lecture 33 - Smooth muscle

Question 1

sarcomeres in smooth muscle?

Answer 1

no such thing.

Question 2

t-tubules in smooth muscle?

Answer 2

there are NO t-tubules, the equivalent is CAVEOLAE

Question 3

regulatory protein

Answer 3

calmodulin (not troponin/tropomyosin)

Question 4

Is there a sarcoplasmic reticulum in smooth muscle?

Answer 4

yes, but it is minimal

Question 5

smooth muscle size and shape

Answer 5

30-200μm x 3-8μm spindle shaped

Question 6

which muscle is the most common in the human body?

Answer 6

smoooooooooooooooooooooth

Question 7

single-unit smooth muscle

Answer 7

group of muscle cells, acting as one unit due to gap junctions between individual cells, innervated by an autonomic neuron

Question 8

multi-unit smooth muscle

Answer 8

each cell woks by itself, innervated by an autonomic neuron mesh

Question 9

Varicosities

Answer 9

bulge-thingies in the autonomic neuron which releases neurotransmitters

Question 10

smooth muscle arrangement in the GI tract

Answer 10

layer of smooth muscle running longitudinal and another layer running transverse at right angles to it.

Question 11

dense bodies

Answer 11

replace z-lines - anchor actin to the sarcolemma

Question 12

intermediate filaments (in the context of smooth muscle)

Answer 12

cytoskeleton element. Wrap the myocyte in a diamond pattern like a mandarin bag

Question 13

how do smooth muscles change in length compared to skeletal muscle?

Answer 13

they can change in length more than skeletal muscle

Question 14

striations in smooth muscle?

Answer 14

nah none of that, but they still contain actin and myosin - less organised though

Question 15

complex electrical behaviour, can be _______, _______, or __________ (=myogenic). For example, - myogenic with _____ _____ of depolarisation (e.g. gut) - ______ induced contraction (e.g. iris)

Answer 15

complex electrical behaviour, can be _neural_, _hormonal_, or _spontaneous_ (=myogenic). For example, - myogenic with _slow_ _wave_ of depolarisation (e.g. gut) - _neurally_ induced contraction (e.g. iris)

Question 16

what is the trigger for contraction in smooth muscle?

Answer 16

contraction in smooth muscle is ALWAYS caused by an increase in cytosol [Ca2+]

Question 17

how do calcium ions enter the cytosol?

Answer 17

• move in from the extracellular fluid by voltage-dependent/-independent (any way it wants) - or from the scant SR via IP3 (NOT Calcium induced)

Question 18

once entering the cytosol, the Ca2+…

Answer 18

binds to and activates calmodulin (1 calmodulin binds 4 Ca2+)

Question 19

activated calmodulin…

Answer 19

activates the myosin light chain kinase enzymes, phosphorylating the light chains (e.g. LC20) located on the neck of the myosin

Question 20

activated MLCK …

Answer 20

activates the myosin ATPases

Question 21

activated myosin …

Answer 21

forms cross-bridges with actin, as per usual

Question 22

maximum rate of cross-bridge formation =

Answer 22

low, therefore slow contractions

Question 23

what happens for the smooth muscle to relax again

Answer 23

1. drop in Ca2+ (caused by CaATPase removing it from the cell) will inactivate MLCK = less phosphorylating 2. Phosphatase is required to dephosphorylate the myosin light chain

Question 24

how is contraction in smooth muscle graded?

Answer 24

by a balance between MLCK and phosphatase enzymes. More Phosphatase = relaxation more MLCK = contraction

Question 25

cAMP impact on contractions in smooth muscle

Answer 25

inhibits MLCK = less contractions

Question 26

how does smooth muscle respond to being stretched?

Answer 26

initially contracts to resist the stretch (stretch activated Ca2+ channels increases force). They then relax slowly, adapting to the change in length (via calcium dependant K+ channels).

Question 27

sumarise each step

Answer 27

1. Increased intracellular Ca²⁺
2. Calmodulin activation
3. Activation of calmodulin dependent protein kinase (myosin light chain kinase)
4. Phosphoralisation of myosin
5. Actin /myosin interaction resulting in contraction