Section 6

Question 1

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

Question 1

Explain smooth muscle excitation contraction coupling.

Question 2

Explain the differences between neurogenic and myogenic smooth muscle.

Question 3

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

Answer 3

Similar to Skeletal Muscle:
- Striated appearance.
- Multinucleation (in some cases).
- Involuntary contractions influenced by neural input.

Differences from Smooth Muscle:
- Presence of striations.
- Involuntary, but control mechanisms more akin to skeletal muscle.
-Intercalated discs with gap junctions for rapid communication.

In essence, cardiac muscle exhibits characteristics resembling both skeletal and smooth muscle, but its striated appearance and involvement of neural control align it more closely with skeletal muscle.

Question 4

Where are smooth muscle cells and cardiac muscle cells typically found in the human body?

Answer 4

Smooth muscle cells are primarily found in the walls of hollow organs and tubes like the digestive system and blood vessels. In contrast, cardiac muscle cells are exclusively located in the heart.

Question 5

How does the structure of smooth muscle cells differ from skeletal muscle cells?

Answer 5

Smooth muscle cells lack sarcomeres.

They have three types of filaments:
- Thick myosin filaments (longer than skeletal muscle).
- Thin actin filaments containing tropomyosin (but not troponin).
- Intermediate filaments for the cytoskeletal framework.

Instead of Z-lines, they have dense bodies serving as anchor points for both the intermediate and contractile filaments.

The thick and thin filaments are not oriented along the cell’s length but form a diamond-like pattern within the cell.

Question 6

What is the key difference between the presence of troponin in skeletal muscle and smooth muscle regarding cross-bridge formation?

Answer 6

In skeletal muscle, troponin blocks cross-bridge formation until Ca2+ is present, but smooth muscle lacks troponin.

Question 7

In smooth muscle, what protein is associated with the myosin head and plays a significant role in cross-bridge formation?

Answer 7

Myosin light chain is associated with the myosin head in both skeletal and smooth muscle, with a more substantial role in smooth muscle.

Question 8

What are the steps for myosin cross-bridge activation in smooth muscle?

Answer 8

1. Ca2+ enters the smooth muscle cell and binds to calmodulin.
2. The Ca2+-calmodulin complex activates myosin light chain kinase.
3. Activated kinase phosphorylates myosin light chain, allowing the myosin cross-bridge to bind to actin.

Question 9

What is the role of calmodulin in smooth muscle contraction?

Answer 9

Calmodulin is a calcium-binding messenger protein that activates myosin light chain kinase during excitation, leading to myosin cross-bridge activation.

Question 10

How do the size and shape of smooth muscle cells compare to skeletal muscle cells?

Answer 10

Smooth muscle cells are smaller, spindle-shaped, and have a single nucleus.

Question 11

In smooth muscle, do cells extend the length of the muscle or form sheets?

Answer 11

Smooth muscle cells form sheets of muscle.

Question 12

What protein found in smooth muscle is crucial for cross-bridge cycling, even though it lacks troponin?

Answer 12

Myosin light chain is imperative for cross-bridge cycling in smooth muscle.

Question 13

How do smooth muscle cells differ from skeletal muscle cells regarding T-tubules and the sarcoplasmic reticulum (SR)?

Answer 13

Smooth muscle cells have no T-tubules and very little SR compared to skeletal muscle cells.

Question 14

What is the primary function of voltage-gated dihydropyridine receptors in smooth muscle cells?

Answer 14

In smooth muscle cells, voltage-gated dihydropyridine receptors serve as calcium channels to allow calcium entry from the extracellular fluid (ECF).

Question 15

How is calcium released from the SR in smooth muscle cells, and what is this process called?

Answer 15

Calcium is released from the SR in smooth muscle cells through a process called calcium-induced calcium release (CICR).

Question 16

How is the equilibrium of calcium levels in the blood maintained in the body, and what processes are involved?

Answer 16

The equilibrium of calcium levels in the blood is maintained through an interplay of calcium absorption from the intestines, movement of calcium into and out of the bones, and the kidney’s reclamation and excretion of calcium into the urine.

Question 17

What is the key difference between single unit smooth muscle and multiunit smooth muscle in terms of their excitation?

Answer 17

Single unit smooth muscle fibers are electrically connected via gap junctions, which allow them to become excited and contract as a single unit. In contrast, multiunit smooth muscle consists of distinct groups or units of smooth muscle cells that are innervated by autonomic nervous system nerves, leading to neurogenic stimulation.

Question 18

Where is single unit smooth muscle primarily found in the body?

