Chapter 8: Excitation and Contraction of Smooth Muscle

Question 1

Which type of smooth muscle fibers is mainly controlled by non-nervous stimuli?

Answer 1

unitary or single-unit muscle fibers

Question 2

Which structures allow free flow of ions from one muscle cell to the next to allow unitary smooth muscle contraction?

Answer 2

gap junctions

Question 3

Where do actin filaments attach in smooth muscle cells?

Answer 3

dense bodies

Question 4

These bonds transmit the force of contraction from one smooth muscle cell to another

Answer 4

intercellular protein bridges between membrane-dense bodies

Question 5

This organization allows smooth muscle cells to contract as much as 80% of their length instead of being limited to less than 30%, as occurs in skeletal muscle.

Answer 5

bidirectional pull of the “side-polar” cross-bridges of myosin filaments

Question 6

The ________ that the cross-bridges remain attached to the actin filaments, which is a major factor that determines the force of contraction, is believed to be greatly increased in smooth muscle.

Answer 6

fraction of time

**the maximum force of contraction of smooth muscle is often greater than that of skeletal muscle, as much as 4 to 6 kg/cm2 cross-sectional area for smooth muscle in comparison with 3 to 4 kilograms for skeletal muscle.

Question 7

What is the reason for the slow cycling time in smooth muscle fibers compared to skeletal muscles?

Answer 7

Smooth muscle fibers have far less ATPase activity than skeletal muscle fibers

Question 8

This mechanism maintains the prolonged tonic contraction in smooth muscle for hours, with little use of energy; hence, little continued excitatory signal is required from nerve fibers or hormonal sources.

Answer 8

latch mechanism

**When smooth muscle has developed full contraction, the amount of continuing excitation can usually be reduced to far less than the initial level even though the muscle maintains its full force of contraction.

Question 9

This phenomenon, occurring in smooth muscle fibers, allows a hollow organ to maintain about the same amount of pressure inside its lumen despite sustained large changes in volume

Answer 9

stress-relaxation or

reverse stress-relaxation

Question 10

This regulatory protein that is activated by calcium ions to cause skeletal muscle contraction is absent in smooth muscle fibers

Answer 10

troponin

Question 11

This regulatory protein counterpart of troponin is present in smooth muscles for binding with calcium ions

Answer 11

calmodulin

Question 12

This is the phosphorylating enzyme activated by the calcium-calmodulin complex

Answer 12

myosin light chain kinase

Question 13

Which chain of smooth muscle myosin filament is phosphorylated for the attachment-detachment of myosin head with the actin filament?

Answer 13

regulatory chain

Question 14

Where is the main source of calcium ions for skeletal and smooth muscles, respectively, for the initiation of action potential?

Answer 14

skeletal muscle = sarcoplasmic reticulum

smooth muscle = extracellular fluid

Question 15

This is the time required for the diffusion of calcium ions from the ECF into the cell before smooth muscle contraction begins

Answer 15

latent period

Question 16

This is a rudimentary analog of the transverse tubule system of skeletal muscles

Answer 16

caveolae

Question 17

This protein channel is required to caused smooth muscle relaxation

Answer 17

calcium pump

Question 18

What enzyme splits the phosphate from the regulatory light chain of the myosin heads to cause cessation of contraction in smooth muscles?

Answer 18

myosin phosphatase

Question 19

What most likely explains the mechanism of the latch state in smooth muscles?

Answer 19

Deactivation of myosin light chain kinase and myosin phosphatase

**as activation of the enzymes decreases, the cycling frequency decreases but, at the same time, the deactivation of these enzymes allows the myosin heads to remain attached to the actin filament for a longer and longer proportion of the cycling period.

Question 20

These structures, formed with autonomic nerve fibers that don’t make direct contact with smooth muscles but branch diffusely on top of a sheet of muscle fibers, secrete their transmitter substance into the matrix coating of most smooth muscles, often a few nanometers to a few micrometers away from the muscle cells

Answer 20

diffuse junctions

Question 21

These are structures of the fine terminal axons of smooth muscles that are counterparts of the typical branching end feet in motor end plate on skeletal muscles.

Answer 21

varicosities

Question 22

What type of neuromuscular junctions in smooth muscles provide more rapid contraction of multi-unit smooth muscle fibers?

Answer 22

contact junctions

Question 23

Although skeletal muscle fibers are stimulated exclusively by the nervous system, smooth muscle can be stimulated to contract by nervous signals, hormonal stimulation, stretch of the muscle, and several other ways. The principal reason for the difference is that the smooth muscle membrane contains:

Answer 23

many types of receptor proteins that can initiate the contractile process

Question 24

Action potentials occur in the same way in skeletal muscles and which type of smooth muscle fibers?

Answer 24

unitary smooth muscle = spike action potentials

Question 25

This is a local property of the smooth muscle fibers that make up the muscle mass and lead to spontaneous generation of action potential; however, these cannot cause muscle contraction themselves

Answer 25

slow wave rhythm

Question 26

What is the approximate threshold that elicit action potential in most visceral smooth muscles?

Answer 26

-35 mV

Question 27

In each peak of the slow wave, one or more action potentials occur. These repetitive sequences of action potentials elicit rhythmical contraction of the smooth muscle mass. Hence, slow waves are called:

Answer 27

pacemaker waves

Question 28

What causes the excitation of visceral smooth muscles upon stretching?

Answer 28

normal slow wave potentials and decrease in overall negativity of the membrane potential

Question 29

In small smooth muscle cells, what do you call the local depolarization caused by the nerve transmitter substance spreading “electrotonically” over the entire fiber and is all that is necessary to cause muscle contraction (even without an action potential)

Answer 29

junctional potential

Question 30

What are the two types of non-nervous and non-action potential stimulating factors often involved in smooth muscle contraction?

Answer 30

1. local tissue factors

2. hormones

Question 31

Give 3 factors for small blood vessels that provides a powerful local feedback control system that controls the blood flow to the local tissue area

Answer 31

Lack of oxygen
Excess Carbon dioxide
Increased hydrogen ion concentration

Question 32

Which hormone inhibits contraction of smooth muscle in the intestine but stimulates contraction of smooth muscle in blood vessels?

Answer 32

norepinephrine