Chapter 8 human physiology

Which of the following does not directly influence motor neurons? a. primary motor cortex b. cerebellum c. brain stem d. afferent neurons (through intervening interneurons) e. None of the above

b. cerebellum

Sarcomeres are arranged end to end as a. Z lines b. myofibrils c. T tubules d. myofilaments e. cross bridges

b. myofibrils

A motor unit refers to a. a single motor neuron plus all of the muscle fibers it innervates b. a single muscle fiber plus all of the motor neurons that innervate it c. all of the motor neurons supplying a single muscle d. a pair of antagonistic muscles e. a sheet of smooth muscle cells connected by gap junctions

a. a single motor neuron plus all of the muscle fibers it innervates

Troponin a. binds with calcium to allow sarcomere shortening b. breaks down myosin cross bridges c. is an enzyme in the sarcomere d. forms the boundaries of a sarcomere e. has an ATPase site

a. binds with calcium to allow sarcomere shortening

The sarcoplasmic reticulum stores ____ when a muscle is relaxed and releases it for binding to ____ during contraction. a. calcium; troponin b. calcium; tropomyosin c. sodium; tropomyosin d. potassium; tropomyosin e. sodium; troponin

a. calcium; troponin

Calcium that enters the cell during smooth muscle excitation binds with a. calmodulin b. inactive myosin kinase c. troponin d. myosin e. actin

a. calmodulin

Considering muscles, bones, and joints as lever systems, a calculated "moment" for the load would be a. force times load arm b. force times power arm c. load arm divided by force d. either a and b e. either b and c

a. force times load arm

Myosin a. is found in the A band b. is found in the I band c. is in the thin filaments d. shortens during muscle contraction because it is one of the contractile proteins e. is found in the Z line

a. is found in the A band

In the body's lever systems, the fulcrums are represented by the a. joints b. long bones c. tendons d. short bones e. skeletal muscles

a. joints

Stretch reflexes are important for a. maintaining balance and posture b. providing afferent information to the primary motor cortex c. determining which skeletal muscles will be used for a certain activity. d. both a and b e. both b and c

a. maintaining balance and posture

Pacemaker activity refers to spontaneous depolarizations resulting from a. shifts in passive ionic fluxes accompanying b. cyclical changes in Na+-K+ pump activity c. automatic changes in channel permeability d. both a and b e. both a and c

a. shifts in passive ionic fluxes accompanying

To pick up something heavier than your pencil, your nervous system could a. stimulate larger motor units b. decrease the frequency of stimulation to allow a more prolonged contraction to occur c. decrease the refractory periods d. block acetylcholinesterase at the neuromuscular junction to allow acetylcholine to function longer e. do all of the above

a. stimulate larger motor units

Fast-oxidative (type IIa) fibers a. contain very few mitochondria b. can be converted into fast-glycolytic fibers by regular resistance activities c. are most abundant in muscles specialized for maintaining low-intensity contractions for long periods of time without fatigue d. have characteristics b and c e. have none of the characteristics

b. can be converted into fast-glycolytic fibers by regular resistance activities

When a muscle atrophies, its muscle fibers a. split lengthwise b. decreases in mass and becomes weaker c. increase in diameter d. undergo mitotic cell division e. dissolve and are replaced by fibrous scar tissue

b. decreases in mass and becomes weaker

Spastic paralysis occurs when a. descending excitatory pathways are destroyed b. excitatory inputs to motor neurons are unopposed because of disruption of an inhibitory system in the brain stem c. muscle spindles are destroyed d. the cerebellum is damaged e. the motor neurons are destroyed

b. excitatory inputs to motor neurons are unopposed because of disruption of an inhibitory system in the brain stem

Z lines a. are formed by the T tubules b. extend down the middle of the I band c. are formed by cross bridges d. are part of the thin filaments e. extend down the middle of the sarcomere

b. extend down the middle of the I band

Skeletal muscle fibers: a. have T tubules within their sarcomeres b. have Z lines extending down the middle of the I bands c. contain gap junctions d. have all of the above characteristics e. have characteristics b and c

b. have Z lines extending down the middle of the I bands

A functional syncytium a. is a pair of antagonistic muscles causing a joint to move in opposite directions b. is able to contract as a unit because action potentials move between adjacent cells through gap junctions c. is a functional junction between a smooth muscle fiber and an autonomic nerve ending d. all of the above e. is none of the above

b. is able to contract as a unit because action potentials move between adjacent cells through gap junctions

