BSC 116 Ch 50 HW/Quiz

9. In a relaxed muscle, the myosin heads of the sarcomeres' thick filaments are extended and ready to bind to the actin strands of the thin filaments. But this binding does not occur until an action potential is triggered in the muscle cell. An action potential results in the release of Ca2+ ions from the sarcoplasmic reticulum into the cytosol of the muscle cell. As the concentration of Ca2+ rises in the cytosol, so does the concentration of Ca2+ in the sarcomeres. In response to changes in the Ca2+ concentration in the sarcomeres, two protein components of the thin filaments, troponin and tropomyosin, control access to actin's myosin-binding sites. In this way, Ca2+ concentration in the cytosol and sarcomeres regulates muscle contraction. Which of the following statements correctly describe(s) the relationship between Ca2+ concentration in the cytosol and the response in the sarcomere? Select all that apply. -Decreasing Ca2+ concentration causes dissociation of Ca2+ from troponin. -Increasing Ca2+ concentration causes movement of tropomyosin, exposing myosin-binding sites on actin. -Increasing Ca2+ concentration causes troponin and tropomyosin to bind to actin. -Decreasing Ca2+ concentration promotes interactions between actin and myosin. -Increasing Ca2+ concentration causes troponin to bind to tropomyosin.

-Decreasing Ca2+ concentration causes dissociation of Ca2+ from troponin. -Increasing Ca2+ concentration causes movement of tropomyosin, exposing myosin-binding sites on actin. (The sequence of events involved in the regulation of skeletal muscle contraction by Ca2+ ions is shown in the figure below. An action potential causes the release of Ca2+ ions from the sarcoplasmic reticulum (SR), resulting in an increase in Ca2+ in the cytosol and sarcomeres. Ca2+ binds to troponin complexes of the thin filaments, causing tropomyosin to move and expose the myosin-binding sites on actin. The myosin heads repeatedly bind and release actin, pulling the thin filaments toward the center of the sarcomere. After the action potential is completed, the Ca2+ channels close and ATP-dependent Ca2+ pumps transport Ca2+ back into the SR, resulting in a decrease in Ca2+ in the cytosol and sarcomeres. The lower cytosolic Ca2+ level causes Ca2+ to be released from troponin. As a result, tropomyosin moves and covers the myosin-binding sites on actin, blocking the myosin heads from binding to actin. This blockage of the myosin-binding sites, in conjunction with ATP binding to the myosin heads, causes the muscle to relax.)

Propagation of an action potential in a skeletal muscle cell links the signal from a motor neuron to contraction of the muscle cell. An action potential in a muscle cell is propagated by the same mechanism as in neurons, the sequential opening and closing of voltage-gated Na+ and K+ channels in the plasma membrane. However, in muscle cells, the topography of the plasma membrane is quite different than in neurons, and this difference is critical to the function of muscle cells. Which of the following statements correctly describe(s) T tubules and their role in conducting action potentials in muscle cells? Select all that apply. -T tubules lack the voltage-gated Na+ and K+ channels that are present in the plasma membrane. -T tubules have receptor proteins that bind neurotransmitters released from the synaptic terminal of the motor neuron. -T tubules are extensions of the sarcoplasmic reticulum that are in contact with the plasma membrane. -Without T tubules, the muscle cell would not be able to contract. -T tubules carry action potentials into the interior of the muscle cell via voltage-gated Na+ and K+ channels. -T tubules are infoldings of the plasma membrane that encircle the myofibrils and are in contact with the sarcoplasmic reticulum.

-Without T tubules, the muscle cell would not be able to contract. -T tubules carry action potentials into the interior of the muscle cell via voltage-gated Na+ and K+ channels. -T tubules are infoldings of the plasma membrane that encircle the myofibrils and are in contact with the sarcoplasmic reticulum. (The T tubules are invaginations of the muscle cell plasma membrane that extend deep into the muscle cell and are in close contact with (but not continuous with) the sarcoplasmic reticulum.The T tubules play two important roles in linking an action potential to muscle contraction. T tubules propagate the action potential from the plasma membrane into the interior of the muscle cell via voltage-gated Na+ and K+ channels. An action potential carried by a T tubule regulates the opening and closing of Ca2+ channels in the sarcoplasmic reticulum. The resulting change in cytosolic Ca2+ concentration triggers contraction of the myofibrils.

Suppose an individual had a mutation that decreased the enzymatic activity of phosphodiesterase in rod cells. What would be the effect of this mutation on the rod cells? -More molecules of transducin would be activated. -Fewer molecules of transducin would be activated. -The ratio of GMP to cGMP in the cytosol would increase. -A smaller hyperpolarization of the rod would occur.

