Muscles: Smooth Muscle

Question 1

Is smooth muscle voluntary

Answer 1

no - it is involuntary

Question 2

Where is smooth muscle found?

Answer 2

Found in the wall of internal organs (gut, blood vessels, airways, bladder, reproductive organs, iris etc)

Question 3

Structural features of smooth muscle

Answer 3

Spindle shaped, uninucleated cell, Not striated

Question 4

Width of smooth muscle

Answer 4

5 µm

Question 5

Length of smooth muscle

Answer 5

100-400 µm

Question 6

Type of junction in single unit smooth muscles

Answer 6

gap junction so that they can act in unison (are electrically coupled)

Question 7

Where are a single unit smooth muscle cells located

Answer 7

Found in most blood vessels and hollow organs (respiratory, digestive, urinary and reproductive tracts)

Question 8

What is a multi-unit smooth muscle

Answer 8

tissue made of discrete bundles of independent cells which are densely innervated and contract only in response to its innervation

Question 9

Examples of multi-unit smooth muscle

Answer 9

vas deferens, iris, piloerection

Question 10

Function of inner “unitary” smooth muscle

Answer 10

It is circular and contracts = they narrow and mush up food

Question 11

Function of outer “unitary” smooth muscle

Answer 11

It is longitudinal and contracts = making food move down tube

Question 12

Does smooth muscle contain T-tubules

Answer 12

no

Question 13

What does smooth muscle contain rather than T-tubules

Answer 13

caveolae

Question 14

What is the function of caveolae

Answer 14

they are “dips” that increase smooth muscles SA to get more AP

Question 15

What is the function of dense bodies in smooth muscle

Answer 15

Dense bodies act like z-lines to “anchor” actin to sarcolemma

Question 16

Does unitary smooth muscle contain gap junctions

Answer 16

yes - to electrically connect cells

Question 17

What is the sarcoplasmic reticulum like in smooth muscle?

Answer 17

it is under/poorly developed (has small volume)

Question 18

Does smooth muscle contain striaitons?

Answer 18

No

Question 19

Does smooth muscle contain actin and myosin filaments

Answer 19

Yes

Question 20

What does decreased organisation of myosin and actin allow smooth muscle to do?

Answer 20

It allows for greater shortening (contract more) and can operate over a large range of lengths (60 - 75% shortening possible)

Question 21

What initiates contraction in smooth muscle

Answer 21

voltage gated Ca2+ channels (relatively few Na+ channels)

Question 22

What triggers the opening of voltage gated Ca2+ channels

Answer 22

increase in intracellular Ca

Question 23

The three types of initiation of smooth muscle

Answer 23

neural, hormonal or spontaneous (myogenic)

Question 24

Smooth muscle calcium source

Answer 24

Extracellular and SR Regulation via voltage, hormones, neurotransmitters and specific ions

Question 25

Why is smooth muscle initiation of contraction a slower process

Answer 25

because it is enzyme regulated: energy conserving as smooth muscle doesn’t need to be pumping all the time

Question 26

What occurs in Stage 1 of the initiation of smooth muscle contraction: Increased intracellular Ca2+

Answer 26

Ca2+ enter the cytosol from the Extracellular fluid (ECF) via voltage- dependent or voltage- independent Ca2+ channels, or from the scant SR

Question 27

What occurs in Stage 2 of the initiation of smooth muscle contraction: Calmodulin (CaM) activation

Answer 27

Ca2+ binds to and activates calmodulin (instead of troponin)

Question 28

What occurs in Stage 3 of the initiation of smooth muscle contraction: Activation of kinase

Answer 28

The activated calmodulin then activates myosin light chain kinase (MLCK). MLCK is an enzyme

Question 29

What occurs in Stage 4 of the initiation of smooth muscle contraction: Phosphorylation of myosin

Answer 29

MLCK activates the myosin by phosphorylating it, which in turn activates the myosin ATPases.

Question 30

What occurs in Stage 5 of the initiation of smooth muscle contraction: Cross bridge cycle

Answer 30

Activated myosin forms cross bridges with actin of the thin filaments and shortening begins in the usual fashion. Contraction occurs by the same actin-myosin interaction as in striated muscle (sliding filament) , but REGULATION IS MYOSIN (NOT ACTIN) BASED. Turning on switch is the phosphorylation of the myosin head. (maximum rate of crossbridge formation is low = slow contractions)

Question 31

When is MLCK active

Answer 31

MLCK is active only in the presence of a small Ca-binding protein, calmodulin (and only when it has Ca bound)

Question 32

Regulatory protein in smooth muscle contraction

Answer 32

In smooth muscle the reg protein is calmodulin, troponin complex is absent.

Question 33

What has to happen for myosin to hydrolyse ATP

Answer 33

Myosin does not hydrolyse ATP (to become energized) unless it is first phosphorylated (on the regulatory light chain, LC20 located on the neck of the myosin)

Question 34

When does a smooth muscle contraction end

Answer 34

Contraction ends when a myosin light chain phosphatase dephosphorylates the myosin light chain

Question 35

What removes Ca2+ from the intracellular space

Answer 35

Ca-ATPase in cytoplasm membrane is the primary mechanism for reducing intracellular Ca++

Question 36

What is Myosin Phosphatase (MLCP)

Answer 36

an enzyme that removes phosphate from myosin head (usually from Ca levels decreasing)

Question 37

The function of enzymes in smooth muscle contraction

Answer 37

Balance of two enzymes (putting/removing Ca) causes contraction (which is dominant)

Question 38

Specific activity of each enzyme in smooth muscle contraction

Answer 38

↑ MLCK activity (Ca2+ regulated) will favour contraction ↑ MLCP activity will favour relaxation When intracellular Ca2+ drop MLCP activity will dominate

Question 39

what MUST be present for contraction to occur

Answer 39

phosphorylation of the myosin head

Question 40

What are diffuse junctions

Answer 40

When autonomic nerve fibres branch and touch the underlying smooth muscle fibres

Question 41

What do varicosities do in autonomic nerve fibres

Answer 41

they release their neurotransmitters into a wide synaptic cleft (a diffuse junction)

Question 42

What happens when a smooth muscle is stretched

Answer 42

Initially contract, effectively resisting the stretch (e.g. blood vessels trying to maintain blood flow constant) -stretch activated calcium channels. But over time it slowly relaxes, adapting to the change in length (e.g. gut) - via calcium-dependent K+ channels, hyperpolarizing the membrane potential