Smooth Muscle

Question 1

Give an example for each type of smooth muscle contraction

Answer 1

Question 2

Why is smooth muscle called such

Answer 2

Lack of striations

Question 3

Describe the common structural organisations of smooth muscles

Answer 3

Question 4

Orientation of Circular & Longitudinal SM in ileum (end of small intestine)

Answer 4

Question 5

Varicosity

Answer 5

Junction between nerves and smooth muscle

Question 6

What are the two major ways in which smooth muscle is arranged

Answer 6

Single Unit vs Multi Unit

Question 7

Compare varicosity in single-unit and multi-unit smooth muscle

Answer 7

Every cell has its own varicosity in multi unit smooth muscle, while one every few has a varicosity in single unit

Question 8

2 Examples of single unit smooth muscle

Answer 8

Arterioles, Lymph, Vessels, Gastrointestinal & Urogenital tracts
Uterus

Question 9

2 Examples of multi unit smooth muscle

Answer 9

Bronchi, Large Arteries, Ciliary body, Iris

Question 10

Does Multi-Unit SM have NMJs

Answer 10

They have no NMJ but diffuse distribution of Neurotransmitter filled bulges along the axons that course through SM - varicosities - that release neurotransmitter into synaptic cleft

Question 11

Does single or multi-unit SM act more in unison

Answer 11

Single Unit SM acts much more in unison to create synchronised contraction & relaxation, e.g. in childbirth as seen in the uniform co-ordinated uterine contractions

Question 12

Visceral Muscle

Answer 12

Single-Unit Muscle

Question 13

How do APs transfer across Single Unit SM

Answer 13

Action potentials can be transferred in single unit cells through gap junctions and behave as a functional syncitium

Question 14

Stretch Relaxation response of Single-Unit SM

Answer 14

**Think of bladder

Tension initially develops on stretch, then SM expands in response and returns to original tone but at new longer length

Question 15

Length-Tension Relationships of SkM vs SM

Answer 15

SkM decreases by a third when contracted and stretch by about 20% when relaxed; smooth muscle can reduce by 2/3s in length and increases up to 2.5x when relaxed

Question 16

Compare Ca2+ concentration in vs outside the cell

Answer 16

Very Low inside (0.0001mM)

Very High outside (2.5mM)

Question 17

How can Ca2+ levels in a cell be elevated

Answer 17

Through voltage gated channel proteins

Or, through the binding of a neurotransmitter which activates a G-protein; leads to activation of Inositol Trisphosphate that moves to the intracellular stores of Ca2+ and interacts with receptors that liberate intracellular stores of Ca2+

Question 18

Describe source of Ca2+ in SM in comparison to SkM

Answer 18

SM has no Transverse Tubule or T-system but SM are small enough that Ca2+ can quickly diffuse to cause changes

It rushes in through voltage gated proteins

Question 19

Role of Ca2+ in SkM vs SM

Answer 19

SkM: Ca2+ has a role in acting on troponin, causing a physical change

SM: Ca2+ binds to calmodulin to cause a chemical change

Question 20

Contraction mechanism in smooth muscle

Answer 20

Intracellular Ca2+ and Calmodulin are elevated; they interact with, and bind to inactive Myosin Light Chain Kinase which phosphorylates MLC using ATP

MLC must be phosphorylated for actin to bind to it and form an X-bridge which thus causes contraction

Question 21

Relaxation mechanism in Smooth Muscle

Answer 21

Reversal of elevated Ca2+ leads to dissociation of Calmodulin from MLCK, inactivating it

Question 22

How does Noradrenaline cause relaxation of SM

Answer 22

NA binds to B Adrenoceptors on Bronchial SM which reduces [Ca2+]

Question 23

Role of Myosin Phosphatase

Answer 23

Dephosphorelates Myosin Light Chain (MLC), rendering it unable to bind to actin which would otherwise cause an X-bridge/contraction

Question 24

Why does myosin phosphatase not affect rates of contractile SM states

Answer 24

Myosin Phosphotase is generally constant so its presence tends not to affect contractile state of the muscle and instead it is the changes of intracellular phosphate ions that has a greater effect