Smooth and Cardiac Muscle

Question 1

What are the three main types of muscle fibres?

Answer 1

Smooth muscle
Cardiac muscle
Skeletal muscle

Question 2

What are the features of smooth muscle?

Answer 2

o An involuntary non-striated muscle
o Provides mechanical control of organ systems (e.g. bladder, uterus)
o Smooth muscle fibres are spindle-shaped (wide in middle and tapered at both ends)
o Fibres have a single nucleus

Question 3

What are the features of cardiac muscle?

Answer 3

o An ‘involuntary muscle’
o Intrinsic myogenic activity responsible for the beating of the heart
o Contractile elements are similar to skeletal muscle (long, thin myofibrils that contract as sarcomere shortens)
o Cardiac myocytes are organised in branched network of fibres running in various directions
o Fibres have centrally located nuclei

Question 4

What are the features of skeletal muscle?

Answer 4

o ‘Voluntary muscle’ anchored by tendons to bone
o Responsible for movements such as locomotion, maintenance of posture and breathing (via contraction of the diaphragm)
o Clear cross striations consisting of actin and myosin
o Multi-nucleated and peripherally located nuclei

Question 5

How do the nuclei of the different muscle types compare?

Answer 5

Skeletal = multinuclear, peripheral location
Cardiac = single/double nucleus, central location
Smooth = single nucleus, central location

Question 6

How does the banding of the different muscle types compare?

Answer 6

Skeletal = actin and myosin form distinct bands
Cardiac = actin and myosin form distinct bands
Smooth = actin and myosin no discreet bands

Question 7

How do the Z discs of the different muscle types compare?

Answer 7

Skeletal = present
Cardiac = present
Smooth = not present (cytoplasmic dense bodies)

Question 8

How do the T tubules of the different muscle types compare?

Answer 8

Skeletal = at A-I junction, triads
Cardiac = at Z-disk, diads
Smooth = none (caveoli)

Question 9

How do the cellular junctions of the different muscle types compare?

Answer 9

Skeletal = none
Cardiac = intercalated discs, gap junctions
Smooth = gap junctions

Question 10

How do the neuromuscular junctions (NMJ) of the different muscle types compare?

Answer 10

Skeletal = present
Cardiac = not present (contraction is intrinsic)
Smooth = not present (contraction is intrinsic, neural or hormonal)

Question 11

How do the calcium-binding proteins of the different muscle types compare?

Answer 11

Skeletal = troponin
Cardiac = troponin
Smooth = calmodulin

Question 12

In cardiac muscle, what is the A band and I band?

Answer 12

A band = thick filaments

I band = Thin filaments

Question 13

What are intercalated discs? What cell-cell junctions do they contain?

Answer 13

Intercalated discs are sites of thickening of sarcolemma (plasma membrane) where the cell are joined together. They connect adjoining cardiac myocytes together.

• Intercalated discs contain three different types of cell-cell junctions:
  1. Fascia adherens or “anchoring junctions” - attach sarcomeres to the cell membrane.
  2. Desmosomes - sites of adhesion, that keep the muscle cells connected when they contract.
  3. Gap junctions – facilitate electrical communication. They permit the passage of ions and enable action potentials to spread between cardiac cells.

Question 14

How do cardiac muscles depolarise to cause contraction/relaxation?

Answer 14

Cardiac muscle does not require action potentials.
Cardiac cells have automaticity - they can spontaneously generate an electrical impulse (depolarise).
Cardiac cells have rhythmicity - they can generate action potentials in a regular and repetitive manner.

Question 15

What are the specialised properties of cardiac muscle?

Answer 15

Cardiac myocytes form an “electrical syncytium” or “functional syncytium”.
Electrical impulses propagate between cells via gap junctions located on the intercalated disk.
Waves of depolarisation propagate to adjacent cells which contract in a synchronous (wavelike) fashion.
This property allows rapid, synchronous depolarisation of the myocardium.
The myocardium functions as a single contractile unit which is important for the pumping action of the heart.

Question 16

What plays an important role in action potentials (AP) in cardiac muscle?

Answer 16

Structure and properties of the tissue (i.e. ion channels) play an important role in determining the electrical behaviour (activity) of the cell

Question 17

What are the key channels involved in the AP?

Answer 17

The AP is shaped by delicate balance between fluxes of ions in and out of the cell.
Depolarising currents:
Na+ and Ca2+ channels into cytosol (travel into cell)
Repolarising currents:
K+ channels out of cell

Question 18

What are the features of action potentials within the heart?

