Module 4 Section 6

Question 1

Describe the contractile filaments in smooth muscle and explain how it is different from skeletal muscle.

Answer 1

• Smooth muscles lack sarcomeres (which skeletal muscles have). Instead, they have 3 types of filaments that participate in contraction:
  1) Thick myosin filaments that are longer than those in skeletal muscle.
  2) Thin actin filaments that contain tropomyosin but not troponin.
  3) Intermediate filaments that don’t directly support contraction but rather the cytoskeletal framework.
• Instead of Z-lines, they have dense bodies which serve as anchor points for both the intermediate and contractile filaments.
• In contrast to skeletal muscles, the thick and thin filaments are not oriented along the length of the cell but rather at angles, forming a diamond-like pattern within the cell.

Question 2

Explain smooth muscle excitation contraction coupling.

Answer 2

• Smooth muscle lacks troponin, which skeletal muscles contain that blocks the cross-bridge formation until Ca is present.
• However, even w/o troponin, actin and myosin in smooth muscle can’t form cross-bridges at rest.
• Both types of muscle have myosin light chain which is associated w/ the myosin head. The protein also aids in cross-bridge formation, but plays a much larger role in smooth muscle than skeletal muscle.

1) During excitation, Ca (entry from ECF and SR) enters the smooth muscle cell and binds to calmodulin.
2) The Ca-calmodulin complex binds to and activates myosin light chain kinase.
3) Once activated, this kinase phosphorylates myosin light chain, which allows the myosin cross-bridge to bind to action.

Question 3

Explain the differences b/w neurogenic and myogenic smooth muscle.

Answer 3

Single unit (myogenic)

• Majority of smooth muscle is single unit.
• The muscle fibres are all electrically connected via gap junctions and, consequently, become excited and contract as a single or functional unit.
• Found in the hollow organs such as the digestive system, reproduction system, urinary tracts and small blood vessels.
• Myogenic = self-excitable and doesn’t require nerve stimulation.
• 2 types of spontaneous depolarization: pacemaker potentials and slow-wave potentials.
• Smooth muscle can be innervated by both branches of the ANS.
• A single neuron can affect a large number of smooth muscle cells, which is in contrast to the motor end plates observed in skeletal muscle.

Multiunit (neurogenic)

• Similar to skeletal muscle since there are distinct groups/units of smooth muscle cells that’re innervated by nerves to contract. They are not motor nerves, but rather, nerves of the autonomic nerve system.
• Found in the walls of the large blood vessels, small airways to the lungs, the base of hair follicles of the skin and in the eye to adjust the lens shape and iris.

Question 4

Explain if cardiac muscle is more similar to skeletal or smooth muscle and why.

Answer 4

Uniquely cardiac

• Action potentials last longer in cardiac muscle than any other muscle type
• Cardiac muscle fibres are joined in a branching network.

Similarities b/w skeletal and cardiac muscles

• Is striated w/ thick and thin filaments organized into sarcomeres
• Contains troponin and tropomyosin as the primary site where Ca activates cross bridge activity
• Has T-tubules and a well defined SR
• Contain lots of mitochondria
• Has a well defined length-tension relationship

Similarities b/w smooth and cardiac muscles

• Ca comes from both the ECF and the SR
• Is interconnected by gap junctions to allow spread of excitation
• Is innervated by the ANS to modify rate and strength of contraction

The cardiac muscle is more similar to the skeletal muscle

Question 5

What arm of the nervous system innervated smooth muscle?

a) Peripheral Nervous System
b) Autonomic Nervous System
c) Somatic Nervous System
d) All of the above

Answer 5

b) Autonomic Nervous System

Question 6

As the smooth muscle is stretched or distended, it can generate stronger contractions to return to its normal length. Smooth muscles can be stretched many times their resting length (from 0.5-2.5x) their resting length and still develop tension.

What are some examples of organs in the body surrounded by smooth muscle that make use of this property? What would happen if optimal length was the normal resting length?

Answer 6

Stomach, intestines and bladder. In these organs, smooth muscle stretching allows for a forceful and purposeful contraction that results in emptying.

If these organs could not stretch past their normal length, they wouldn’t be able to generate enough tension to contract and empty.