Smooth muscle contraction

Question 1

What does smooth muscle look like under a microscope?

Answer 1

Sheet like layers
No striations
Single nuclei
Spindle shaped cells

Question 2

What differs the arrangement of Smooth muscle to Skeletal muscle?

Answer 2

Smooth muscle’s thick and thin filaments are NOT organised in myofibrils and sarcomeres like Skeletal muscle has - hence no banding pattern

Question 3

How does contractions of Skeletal muscle occur?

Answer 3

By the sliding-filament mechanism and cross bridge mechanism

Question 4

Where are smooth muscles found?

Answer 4

Surrounding. hollow structures and organs that. undergo large changes in the lengths of the smooth muscle fibres in their walls

Question 5

What does the absence of cross striations indicate about Smooth muscle?

Answer 5

That Smooth muscle lack myofibrillar structure

Question 6

What percentage of Actin and Myosin is in Smooth muscles compared to striated muscles?

Answer 6

10% of their Myosin and Actin

Question 7

What is Smooth muscles Actin : Myosin ratio?

Answer 7

Actin : Myosin

15:1

Question 8

How is Actin and Myosin held within Smooth muscle?

Answer 8

Actin filaments - attached to dense bodies and radiate out from them, functioning in the same way as the Z-lines do

Myosin filaments - sit in between Actin

Question 9

How does Smooth muscle change from relaxed too contracted?

Answer 9

Pulls thick filaments and cell becomes shorter and wider

Question 10

What 2 other things give structure to Smooth muscle cells?

Answer 10

intermediate filaments and membrane dense area (attachment plaques)

Question 11

What is the Sarcoplasmic reticulum like in Smooth muscle cells?

Answer 11

Under-developed Sarcoplasmic reticulum

Caveoli acts like T-tubules and since it has no T-tubules its sat at the plasma membrane

Question 12

Where is Calcium stored in the Smooth muscle?

Answer 12

Extracellular space near Caveoli

Question 13

What is cross-bridge cycling in Smooth muscle controlled by and what does this do?

Answer 13

Calcium regulated enzyme - MLCK (Myosin light chain kinase) phosphorylates Myosin, and only the phosphorylated version can bind to Actin and undergo cross-bridge cycling

Question 14

What must happen to relax a contracted smooth muscle?

Answer 14

Myosin must be dephosphorylated by the enzyme MLCP (Myosin light chain phosphatase) because dephosphorylated Myosin is unable to bind to Actin

Question 15

What are the 6 steps involved in contracting a Smooth muscle?

Answer 15

1. Initiated by calcium from Extracellular fluid or Sarcoplasmic reticulum
2. Calcium binds to
   calmodulin (instead of troponin as in skeletal muscle)
3. Ca-caImoduIin-MLCK
   complex leads to phosphorylation of MLC (requires 1 ATP)
4. MLC is part of myosin head
5. Phosphorylated myosin head binds to actin and power stroke occurs automatically
6. A second ATP is required to release
   myosin head from actin

MLC = Myosin light chain 
MLCK = Myosin light chain kinase

Question 16

What are the 3 steps involved in Smooth muscle relaxation?

Answer 16

1). Calcium concentration dips
below a critical level due to
being pumped out of the cell or
into the SR

2). Calcium is released from calmodulin

3). MLCP removes phosphate from the MLC, causing detachment of
the myosin head from the actin filament, causing relaxation.

MLCP = Myosin light chain phosphatase

Question 17

What are the 2 sources of Calcium that increase cytosolic Calcium to initiate Smooth muscle contraction?

Answer 17

The Sarcoplasmic reticulum

Extracellular Calcium entering the cell through plasma-membrane Calcium channels (Via voltage sensitive pathways or extracellular messengers)

Question 18

Where does Calcium have to go in order to relax a muscle?

Answer 18

Calcium must be removed back to the Sarcoplasmic reticulum or extracellular fluid

Question 19

What 5 factors influence Smooth muscles contractile activity ?

Answer 19

• Spontaneous electrical activity in the plasma
membrane of the muscle cell

• Neurotransmitter released by the autonomic
neurons

• Hormones

• Locally induced changes in the chemical
composition (paracrine factors, acidity, oxygen,
osmolarity, ion concentrations) of the extracellular
fluid surrounding the cell

• Stretch

Question 20

How can Smooth muscle cells generate action potentials in the absence of any neural or hormonal input ?

Answer 20

• The membrane potential change during the spontaneous depolarization to threshold is known as a
pacemaker potential.

Question 21

How do Smooth muscle pacemaker cells vary?

Answer 21

They vary as their membrane potentials drifts up and down due to regular variation in ion flux across the membrane

These periodic fluctuations are called slow waves.

Question 22

Where about can Smooth muscle Pacemaker cells be found?

Answer 22

Throughout the gastrointestinal

tract, and this is why the gut smooth muscle tends to contract rhythmically even in the absence of neural input

Question 23

What do Smooth muscle cells have instead of the specialised motor end-plate regions found in Skeletal muscles?

Answer 23

Swollen regions known as varicosities which contains many vesicles filled with neurotransmitter, of which some. are released when an action potential passes the varicosity

Question 24

How many Smooth muscle cells are influenced by the neurotransmitters of a single neuron?

Answer 24

Multiple

Question 25

How many neurons can influenced a single Smooth muscle cell ?

Answer 25

Neurotransmitters from more than 1 neuron

Question 26

How do neurotransmitters affect contractile activity?

Answer 26

Some enhance contractile activity while others decrease contractile activity

Question 27

How many the same neurotransmitters function vary when released in different areas?

Answer 27

A neurotransmitter can have a different function depending on what Smooth muscle it is release into

I.e - Noradrenaline enhances most vascular smooth muscles but relaxes airway smooth muscle

Question 28

What is the type of neurotransmitter response dependent upon?

Answer 28

The type of response (excitatory or inhibitory) depends on the receptors the neurotransmitter is binding to and the intracellular signalling mechanisms those receptors activate

Question 29

What does Nitric oxide, a local factor do to a Smooth muscle?

Answer 29

it relaxes it, acting in a paracrine manner

Question 30

How can some Smooth muscles respond when stretched?

Answer 30

They can contract when they are stretched, as stretching opens mechxnosensitive ion. channels, leading to a further membrane depolarisation

Question 31

What are the 2 types of Smooth muscle?

Answer 31

• Unitary, single unit, visceral
— sheets of electrically coupled cells which act in unison - often spontaneously active (a ‘syncytium’ e.g., gut and
blood vessels)

Multiunitary, multi unit 
— tissue made of discrete 
bundles of cells which are 
densely innervated and 
contract only in response to 
its innervation (e.g., vas 
deferens, iris, piloerectors)