The muscle cells of a marathon runner's legs would exhibit all these characteristics except a. high resistance to fatigue b. low myoglobin content c. low glycogen content d. many mitochondria e. slow speed of contraction

b. low myoglobin content

Repaying the oxygen deficit after strenuous exercise involves a. formation of lactate in the muscle cells b. replenishing stores of creatine phosphate and glycogen c. unloading oxygen from myoglobin d. all of the above e. both b and c

b. replenishing stores of creatine phosphate and glycogen

According to the sliding-filament mechanism, the a. A bands slide between the I bands b. thin filaments slide inward toward the center of the A band c. Z lines slide between the T tubules d. contractile proteins shorten, thus shortening the sarcomere e. filaments slide past the lateral sacs of the sarcoplasmic reticulum

b. thin filaments slide inward toward the center of the A band

During contraction of skeletal muscle fibers, the a. contractile proteins shorten b. thin filaments slide inward toward the A band's center as a result of cycles of cross-bridge binding and bending c. thick and thin filaments become tightly coiled, thus shortening the sarcomere d. I bands slide in between the A bands e. lateral sacs of the sarcoplasmic reticulum shorten, pulling the Z lines closer together

b. think filaments slide inward toward the A band's center as a result of cycles of cross-bridge binding and bending

Which of the following is involved in the relaxation of a muscle cell? a. Synthesis of ACh by acetylcholinesterase b. Initiation of action potentials c. The troponin-tropomyosin complex slipping back into its blocking position d. Detachment of actin from tropomyosin e. T tubules actively absorbing Ca2+

c. the troponin-tropomyosin complex slipping back into its blocking position

Identify the characteristic(s) shared by all types of muscle tissues? a. They are neurogenic. b. They have T tubules. c. They have tropomyosin. d. They attach to tendons. e. They have neuromuscular junctions.

c. they have tropomyosin

Each I band in a skeletal muscle fiber includes a. overlapping thin and thick filaments b. thick filaments and one Z disc c. thin filaments and one Z disc d. thin filaments and two Z discs e. actin, troponin, tropomyosin and two Z discs

c. thin filaments and one Z disc

Multi-unit smooth muscle is a. neurogenically activated b. under ANS control c. found in the iris of the eye d. all of the above e. none of the above

d. all of the above

The stretch receptors in the central portion of the muscle spindle can be activated by a. passive stretch of the whole muscle, including stretch of the muscle spindle b. contraction of the end portions of the muscle spindle c. gamma motor neuron stimulation of the muscle spindle d. all of the above e. none of the above

d. all of the above

Which of the following is involved in muscle relaxation? a. Acetylcholinesterase removes ACh and the muscle cell returns to resting potential. b. Ca2+ is actively taken up by the lateral sacs when there is no longer an action potential. c. The cross bridges pull the filaments back to their original resting positions. d. Both a and b e. Both b and c

d. both a and b

Which of the following muscle types are myogenic? a. cardiac muscle b. single-unit smooth muscle c. multi-unit smooth muscle d. Both a and b e. Both b and c

d. both a and b

Energy sources available to form ATP in muscle fibers in the absence of oxygen include a. creatine phosphate b. chemiosmosis c. glycolysis d. creatine phosphate and glycolysis e. chemiosmosis and glycolysis

d. creatine phosphate and glycolysis

Cardiac muscle tissue a. has well developed sarcoplasmic reticulum b. has fast myosin ATPase activity c. contracts only when stimulated neurogenically d. has gap junctions e. stores calcium in its T tubules

d. has gap junctions

Muscular fatigue can be caused by a. lactic acid accumulation b. depletion of ATP c. depletion of ACh d. only two of the above e. a, b, or c

e. a, b, or c

Calcium turns on cross bridges by physically repositioning the troponin-tropomyosin complex to uncover the actin cross-bridge binding sites in a. skeletal muscle b. cardiac muscle c. smooth muscle d. striated muscle e. all except c

e. all of the above except c

Which statement below is characteristic of most of the body's lever systems? a. They work at mechanical advantage. b. They work at a mechanical disadvantage. c. Muscles must exert greater forces than the load. d. Both a and b e. Both b and c

e. both b and c

Intrafusal muscle fibers a. are supplied by alpha motor neurons b. are found within muscle spindles c. contain sensory nerve endings that are activated by stretch d. characteristics a and c e. characteristics b and c

e. characteristics b and c

Which one of the following statements about cardiac muscle is incorrect? a. It contains gap junctions. b. It is found only in the heart c. It is self-excitable. d. It is striated with intercalated disks. e. It lacks tropomyosin.

