eyes located at the front of the head; the overlapping images coming into both eyes are processed together in the brain to form one perception?

This type of vision enables depth perception.

Answer 47

Binocular (or steroscopic) vision

Question 48

What is a type of vision in animals that have eyes on the sides of the head; the animal sees a wide area at one time, though depth perception is reduced?

Answer 48

Monocular vision

Question 49

[Start 44.6 Chemoreception]

What includes the senss of smell (olfaction) and the taste (gustation), both of which involve detecting chemicals in air, water, and food?

Answer 49

Chemoreception

Question 50

T/F A single molecule binding to a chemoreceptor can be perceived as an odor.

Answer 50

True.

Question 51

Where are the chemoreceptor neurons located on sensory hairs of insects?

Answer 51

Located on the proboscis (coiled tounge), legs, feet, and antennae.

Question 52

How many chemoreceptors are on the hair cell of a bow fly’s foot?

Answer 52

4 different receptors and they respond to different molecules.

Receptors on dendrites of the chemoreceptor cells inside the pore bind to the molecules and initiate a sensory transduction pathway that opens ion channels in the membrane.

This depolarizes the plasma membrane of the chemoreceptor cell and generates action potentials, which are sent to the brain for interpretation.

Question 53

What is a fun fact about taste and smell that can be related to symptoms of COVID?

Answer 53

Taste and smell are closely related. The distinction is largely meaningless for aquatic animals, because for them all chemoreception comes through the water.

Even in terrestrial animals, about 80% of the perception of taste is actually due to activation of olfactory receptors. (This is why food loses its flavor when the sense of smell is impaired, such as when you have a cold.)

Question 54

What does mammals’ sensitivity of their olfactory ability depend on?

Answer 54

Their supply of olfactory receptor cells, which can range from 5 - 6 million in humans to 100 million in rabbits and 220 million in dogs.

Question 55

Where are the mammalian olfactory sensory receptor neurons located?

Answer 55

The epithelial tissue at the upper part of the nasal cavity.

These cells are surrounded by two additional cell types: supporting cells and basal cells.

Question 56

Where are the supporting cells of the olfactory sensory receptors located and what is their job?

Answer 56

That are located between the receptor cells and provide physical support for the olfactory receptors.

Question 57

What do the cubodial basal cells accomplish?

Answer 57

The basal cells differentiate into new olfactory receptors every 30–60 days, replacing those that have died after prolonged exposure of their cell endings.

Question 58

What part of the olfactory sensory receptors have long, thin extensions called cilia that extend into the mucous layer that covers the epithelium?

Answer 58

The dendrites

Despite the superficial similarity in structure, these cells do not function like the mechanoreceptor hair cells of the auditory and vestibular systems.

Question 59

What is located within the plasma membrane of the cilia, which provides a large surface area, and when airborne molecules are dissolved in the muscus, binds to them?

Answer 59

Receptor proteins.

When an odor molecule binds to its receptor protein, it initiates a signal transduction pathway that ultimately opens Na+ channels in the plasma membrane.

The subsequent depolarization results in action potentials being transmitted to the next series of cells located in the olfactory bulbs of the brain.

Question 60

What are the olfactory bulbs of the brain?

Answer 60

The olfactory bulbs are a collection of neurons that act as an initial processing center of olfactory information and relay it to the cerebrum for further processing and interpretation.

The relative size of the olfactory bulbs correlates with the importance of olfaction to a given species.

In humans, the olfactory bulbs make uponly about 5% of the weight of the brain, whereas in nocturnal animals like rats and mice, they can comprise as much as 20%.

Even with their relatively limited olfactory sensitivity, however, humans have the capacity to detect 10,000 or more different odors. Recent evidence suggests that this number could in principle be as high as 1 trillion! The mechanism by which mammals detect so many different odors remained a mystery until 1991, when two scientists uncovered the molecular basis of olfaction.

Question 61

What are structures located in the mouth and tongue of vertebrates that contain the sensory cells, supporting cells, and associated neuronal endings that contribute to taste sensation?

Answer 61

Taste buds

Question 62

What are the bumps that you see on your tounge?

Answer 62

They are not taste buds but papillae, which are elevated structures on the tounge that collect food molecules.

Question 63

What is the depressions in between papillae that contain many taste buds with sensory receptor cells?

Answer 63

Taste pores

Question 64

What do the sensory receptor cells, within the taste pores, have that extend into the taste pore itself?

Answer 64

Microvilli (another example of an anatomical sensory adaptation that increases surface area)

Question 65

Food that has dissolved in saliva attach TO WHAT and TO WHERE?

Answer 65

Receptor proteins on the microvilli.

This binding triggers intracellular signals that alter ion permeability and membrane potentials. The sensory receptor cells then release neurotransmitters onto underlying sensory neurons.

