Ch8 Excitation and Contraction of Smooth Muscle

Question 1

What is the diameter range of smooth muscle fibers?

Answer 1

1 to 5 micrometers

Question 2

What are the two major types of smooth muscle?

Answer 2

• Multi-unit smooth muscle
• Unitary (or single-unit) smooth muscle

Question 3

What characterizes multi-unit smooth muscle?

Answer 3

Composed of discrete, separate smooth muscle fibers that operate independently

Question 4

How is control mainly exerted in multi-unit smooth muscle?

Answer 4

By nerve signals

Question 5

What is unique about unitary smooth muscle?

Answer 5

Fibers contract together as a single unit due to interconnections

Question 6

What are dense bodies in smooth muscle analogous to in skeletal muscle?

Answer 6

Z disks

Question 7

What is the role of calcium ions in smooth muscle contraction?

Answer 7

Activates the contractile process

Question 8

What is the energy source for smooth muscle contraction?

Answer 8

Adenosine triphosphate (ATP)

Question 9

What is the contraction duration of smooth muscle compared to skeletal muscle?

Answer 9

Prolonged tonic contraction lasting hours or days

Question 10

What is the latch mechanism in smooth muscle?

Answer 10

Allows maintenance of contraction with minimal energy use

Question 11

What is the typical onset time for smooth muscle contraction?

Answer 11

50 to 100 milliseconds after excitation

Question 12

What is stress-relaxation in smooth muscle?

Answer 12

Ability to return to original force of contraction after being stretched

Question 13

What is the difference in energy requirement for maintaining contraction between smooth and skeletal muscle?

Answer 13

Smooth muscle requires 1/10 to 1/300 as much energy

Question 14

How does the physical organization of smooth muscle differ from skeletal muscle?

Answer 14

Smooth muscle lacks a striated arrangement of actin and myosin filaments

Question 15

What causes the slow cycling of myosin cross-bridges in smooth muscle?

Answer 15

Lower ATPase activity compared to skeletal muscle

Question 16

What stimulates smooth muscle contraction?

Answer 16

Increase in intracellular calcium ions

Question 17

True or False: Smooth muscle contains the troponin complex.

Answer 17

False

Question 18

What happens to the pressure in the urinary bladder as it stretches?

Answer 18

Initially increases, then returns to nearly original level

Question 19

Fill in the blank: Smooth muscle can contract as much as _______ of their length.

Answer 19

80%

Question 20

What is the maximum force of contraction in smooth muscle compared to skeletal muscle?

Answer 20

Greater, often 4 to 6 kg/cm² for smooth muscle versus 3 to 4 kg for skeletal muscle

Question 21

What are the intercellular connections that allow ion flow in unitary smooth muscle?

Answer 21

Gap junctions

Question 22

What is the relationship between the amount of myosin filaments and the force of contraction in smooth muscle?

Answer 22

Maximum force can be greater despite fewer myosin filaments

Question 23

What initiates the contraction in smooth muscle?

Answer 23

Nerve stimulation, hormonal stimulation, stretch, or chemical changes

Question 24

What triggers the increase in intracellular calcium ion (Ca2+) concentration in smooth muscle?

Answer 24

Ca2+ enters through calcium channels or is released from the sarcoplasmic reticulum

The increase in Ca2+ concentration activates contraction processes.

Question 25

What complex is formed when calcium ions bind to calmodulin (CaM)?

Answer 25

Ca2+-CaM complex ## Footnote This complex activates myosin light chain kinase (MLCK).

Question 26

What does active myosin light chain kinase (MLCK) do?

Answer 26

Phosphorylates the myosin light chain ## Footnote This phosphorylation allows myosin heads to bind with actin filaments.

Question 27

What is the role of calmodulin in smooth muscle contraction?

Answer 27

Activates myosin cross-bridges ## Footnote Calmodulin is crucial for initiating contraction in smooth muscle.

Question 28

What is the primary source of calcium ions for smooth muscle contraction?

Answer 28

Extracellular fluid ## Footnote Unlike skeletal muscle, smooth muscle relies on calcium from outside the cell.

Question 29

What is the latent period before contraction in smooth muscle?

Answer 29

200 to 300 milliseconds ## Footnote This period is significantly longer than that in skeletal muscle.

Question 30

What structure in smooth muscle is analogous to the transverse tubule system in skeletal muscle?

Answer 30

Caveolae ## Footnote Caveolae are small invaginations that facilitate calcium ion release.

Question 31

How does extracellular calcium ion concentration affect smooth muscle contraction?

Answer 31

Smooth muscle contraction ceases when calcium concentration decreases to about 1/3 to 1/10 normal ## Footnote This contrasts with skeletal muscle, where contraction is less affected by extracellular calcium levels.

Question 32

What is required for smooth muscle relaxation after contraction?

Answer 32

Calcium pump ## Footnote This pump removes calcium ions from the intracellular fluid.

Question 33

What enzyme is responsible for the cessation of contraction in smooth muscle?

Answer 33

Myosin phosphatase ## Footnote This enzyme splits phosphate from the regulatory light chain of myosin.

