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PA10315 COPY > Smooth muscle > Flashcards

Smooth muscle Flashcards

1
Q

Where is smooth muscle found?

A
  1. Hollow organs and blood vessels
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2
Q

Give examples of where smooth muscle is found.

A
  1. The intestine
  2. uterus
  3. airways
  4. hairs in the skin
  5. eyes.
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3
Q

Where is smooth muscle found in arteries and veins?

A
  1. Blood vessels have inner layer of endothelial cells (tunica intima - in contact with blood)
  2. Tunica media is under endothelial cells, it is a layer of smooth muscle
  3. Tunica externa is fibrous connective tissue surrounding blood vessels
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4
Q

What is the typical diameter of a smooth muscle cell?

A
  1. 2-10um.
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5
Q

What is the typical length of a smooth muscle cell?

A
  1. 50-400um.
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6
Q

What is absent in smooth muscle compared to skeletal muscle?

A
  1. There is no troponin.
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7
Q

What are some of the properties of smooth muscle?

A
  1. Small cell types
  2. Nerves spread across the smooth muscle
  3. No striations (smooth appearance)
  4. Spindle shaped - bigger in centre and tapered at the end
  5. Diverse - can vary in length
  6. Can be interconnected to form a sheet of smooth muscle
  7. Nucleated
  8. It is involuntary
  9. less well developed sarcoplasmic reticulum compared to skeletal muscle.
  10. there is slow myosin ATPase compared to skeletal
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8
Q

What are the two sources of calcium in smooth muscle contraction?

A
  1. Internal calcium stores in the sarcoplasmic reticulum

2. voltage gated calcium ion channels in the plasma membrane.

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9
Q

What starts the contraction in smooth muscle?

A
  1. Calmodulin binds to calcium.

2. Calmodulin is the calcium binding protein in smooth muscle

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10
Q

What is cross-bridge activation?

A
  1. Phosphorylation of myosin to allow it to undergo cross-bridge cycling.
  2. This is controlled by a Ca+ -regulated enzyme.
  3. Myosin light-chain phosphatase is an enzyme which dephosphorylates the myosin head group stopping cross-bridge activation
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11
Q

What are the key steps in cross-bridge activation?

A
  1. Calcium level increases and binds to calmodulin activating it
  2. This complex binds to myosin light-chain kinase and activates the enzyme.
  3. Kinase is an enzyme - adds a phosphate group onto a specific point in a protein
  4. Phosphorylates myosin light chain in the head group of the myosin using ATP as a source of phosphate
  5. No troponin so all myosin binding sites are exposed
  6. Control occurs at the myosin filament
  7. When myosin group is unphosphorylated, the myosin is not in the energised state (not ready to bid)
  8. When phosphorylated, the myosin head is forced towards the thin filament
  9. Conformational change moves the cross bridge towards the actin filament so that cross bridge cycling can occur
  10. Two molecules of ATP are used - one to phosphorylate head group and one for the cross bridge cycling (same as in skeletal muscle)
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12
Q

What factors influence smooth muscle contraction?

A
  1. Spontaneous electrical activity of the muscle cell,
  2. neurotransmitter release from autonomic neurons,
  3. circulating hormones,
  4. local environment changes in the fluid surrounding the cells
  5. mechanical stretch.
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13
Q

Why is smooth muscle shortening much slower than in skeletal muscle?

A
  1. Smooth muscle has a low rate of ATPase activity.
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14
Q

Why does smooth muscle not undergo fatigue during prolonged periods of activity?

A
  1. Slow rate of energy usage due to the low rate of ATPase activity.
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15
Q

What are the two uses of ATP in smooth muscle?

A
  1. Hydrolyzing one ATP to transfer a phosphate onto a myosin light chain to start cross-bridge cycling
  2. then one ATP is used per cycle to provide the energy for force generation.
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16
Q

What is a varicosity

A
  1. A swollen region at the end of a branch from the axon of a postganglionic autonomic neuron.
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17
Q

What do varicosities contain?

A
  1. Many vesicles filled with neurotransmitter

2. Some of which are released when an action potential passes the varicosity.

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18
Q

How can a number of smooth muscle cells be influenced by neurotransmitters from a single neuron?

A
  1. The varicosities from a single axon may be located on multiple muscle cells.
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19
Q

How can a single muscle cell be influenced by neurotransmitters from more than one neuron?

A
  1. A single muscle cell may be located may be located near to varicosities from sympathetic and parasympathetic neurones.
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20
Q

What are the two types of acetylcholine receptors?

A
  1. Nicotinic- ligand gated ion channel

2. Muscarinic- g protein coupled receptors

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21
Q

Where are the two types of acetylcholine receptors found?

A
  1. Nicotinic in the skeletal muscle neuromuscular junction

2. muscarinic in the smooth muscle.

22
Q

What do alpha-adrenergic receptors do?

A
  1. Mediate constriction in most vascular smooth muscle.
23
Q

What do beta-adrenergic receptors do?

