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Physiological concepts > Wk4 - SM pacemakers > Flashcards

Wk4 - SM pacemakers Flashcards

1
Q

how is smooth muscle classified?

A

how muscle fibres can be excited

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2
Q

what’re the 2 types of smooth muscle (SM)?

A
  1. Multiunit (neurogenic)

2. Singleunit (myogenic; self excitable)

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3
Q

2 types of smooth muscle - multiunit (general info)

A
  • neurogenic (stimulated by nervous output)

- respond to stimulus from ANS in similar fashion to skeletal muscle

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4
Q

where is multiunit SM found?

A
  • walls of large BV
  • bronchioles of lung
  • iris and lens muscles of eye
  • hair follicles
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5
Q

2 types of SM - single unit (general info)

A
  • myogenic (self excitable); can form AP without external stimulation
  • AKA visceral smooth muscle
  • functions as syncytium; cells interconnected by gap junctions and so a wave of excitation passes through the tissue to bring about contraction
  • doesn’t maintain constant RMP (more in later flashcards)
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6
Q

where is single unit SM found?

A
  • walls of hollow viscera

- this means the GIT, reproductive tract etc

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7
Q

give an example of single unit SM functioning?

A
  1. uterus contracting during labour

2. peristalisis of food stuffs through GIT

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8
Q

does single unit SM maintain a constant RMP?

A

NO, this is why it is able to generate an AP without external stimulation (similar to the pacemakers of the heart!)

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9
Q

the membrane potential of single unit SM oscillates between what 2 types of ‘potentials’?

A
  1. Slow wave potentials

2. Pacemaker (pacesetter) potentials

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10
Q

what’re slow wave potentials?

A
  • exhibited by single unit SM cells
  • wave like fluctuations in membrane potential
  • cyclically bring membrane potential closer or further from threshold level
  • never cause AP by themselves, however will induce regular AP’s when other factors facilitate this
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11
Q

slow wave potentials AKA

A

Basic Electrical Rhythm

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12
Q

Are slow wave potentials action potentials? Do they initiate muscle contraction?

A

Not AP’s and therefor do not induce muscle contraction (not directly atleast)

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13
Q

Do pacemaker potentials reach threshold, and thus induce an AP and muscle contraction?

A

Yes, this is what enables them to create regular rhythms of contraction

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14
Q

SM cells are connected by ___ ______ which provide ___ ______ pathways

A

SM cells connected by gap junctions which provide low resistance electrical pathways

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15
Q

What causes slow wave potentials?

A

cyclical variation in the rate at which the Na+ pump pushes Na+ out of the cell

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16
Q

what happens should the slow ‘waves’ of electrical potential actually manage to reach threshold level?

A

a burst of AP’s are initiated; which in turn results in the repeating, rhythmical cycles of smooth muscle contraction in tissues

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17
Q

what determines whether or not the minor oscillations caused by slow wave potentials reaches threshold level, and therefor actually induces the contraction of smooth muscle? Why?

A 
several interrelated factors...
-mechanical stimulation 
-nervous stimulation 
-hormonal input 
These determine whether or not threshold is reached as they decipher the resting potential (starting point) around which slow wave potentials oscillate.
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18
Q

The small intestine is a site where slow wave potentials contribute to the contraction of SM surrounding the gut tube. It is the ___ ____ which causes the resting potential of SM cells to be raised and thus regular AP’s to be induced depending upon the frequency of the slow wave potentials.

A

mechanical stimulation

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19
Q

the waves of contraction brought about by slow wave Electrical potentials in the small intestine are ____ __ ____ segments, in an effort to ensure segmentation of food stuffs. A wave of contraction rarely travels more then ___cm, allowing adequate time for digestion and absorption.

A

localised to short; 10cm

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20
Q

BER

A

?????????

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21
Q

The BER of the small intestine is independent of ____ _____

A

extrinsic innervation

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22
Q

How is the small intestines excitability influenced by the different divisions of the ANS?

A

increased excitability –> PNS

decreased excitability –> SNS

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23
Q

The excitability of the small intestine is influenced by what 3 factors?

A
  1. ANS
  2. circulating hormones
  3. enteric neurones
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24
Q

The force and duration of muscle contraction is directly related to the ____ and ____ of action potentials

A

amplitude and frequency

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25
Q

Pacesetter (pacemaker) SM cells depolarise by themselves due to what?

A

changes in ion channel permeabilities

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26
Q

What structural feature ensures that the AP brought about by pacesetter SM cells spreads to the surrounding SM tissue?

A

gap junctions

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27
Q

The pace (rate) of rhythmic contractile activities in the gut (such as peristalsis) depend upon what?

A

The inherent rate established by pacesetter cells

28
Q

The intensity of contraction depends upon the ___ ___ ____ that occur when the pacesetter potential reaches threshold.

A

number of AP’s

29
Q

why is it that the greater the number of AP’s the more intense the strength of contraction in SM?

A

greater #AP’s –> greater [Ca+2] in cytosol –> greater amount of crossbridges form –> more intense contraction strength

30
Q

t/f: pacesetter cells initiating AP’s do not occur along the whole length of the small intestine

A

False, they do occur along the whole length of the small intestine however their activity does vary along the length

31
Q

describe the activity of pacesetter cells along the length of the small intestine

A

pacesetter cell activity declines along the length of the SI, thus highest frequency in duodenum (11-13/ min) and lowest frequency in terminal ileum (8-9/min)

32
Q

Define peristalsis. What controls this event - and where is such structure found?