Question 34

What phenomenon allows smooth muscle to maintain tone without much energy expenditure?

Answer 34

Latch phenomenon ## Footnote This mechanism allows myosin heads to stay attached to actin with minimal ATP usage.

Question 35

What types of stimuli can cause smooth muscle contraction?

Answer 35

* Nervous signals * Hormonal stimulation * Stretch of the muscle * Other means ## Footnote This is in contrast to skeletal muscle, which is only stimulated by the nervous system.

Question 36

How are neurotransmitters secreted at the smooth muscle neuromuscular junction?

Answer 36

From autonomic nerve varicosities ## Footnote These varicosities secrete neurotransmitters into the matrix coating the muscle.

Question 37

What are the two main neurotransmitters involved in smooth muscle contraction?

Answer 37

* Acetylcholine * Norepinephrine ## Footnote These neurotransmitters can have excitatory or inhibitory effects depending on the receptor type.

Question 38

What determines whether smooth muscle is excited or inhibited?

Answer 38

Type of receptor protein on the muscle cell membrane ## Footnote Receptors can be either excitatory or inhibitory.

Question 39

What is the typical resting membrane potential of smooth muscle?

Answer 39

−50 to −60 millivolts ## Footnote This is less negative compared to skeletal muscle.

Question 40

What primarily generates the action potential in smooth muscle?

Answer 40

Flow of calcium ions ## Footnote Smooth muscle has more voltage-gated calcium channels than sodium channels.

Question 41

How does the action potential in smooth muscle differ from that in skeletal muscle?

Answer 41

Calcium channels open more slowly and remain open longer ## Footnote This results in prolonged plateau action potentials in smooth muscle.

Question 42

What is the main role of fiber in action potentials?

Answer 42

The fiber is mainly responsible for the action potential ## Footnote This refers to the electrical signals that facilitate communication in nerve and muscle fibers.

Question 43

How do calcium channels compare to sodium channels in terms of opening speed?

Answer 43

Calcium channels open many times more slowly than sodium channels ## Footnote This characteristic contributes to the prolonged plateau action potentials in smooth muscle fibers.

Question 44

What are the two forms of action potentials in visceral smooth muscle?

Answer 44

1. Spike potentials 2. Action potentials with plateaus

Question 45

What is the duration range of typical spike action potentials in unitary smooth muscle?

Answer 45

10 to 50 milliseconds ## Footnote This duration allows for rapid contractions in response to various stimuli.

Question 46

What is the significance of action potentials with plateaus in smooth muscle?

Answer 46

They can account for prolonged contraction in some types of smooth muscle ## Footnote Examples include the ureter and uterus.

Question 47

What is the role of calcium ions during action potentials in smooth muscle?

Answer 47

Calcium ions act directly on the smooth muscle contractile mechanism to cause contraction ## Footnote This dual role of calcium is crucial for muscle function.

Question 48

What are slow wave potentials in unitary smooth muscle?

Answer 48

They are basic slow wave rhythms of membrane potential that can lead to spontaneous action potentials ## Footnote Slow waves themselves do not cause contraction but can initiate action potentials.

Question 49

What triggers the generation of action potentials in response to slow waves?

Answer 49

When the peak of the negative slow wave potential rises to about -35 millivolts ## Footnote This is the approximate threshold for eliciting action potentials in most visceral smooth muscle.

Question 50

What can cause spontaneous action potentials in visceral smooth muscle?

Answer 50

Stretching of the muscle, combined with normal slow wave potentials ## Footnote This allows for automatic contractions in response to distension.

Question 51

What is the effect of local tissue factors on smooth muscle contraction?

Answer 51

They can cause smooth muscle relaxation and vasodilation ## Footnote Examples include lack of oxygen and excess carbon dioxide.

Question 52

What hormones influence smooth muscle contraction?

Answer 52

1. Norepinephrine 2. Epinephrine 3. Angiotensin II 4. Endothelin 5. Vasopressin 6. Oxytocin 7. Serotonin 8. Histamine

Question 53

How do hormones affect smooth muscle contraction?

Answer 53

They can either stimulate contraction or inhibit it depending on the type of receptors present ## Footnote Excitatory receptors cause contraction, while inhibitory receptors prevent it.

Question 54

What is the mechanism of smooth muscle excitation by hormones?

Answer 54

Hormones can open sodium or calcium channels, causing depolarization and contraction ## Footnote In some cases, this can occur without action potentials.

Question 55

What is hyperpolarization in smooth muscle?

Answer 55

It is an increase in negativity inside the muscle cell that inhibits contraction ## Footnote This occurs when sodium and calcium channels are closed or potassium channels are opened.

Question 56

What is the role of second messengers like cAMP and cGMP in smooth muscle?

Answer 56

They change the degree of phosphorylation of enzymes that indirectly inhibit contraction ## Footnote This can occur without directly changing the membrane potential.

Question 57

What is the relationship between norepinephrine and smooth muscle in different locations?

Answer 57

Norepinephrine can inhibit contraction in the intestine but stimulate contraction in blood vessels ## Footnote This illustrates the diversity of responses to the same substance in different tissues.