smooth muscle

Question 1

all smooth muscles lack

Answer 1

the striated banding pattern found in cardiac and skeletal fibers

Question 2

neurons to all smooth muscles are derived from the

Answer 2

autonomic nervous system (ANS)

Question 3

characteristics of each smooth muscle

Answer 3

spindle-shaped

has a single nucleus

lacks striations

much smaller (especially shorter) than a skeletal muscle cell

often interconnected to form sheet-like layers of cells

Question 4

thick (myosin) and thin (actin) filaments are not organized into

Answer 4

myofibrils or aligned into sarcomeres

Question 5

dense bodies

Answer 5

anchor the thin (actin) filaments to the cell’s plasma membrane or to structures in the cytoplasm, and serve the same role as the Z discs of skeletal muscle

Question 6

single-unit (unitary; visceral) smooth muscle

Answer 6

an entire group of muscle fibers responds to stimulation as a single unit (electrical and contractile activity is synchronous in all cells)

1. occurs because each cell is connected to adjacent cells by gap junctions
2. some of the cells have pacemaker activity and spontaneously generate action potentials, which spread to other cells via the gap junctions
3. nerves (not primarily), hormones and local factors (primarily) can alter the contractile activity of the cells
4. the activity of the entire muscle can be regulated by controlling the frequency with which the pacemaker fires
5. a contraction can often be induced by stretching the muscle

Question 7

examples of single-unit (unitary; visceral) smooth muscles

Answer 7

smooth muscles of the stomach and intestines

the bile ducts

the ureters

the uterus

muscle in many blood vessels

Question 8

multi-unit smooth muscle

Answer 8

contain few or no gap junctions, and each cell thus responds independently

1. richly innervated by the ANS
2. the contractile response of the entire muscle depends on the number of fibers that are activated and the frequency of nerve stimulation
3. although stimulation by axons/transmitter leads to some degree of depolarization and a contractile response, the muscle cells do not typically make action potentials
4. may respond to hormones, but not usually stretch

Question 9

examples of multi-unit smooth muscles

Answer 9

smooth muscles in the eyes

large airways

large arteries

attached to the hairs in the skin

Question 10

in smooth muscle, a myosin molecule is formed from

Answer 10

6 polypeptide chains: 2 heavy and 4 light

Question 11

a myosin molecule’s tail is formed by

Answer 11

most of the two heavy chains wrapped into a double helix

Question 12

a myosin molecule’s 2 heads are each formed by

Answer 12

the folded end of one heavy chain plus 2 of the light chains

Question 13

smooth muscles cells lack _____, meaning ______ is never in a position where it blocks myosin’s access to actin

Answer 13

troponin

tropomyosin

Question 14

during contraction, inputs/signals to the cell increase Ca2+ concentration in the cell’s cytoplasm by

Answer 14

opening (maybe not directly) membrane Ca2+ channels that allow influx of extracellular Ca2+

OR

by stimulating the cell’s sarcoplasmic reticulum to release stored Ca2+

Question 15

during contraction, Ca2+ binds reversibly

Answer 15

within the cell with the protein called calmodulin

Question 16

during contraction, the calcium-calmodulin complex binds to and activates

Answer 16

myosin light chain kinase (MLCK)

Question 17

during contraction, MLCK uses

Answer 17

ATPs to phosphorylate one of the light chains of each myosin head

(2 of 4 light chains are phosphorylated)

Question 18

during contraction, the phosphorylated myosin head can now bind

Answer 18

repetitively with the actin (thin) filament and go through the entire cycling process of intermittent “pulls” (as in skeletal muscle), thus causing contraction

Question 19

during relaxation, myosin must be dephosphorylated so that it is unable to bind to actin, this happens by:

Answer 19

a slowly-acting ATP-requiring pump transports calcium back into the sarcoplasmic reticulum or extracellular fluid, so that MLCK is no longer being activated

the activity level of the dephosphorylating enzyme, myosin light chain phosphatase, which is continuously active in smooth muscle cells, now outpaces that of MCLK and the amount of phosphorylated myosin declines, producing relaxation