Action potentials travel from these neurons to the thalamus and other regions of the forebrain, where the taste is perceived.

Question 66

Taste Facts or Info

There are a number of different types of taste cells, and they are distributed on the tongue in different areas that overlap considerably.

Each type of cell has a specific transduction mechanism that allows it to detect specific chemicals present in ingested foods and fluids. Their activation results in the perception of sweet, sour, salty, and bitter tastes.

In addition, a recently recognized fifth taste perception called umami (after a Japanese word for delicious) is associated with the presence in ingested food of glutamate and other similar amino acids, and is usually described as making food flavorful.

It may account for the widely recognized effects of monosodium glutamate (MSG) in enhancing the flavor of food.

The senses of taste and smell are enhanced when we are hungry, a phenomenon that most likely occurs in other animals as well. Once we have eaten, we are less aware of the smell and taste of food.

The importance of this for survival is clear: A hungry animal needs to eat. An enhanced sense of smell aids in locating food, and a heightened sense of taste encourages an animal to eat.

Afterward, these senses become temporarily dampened so as not to distract an animal from its other needs. The mechanism by which these changes occur is uncertain, but it may involve a temporary alteration in the number of smell and taste receptors, or in their ability to bind ligands.

Answer 66

N/A

Question 67

Taste Facts or Info

There are a number of different types of taste cells, and they are distributed on the tongue in different areas that overlap considerably.

Each type of cell has a specific transduction mechanism that allows it to detect specific chemicals present in ingested foods and fluids. Their activation results in the perception of sweet, sour, salty, and bitter tastes.

In addition, a recently recognized fifth taste perception called umami (after a Japanese word for delicious) is associated with the presence in ingested food of glutamate and other similar amino acids, and is usually described as making food flavorful.

It may account for the widely recognized effects of monosodium glutamate (MSG) in enhancing the flavor of food.

The senses of taste and smell are enhanced when we are hungry, a phenomenon that most likely occurs in other animals as well. Once we have eaten, we are less aware of the smell and taste of food.

The importance of this for survival is clear: A hungry animal needs to eat. An enhanced sense of smell aids in locating food, and a heightened sense of taste encourages an animal to eat.

Afterward, these senses become temporarily dampened so as not to distract an animal from its other needs. The mechanism by which these changes occur is uncertain, but it may involve a temporary alteration in the number of smell and taste receptors, or in their ability to bind ligands.

Answer 67

N/A

Question 68

[Start 44.7 Impact on Public Health]

T/F Sensory disorders are among the most common neurological problems found in humans and range from mild (needing eyeglasses or a hearing aid) to severe (blindness or deafness).

Answer 68

True

Question 69

What are the three disorders that affect vision loss the most?

Answer 69

Glaucoma, macular degeneration, and cataracts.

Question 70

What is a condition in which drainage of aqueous humor in the eye becomes blocked and the pressure inside the eye increases. If untreated, this pressure damages cells in the retina and leads to irreversible loss of vision?

Answer 70

Glaucoma

Causes are not always known, but severe trauma or infection, chronic use of certain medicines, blood vessel disease of the eyes, or diseases such as diabetes.

Accounts for roughly 10% of all cases of blidness in the U.S.

Question 71

What is an eye condition in which photoreceptor cells in and around the macula (which contains the fovea of the retina) are lost; one of the leading causes of blindness in the U.S?

Answer 71

Macular degeneration.

Because so many cones are in this region, sharpness and color are lost. Mostly occurs after 60.

Risk factors are heredity, smoking, hypertension, elevated blood cholesterol, and obesity.

Accounts for roughly 25% of all blindness cases. No cure, but there are treatment options that can slow it.

Question 72

What is an accumulation of protein in the lens of the eye; causes blurring and poor night vision?

Answer 72

Cataracts

By age 65, as many as 50% of all people have one or more cataracts in either eye, and jumps to 70% by age 75.

Tramua, chronic used of certain medicinal drugs such as steroids, hypertension, diabetes, and heredity.

Also associated with alcohol abuse, excessive exposure to UV radiation from the sun, and smoking.

Question 73

What is hearing loss, usually cause by damage to the hair cells within the cochlea?

Answer 73

Deafness

Some cases result from functional problems in brain areas that process sound or in the nerves that carry information from the hair cells to the brain.

Hearing loss is the leading occupational disorder in the U.S. and accounts for 7 - 21% of all cases of hearing loss worldwide.

Chronic exposure to loud sounds appears to produce a state of metabolic exhaustion in the hair cells of the cochlea. As a result, these cells become fatigued and are unable to maintain normal biochemical processes.

One consequence of this is the build up of free radicals and they oxidize lipids in cellular membranes, damaaging them. Mitochondrial membranes appear to be the most susceptable and when they die, the cell cannot produce ATP needed to fulfill energy demands is compromised.