Smooth Muscle

Question 1

Describe the 2 types of smooth muscle.

Answer 1

• multi unit smooth muscle
• single unit ( visceral ) smooth muscle

Question 2

What is multi unit smooth muscle?

Answer 2

• discrete
• separate smooth muscle fibers
• each muscle fiber contracts independently
• independently innervated
• located :
  –> ciliary muscles
  –> iris muscle of eyes
  –> base of hair follicles
  –> smaller airways of lungs
  –> walls of large blood vessels
• varicosity intertwined between cells

Question 3

What are single unit smooth muscle?

Answer 3

-also called visceral, syncytial or unitary smooth muscle
- fibers are arranged in sheets or bundles
–> cell membranes are adherent to one another
–> force generated in one muscle fiber can be transmitted to next
–> contract together as a single unit
- cell membranes are joined by many gap junctions
–> ions can flow freely from one muscle cell to next
–> fibers contract together
- located :
–> GI tract
–> bile ducts
–> ureters
–> uterus
–> many blood vessels

Question 4

Describe particularities of smooth muscle.

Answer 4

• does not have same striated arrangement of actin and myosin filaments
• large # of actin filaments attached to DENSE BODIES ( similar to Z-disks )
  —** some bodies bonded together by intracellular protein bridges
  —> NO troponin-tropomyosin complex
• myosin filaments intercalated among actin filaments
• myosin filaments have side polar cross bridges
• SR is slightly developed
  –* not a major source of Ca for smooth muscle contraction ECF IS
  —> lies near cell membrane
  —> small invaginations = Caveolae
  —–* rudimentary T Tubules and excited Ca release

Question 5

What is a side polar cross bridge?

Answer 5

• arranged so that bridges on one side bend in one direction and those on other side bend in opposite direction
• configuration allows myosin to pull actin filament simultaneously in opposite directions
• smooth muscle can contract as much as 80% of their length
  —-> skeletal muscle only 30%

Question 6

List 4 factors that can initiate contraction in smooth muscle.

Answer 6

1. Nervous Stimulation
2. Hormonal Stimulation
3. Local Tissue chemical factors
4. self excitation

Question 7

What is a spike potential?

Answer 7

• similar to those seen in skeletal muscle
  –> can be elicited by electrical stimulation, hormones, stretch or spontaneously.
• some smooth muscle cells are SELF EXCITATORY, AP arise w/out extrinsic stimulus
  –> associated w/ basic slow wave rhythm of membrane potential
  –> when strong enough AP initiated

Question 8

What is an action potentials w/ plateau?

Answer 8

• onset is similar to that of typical spike potential
• instead of rapid repolarization, muscle fiber undergoes a delayed repolarization phase
• importance of PLATEAU is that it can account for PROLONGED CONTRACTION that occurs in some types of smooth muscles

Question 9

Describe 5 steps of smooth muscle contraction.

Answer 9

1. Calcium concentration in cytosol of smooth muscle cell increases when Ca enters cell and released from SR
2. Calcium ions bind reversibly w/ calmodulin
3. Ca-calmodulin complex joins w/ and activated myosin light chain kinase (MLCK)
4. One of light chains of each myosin head is phosphorylated increasing myosin ATPase activity
5. Concentration of smooth muscle

Question 10

Describe how Ca pump and Myosin phosphatase act to stop smooth muscle contraction.

Answer 10

• calcium pump is required to cause smooth relaxation
  –> Ca pump move calcium ions back to ECF or SR
  —-> requires ATP
  —-> slow acting
  —-> contraction last longer than skeletal muscle
• after Ca channels close and Ca pump move Ca ion out of cell
  –> calcium contraction falls
  –> Myosin Phosphatase is activated