Chapter 8 - Excitation And Contraction Of smooth Muscle

Question 1

What 2 major types can smooth muscles be divided into?

Answer 1

1. Multi unit smooth muscle
2. Unitary (or single unit) smooth muscle

Question 2

What is the most important characteristic of multi unit smooth muscle fibers?

Answer 2

Each fibre can contract independently of the others and their control is exerted mainly by nerve signals.

Question 3

What characterises unitary smooth muscle?

Answer 3

A mass of hundreds to thousands of smooth muscle fibres that contract together as a unit.

Question 4

Does smooth muscle contain actin, myosin and troponin complex?

Answer 4

It contains both actin and myosin filaments having chemical characteristics that are similar to those in skeletal muscle.

It does NOT contain the normal troponin complex so the mechanism for contraction is different.

Question 5

What does smooth muscle contain in place of troponin and how does it work?

Answer 5

Calmodulin

It initiates contraction by activating the myosin cross bridges.

The calcium ions binds with calmodulin.

The calmodulin-calcium complex then joins with and activates myosin light chain kinase - a phosphorylating enzyme.

One of the light chains of each myosin head, called the regulatory chain becomes phosphorylated in response to the myosin kinase.

Question 6

How does smooth muscle differ from skeletal muscle regarding contraction?

Answer 6

-Slow cycling of the myosin cross bridges

-Low energy requirement to sustain smooth muscle contraction

-Slowness of onset of contraction and relaxation of the total smooth muscle

-Maximum force of contraction is often greater in smooth muscle than in skeletal muscle

-“Latch mechanism” facilitates prolonged holding of contractions of smooth muscle. This means that once smooth muscle has developed full contraction, the amount of continuing excitation can usually be reduced to far less than the initial level yet the muscle maintains its full force of contraction.

-There is a stress relaxation of smooth muscle

Question 7

What is the principal reason that smooth muscle can be stimulated by nervous signals, hormonal stimulation, by stretch of the muscle etc. contrary to skeletal muscle that is stimulated exclusively by the nervous system?

Answer 7

The reason for this is that the smooth muscle membrane contains many types of receptor proteins that can initiate the contractile process.

Question 8

What is the physiologic anatomy of smooth muscle neuromuscular junctions?

Answer 8

The autonomic nerve fibers that innervate smooth muscles generally branch diffusely on top of a sheet of muscle fibres. In most instances, these fibres do not make direct contact with the smooth muscle fibre cell membranes but instead form diffuse junctions that secrete their transmitter substance into the matrix coating of the smooth muscle and the substance then diffuses to the cells.

Question 9

How do the vesicles of the autonomic nerve fibre endings differ from skeletal muscle junctions?

Answer 9

Skeletal muscle junctions always contain acetylcholine but the vesicles of the autonomic nerve fiber endings contain acetylcholine in some fibers and norepinephrine in others.

Question 10

When acetylcholine excites a muscle fiber, norepinephrine inhibits it. Why are the responses different?

Answer 10

Both acetylcholine and norepinephrine excite of inhibitors smooth muscle by first binding with a receptor protein on the surface of the muscle cell membrane. Some of the receptor proteins are excitatory receptors whereas others are inhibitory receptors.

Question 11

What two formes do the action potential of visceral smooth muscle occur in?

Answer 11

1. Spike potentials
2. Action potentials with plateaus

Question 12

What ion channel is important in generating smooth muscle action potential?

Answer 12

Calcium channels

Flow of calcium ions to the interior of the fibre is mainly responsible for the action potential. The calcium ions acts directly on the smooth muscle contractile mechanism to cause contraction.

Sodium participates little in the generation of the action potential in most smooth muscle.

Question 13

What can slow wave potentials in unitary smooth muscle lead to?

Answer 13

Spontaneous generation of action potentials.

Question 14

In the normal resting state, many of the small blood vessels remain contracted, but when extra blood flow to the tissue is necessary, multiple factors can relax the vessel wall allowing for increased flow. What dad some specific control factors?

Answer 14

1. Lack of oxygen in the local tissues causes smooth muscle relaxation and therefor vasodilation
2. Excess carbon dioxide causes vasodilation
3. Increased hydrogen ion concentration causes vasodilation

Question 15

How does smooth muscle inhibition occur by hormones?

Answer 15

It occurs when the hormone or other tissue factor closes the sodium and calcium channels to prevent entry of these positive ions.

Inhibition also occurs if the normally closed potassium channels are opened allowing potassium ions to diffuse out of the cell.

Question 16

Sometimes smooth muscle contraction of inhibition is initiated by hormone ms without directly causing any change in the membrane potential. What happens in these instances?

Answer 16

The hormone may activate a membrane receptor that does not open any ion channels but instead causes an internal change in the muscle fibre such as release of calcium ions from the intracellular sarcoplasmic reticulum- the calcium then induces contraction.

Question 17

Although the contractile process in smooth muscle as in skeletal muscle is activated by calcium ions - the source of the ions is different. What is the difference?

Answer 17

The sarcoplasmic reticulum which provides virtually all the calcium ions for skeletal muscle contraction is only slightly developed in smooth muscle. Instead most of the calcium that cause contraction enter the muscle cell from the ECF at the time of the action potential or other stimulus.

Question 18

To cause relaxation of smooth muscle after it has contracted, the calcium ions must be removed from the ICF. How is this removal achieves?

Answer 18

By a calcium pump that pumps calcium ions out of the smooth muscle fibre back into the ECF or into a sarcoplasmic reticulum if it is present.