A
  • forward propelling movt of chyme
  • due to progressive contraction of circular muscles whilst longitudinal relaxes
  • periodicity of these movements controlled by pacemaker (pacesetter) potentials of smooth muscle cells in the muscularis externa
33
Q

What is the most common movement in the small intestine?

A

segmentation

34
Q

Define and describe segmentation.

A
  • local mixing movement of chyme with intestinal juices

- caused by closely spaced alternate contraction and relaxation of circular muscle layer

35
Q

Enteric reflexes in small intestine respond to the presence of what? What is the effect of this?

A

presence of chyme –> increase intestinal motility

36
Q

Segmentation depends on ____, which sends impulses back to the enteric plexus and CNS.

A

distension (stretch)

37
Q

Where are the pacesetter SM cells of the small intestine located?

A

in the musculus externa of the GIT

38
Q

What does MMC stand for?

A

Migrating Myoelectric Complex

39
Q

Briefly explain what the MMC is.

A

The Migrating Myoelectric Complex is a process which occurs in fasting individuals. It involves the motility of the small intestine following a different pattern, characterised by bursts of intense electrical and contractile activity - which is then followed by periods of inactivity.

40
Q

Waves of activity due to the MMC appear, and propagate (progress) toward which locations?

A

Appear in the duodenum and progress toward the terminal ileum.

41
Q

How often do waves of activity due to the MMC repeat?What does this time period represent?

A

Every 75-90 minutes. This is the time it takes for the “wave” to travel from duodenum to terminal ileum.

42
Q

How many phases are there in the MMC?

A

4

43
Q

What’re the 4 phases of the MMC?

A
  1. Quiescent phase - slow waves, few AP’s/ contractions.
  2. Irregular AP’s & contraction - gradual increase of both intensity and frequency of AP’s.
  3. Intense activity - for 3-6 mins
  4. Rapid decline of activity - blends into phase 1
44
Q

T/f: A new MMC only begins as the previous finishes.

A

True; which is why the time for a singular wave to travel from duodenum to terminal ileum is the same as the frequency of MMC’s occurring.

45
Q

What is the primary function of the MMC?

A

Sweep the bowel clean

46
Q

What is a secondary function of the MMC?

A

The MMC inhibits the migration of colonic bacteria into the terminal ileum.

47
Q

What hormone is responsible for MMC?

A

Motilin

48
Q

What event abolishes the MMC?

A

Eating

49
Q

What’re the 3 major functions of gastric motility?

A
  1. Allow stomach to act as food reservoir.
  2. Fragment food into smaller pieces and mix it with digestive secretions.
  3. Empty gastric contents into the duodenum at a CONTROLLED rate.
50
Q

This phrase relates to the stomach.

Contractions in the ____ and ____ regions are weak, whereas in the ____ they are vigorous.

A

fundus and body; antrum

51
Q

Detail the layers of smooth muscle surrounding the stomach.

A
  1. Outer - longitudinal
  2. Middle - circular
  3. Inner - oblique
52
Q

How often to gentle mixing waves occur in the stomach?

What is the purpose of these waves?

A

Every 15-20 seconds. The purpose of gentle mixing waves is to mix bolus with gastric juice, forming chyme.

53
Q

Vigorous waves of contraction occur in the smooth muscle of the stomach, in which direction?

A

From the body of the stomach toward the pyloric region

54
Q

Where do ‘intense’ waves of contraction occur in the stomach? What is the purpose of these?

A

near the pylorus (exit). The purpose of these intense contractions is to open the pyloric sphincter and squirt a a small amount of chyme into the duodenum with each wave.

55
Q

Where do gastic contractions of the stomach begin?

A

In the middle of the body of the stomach - this region is known as the pacemaker zone.

56
Q

What is an ‘antrum’?

A

a natural cavity, thus all the space in the stomach is the ‘antrum’ of the stomach.

57
Q

How often does ‘major reflex mixing’ occur in the stomach?

A

3 times/ minute

58
Q

Gastric potentials are generated by a ____ zone.

A

pacemaker

59
Q

The shape of a gastric electrical potential resembles the electrical potential of what?

A

cardiac muscle. However - the key difference is that the timebase for the gastric electrical potential is in seconds rather then ms!

60
Q

In gastric smooth muscle, the longer the cell remains ______ ___ ______ - the greater the force of contraction.

A

depolarised above threshold

61
Q

The following refers to the electrical activity underlying gastric contractions:
What causes the steep depolarisation?

A

Calcium influx through voltage gated calcium channels.

62
Q

The following refers to the electrical activity underlying gastric contractions:
What causes the slow plateau phase post depolarisation?

A

Calcium and sodium influx through slower voltage gated channels.

63
Q

The following refers to the electrical activity underlying gastric contractions:
What causes repolarisation of the gastric SM cell?

A

K+ efflux.

64
Q

Gastric emptying is regulated by both ____ and ____ mechanisms.

A

neural and hormonal

65
Q

Which cells are responsible for the detection of acidity, osmotic pressure and fat content of chyme?

A

Duodenal and jejunal mucosal cells.

66
Q

Stimulation of the duodenal and jejunal mucosal cells (indicating presence of fat/ acidity) will _____ the rate of gastric emptying.

A

Decrease.
This is as….
1. fat is not emptied into the duodenum at a rate which exceeds emulsification
2. acid is not dumped from the body too quickly and can be neutralised by duodenal secretions.

Physiological concepts (12 decks)

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