e. it lacks tropomyosin

The origin of a muscle is its a. main, thickest part b. middle, thinner part c. movable end of attachment d. source of development in the fetus e. stationary end of attachment

e. stationary end of attachment

Cross bridge interaction between actin and myosin in skeletal muscle is directly blocked by a. acetylcholine b. triads c. Z lines d. calcium e. tropomyosin

e. tropomyosin

Binding of ____ to myosin permits the cross-bridge to _____. a. ATP; bind with actin b. ATP; detach from actin c. calcium; bind with actin d. calcium; detach from actin e. None of the above

b. ATP; detach from actin

Which of the following changes in banding pattern occur during muscle relaxation? a. Thin filaments move away from the Z lines. b. I bands get wider. c. H zones get narrower. d. Thick filaments become longer. e. Both b and c occur.

b. I bands get wider

Order the following muscle cell components from larger to smaller 1. troponin 2. myofibril 3. sarcomere 4. thin filament 5. muscle fiber

5, 2, 3, 4, 1

An action potential rapidly spreads to the central portions of a muscle cell by means of a. Z lines b. sarcoplasmic reticulum c. T tubules d. lateral sacs e. both a and c

c. T tubules

Dihydropyridine receptors are most closely associated with a. neuromuscular junctions b. axon terminals c. T tubules d. myosin-binding sites e. ATPase sites

c. T tubules

Which of the following is most closely associated with a skeletal muscle fiber's latent period? a. hyperpolarization b. relaxation c. diffusion of calcium out of the SR's lateral sacs d. development of tension e. shortening of the muscle fiber

c. diffusion of calcium out of the SR's lateral sacs

Fatigue is the failure of a muscle fiber to maintain ____ as a result of previous contractile activity. a. excitability b. muscle mass c. tension d. sarcomere number e. mitochondria

c. tension

Sequence the following events involved in muscle cell contraction: 1. Sodium channels open and sodium flows in. 2. Impulse reaches the axon's terminal. 3. ACh binds to receptors on the muscle cell membrane. 4. Synaptic vesicles fuse with neuron's membrane and release ACh via exocytosis. 5. If enough sodium moves in to the muscle cell, an impulse (action potential) develops.

2, 4, 3, 1, 5

The region between two Z lines is a a. muscle fiber b. myofibril c. myofilament d. sarcomere e. sarcoplasmic reticulum

D. sarcomere

Skeletal muscle fibers a. are some of the longest cells in the body b. are attached end-to-end within a muscle c. contain Z lines within their A bands d. contain only one nucleus e. exhibit characteristics b and c

a. are some of the longest cells in the body

Actin a. does not perform a power stroke during muscle contraction b. contains a globular head that forms a cross bridge between the thick and thin filaments c. is not found within A bands d. is inhibited by Ca2+ e. does not attach to the Z lines

a. does not perform a power stroke during muscle contraction

Muscle tension a. is created when tension generated by sarcomeres is transmitted via the connective tissue and tendons to the bones b. is the force exerted on a muscle by the weight of an object c. is greater than the load during an isometric contraction d. is all of the above e. is none of the above

a. is created when tension generated by sarcomeres is transmitted via the connective tissue and tendons to the bone

Which statement is incorrect about smooth muscle? a. It develops less tension per unit cross-sectional area compared to skeletal muscle. b. It can maintain tension with comparatively less ATP consumption than skeletal muscle. c. It lacks troponin. d. The range of lengths over which it can develop near maximal tension is greater than for skeletal muscle. e. In the walls of a hollow organ it can accommodate variable volumes with little change in the pressure exerted on the organ's contents.

a. it develops less tension per unit cross-sectional area compared to skeletal muscle

