Smooth and Cardiac Muscle

Question 1

Describe cardiac muscle cells

Answer 1

Fibres have central nuclei, have desmosomes and gap junctions (connect cells electrically)

Question 2

How do cardiac cells display electrical coupling?

Answer 2

Gap junctions allow propagation of action potentials between cells

Question 3

Outline the process of cardiac muscle contraction

Answer 3

1) Depolarisation (sodium influx) causes voltage-gated calcium channels to open
2) Calcium affects troponin in sarcomere –> contraction
3) Calcium released from sarcomere and taken back into SR by SERCA –> diastole
4) Rise in intracellular calcium triggers further calcium release from SR (ryanodine receptor)

Question 4

What type of calcium channel is present on cardiac cells?

Answer 4

L-type

Question 5

Define ‘automaticity’

Answer 5

Ability to initiate their own beat (cells)

Question 6

How does the sympathetic nervous system affect cardiac muscle?

Answer 6

Noradrenaline –> B1 adrenoreceptor –> increase HR and contraction force

Question 7

How does the parasympathetic nervous system affect cardiac muscle?

Answer 7

ACh –> M2 receptors –> decreased HR

Question 8

Describe the structure of smooth muscle

Answer 8

Covered by oblique loose lattice of thick and thin filaments, dense bodies (attachment for thick/thin filaments) and mechanical attachments and gap junctions

Question 9

What are the two types of smooth muscle?

Answer 9

Multi unit and single unit smooth muscle

Question 10

Describe multi-unit smooth muscle

Answer 10

Each smooth muscle cell contracts independently - fine control and gradual responses e.g. intrinsic eye muscles

Question 11

Describe single-unit smooth muscle

Answer 11

SM cell spreads action potential via gap junctions to form functional syncytium –> slow and steady contractions

Question 12

What is a ‘varicosity’?

Answer 12

Where the ANS makes contact with smooth muscle cells

Question 13

What do varicosities do?

Answer 13

Release neurotransmitter into space surrounding muscle when innervated by autonomic post-ganglionic fibres

Question 14

How can intracellular calcium level rise be generated?

Answer 14

Depolarisation (opens voltage-gated Ca2+ channels) or agonist of PLC which secretes IP3 which triggers Ca2+ release from SR

Question 15

Describe the role of calcium in smooth muscle contraction

Answer 15

Calcium influx –> binds to calmodulin –> binds to myosin light-chain kinase (MLCK) –> phosphorylates regulatory region of myosin light chains –> increases ATPase activity of myosin -> allows myosin head to group and undergo cross-bridge cycling (contraction)

Question 16

Describe what happens in the contraction of smooth muscle on a macromolecular scale

Answer 16

Thin filament slides past thick filament –> pulls on dense bodies (connected to sarcolemma) –> dense bodies pull on intermediate filament –> entire muscle fibre contraction (midsection bulge appearance)

Question 17

Which smooth muscle receptors are involved in vasoconstriction?

Answer 17

a1 (adrenergic), M2, endothelin receptor, thromboxane receptor, P2x

Question 18

Which smooth muscle receptors are involved in vasodilation?

Answer 18

B2 (adrenergic), histamine receptor, adenosine receptor, PGI2, P2y

Question 19

How does the parasympathetic nervous system act on bronchial smooth muscle?

Answer 19

Constriction and mucus secretion; ACh/M3 type muscarinic receptors

Question 20

How does the sympathetic nervous system act on bronchial smooth muscle?

Answer 20

Bronchial dilation; Adrenalin/Noradrenaline/B2 adrenergic receptor

Question 21

How does calcium binding differ in smooth muscle compared to skeletal and cardiac muscle?

Answer 21

Calcium binds to calmodulin in SM not troponin