Chapter 8 - Guyton

Question 1

General characteristics of smooth muscle.

Answer 1

smooth muscle is composed of smaller fibers as compared to skeletal muscle, same contractive forces (actin and myosin) that cause skeletal muscle to contract also cause smooth muscle contraction

Question 2

Smooth muscle in each organ is distinct from other organs in these ways:

Answer 2

1. physical dimensions
2. organization into bundles or sheets
3. response to different types of stimuli
4. characteristics of innervation
5. function

Question 3

Explain multi-unit smooth muscle.

Answer 3

composed of separate smooth muscle fibers that operate independently and is innervated by a single nerve ending, control is exhibited mainly by nerve signals, outer surfaces of the fibers are covered by a membrane-like substance
a mixture of fine collagen and glycoprotein that helps insulate the separate fibers from one another, Ex. ciliary and iris muscles of eye and piloerector muscles that cause the erection of hairs on your body

Question 4

Explain single-unit smooth muscle.

Answer 4

aka syncytial smooth or visceral smooth muscle, control is exerted by non-nervous stimuli, hundreds of thousands of smooth muscle fibers contract together as a single unit, fibers are arranged in sheets/bundles, cell membranes are adherent to each other at multiple point which allows force generated at one muscle fiber to be transmitted to the next and are joined by gap junctions

Question 5

Does smooth muscle contain troponin?

Answer 5

no

Question 6

How is the force contraction in smooth muscle transmitted between one cell and the next?

Answer 6

intercellular protein bridges

Question 7

What is the function of the dense bodies in smooth muscle?

Answer 7

dense bodies serve the same role as the Z discs in skeletal muscle

Question 8

Why can smooth muscle cells contract as much as 80% of their length instead of being limited to <30% like in skeletal muscle?

Answer 8

myosin filaments have “side polar” crossbridges, this arrangement allows bridges on side to hinge in one direction and those on the other side to hinge in the opposite direction, the benefit of this is that it allows myosin to pull actin in both directions

Question 9

What is the reason for the prolonged, tonic contraction in smooth muscle as compared to skeletal muscle?

Answer 9

slow cycling of the myosin crossbridges, low energy requirement to maintain contraction, slowness of onset of contraction and relaxation, maximum force of contraction is greater, “latch” mechanism, stress-relaxation of smooth muscle

Question 10

Slow Cycling of the Myosin Cross-Bridges

Answer 10

the cross-bridge heads have far less ATPase activity than in skeletal muscle, degradation of the ATP that energizes the movements of the cross-bridge heads is greatly reduced

Question 11

Low Energy Requirement to Sustain Smooth Muscle Contraction

Answer 11

only 1/10 to 1/300 as much energy is required to sustain the same tension of contraction in smooth muscle as in skeletal muscle

Question 12

Slowness of Onset of Contraction and Relaxation of the Total Smooth Muscle Tissue

Answer 12

the slow onset of contraction, as well as prolonged contraction, is caused by the slowness of attachment and detachment of the cross-bridges with the actin filaments, the initiation of contraction in response to calcium ions is much slower than in skeletal muscle

Question 13

Maximum Force of Contraction is Often Greater in Smooth Muscle than in Skeletal Muscle

Answer 13

this great force of smooth muscle contraction results from the prolonged period of attachment of the myosin cross-bridges to the actin filaments

Question 14

What is the “latch” mechanism?

Answer 14

smooth muscle can maintain prolonged tonic contraction for hours with little use of energy
little continued excitatory signal is required from nerve fibers or hormonal sources

Question 15

What is the significance of the stress-relaxation of smooth muscle?

Answer 15

smooth muscle has the ability to return to nearly its original force of contraction seconds or minutes after it has been elongated or shortened; when the volume is suddenly decreased, the pressure falls drastically at first but then rises in another few seconds or minutes to or near the original level and this allows a hollow organ to maintain about the same amount of pressure inside its lumen despite long-term, large changes in volume

Question 16

General mechanism of smooth muscle contraction.

Answer 16

calcium ions combine with calmodulin to cause activation of myosin kinase and phosphorylation of the myosin head

Question 17

How does calmodulin initiate contraction in smooth muscle?