Answer 18

Single unit smooth muscle is predominantly found in the walls of hollow organs such as the digestive system, reproductive system, urinary tracts, and small blood vessels.

Question 19

Where is multiunit smooth muscle found in the body?

Answer 19

Multiunit smooth muscle is found in the walls of 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 20

What is the key difference between single unit and multiunit smooth muscle in terms of excitation?

Answer 20

Single unit smooth muscle is myogenic, meaning it can self-generate action potentials and does not require nerve stimulation, whereas multiunit smooth muscle is neurogenic and requires autonomic nervous system nerves for stimulation.

Question 21

What is a characteristic of myogenic single unit smooth muscle?

Answer 21

Single unit smooth muscle consists of clusters of specialized smooth muscle cells that are automatic or spontaneously depolarize to generate action potentials that can spread throughout the muscle.

Question 22

What are the two types of spontaneous depolarization in single unit smooth muscle?

Answer 22

The two types of spontaneous depolarization in single unit smooth muscle are pacemaker potentials and slow-wave potentials.

Question 23

How do pacemaker potentials work in single unit smooth muscle?

Answer 23

Pacemaker potentials involve a gradual membrane depolarization until it reaches threshold, at which point it fires an action potential. These potentials are generated by If channels that are permeable to Na+ and K+ ions and open to depolarize the cell. When the membrane potential becomes more positive, If channels close, and Ca2+ channels open to continue depolarizing the cell.

Question 24

What causes slow-wave potentials in single unit smooth muscle?

Answer 24

Slow-wave potentials are caused by the active transport of Ca2+ ions across the membrane. These transported ions create an oscillating wave of alternating hyperpolarization and depolarization, and if the threshold is reached during the depolarization phase, an action potential is generated.

Question 25

What differentiates the gradation of contraction between skeletal muscle and single unit smooth muscle?

Answer 25

In skeletal muscle, gradation of contraction is under neural control and involves motor unit recruitment and twitch summation. In single unit smooth muscle, gradation of contraction occurs by modulating cytosolic Ca2+ concentration, as all muscle cells contract together due to gap junctions.

Question 26

What is the term for the degree to which single unit smooth muscle is contracted at any given time, and how can it be modified?

Answer 26

The term for the degree of contraction in single unit smooth muscle is called its “tone.” It can be modified by factors that influence cytosolic Ca2+ concentration.

Question 27

Which branch of the nervous system innervates smooth muscle?

Answer 27

The autonomic nervous system

Question 28

How is smooth muscle innervated by the autonomic nervous system different from the innervation of skeletal muscle?

Answer 28

In smooth muscle, postganglionic autonomic neurons travel across the surface of many muscle cells and release neurotransmitters from specialized areas called varicosities, allowing a single neuron to affect a large number of smooth muscle cells. This is different from the motor end plates observed in skeletal muscle.

Question 29

What is the length-tension relationship in smooth muscle, and why is it important for the functioning of organs surrounded by smooth muscle?

Answer 29

Smooth muscle at rest is far below the optimal tension, and it can generate stronger contractions as it is stretched or distended, returning to its normal length. The length-tension relationship in smooth muscle is not as defined as in skeletal muscle, allowing smooth muscles to be stretched many times their resting length. Organs like the stomach, intestines, and bladder make use of this property, allowing for forceful and purposeful contractions that result in emptying. If optimal length were the normal resting length, these organs wouldn’t be able to generate enough tension to contract and empty.

Question 30

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 30

Some examples of organs are the stomach, intestines, and bladder. In these organs, smooth muscle
stretching allows for a forceful and purposeful contractions that result 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.

Question 31

How do the action potentials in cardiac muscle compare to other muscle types?

Answer 31

Action potentials last much longer in cardiac muscle than in other muscle types.

Question 32

What is unique about the fiber arrangement in cardiac muscle?

Answer 32

Cardiac muscle fibers branch, which is distinct from other muscle types.

Question 33

What is similar between skeletal and cardiac muscle?

Answer 33

1. Is striated with thick and thin filaments organized into sarcomeres
2. Contains troponin and tropomyosin as the primary site where Ca2+ activates cross bridge activity
3. Has T-tubules and a well defined SR
4. Contain lots of mitochondria
5. Has a well defined length-tension relationship

Question 34

What is similar between smooth muscle and cardiac muscle?

Answer 34

1. Ca2+ comes from both the ECF and the SR
2. Is interconnected by gap junctions to allow spread of excitation
3. Is innervated by the ANS to modify rate and strength of contraction