Question 20

the rates of ATP splitting and cross bridge cycling during contraction are much _____ in smooth muscle than in skeletal muscle

Answer 20

slower

Question 21

the velocity of smooth muscle shortening is _____ in smooth muscle than in skeletal muscle

Answer 21

lower

Question 22

smooth muscle does not undergo _____ during prolonged periods of activity

Answer 22

fatigue

Question 23

latch state

Answer 23

once smooth muscle has developed full contraction, the tension it generates can often be maintained near the full level despite fairly dramatic declines in the level of energy consumption

Question 24

the controlling inputs to smooth muscle cells that increase the cell’s cytosolic Ca2+ concentration include:

Answer 24

transmitters released by autonomic neurons

hormones

local factors such as: acidity, osmolarity, O2/CO2 concentration, and paracrines from nearby cells

stretch of the fiber

spontaneous electrical activity in the fiber’s plasma membrane

Question 25

in some smooth muscles, action potentials (membrane depolarizations) are not necessary to release Ca2+ from the sarcoplasmic reticulum; instead,

Answer 25

internal second messenger molecules generated in response to the binding of external chemical messengers trigger the release of sarcoplasmic reticulum Ca2+, meaning the cell can contract in the absence of a membrane potential change

Question 26

in smooth muscles that contract in response to the production of action potentials, what ions is responsbile for carrying positive charge into the cell during the rising phase of the action potential

Answer 26

Ca2+ (not Na+)

(a graded, threshold-reaching membrane depolarization in response to an input opens voltage-gated Ca2+ channels, and Ca2+ entry produces the rising phase)

Question 27

some smooth muscle cells have a resting membrane potential that depolarizes slowly to threshold (that is that produces ______ ); thus these cells generate action potentials (and contract) spontaneously and rhythmically in the absence of

Answer 27

pacemaker potentials

neural, hormonal, or local inputs

Question 28

some smooth muscle pacemaker cells have membrane potentials that drift up and down (due possibly to regular variation in ion flux across the membrane) to produce

Answer 28

slow waves

Question 29

if any given slow wave is large enough to reach threshold, one action potential or often a

Answer 29

short burst of action potentials is produced

Question 30

because slow waves are usually found in single-unit smooth muscle, when an action potential is produces, the action potential

Answer 30

spreads over the entire syncytial group of cells, and the group contracts as a unit

Question 31

neuronal and hormonal regulation of contraction:

unlike in skeletal muscle, smooth muscle fibers do not have a

Answer 31

motor end-plate region

Question 32

neuronal and hormonal regulation of contraction:

as the axon of a postganglionic autonomic neuron approaches, it divides into many branches, with each branch containing many swollen regions called

Answer 32

varicosities

Question 33

neuronal and hormonal regulation of contraction:

transmitter-filled vesicles are within each varicosity, and some vesicles are discharged as

Answer 33

an action potential passes the varicosity

Question 34

neuronal and hormonal regulation of contraction:

varicosities from a single axon may occur along (and thus influence)

Answer 34

several muscle fibers

Question 35

neuronal and hormonal regulation of contraction:

a single muscle fiber may be near (and influenced by) varicosities of

Answer 35

postganglionic fibers of both sympathetic and parasympathetic neurons

Question 36

neuronal and hormonal regulation of contraction:

some transmitters _______ contractile activity

Answer 36

enhance and others inhibit

Question 37

neuronal and hormonal regulation of contraction:

a given transmitter can increase tension in one smooth muscle and

Answer 37

decrease tension in another

e.g. norepinephrine (binding to alpha-adrenergic receptors) enhances contraction of most blood vessel smooth muscle, but it relaxes bronchiole smooth muscle (by binding to beta2-adrenergic receptors)

Question 38

neuronal and hormonal regulation of contraction:

smooth muscle plasma membranes contain receptors for a variety of hormones including:

Answer 38

epinephrine

cholecystokinin

angiotensin II

vasopressin