Answer 17

calcium ions bind with calmodulin, calmodulin-calcium complex joins with and activates myosin light chain kinase (phosphorylating enzyme), one of the light chains (regulatory chains) of each myosin head becomes phosphorylated in response to ts myosin kinase; when this chain is not phosphorylated, the attachment-detachment cycling of the myosin head with the actin filament does not occur; when it is phosphorylated, it is able to bind repetitively with the actin filament proceeding through the entire cycling process of intermittent “pulls” (just like in skeletal muscle) and cause contraction

Question 18

What is the role of myosin phosphatase in smooth muscle?

Answer 18

located in the cytosol of the smooth muscle cell, it splits the phosphate from the regulatory light chain, the cycling stops and contraction ceases, the time required for relaxation is determined to a great extent by the amount of active myosin phosphatase in the cell

Question 19

Possible mechanism for regulation of the latch phenomenon?

Answer 19

as the myosin kinase and myosin phosphatase activation decreases, the cycling frequency decreases; 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 portion of the cycling period

Question 20

What is one distinguishing feature of the NMJ of smooth muscle?

Answer 20

autonomic fibers that innervate smooth muscle branch diffusely on top of a sheet of muscle fibers, fibers do not make direct contact with the smooth muscle fiber membranes; instead they form diffuse-junctions

Question 21

The vesicles in skeletal muscle contain Ach. In smooth muscle, the vesicles may contain Ach or?

Answer 21

norepinephrine

Question 22

Can Ach and NE be secreted by the same nerve fibers?

Answer 22

no, Ach is excitatory and NE is inhibitory

Question 23

Two forms of visceral smooth muscle contraction?

Answer 23

spike potentials and APs with plateaus

Question 24

Does smooth muscle have more or less voltage-gated calcium channels than skeletal muscle?

Answer 24

more, less sodium channels

Question 25

What is the cause of spontaneous APs in visceral smooth muscle when it is stretched?

Answer 25

the normal slow wave potentials and decrease in overall negativity of the membrane potential caused by the stretch itself; this stretch response allows the gut wall for example, when excessively stretched, to contract automatically and rhythmically

Question 26

How do multi-unit smooth muscle fibers contract without APs?

Answer 26

contract in response to nerve stimuli, the nerve endings secrete Ach or NE which cause depolarization of the smooth muscle membrane, elicits a contraction; APs usually do NOT develop because the fibers are too small to generate them

Question 27

How do small arterioles, which are normally in constant contraction, relax in order to allow blood flow?

Answer 27

a powerful local feedback control system controls the blood flow to the local tissue area by the following ways: 1. lack of oxygen in the local tissues causes smooth muscle relaxation, and therefore, vasodilation 2.excess carbon dioxide causes vasodilation 3. increased hydrogen ion concentration causes vasodilation 4. adenosine, lactic acid, increased potassium ions, diminished calcium ion concentration, and increased body temp can all cause local vasodilation

Question 28

Which hormones can cause smooth muscle contraction?

Answer 28

NE, epinephrine, Ach, angiotensin, endothelin, vasopressin, oxytocin, serotonin, histamine

Question 29

How can hormones initiate or inhibit an AP?

Answer 29

by acting on the receptor affecting sodium and potassium channels

Question 30

Can a hormone affect muscle contraction without causing a change in membrane potential?

Answer 30

yes hormone may activate the membrane receptor that causes an internal change in the muscle fiber, such as release of calcium from the sarcoplasmic reticulum (SR), calcium then induces the contraction

Question 31

How does the cell inhibit contraction initiated by hormones affecting the SR and resulting in increased calcium?

Answer 31

activate adenylate cyclase (cAMP) or guanylate cyclase (cGMP) in the cell membrane, these changes the degree of phosphorylation of several enzymes that indirectly inhibit contraction, pump that moves Ca into the SR is activated, as well as the cell membrane that moves calcium out of the cell itself

Question 32

The force of contraction of smooth muscle cells is usually dependent on:

Answer 32

extracellular fluid calcium concentration (SR is only slightly developed in smooth muscle so it is not primarily coming from that source)