Answer 18

Size and shape of the APs can differ between cells.
The shape of the cardiac action potential relates to its function within the heart.
Voltage-dependent ion channel proteins in the plasma membrane generate the action potentials.
Cells have different kinds of voltage-dependent ion channels

Question 19

How do the pacemaker cells depolarise?

Answer 19

Cells of the pacemaker tissues (sinoatrial and atrioventricular nodes) depolarise spontaneously (automaticity).
Note: atrial and ventricular cells only display automaticity in disease (NOT normally).

Question 20

What are the activities of the ventricular cells (Purkinje network, atrium, Bundle of His and ventricle)?

Answer 20

Ventricular cells also termed “work cells” have different shaped APs.
They contract in a coordinated fashion to pump blood around the body.
Refractory periods - during which the ion channels are inactivated and the muscle is unresponsive.
This means that however hard the heart is stimulated individual contractions cannot fuse into a maintained tetanic contraction (as happens in skeletal muscle).
The heart has to beat rhythmically.

Question 21

What is the process of calcium signalling during cardiac muscle contraction?

Answer 21

Depolarisation of the membrane (influx of sodium via sodium channelx) opens voltage-gated calcium channels
Influx of calcium through voltage-gated (L-type) calcium channels (LTCC) in the cell membrane
The rise in intracellular calcium triggers further calcium release from the sarcoplasmic reticulum (SR) by the ryanodine receptor (RyR)
Calcium then associates with troponin C in the sarcomere to initiate contraction in the cardiac muscle (systole)
These events are terminated by release of calcium from the sarcomere (causing relaxation. diastole) and its reuptake into the sarcoplasmic reticulum

Question 22

Why are calcium (and ATP) key for striated muscle contraction?

Answer 22

Calcium is important for actin and myosin interaction

ATP hydrolysis provides the energy to drive filament sliding

Question 23

What is the heart modulated by?

Answer 23

The autonomic nervous system

Question 24

What is the effect of the sympathetic nervous system on the heart? What about the parasympathetic nervous system?

Answer 24

Sympathetic:
Increased heart rate and force of contraction
Secretion of noradrenalin and activation of beta1-adrenoreceptor
Parasympathetic:
Decreased heart rate
Secretion of acetylcholine and activation of muscarinic receptors (M2)

Question 25

What is smooth muscle?

Answer 25

A heterogenous group of muscle with a range of physiological properties

Question 26

What is the structure of smooth muscle?

Answer 26

Loose lattice of thick and thin filaments that run obliquely across the muscle
Cytoskeletal intermediate filaments assist in the transmission of force generated by contraction
Dense bodies serve as attachments for the thick and thin filaments – considered functionally equivalent to z lines of striated muscle
Two types of attachments:
o Mechanical attachments between cells
o Gap junctions that provide a pathway for the passage of electrical signals between cells.
Filaments of the contractile proteins are attached to the plasma membrane at junctional complexes.

Question 27

Where is smooth muscle found?

Answer 27

Within the walls of organs and structures (e.g. bladder, oesophagus, stomach, intestines, uterus, blood vessels, bronchi, urethra, bladder, erector pili in the skin)

Question 28

What are the functions of smooth muscle?

Answer 28

Regulation of the diameter of blood vessels
Regulating diameter of airways
Propulsion of food through GI tract
Contraction of the uterus, delivery of baby

Question 29

What are the possible shapes of action potentials in smooth muscles?

Answer 29

A simple spike (top image on previous slide).
A spike followed by a plateau (not shown).
Spikes on top of slow waves (bottom image).

Question 30

What is the function of pacemaker cells within smooth muscle?

Answer 30

Many smooth muscle cells are capable of initiating spontaneous electrical activity (generated by pacemaker currents).
This can generate regular and repetitive oscillations in the membrane potential (Vm), slow waves.

Question 31

What are the characteristics of action potential in the smooth muscle?

Answer 31

Action potential in smooth muscle (10-50ms) lasts much longer than that in skeletal muscle (approx 2ms)
Depolarisation depends mainly on opening of voltage gated calcium channels (but sodium can also contribute)
Calcium channels open more slowly than sodium channels, thus slower upstroke (and longer duration) of action potentials compared to skeletal muscle
Contribution of each ion depends on type of muscle
E.g. influx of calcium ions in smooth muscle in gut versus influx of sodium in smooth muscle in bladder

Question 32

What are the two mechanisms by which intracellular calcium can rise in excitation?