The regulation of smooth muscle contraction is mediated by the phosphorylation of ____ in response to calcium binding to ____. a. myosin; calmodulin b. actin; calmodulin c. troponin; calmodulin d. myosin; troponin e. actin; troponin

a. myosin; calmodulin

Twitch summation a. results from increases in cytosolic calcium levels b. results from pumping of calcium into the sarcoplasmic reticulum c. can be enhanced by allowing a cell to completely relax d. rarely results in tetany e. none of the above

a. results from increases in cytosolic calcium levels

During a cross-bridge cycle in skeletal muscle, a. the cross bridge is energized when it splits ATP into ADP and P b. the cross bridge must bind with ATPase before a power stroke will occur c. Ca2+ causes tropomyosin to move off of the myosin's actin-binding sites d. ADP binds to the cross bridge at the end of the power stroke e. both a and b occur

a. the cross bridge is energized when it splits ATP into ADP and P

Which statement is not true about cross bridges? a. They are a component of thin filaments. b. They are comprised of myosin. c. They have an actin binding site. d. They have an ATPase binding site. e. They provide power stroking.

a. they are a component of thin filaments

Of the steps in excitation-contraction below, which step occurs before the others? a. Exposed actin sites bind with myosin cross bridges. b. Sodium channels open in the fiber's membrane. c. Calcium is released from the sarcoplasmic reticulum. d. Troponin binds calcium. e. Attachment of ATP allows for cross bridge detachment.

b. Sodium channels open in the fiber's membrane

In a muscle fiber undergoing maximal tetanic stimulation, the velocity of shortening ____ as the load ____. a. decreases; decreases b. decreases; increases c. increases; increases d. remains constant; increases e. remains constant; decreases

b. decreases; increases

Muscles developing tension while lengthening are performing ____ contractions. a. concentric b. eccentric c. isometric d. fatiguing e. oscillating

b. eccentric

With eccentric muscle contractions, a. development of tension occurs at constant muscle length b. muscles lengthen while contracting c. muscles shorten while contracting d. muscle length and tension vary throughout a range of motion e. none of the above occur

b. muscles lengthen while contracting

Which of the following statements about the different types of muscle fibers is incorrect? a. The higher the ATPase activity, the faster the speed of contraction. b. Muscles with a high glycolytic capacity and large glycogen stores are more resistant to fatigue. c. Muscles with high ATP-synthesizing ability are more resistant to fatigue. d. Oxidative types of muscle fibers contain myoglobin. e. Muscle fibers containing large amounts of myoglobin have a dark red color in comparison to the paler fibers, which have little myoglobin.

b. muscles with a high glycolytic capacity and large glycogen stores are more resistant to fatigue

The T tubules a. store Ca2+ b. provide a means of rapidly transmitting the action potential from the surface into the central portions of the muscle fiber c. store ATP d. run longitudinally between the myofibrils e. have expanded lateral sacs

b. provide a means of rapidly transmitting the action potential from the surface into the central portions of the muscle fiber

During coactivation, a. all muscle fibers in a skeletal muscle are activated simultaneously b. the gamma motor-neuron and alpha motor-neuron systems to a skeletal muscle are activated simultaneously c. all cross bridges within a single skeletal muscle are activated simultaneously d. the primary (annulospiral) and secondary (flower-spray) endings within a muscle spindle are activated simultaneously e. none of the above occur

b. the gamma motor-neuron and alpha motor-neuron systems to a skeletal muscle are activated simultaneously

The energy for cross bridge cycling during muscle contraction is provided by: a. acetylcholine b. Ca2+ c. ATP d. myosin e. actin

c. ATP

Enlargement of muscle due to weight lifting is primarily a result of a. enlargement of individual myofilaments b. hyperplasia c. increased production of actin and myosin d. increases in the number of cells e. Both b and d

c. increased production of actin and myosin

Choose the false statement about smooth muscle. a. It is under involuntary control. b. It does not have troponin. c. Its contraction is initiated neurogenically only. d. It is found in walls of hollow tube-like organs. e. It is innervated by the ANS.

c. its contraction is initiated neurogenically only

Submaximal isometric contractions are important for a. moving large objects b. walking c. maintaining posture d. writing e. both a and b

c. maintaining posture

What is responsible for initiating contraction of smooth muscle? a. Stimulation by motor neurons b. Inhibition of acetylcholinesterase c. Membrane potential drifting to threshold as a result of automatic changes in ion movement across the membrane d. Excitation of the gap junctions by a transmitter substance e. Stimulation by the autonomic nervous system

c. membrane potential drifting to threshold as a result of automatic changes in ion movement across the membrane

