ET : M - Smooth Muscle

Question 1

What is the structure of smooth muscles?

Answer 1

Very long and thin (typically 100-400 μm long and 5-10 μm in diameter), with a single central nucleus and is tapered at the ends

Question 2

What is the cell shape?

Answer 2

Spindle

Question 3

Examples of where smooth muscles are found

Answer 3

• Airways
• Bladder and reproductive organs
• Blood vessels
• Iris and ciliary muscles in the eye

Question 4

What is the structure of single-unit (unitary/visceral) smooth muscle tissues?

Answer 4

Sheets of cells that are electrically connected by ‘dense bodies’ or gap junctions (action potential can spread from cell to cell) and act in unison (i.e. as one unit), often spontaneously active

Question 5

Where are single-unit smooth muscle tissues mainly found?

Answer 5

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

Question 6

What is the structure of multi-unit muscle tissues?

Answer 6

Tissue made of discrete bundles of independent cells which are densely innervated and contract only in response to its innervation (each cell is electrically isolated)

Question 7

Generally, what are smooth muscles richly innervated by?

Answer 7

Automatic nervous system whose nerves form varicosities, releasing various neurotransmitters on many target cells

Question 8

How can motor units arise?

Answer 8

From either a single nerve innervating several cells or from the spread of excitation from one cell to another

Question 9

What is poorly organised?

Answer 9

The contractile machinery, with actin and myosin filaments spread throughout the cell with no obvious pattern

Question 10

What’s an advantage of having a poorly organised contractile machinery?

Answer 10

Greater shortening can occur without filaments colliding with each other

Question 11

Since smooth muscles don’t have sarcomeres, what does it allow?

Answer 11

A cell can contract from a relaxed length of ~150μm to ~30μm, which allows smooth muscle cells to change the diameter of hollow organs over a wide range (more contraction)

Question 12

What is poorly developed?

Answer 12

SR and is spread througout the cell

Question 13

What do smooth muscles not have?

Answer 13

Troponin and T-tubules (have caveolae instead which acts to increase surface area and helps to get a little bit more action potential into the centre)

Question 14

What do smooth muscles have relatively of?

Answer 14

Few mitochondria spread throughout the cell

Question 15

What is the intermediate filament?

Answer 15

Cytoskeleton element

Question 16

What do dense bodies act like?

Answer 16

Z discs to “anchor” actin to sarcolemma (it’s where all the filaments join)

Question 17

Do smooth muscles contain striations?

Answer 17

No (no “banded” appearance)

Question 18

How is contraction initiated?

Answer 18

Involuntary (myogenic - neural, hormonal, spontaneous)

Question 19

What are Ca2+ sources in smooth muscles?

Answer 19

Extracellular and SR (opening of channels regulated by voltage, hormones, neurontransmitters and specific ions)

Question 20

How can Ca2+ enter the cell in smooth muscles?

Answer 20

Ca2+ enters the cell via voltage-gated Ca2+ channels or voltage-independent Ca2+ channels or from the SR

Question 21

What is contraction initiated by?

Answer 21

An increase in intracellular Ca2+, which is supplied by Ca2+ channels in the surface membrane and/or Ca2+ release from the SR

Question 22

How are the Ca2+ channels in the surface membrane activated and regulated?

Answer 22

Activated by depolarisation, tends to be regulated by second messengers (some may be regulated directly by certain neurotransmitters). A lot of Ca2+ within the cell is released due to hormones which, trigger receptors through second messenger pathway, usually involving IP3, causing Ca2+ to be released from the SR

Question 23

What does Ca2+ bind to and activate?

Answer 23

Calmodulin

Question 24

What does the activated calmodulin activate?

Answer 24

Calmodulin-dependent protein kinase (Myosin Light Chain Kinase (MLCK) - an enzyme)

Question 25

What allows myosin to interact with actin?

Answer 25

MLCK phosphorylating myosin light chains, thus, hydrolysing ATP (to become energised) and activating myosin ATPases

Question 26

How does the cross-bridge cycle occur?

Answer 26

MLCK acts on myosin heads as it comes along and interacts with the myosin neck region where all the bending/enzyme activity occurs. Activated myosin (phosphate from ATP added to light chain) forms cross-bridges with actin of thin filaments and contraction begins

Question 27

Why is MLCK myosin regulated?

Answer 27

All the action is on myosin (thick filaments)

Question 28

The max. cross bridge cycling rate is ____ and contraction is quite ____

Answer 28

The max. cross bridge cycling rate is low and contraction is quite slow

Question 29

When does contraction end (relaxation)?

Answer 29

A Myosin Light Chain Phosphatase (MLCP) dephosphorylates the light chains (by getting rid of phosphate on myosin head)

Question 30

What is the primary mechanism for reducing intracellular Ca2+?

Answer 30

Ca2+ ATPase in the cytoplasm

Question 31

Since smooth muscle contraction is ___ regulated, it’s ____ but _____

Answer 31

Since smooth muscle contraction is enzyme regulated, it’s slow but efficient

Question 32

What is the contractile state of the cell dependent on?

Answer 32

Ca2+ levels within the cell and the activity of MLCK and phosphatase. The contractile state is therefore, the balance of both MLCK and phosphatase activities and contraction can be graded by differential modulation of these processes

Question 33

Increased MLCK activity (Ca2+ reguluated) favours?

Answer 33

Contraction

Question 34

Increased MLCP activity favours?

Answer 34

Relaxation

Question 35

How can smooth muscle contraction be modulated?

Answer 35

• Neurotransmitters
• Hormones
• Environment (pH, O2, etc.)
• Histamine
• Adenosine
• Prostacyclin
• Nitric Oxide
• Stretch
  These all can affect either the cytosol Ca2+, MLCK or MLCP activity (and if any of one of them changes, it will change the tone of blood vessel or gut etc.)

Question 36

What innervates most smooth muscle fibres>

Answer 36

Automatic nerve fibres

Question 37

What do automatic nerve fibres do?

Answer 37

Branch and form “diffuse junctions” with underlying smooth muscle fibres

Question 38

What do varicosities in the terminal axon do?

Answer 38

Contain and release neurotransmitters into a wide synaptic cleft (a diffuse junction), it leaks the hormones out and can affect the neighbouring cells (in response to action potentials).

Question 39

Where is neurotransmitters secreted and diffused?

Answer 39

Secreted into the matrix coating and diffuses to the muscle cells

Question 40

When you stretch a smooth muscle, it will generally?

Answer 40

Initially contract, effectively resisting the stretch - it constricts so there’s a lot of stress and tension (e.g. blood vessels trying to maintain blood flow constant). This mechanism appears to reside in the presence of ‘stretch activated’ ion channels, which alow Ca2+ to enter the cell

Question 41

What happens to smooth muscles after stretch overtime?

Answer 41

Slowly relaxes, adapting to the change in length (e.g. gut), via Ca2+ dependent K+ channels which, when activated, hyperpolarises the membrane potential and shut down Ca2+ influx via voltage-dependent Ca2+ channels