Smooth Muscle Contraction

Question 1

What does smooth muscle not contain that skeletal and cardiac muscle does

Answer 1

the thick and thin filaments are not arranged into myofibrils (no striations = no myofibrillar structure) and there are NO SARCOMERES

Question 2

How does contraction occur in smooth muscle

Answer 2

sliding-filament mechanism

Question 3

where is smooth muscle found

Answer 3

surrounds hollow structures and organs that undergo changes in volume with accompanying changes in the lengths of the smooth muscle fibres in their wall

Question 4

What shape are smooth muscles

Answer 4

spindle shaped
20-200 um in length
2-20 um in diameter

Question 5

what is the actin:myosin ratio

Answer 5

15:1

Question 6

Tell me about Caveoli and the sarcoplasmic reticulum in smooth muscle

Answer 6

rudimentary SR
No t-tubules
caveoli at the plasma membrane (little pockets, invaginations)

Question 7

Where is Ca2+ stored/hidden

Answer 7

in the extracellular space near caveoli

Question 8

In smooth muscle how is cross-bridge cycling controlled

Answer 8

by Ca2+ regulated enzyme that phosphorylates myosin (Myosin Light Chain Kinase MLCK). As only the phosphorylated form of smooth muscle myosin can bind and undergo cross-bridge cycling

Question 9

in smooth muscle how does relaxation of the contracted muscle occur

Answer 9

myosin must be dephosphorylated as in this new form it is unable to bind to actin
MEDIATED BY Myosin Light chain Phosphatase MLCP

Question 10

What are the two sources of Ca2+

Answer 10

• SR

- Extracellular Ca2+ entering the cell through the plasma membrane Ca2+ channels

Question 11

what has to happen to Ca2+ for the smooth muscle to relax

Answer 11

the ca2+ has to be removed either to the SR or back to the extracellular fluid

Question 12

What inputs influence smooth muscle contractile activity

Answer 12

spontaneous electrical activity in the plasma membrane of the muscle cell
neurotransmitter released by the autonomic neurones
hormones
locally induced changes in the chemical composition (almost always results in relaxation
stretch

Question 13

examples of locally induced changes in the chemical composition

Answer 13

paracrine factors, acidity, oxygen, osmolarity, ion concentrations

Question 14

spontaneous electrical activity in the plasma membrane of the muscle cell

Answer 14

this is in the absence of any neural or hormonal input

Question 15

what is the pacemaker potential

Answer 15

The membrane potential change occurring during the spontaneous depolarization to threshold

Question 16

what causes the membrane poetical to drift up and down

Answer 16

regular variation in ion flux across the membrane

THIS IS KNOWN AS SLOW WAVES

Question 17

why can the GI tract rhythmically

Answer 17

there are pacemaker cells found throughout the GI tract therefore can contract rhythmically even in the absence of neural input

Question 18

instead of a specilizes motor end plate region what does smooth muscle have

Answer 18

swollen regions known as varicosities, these contain many vesicles filled with neurotransmitters, some of which are released when an AP passes the varicosity

Question 19

effects of noradrenaline (having opposite effects un different muscle tissues

Answer 19

if released from a postganglionic sympathetic neurone, enhances contraction of most vascular smooth muscle by acting on alpha - adrenergic receptors, but produces relaxation of airway (bronchiolar) smooth muscle by acting in beta-2adrenergic receptors.

Question 20

What dermines the type of response by neurotransmitters

Answer 20

the receptors the chemical messenger binds to in the membrane on the intracellular signalling mechanisms those receptors have

Question 21

What are some local factors which can effect smooth muscle tension

Answer 21

paracrine signals 
acidity 
O2 and CO2 levels
osmolarity 
ion composition of the extracellular fluid

Question 22

What produces smooth muscle relaxation acting in a paracrine manner

Answer 22

nitric oxide (NO)

Question 23

What happens during stretching

Answer 23

opens mechxnosensitive ion channels, leading to membrane depolarisation
the resulting contraction opposes the forces acting to stretch the muscle

Question 24

Unitary, single unit, visceral

Answer 24

heets of electrically coupled cells which acts in unison - often spontaneously active (a ‘syncytium’ e.g., gut and blood vessels)

Question 25

Multiunitary, multi unit

Answer 25

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)