Thick filaments in skeletal muscle are composed of a. actin b. troponin and tropomyosin c. myosin d. all of the above e. only b and c

c. myosin

The striated appearance of skeletal muscle is due to the a. orderly arrangement of the T tubules b. orderly arrangement of the lateral sacs of the sarcoplasmic reticulum c. orderly arrangement of the thick and thin filaments into A and I bands d. orderly arrangement of the motor units e. presence of white and red muscle fibers within the muscle

c. orderly arrangement of the thick and thin filaments into A and I bands

During aerobic exercise, the primary means for ATP production in muscle fibers involves a. creatine phosphate b. fermentation c. oxidative phosphorylation d. glycolysis e. myoglobin

c. oxidative phosphorylation

During contraction, asynchronous cycling of cross bridges a. prevents tetany b. prevents thick filaments from slipping backwards c. prevents thin filaments from slipping backwards d. utilizes less ATP e. none of the above

c. prevents thin filaments form slipping backwards

During muscle contraction, a. the Z lines move farther away from the thick filaments b. the A band becomes narrower c. the H zone becomes narrower or disappears d. the I band remains unchanged e. both b and c occur

c. the H zone becomes narrower or disappears

During contraction of a sarcomere, a. the Z discs move farther apart b. the A bands become thicker c. the I bands become narrower d. the thin filaments become shorter e. events c and d occur

c. the I bands become narrower

Conscious initiation of muscle contraction is controlled by a. the spinal cord b. the brain stem c. the cerebral cortex d. the thalamus e. none of the above

c. the cerebral cortex

A function of tropomyosin is a. to bind Ca2+ b. to form attachments with myosin cross bridges c. to prevent myosin cross bridges from attaching to the thin filaments d. to act as an ATPase e. none of the above

c. to prevent myosin cross bridges from attaching to the thin filaments

Which of the following are involved in the regulation of cross bridge attachment activity? a. calcium ions b. troponin c. tropomyosin d. all of the above e. only b and c

d. all of the above

During excitation-contraction coupling, a. the action potential travels down the T tubules b. Ca2+ is released from the sarcoplasmic reticulum c. Ca2+ is taken up by the sarcoplasmic reticulum d. both a and b occur e. both a and c occur

d. both a and b occur

Myoglobin a. can store small amounts of O2 b. increases the rate of O2 transfer from the blood into muscle fibers c. is abundant in fast-glycolytic fibers d. can perform a and b e. has characteristics a and c

d. can perform a and b

Cross bridges a. bind to actin during muscle contraction b. are globular heads made of myosin c. do not bend during muscle contraction d. have characteristics a and b e. have characteristics b and c

d. have characteristics a and b

Foot proteins a. span the gap between a lateral sac of the sarcoplasmic reticulum and a T tubule b. are believed to serve as Ca2+ channels c. bind to and pull the thin filaments inward toward the A band's center during contraction d. have characteristics a and b e. are none of the above

d. have characteristics of a and b

Select the correct statement regarding smooth muscle. a. It composes the walls of the heart. b. It is absent in the walls of hollow organs. c. Its cells are multinucleated. d. Its cells are spindle-shaped. e. Its cells lack actin and myosin.

d. its cells are spindle-shaped

During an isometric contraction, the muscle a. maintains a constant tension b. shortens c. moves a body part d. maintains a constant length e. tension is greater than the load

d. maintains a constant length

Picking up a book at a constant speed requires that a. muscle tension be equal to the book's weight b. the muscle perform an eccentric contraction c. the muscle perform an isometric contraction d. the muscle perform a concentric contraction e. both a and d occur

d. the muscle perform a concentric contraction

Based on the length-tension relationship, a. stretching a skeletal muscle 30% longer than the lo results in a greater contraction b. varying the amount of overlap of thick and thin filaments does not greatly affect contraction force as long as tension remains the same c. more tension develops if a muscle is 30% shorter than its resting length d. muscle tension remains the same as long as the muscle's length is not more than 30% of the resting length. e. a resting muscle that is shorter or longer than its lo will generate less tension at the onset of contraction.

e. a resting muscle that is shorter or longer than its lo will generate less tension at the onset of contraction