Answer 32

1. Depolarisation of the membrane opens voltage-gated calcium channels and calcium enters through voltage-gated calcium channels.
2. Agonist induced release of calcium via IP3

Question 33

How do calmodulin and myosin-light chain kinase regulate contraction?

Answer 33

Calcium binds to calmodulin and the Ca2+-calmodulin complex then activates an enzyme called myosin (light chain) kinase (MLCK)
MLCK in turn activates the myosin heads by phosphorylating them (converting ATP to ADP and Pi, with the Pi attaching to the head)
Phosphorylation of myosin light chain increases ATPase activity and allows the myosin head groups to bind actin and undergo cross-bridge cycling

Question 34

What happens in smooth muscle contraction?

Answer 34

The thin filaments slide past the thick filaments pulling on the dense bodies (connected to the sarcolemma)
The dense bodies pull on the intermediate filaments’ networks throughout the sarcoplasm.
This arrangement causes the entire muscle fibre to contract, the ends are pulled toward the centre, causing the midsection to bulge.

Question 35

What is involved in the regulation of smooth muscle?

Answer 35

Smooth muscle has a broad range of functions, and therefore regulation is also diverse.
The triggers for smooth muscle contraction include hormones, neural stimulation by the ANS, and local factors.
In certain locations, such as walls of visceral organs, stretching the muscle can trigger its contraction (the stretch-relaxation response)

Question 36

What are varicosities and what do they do?

Answer 36

Release neurotransmitter into space surrounding the muscle. Neurotransmitter receptors are widely spread across the postsynaptic membrane.

Question 37

What do smooth muscle gap junctions permit?

Answer 37

Electrical coupling between cells

The degree by which these two forms (neuronal innervation and gap junction connections) are used in tissue is specific

Question 38

What are the two types of smooth muscle organisation?

Answer 38

Generally 2 types:

1. Multi-unit smooth muscle
2. Single-unit smooth muscle

Question 39

What happens in multi-unit smooth muscle?

Answer 39

In multiunit smooth muscle each cell receives synaptic input and there is little electrical coupling.
Each smooth muscle cell can contract independently of its neighbour.
Allows for fine control and gradual responses.
Similar to motor unit recruitment in skeletal muscle.
Intrinsic muscles of the eye (smooth muscle of the iris), and smooth muscle of the larger blood vessels.

Question 40

What happens in single-unit smooth muscle? Where is this type of muscle found?

Answer 40

Autonomic nervous system innervates a single cell within a sheet or bundle
Action potential is propagated by gap junctions to neighbouring cells
Whole bundle or sheet contracts as a functional syncytium.
This type of smooth muscle is found in the walls of all visceral organs except the heart (which has cardiac muscle in its walls), and so it is commonly called visceral muscle.
In general, visceral smooth muscle produces slow, steady contractions that allow substances, such as food in the digestive tract, to move through the body.

Question 41

What is vascular smooth muscle?

Answer 41

A particular type of smooth muscle found within the walls of blood vessels

Question 42

What receptors control contraction (vasoconstriction) in vascular smooth muscle cells?

Answer 42

Adrenergic receptor (alpha-1)
Muscarinic receptor (M2)
Endothelin receptor
Thromboxane receptor
Purinergic receptor (P2x)

Question 43

What receptors control relaxation (vasodilation) in vascular smooth muscle cells?

Answer 43

Adrenergic receptor (beta-2)
Histamine receptor 
Adenosine receptors
Prostacyclin (PGI2)
Purinergic receptor (P2y)

Question 44

What is bronchial smooth muscle?

Answer 44

Smooth muscle down to terminal bronchioles - under autonomic control

Question 45

How is bronchial smooth muscle controlled by the parasympathetic nervous system?

Answer 45

Action: bronchial CONSTRICTION and mucus secretion
Mechanism: acetylcholine/M3 type muscarinic receptors
Treatments: short-acting muscarinic antagonists (SAMAs) such as ipratropium. Long-acting muscarinic antagonists (LAMA) such as tiotroium.

Question 46

How is bronchial smooth muscle controlled by the sympathetic nervous system?

Answer 46

Action: bronchial DILATION
Mechanism: adrenaline/noradrenaline/beta-2 adrenergic receptor
Treatments: short acting beta agonists (SABAs) such as salbutamol which typical lasts 4-6 hours. Long-acting beta agonists (LABAs) such as salmeterol which lasts 12 hours, cause dilation.