A smaller hyperpolarization of the rod would occur. (If phosphodiesterase activity decreases, the concentration of cGMP in the cytosol will increase. An increased concentration of cGMP would result in more sodium channels in the rod cell membrane remaining open, and the change in membrane potential after light stimulation would be smaller. This could result in the individual having a decreased sensitivity to light and poor vision under dim light conditions.)

Myosin heads have binding sites for _____. See Concept 50.5 (Page 1124) -tropomyosin and actin -tropomyosin and troponin -ATP and calcium -ATP and actin -actin and calcium

ATP and actin

True or false? Cone cells are color photoreceptors that use different retinal molecules to absorb different wavelengths of light. -True -False

False (Although it is true that cone cells are the color photoreceptors in the eye, they use different opsin (not retinal) molecules in their opsin-retinal complexes to absorb different wavelengths of light.)

Which part of the eye contains the densest concentration of cone cells? -Lens -Opsin -Cornea -Fovea

Fovea (The fovea, located in the center of the retina, has the densest concentration of cone cells.)

Which of the following statements does not describe a vertebrate eye? -It contains many light-sensing columns called ommatidia. -It acquires images by focusing light on receptor cells. -It contains cells that are sensitive to either dim light or color. -It contains photoreceptors that are either rod- or cone-shaped.

It contains many light-sensing columns called ommatidia. (Ommatidia are only found in compound eyes.)

Which part of the eye bends light to focus it on the retina? -Sclera -Lens -Optic Nerve -Fovea

Lens (The lens focuses light onto the retina.)

In mammals, the eardrum transmits sound waves to three small bones, which transmit the waves to fluid waves within the cochlea. Where are the three small bones located? -Outer ear -Middle ear -Inner ear

Middle ear (The middle ear holds these tiny bones, the malleus, incus, and stapes.)

Which part of the eye is the opening through which light initially passes? -Fovea -Retina -Pupil -Sclera

Pupil (The pupil is the small opening in the center of the iris through which light initially passes.)

Which of the following statements about photoreception is true? -Rod cells are responsible for color vision. -Rhodopsin is found in cone cells. -Cone cells are sensitive to dim light. -The retinal molecule changes shape when it absorbs light.

The retinal molecule changes shape when it absorbs light. (The retinal molecule is the light-sensitive portion of the retinal-opsin complex.)

Suppose a mutation greatly increases the affinity of a rod's sodium channels for cGMP, such that once cGMP binds to a channel, it remains bound for much longer than normal. What would be the effect of this mutation on the rod's sodium channels? -The sodium channels would open and close randomly, regardless of the presence of light. -The sodium channels would remain open in the dark and close rapidly in the light. -The sodium channels would remain open longer, even when cGMP decreases after light stimulation. -The sodium channels would remain closed in the light and the dark, even when cGMP decreases after light stimulation.

The sodium channels would remain open longer, even when cGMP decreases after light stimulation.

True or false? The "blind spot" in vertebrate eyes is a region of the retina that contains no photoreceptors. -True -False

True

Skeletal muscle contraction is a graded process, meaning that you can voluntarily alter the strength and extent of contraction of your skeletal muscles, such as your biceps. Increasing the strength and extent of contraction occurs by increasing the number of muscle cells that receive action potentials. In addition, increasing the number of action potentials sent to a muscle cell can also increase muscle tension, as shown in the graph. A tiny muscle "twitch" is caused by a single action potential. Two or more closely spaced action potentials have an additive effect because the muscle does not have sufficient time to relax between action potentials. A long series of closely spaced action potentials results in a sustained, maximum contraction, called tetanus. Which of the following statements correctly describes why a series of closely spaced action potentials causes a sustained contraction rather than a series of closely spaced twitches? -Release of Ca2+ from the sarcoplasmic reticulum through channels is slow compared to the uptake of Ca2+ into the SR via ATP-dependent pumps, resulting in Ca2+ slowly trickling into the sarcomeres between closely spaced action potentials. -When a series of action potentials is closely spaced, there is not sufficient time for Ca2+ uptake into the sarcoplasmic reticulum between action potentials, and Ca2+ remains bound to troponin throughout the series. -Ca2+ ions are released quickly from troponin, keeping the Ca2+ concentration in the cytosol high between closely spaced action potentials. -Fewer Ca2+ ions are released from the sarcoplasmic reticulum as a result of several closely spaced action potentials than as a result of a single action potential.

