smooth muscle

Question 1

function of smooth muscle

Answer 1

• propels contents
• maintains pressure
• regulates internal flow (by changing tube diameter)

Question 2

describe a smooth muscle cell

Answer 2

spindle-shaped cells with single nucleus

Question 3

how are smooth muscle cells arranged

Answer 3

sheets in a diamond shaped lattice

*there are no striations (No sarcomeres)

Question 4

compare myosin and actin filaments between smooth and skeletal muscle

Answer 4

• thick myosin filaments are longer than those in skeletal muscle
• thin actin filaments contain tropomyosin but lack troponin
• smooth muscles also contain filaments of intermediate size that form part of cytoskeletal framework that supports cell shape

Question 5

smooth muscle innervation is delivered via what mechanism

Answer 5

varicosity: vesicle containing neurotransmitter

*no motor end plates

Question 6

name the two major types of smooth muscle

Answer 6

• single-unit smooth muscle (unitary)
• multiunit smooth muscle

Question 7

characteristics of multiunit smooth muscle

Answer 7

• each fiber behaves seperately (like a seperate motor unit)
• little or no coupling between cells
• units must be seperately stimulated by nerves to contract

Question 8

characteristics of single-unit smooth muscle

Answer 8

• myogenic: self-excitable (does not require nervous stimulation for contraction)
• also called visceral smooth muscle
• fibers become excited and contract as a single unit
• cell are electrically linked by gap junction

Question 9

Which type of smooth muscle contains gap junctions? Function of gap junctions in smooth muscle.

Answer 9

• found in single-unit smooth muscle
• allow for “full recruitment”
• conduct Na+ and Ca2+ electrically couple cells

Question 10

action potential of smooth muscle cells are dependent on what ion

Answer 10

calcium

Question 11

what type of smooth muscle cells fire action potentials

Answer 11

single unit smooth muscle cells

• spontaneous depolarization

Question 12

pacemaker potential

Answer 12

membrane potential gradually depolarizes until it reaches threshold for firing a single action potential

Question 13

slow wave potential

Answer 13

membrane potential alternately depolarizes and hyperpolarizes. When the threshold is reached, the cell fires a burst of action potentials

Question 14

unlike skeletal muscle, regulation of crossbridge cycling in smooth muscle occurs where?

Answer 14

on the thick myosin filament

Question 15

difference between contraction of smooth muscle and skeletal muscle

Answer 15

smooth muscle lacks

• distinct striations
• T-tubules
• sarcomeres
• troponin
• contains fewer SR

Question 16

list steps of calcium activation of myosin cross bridge in smooth muscle

Answer 16

1. Ca2+ binds to calmodulin -> CA2+-calmodulin
2. CA2+-calmodulin binds to inactive myosin kinase -> active myosin kinase
3. active myosin kinase phosphorylates inactive myosin
4. phosphorylated myosin can bind to actin

Question 17

a single excitation in smooth muscle does not fully activate smooth muscle cells. What must happen to increase tension

Answer 17

as CA2+ concentration increases, more cross-bridges are brought into play and greater tension develops

Question 18

name the 3 different mechanisms of increasing intracellular calcium for usein smooth muscle cell

Answer 18

1. G protein; PLC; IP3 causes CA2+ release from SR
2. ligand gated CA2+ channel -> CA2+ induced CA2+ release from SR
3. voltage gated CA2+ channel -> CA2+ induced CA2+ release from SR

Question 19

list steps for relaxation of smooth muscle

Answer 19

1. decrease in cytosolic calcium concentration
2. MLCK(Myosin light-chain kinase) returns to inactive form
3. enzyme myosin phosphatase removes phosphate from myosin
4. cross-bridge is inhibited

Question 20

name the 3 mechanisms for decreasing intracellular calcium in smooth muscle

Answer 20

1. SERCA (SR CA2+ ATPase)
2. Sarcolemmal Na+/Ca2+ exchanger
3. Sarcolemmal Ca2+ ATPase

Question 21

how does the Sarcolemmal Na+/Ca2+ exchanger work

Answer 21

• energy for the extrusion of calcium against its concentration gradient is from the inward driving force for Na+
• the Na+/K+ ATPase maintains the sodium gradient