Single-unit smooth muscle a. contains an abundance of gap junctions b. is self-excitable c. is found in the walls of the digestive, reproductive, and urinary tracts and small blood vessels d. forms functional syncytia e. all of the above

e. all of the above

Twitch summation a. is a means by which gradation of muscle contraction may be accomplished b. results from the additional release of Ca2+ within the cytosol of muscle fibers c. results from increasing the frequency at which motor units are firing within a muscle d. results from more cross bridges binding to actin within muscle fibers e. all of the above

e. all of the above

With the type of lever system exemplified by flexion of the elbow joint, when an object is held in the hand, the a. power arm of the lever is the distance between the elbow joint and the insertion of the biceps muscle b. load arm of the lever is the distance between the elbow joint and the hand c. velocity and distance moved by the hand is amplified at the expense of the biceps muscle having to exert considerably greater force than the actual load that is moved d. answers a, b, and c e. answers a and b

e. answers a and b

Muscle fibers develop tension by a. contraction of sarcomeres b. shortening c. the pushing of myosin by actin d. all of the above e. both a and b

e. both a and b

A lack of ATP in a contracted skeletal muscle cell would most likely cause a. cross bridges to detach from actin b. the sarcomeres to remain shortened c. an inability to relax d. both a and c e. both b and c

e. both b and c

The functional unit of skeletal muscle a. is the smallest contractile component of a muscle fiber b. is the area between two Z lines c. is the sarcomere d. contains only one A band e. exhibits all of the above characteristics

e. exhibits all of the above characteristics

In twitch summation, the muscle fiber a. is stimulated again before the fibers returns to resting potential. b. is stimulated again before it completely returns to resting position. c. contracts stronger but stronger action potentials do not occur. d. experiences a and b e. experiences b and c

e. experiences b and c

Actin a. has ATPase activity b. are spherical c. forms a helical chain that forms the main structural component of the thin filaments d. has characteristics a and b e. has characteristics b and c

e. has characteristics b and c

The first means by which ATP is produced at the onset of contractile activity is a. transfer of energy and phosphate from creatine phosphate to ADP b. oxidative phosphorylation c. fatigue rapidly d. contain myoglobin e. have all of the above characteristics

e. have all of the above characteristics

Fast-glycolytic (type IIx) muscle fibers a. have high myosin-ATPase activity b. can carry out oxidative phosphorylation c. fatigue rapidly d. contain myoglobin e. have all of the characteristics

e. have all the above characteristics

The H zone a. is the area within the middle of the A band where the thin filaments do not reach b. shortens or disappears during contraction c. contains only thick filaments d. does not contain cross bridges e. is all of the above

e. is all the above

If the load on a muscle is increased, eventually a load will be reached at which the velocity of shortening becomes zero. At this point, the muscle contraction is referred to as a. concentric b. eccentric c. isotonic d. isokinetic e. isometric

e. isometric

Myosin is a. found in the I band b. attached to tropomyosin c. a regulatory protein d. all of the above e. none of the above

e. none of the above

During an isotonic contraction, a. sarcomeres do not shorten in the muscle b. movement does not occur inside the muscle c. the muscle does not change length d. the muscle's tension does not overcome a load e. the muscle's tension remains constant

e. the muscles tension remains constant

Whole muscle tension depends on all of the following except a. number of muscle fibers contracting b. tension produced by each contracting fiber c. extent of motor-unit recruitment d. frequency of stimulation e. the proportion of each motor unit used at any given time

e. the proportion of each motor until used at any given time

Select the correct statement about the summation of simple twitches. a. They can occur because of the long duration of the action potential in a muscle fiber. b. They reduce the tension in a muscle. c. They result from the slow stimulation of a muscle fiber. d. The effect is unrelated to the refractory periods of action potentials. e. The twitches resulting from separate action potentials.

e. the twitches resulting from separate action potentials

Which statement is incorrect about cross bridges? a. They are globular myosin heads that protrude from the thick filaments. b. they bend during muscle contraction. c. They attach and detach from actin during muscle contraction. d. They are not found in the I band. e. They bind with troponin during contraction.

e. they bind with toponym during contraction

____ is not required for contraction of smooth muscle fibers. a. Calcium b. Calmodulin c. Phosphate d. ATP e. Troponin

e. troponin

A functional syncytium of cardiac muscles cells means that they a. are striated b. exhibit muscle tone c. have a short refractory period d. lack the stimulation of a pacemaker e. work as a unit mechanically and electrically

e. work as a unit mechanically and electrically