When a series of action potentials is closely spaced, there is not sufficient time for Ca2+ uptake into the sarcoplasmic reticulum between action potentials, and Ca2+ remains bound to troponin throughout the series. (In response to a series of closely spaced action potentials, sustained muscle contraction (tetanus) can only occur if the Ca2+ level in the sarcomere remains high throughout the contraction. If sufficient Ca2+ were taken up into the sarcoplasmic reticulum (SR) to lower the Ca2+ level in the sarcomere significantly between each action potential, tropomyosin would cover the myosin-binding sites on the actin, and the muscle would relax between each action potential. The Ca2+ channels in the SR conduct ions much more rapidly than the Ca2+ pumps in the SR. So even though the Ca2+ channels close between closely spaced action potentials, the Ca2+ concentration in the cytosol and the sarcomeres remains high. Consequently, Ca2+ remains bound to troponin between action potentials, the myosin-binding sites remain exposed, and muscle contraction continues until tetanus is achieved.)

Sensory adaptation is apparent when _____. See Concept 50.1 (Page 1107) -a person is no longer aware of a heavy necklace that was put on earlier in the day -your initial dive into a cold swimming pool gives your skin "goose bumps" -sitting in a hot room causes you to sweat -individuals who live in cold climates have fewer cold receptors in their skin -water above a certain temperature stimulates pain receptors rather than temperature receptors

a person is no longer aware of a heavy necklace that was put on earlier in the day (Sensory adaptation is a decrease in responsiveness during continued stimulation.)

Which component of the signal transduction pathway in rod cells is found in the cytosol of the cell? -Retinal -cGMP -Rhodopsin -Transducin

cGMP (Retinal, rhodopsin, and transducin are all embedded in the membranes of the disks in the outer segment of a rod cell. cGMP acts as a signal molecule in the cytosol, connecting the activation of proteins in the membranous disks to the closing of sodium ion channels in the plasma membrane.)

Artificial electrical stimulation of a human's menthol-sensitive neurons would likely produce the sensation of _____. -hot temperature -cold temperature -deep pressure -odor of pepper

cold temperature

Elephants hear sounds that are too low for humans to hear. This sensitivity is primarily due to the differences in the _____. -flexibility of the basilar membrane in the cochlea -size and shape of the outer ear -arrangement and shape of the ossicles -size and flexibility of the tympanic membrane (eardrum)

flexibility of the basilar membrane in the cochlea

Artificial electrical stimulation of a human's capsaicin-sensitive neurons would likely produce the sensation of _____. -tactile stimulus -cold temperature -hot temperature -deep pressure

hot temperature

Statocysts contain cells that are _____. -chemoreceptors used in selecting migration routes -thermoreceptors used in prey detection -photoreceptors used in setting biological rhythms -mechanoreceptors used to detect orientation relative to gravity

mechanoreceptors used to detect orientation relative to gravity

Sensory transduction in the auditory system is much like transduction of _____. See Concept 50.2 (Page 1110) -hormones binding to receptor proteins -visual stimuli by rods in the retina -mechanosensory stimuli -odorant molecules binding to receptor proteins on olfactory neurons -sweet-tasting molecules binding to receptor proteins on the tongue

mechanosensory stimuli

During signal transduction in a rod cell, the sensory signal is amplified when -transducin activates a phosphodiesterase. -a shape change in retinal activates rhodopsin. -a photon converts cis-retinal to trans-retinal. -phosphodiesterase catalyzes cGMP hydrolysis.

phosphodiesterase catalyzes cGMP hydrolysis. (After activation by transducin, phosphodiesterase hydrolyzes many cGMP molecules, resulting in amplification of the signal and the hyperpolarization of the cell.)

Unlike signal transduction in many other sensory receptors, signal transduction in rods -results in hyperpolarization of the sensory cell. -does not involve the participation of a G protein. -takes place entirely in the plasma membrane. -involves a second messenger.

results in hyperpolarization of the sensory cell. (In a rod cell, sensory transduction results in hyperpolarization and a decrease in neurotransmitter release. In other sensory cells, such as olfactory receptor cells, sensory transduction instead results in depolarization and an increase in neurotransmitter release.)

Immediately after putting on a shirt, your skin might feel itchy. However, this perception soon fades due to _____. -reduced motor unit recruitment -accommodation -reduced receptor amplification -sensory adaptation

sensory adaptation When the mammalian brain compares the actual temperature of the body to the preferred temperature of the body, which general component is being used? -sensor -effector -integrator -motor !integrator

The energy for sensory transduction by the lateral line system in fish comes from _____. See Concept 50.2 (Page 1110) -electrical signals in the water -water movements -high-frequency sound waves transmitted in water -thermal energy -odors of prey

water movements (The neural signals evoked in the lateral line system provide all fish with feedback on their swimming movements and on other objects, including other fish, in the water.)