A
  1. Mediate dilation of vascular smooth muscle and lung airway smooth muscle. (bronchodilation)
  2. Can use b adrenergic receptors as a target to treat asthma
24
Q

What do muscarinic receptors (M3) do in the bronchial smooth muscle?

A
  1. Mediate bronchoconstriction.
25
Q

What do beta-adrenergic receptors do in the bronchial smooth muscle?

A
  1. They mediate smooth muscle relaxation and bronchodilation.
26
Q

What is the volume of an empty stomach?

A

50ml.

27
Q

What smooth muscles relax before the arrival of food to increase the volume of the stomach?

A

The muscles in the fundus and body.

28
Q

What is the maximum volume of the stomach?

A

1.5 litres.

29
Q

What is receptive relaxation?

-

A
  1. Relaxation of the smooth muscles of the stomach (fundus and body) when food is swallowed - mediated by parasympathetic nerves in the enteric nerve plexuses.
30
Q

What neurotransmitters mediate receptive relaxation?

A

Nitrous oxide and serotonin.

31
Q

What are peristaltic waves?

A
  1. Waves produced in the stomach in response to arriving food.
32
Q

Where do peristaltic waves begin and what path do they follow?

A
  1. They begin in the body of the stomach and produces only a ripple as it proceeds towards the antrum.
33
Q

What happens as the wave approaches the larger mass of wall muscle surrounding the antrum?

A
  1. A more powerful contraction is produced which mixes the luminal contents and closes the pyloric sphincter.
34
Q

What happens after peristaltic waves begin and how is there a regular contraction

A
  1. Stretch of stomach muscle generates additional electrical activity
  2. Drifting resting potential reaches threshold and causes a spike of action potential
  3. Generation of slow waves in resting muscle, doesn’t hit threshold
  4. Excitatory stimulus, slow waves are elevated and reach threshold and action potentials are generated
  5. Regular contraction due to the slow waves
35
Q

What happens to the pyloric sphincter muscles as a peristaltic wave arrives?

A

It contracts.A ring of smooth muscle and connective tissue between between the antrum and duodenum.

36
Q

What is retropulsion?

A

Backward motion of chyme.

37
Q

What does retropulsion do?

A

It generates strong shear forces that help to disperse the food particles and improve mixing of the chyme.

38
Q

How is stomach contents stopped from entering the esophagus.

A

The lower esophageal sphincter prevents the retrograde movement of the stomach contents into the esophagus.

39
Q

How are gastric peristaltic waves created?

A
  1. Pacemaker cells in the longitudinal smooth muscle layer

2. They generate slow waves of electrical activity

40
Q

What is the basic electrical rhythm of the stomach?

A

Spontaneous depolarisation-repolarisation cycles. These are slow waves.

41
Q

Where are slow waves conducted?

A
  1. They are conducted through gap junctions along the stomach’s longitudinal muscle layer and also induce similar slow waves in the overlying circular muscle layer.
  2. Arrangements of proteins in one cell which form a channel and attach to the same chain of proteins in another cell, channels linking two cells together which allow waves of depolarisation to quickly spread between cells
42
Q

Why are neural or hormonal inputs needed for slow waves to be conducted?

A
  1. Without these inputs, the depolarisations are too small to cause significant contractions.
43
Q

How is the membrane further depolarised for the gastric peristaltic waves?

A
  1. Excitatory neurotransmitters and hormones act upon the smooth muscle.
44
Q

What is single-unit smooth muscle?

A
  1. Smooth muscle that responds to stimulation as a single unit because gap junctions join muscle fibres, allowing electrical activity to pass from cell to cell.
  2. Pathway of low resistance between cells, so the cells are electrically coupled
  3. fibres aggregated into sheets/bundles
  4. Need whole visceral organ to contract synchronously
  5. Synchronous activity is mediated by gap junctions
45
Q

What are examples of single unit smooth muscles?

A
  1. The intestine
  2. bladder
  3. small diameter blood vessels.
46
Q

What are multi-unit smooth muscle cells?

A
  1. They have no or jew gap junctions

2. each cell responds independently and the muscle behaves as multiple units.

47
Q

What are some features of multi-unit smooth muscle?

A
  1. They are not activated by stretch
  2. They resemble striated muscle
  3. No electrical coupling - allows fine control and gradual responses
  4. APs do not occur often
  5. Nerve stimulation is important
48
Q

What are some examples of multi-unit smooth muscle?

A
  1. Large arterie
  2. large airways in lungs
  3. pilomotor muscles.
49
Q

shape of smooth muscle cells

A

spindle shaped

50
Q

how does the organisation of actin and myosin in smooth muscle compare to skeletal

A
  1. Contain actin and myosin
  2. The arrangement of actin and myosin is different compared to skeletal
  3. Stretch across the cell
  4. Thin filaments anchored into dense bodies (instead of Z discs)
  5. Thick filaments are in between thin filaments from two different dense bodies
  6. Troponin is absent (another protein for calcium)
  7. Less organised

PA10315 COPY (48 decks)

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