Chapter 8 - Excitation And Contraction Of Smooth Muscles

Question 1

Multi unit smooth muscle.

Answer 1

• Composed of Discrete, separate smooth muscle fibers.
• Operates independtly.
• Innervated by a single nerve ending.
• example: ciliary muscle of the eye, iris muscle of the eye, piloerector muscle

Question 2

Unitary smooth muscle

Answer 2

• aka SYNCYTIAL SMOOTH MUSCLE / VISCERAL SMOOTH MUSCLE
• hundred of fibers contract at the same time.
• examples: gastrointestinal tract, bile ducts, ureters, uterus and many blood vessels.

Question 3

Smooth muscle

Answer 3

Smooth muscle

• does not have troponin complex.
• Z disk is called “ dense bodies “.
• most myosin filaments have “ sidepolar “ cross - bridge
• this allows the myosin to pull the actin filament in one direction on one side and SIMULTANEOUSLY pulling another actin filament in the opposite direction
• prolonged tonic contraction.
• sarcoplasmic reticulum is slightly developed
• presence of LATENT PERIOD - period before contractin begins, 50x greater than skeletal muscle

Question 4

Slow cycling of myosin cross- bridges.

Answer 4

• 1/10 to 1/100th compared to the skeletal muscle.
• REASON: less activity of ATPase enzyme, decrease in the degradation of the ATP that energizes the movement of the cross bridges heads is greatly reduced.

Question 5

Low energy requirement to sustain smooth muscle contraction

Answer 5

• 1/10 to 1/300 as much energy is required.

- IMPORTANT to the overall energy economy of the body, because some organ maintain tonic contraction.

Question 6

Slowness of onset of contraction and relaxation of the toal smooth muscle tissue.

Answer 6

• 30x as long as a single contraction of a skeletsl muscle.
• REASONS: (1) because of slow cycling of the myosin cross bridges. (2) initiation of contraction in respone to calcium ion is much slower than in skeletal muscle.

Question 7

The maximum force of contraction is often greater in smooth muscle than in skeletal muscle.

Answer 7

• 4 to 6kg/cm2 cross sectional area for smooth muscle
• 3 to 4 kg for skeletal muscle.
• this great force of smooth muscle contraction results from the prolonged period of attachment of the myosin cross bridges to the actin filaments.

Question 8

The “Latch” Mechanism facilitates prolonged holding of contractions of smooth muscle

Answer 8

• amount of energy needed to maintain contraction is less than the initial contraction
• IMPORTANT: it can maintain prolonged tonic contraction in smooth muscle for hours with little use of energy.

Question 9

Stress- Relaxation of smooth muscle

Answer 9

-IMPORTANCE: allows hollow organ to maintain about the same amount of pressure inside the lumens despite sustained, large changes in volumes.

Question 10

Increase of intracellular calcium ion is caused by:

Answer 10

1. Nerve stimulation
2. Hormonsl stimulation
3. Stretch of the fiber.
4. Change in the chemical environment of the fiber.

Question 11

Regalutory protein of smooth muscle that controls contraction?

Answer 11

Calmodulin

Question 12

Sequence in the contraction of smooth muscle

Answer 12

1. ) Calcium levels increases due (1) influx of calcium through the calcium channel, (2) release of calcium from the sarcoplasmic reticulum.
2. ) Calcium binds reversibly to CALMODULIN
3. ) Calcium - Calmodulin complex JOINS and ACTIVATES the MYOSIN LIGHT CHAIN KINASE * phosphorylating agent.
4. ) One of the light chain in the myosin head becomes phosphorylated due to myosin kinase, attachment and detachment of the cycling of the myosin head occurs

Question 13

Smooth muscle contraction is dependent to?

Answer 13

Extracellular Calcium Ion Concentration

Question 14

It is important in the cessation of contraction of smooth muscle

Answer 14

Myosin phosphatase

Question 15

Two enzymes inhibited in “latch” mechanism

Answer 15

1. ) myosin kinase

2. ) myosin phosphatase

Question 16

An excitatory transmitter substance for smooth muscle

Answer 16

Acetylcholine

Question 17

Inhibits smooth muscle excitation

Answer 17

Norepinephrine

Question 18

Membrane potential of smooth muscle

Answer 18

-50 to -60 mV