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Principles of Physiology and Pharmacology > Cardiac and Smooth Muscle > Flashcards

Cardiac and Smooth Muscle Flashcards

1
Q

Cardiac muscle is ‘quiescent’. What does this mean?

A

The muscle does not pulse unless it receives an electrical impulse from the sinoatrial node (SAN).

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2
Q

What is a sarcomere?

A

Unit of muscle (skeletal and cardiac). A-bands made up of myosin. I-band made up of actin.
Z-lines repeat every 2 microns.

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3
Q

What is the structure of cardiac muscle filaments?

A

Regular lattice.

One thick filament surrounded by a ring of thin filaments.

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4
Q

What is the sarcoplasmic reticulum?

A

A membrane bound structure in muscle cells which contain large concentrations of calcium ions.

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5
Q

What are T-tubules and what is their function?

A

T-tubules are infoldings of the plasma membrane. Their function is to carry action potentials down into the centre of the cell to ensure co-ordinated contraction.

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6
Q

What is the terminal cisterna?

A

Where the sarcoplasmic reticulum associate with the T-tubules.

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7
Q

Give one feature of cardiac muscle that allows it to never fatigue.

A

Contains many mitochondria.

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8
Q

How long does cardiac action potential last? How is this different to skeletal muscle?

A

200-400ms.
This is much longer than in skeletal muscle - ensure spatial summation cannot occur and the refractory period is long.
This prevents tetanisation (continuous contraction) of the cardiac muscle - prevent arrhythmias.

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9
Q

How does excitation cause a release of calcium in cardiac muscle? (5 steps)

A

Calcium-induced calcium release.

  1. AP travels down T-tubule to L-type (voltage-gated) Ca2+ channels.
  2. Ca2+ channels open, extracellular Ca2+ enters cell.
  3. Ca2+ binds to calcium release channels on the sarcoplasmic reticulum.
  4. Ca2+ released from sarcoplasmic reticulum.
  5. Ca2+ binds to muscle filaments to stimulate contraction.
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10
Q

Where does the calcium required for skeletal muscle contraction come from?

A

The sarcoplasmic reticulum only.

The calcium ion channels of the SR are voltage-gated in skeletal muscle.

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11
Q

Where does the calcium required for cardiac muscle contraction come from?

A

Extracellular and intracellular(from the sarcoplasmic reticulum).
Extracellular calcium activates calcium-release channels on the SR.

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12
Q

What is a DHP receptor?

A

Voltage-gated calcium ion channel in T-tubule membrane.

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13
Q

How does relaxation of cardiac muscle occur?

A

Ca2+ is reuptaken into the sarcoplasmic reticulum through ATP-driven ion pumps SERCA (sarcoplasmic reticulum calcium ATP-ase).
Na+/Ca+ exchanger pumps Ca2+ out of the cell.

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14
Q

How does Ca2+ stimulate contraction?

A

Binds to troponin C which moves tropomyosin away from actin, exposing myosin binding site.

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15
Q

What happens when Ca2+ binds to troponin C?

A

Tropomyosin moves away from the thin actin filament, exposing the myosin binding site.

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16
Q

What is the length tension relationship in skeletal muscle?

A

Up to a point, increased length = increased force.

Overstretching will cause damage, thus decrease force.

17
Q

What is the length-tension relationship in cardiac muscle?

A

Increased length = Increased tension/force

This occurs as increasing sarcomere length increases Ca2+ sensitivity and maximal activated force.

18
Q

What is the force-frequency relationship in cardiac muscle?

A

Increasing the rate of cardiac contraction increases tension.
Frequency increases > SR Ca2+ content increases > force of contraction increases

19
Q

Where is smooth muscle located?

A

In the walls of hollow organs (stomach, intestines, bladder, blood vessels).

20
Q

What nervous system innervates smooth muscle?

A

The autonomic nervous system.

21
Q

What are contractions of the GI tract initiated by?

A

Pacemaker cells, the Interstitial Cells of Cajal.

22
Q

Give five characteristic features of smooth muscle.

A
  1. Non-striated
  2. Elongated shape
  3. Presence of dense bodies which anchor actin filaments
  4. Sarcoplasmic reticulum
  5. Gap junctions between cells to allow current and small molecules to flow.
23
Q

What are autocoids?

A

Physiologically active factor released by cells. Typically act locally and briefly on other cells.

24
Q

What neurotransmitter do sympathetic nerves release to arteries?

A

Noradrenaline

25
Q

Name an important vasoconstrictor.

A

Angiotensin II

26
Q

How does vascular smooth muscle contraction occur? (7 steps)

A
  1. Noradrenaline/angiotensin II binds to α1 adrenergic receptor.
  2. G-protein is activated - activates phospholipase C.
  3. Phospholipase C converts PIP2 > DAG +IP3
  4. IP3 causes Ca2+ release from sarcoplasmic reticulum.
  5. DAG opens receptor-gated channels - Na+ and Ca2+ enter cell.
  6. Na+ influx causes membrane depolarisation - voltage-gated Ca2+ channels open on membrane.
  7. Ca2+ triggers contraction of smooth muscle.
27
Q

What are the two main vasoconstrictors?

A

Noradrenaline and angiotensin II

28
Q

Outline nitric oxide mediated vasodilation. (6 steps)

A
  1. Endothelial cells release nitric oxide.
  2. NO activates guanylate cyclase which converts GTP > cGMP.
  3. cGMP activates protein kinase G
  4. Protein kinase G opens K+ channels in membrane causing membrane hyperpolarisation.
  5. cGMP also activates SERCA and PMCA which pump Ca2+ out of the cytoplasm.
  6. cGMP causes calcium desensitisation.
29
Q

How does cAMP act as a vasodilator?

A

When adrenaline binds to B1 receptors - cAMP is produced which acts as a vasodilator.

30
Q

What is cross-bridge cycling in smooth muscle?

A

Myosin-phosphate binds to actin to form actin-myosin-phosphate complex which dessociates back to myosin-phosphate.
Slow cycling enables indefinate contraction.

31
Q

How is phosphorylation important in smooth muscle contraction?

A

Myosin can only form a complex with actin when it has been phosphorylated. Myosin phosphtase removes phosphate.

32
Q

What inhibits myosin-phosphatase? (promotes contraction)

A

Rho Kinase

- causes calcium sensitisation. Promotes contraction.

33
Q

What activates myosin-phosphatase? (promotes relaxation)

A

Nitric oxide, via cGMP.

- causes calcium desensitisation.

34
Q

What ion causes action potential upstroke in skeletal muscle?

A

Influx of Na+

35
Q

What ion causes action potential upstroke in smooth muscle?

A

Ca2+

Principles of Physiology and Pharmacology (13 decks)

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