Smooth muscle physiology

Question 1

Characteristics of smooth muscle

Answer 1

1. No striations
2. No myofibrils, no sarcomeres
3. Contains actin and myosin
4. Spindle-shaped body
5. 1/10 of the size of skeletal muscle
6. Dense bodies
7. Involuntary control of the Autonomic Nervous System
8. Uninucleated

Question 2

What are dense bodies?

Answer 2

Anchoring sites for actin filaments similar to the function of Z disks in striated muscle

Dense bodies are anchored to the sarcolemma by attachment plagues

Allows for the attachment of intermediate filaments which form an overall net-like structure on the muscle cell

Question 3

What is the orientation of the thick and thin filaments?

Answer 3

These are dispersed throughout the sarcoplasm of the cell with the thin filaments made up of F-actin and tropomyosin are anchored by dense bodes and in between each thin filament is the short thick filaments

When contractions take place the cell actually twists or contorts

Question 4

Where is smooth muscle located

STOVE

Answer 4

S → Skin (arrector pili muscles that cause hairs to raise)

T → Tracts of the reproductive, respiratory (around bronchi), and urinary system

O → hollow organs (intestines, bladder, uterus, stomach

V → vessels (blood vessels constriction)

E → eyes (iris contraction/dilation, and lens movement)

Question 5

Smooth muscle contractions in terms of dense bodies and filaments

Answer 5

As the myosin filament heads slide the actin filaments forward they also pull on the dense bodies, which pulls on the network of intermediate filaments running throughout the cell causing it to shorten/ squeeze.

Question 6

What are the two subtypes of smooth muscle?

Answer 6

1. Single unit
2. Multiunit

Question 7

What is a single unit smooth muscle?

Answer 7

This is when only one or two nerves are required to activate an entire sheet of smooth muscle as the gap junctions that the cells use to communicate allow for the transmission of the action potential rapidly so the entire sheet acts together.

eg. a single plug powers an entire string of Christmas lights

Question 8

Where is single unit smooth muscle found

Answer 8

Found in hollow organs

Intestinal organs

blood vessels

respiratory tract

Question 9

What is a multi-unit smooth muscle?

Answer 9

Contain fewer to no gap junctions between these cells so it requires multiple nerve fibers to transmit signals for the smooth muscle cells to contract.

Recruitment of muscle fibers do occur because each fiber acts individually

Question 10

Where are multi-unit smooth muscle found?

Answer 10

Large airways

Arteries

Eye (ciliary muscle, iris)

Question 11

What does it mean that single-unit smooth muscles are pacemakers?

Answer 11

Similar to cardiac cells in the SA and AV node some smooth muscle cells have a myogenic activity which means they can contract on their own without stimulation from the autonomic nervous system through spontaneous depolarizations.

Question 12

Are multi-unit muscle cells self excitable?

Answer 12

No

They rely on a neuron to excite them similar to skeletal muscle cells

Question 13

What are varicosities?

Answer 13

These are bulbs at the end of the axon terminal which contains various neurotransmitters

Question 14

What kind of control is multi-unit for?

Answer 14

Fine control as the muscles act separately

Question 15

What kind of control is single-unit smooth muscle?

Answer 15

Gross control as these cells spread the signal so they can work together or a mass scale

Question 16

General steps for Smooth muscle excitation-contraction

Answer 16

1. Ca²⁺ comes into the muscle cell gradually increases in membrane potential so it is more positive
2. Voltage-gates Ca2+ channels in the plasma membrane will open causing a flood of positive charge
3. Ca2+ triggers the release of Ca2+ from the SR
4. Ca2+ binds to calmodulin
5. Ca2+ - calmodulin activates myosin light-chain kinase (MLCK)
6. Myosin ATPase is activated via phosphorylation
7. Crossbridge cycling
8. Contraction

Question 17

How are slow-wave potentials formed?

Answer 17

As Ca ²⁺ leaks into the cell the membrane potential increases

If the [Ca²⁺] is not high enough to reach the threshold potential the potential will go down due to K⁺ channels releasing cations causing the membrane potential to become more negative.

Question 18

How does a Smooth muscle contract

Answer 18

Ca2+ can leak into the cell through leaky Ca2+ which can gradually make the membrane potential high enough to reach threshold potential

Once threshold potential is reached this will cause voltage gated Ca2+ channels to open which will causing a flood of Ca2+ to enter the cell

The release of Ca2+ from the SR is then stimulated

The Ca2+ will then bind to calmodulin activating it

Ca2+ - calmodulin complex will then activate myosin light chain kinase

MLCK will then phosphorylate the myosin light chain causing Myosin ATPase to be active.

The myosin ATPase will then hydrolyze the ATP into ADP and Pi resulting in an active cocked back myosin

Myosin will then bind to the actin filamernt myosin binding sites and

Question 19

How is the crossbridge cycle regulated?

Answer 19

1. No troponin, tropomyosin, and actin complex
2. Tropomyosin and calponin bind to the thin filament to prevent the binding of the myosin before needed
3. Caldesmen is on the myosin
4. Calponin and caldesmon inhibit the ATPase activity of the myosin head so it cannot hydrolyze ATP into ADP and Pi initiating cross bridge
5. Ca2+/ Calmodulin when formed inhibits the calponin and caldesmon from inhibiting the ATPase activity and hinder the myosin and actin-binding
6. Ca2+/ Calmodulin will activate MLCK to add a phosphate to the neck of the myosin on the regulatory light chain to stimulate the myosin ATPase activity
7. Hydrolyzes ATP so that the myosin cocks back and attaches to a myosin head

Question 20

Why does it take so long for the smooth fibers to contract?

Answer 20

10-100x slower than skeletal muscle

Each of the myosin necks needs to be phosphorylated in order for the ATPase activity to be active.

Question 21

How does the smooth muscle relax?

Answer 21

• Myosin Light Chain Phosphatase removes the phosphate groups from each of the myosin necks
• Na⁺ -Ca²⁺ counter-transporters will remove the Ca²⁺ from the sarcoplasm into the SR
• As the [Ca²⁺] goes down this will cause Calmodulin to become inactive so it can no longer inhibit Calponin and caldesmon.
• Calponin and caldesmon will then go back to complexing with actin and tropomyosin blocking myosin from binding to actin and inhibiting ATPase activity.

Question 22

How does the smooth muscle relax?

Answer 22

• Myosin Light Chain Phosphatase removes the phosphate groups from each of the myosin necks
• Na⁺ -Ca²⁺ counter-transporters will remove the Ca²⁺ from the sarcoplasm into the SR
• As the [Ca²⁺] goes down this will cause Calmodulin to become inactive so it can no longer inhibit Calponin and caldesmon.
• Calponin and caldesmon will then go back to complexing with actin and tropomyosin blocking myosin from binding to actin and inhibiting ATPase activity.

Question 23

Describe the neural regulation of the smooth muscle?

Answer 23

Smooth muscle is involuntary so it is under the control of the Autonomic Nervous System (Sympathetic and/or Parasympathetic)

Neural regulation can either be inhibitory or excitatory to the muscle, depends on the receptor type and neurotransmitters

The reaction is dependent on where the muscle is located