Wk4 - SM pacemakers Flashcards
how is smooth muscle classified?
how muscle fibres can be excited
what’re the 2 types of smooth muscle (SM)?
- Multiunit (neurogenic)
2. Singleunit (myogenic; self excitable)
2 types of smooth muscle - multiunit (general info)
- neurogenic (stimulated by nervous output)
- respond to stimulus from ANS in similar fashion to skeletal muscle
where is multiunit SM found?
- walls of large BV
- bronchioles of lung
- iris and lens muscles of eye
- hair follicles
2 types of SM - single unit (general info)
- 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)
where is single unit SM found?
- walls of hollow viscera
- this means the GIT, reproductive tract etc
give an example of single unit SM functioning?
- uterus contracting during labour
2. peristalisis of food stuffs through GIT
does single unit SM maintain a constant RMP?
NO, this is why it is able to generate an AP without external stimulation (similar to the pacemakers of the heart!)
the membrane potential of single unit SM oscillates between what 2 types of ‘potentials’?
- Slow wave potentials
2. Pacemaker (pacesetter) potentials
what’re slow wave potentials?
- 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
slow wave potentials AKA
Basic Electrical Rhythm
Are slow wave potentials action potentials? Do they initiate muscle contraction?
Not AP’s and therefor do not induce muscle contraction (not directly atleast)
Do pacemaker potentials reach threshold, and thus induce an AP and muscle contraction?
Yes, this is what enables them to create regular rhythms of contraction
SM cells are connected by ___ ______ which provide ___ ______ pathways
SM cells connected by gap junctions which provide low resistance electrical pathways
What causes slow wave potentials?
cyclical variation in the rate at which the Na+ pump pushes Na+ out of the cell
what happens should the slow ‘waves’ of electrical potential actually manage to reach threshold level?
a burst of AP’s are initiated; which in turn results in the repeating, rhythmical cycles of smooth muscle contraction in tissues
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?
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.
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.
mechanical stimulation
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.
localised to short; 10cm
BER
?????????
The BER of the small intestine is independent of ____ _____
extrinsic innervation
How is the small intestines excitability influenced by the different divisions of the ANS?
increased excitability –> PNS
decreased excitability –> SNS
The excitability of the small intestine is influenced by what 3 factors?
- ANS
- circulating hormones
- enteric neurones
The force and duration of muscle contraction is directly related to the ____ and ____ of action potentials
amplitude and frequency
Pacesetter (pacemaker) SM cells depolarise by themselves due to what?
changes in ion channel permeabilities
What structural feature ensures that the AP brought about by pacesetter SM cells spreads to the surrounding SM tissue?
gap junctions
The pace (rate) of rhythmic contractile activities in the gut (such as peristalsis) depend upon what?
The inherent rate established by pacesetter cells
The intensity of contraction depends upon the ___ ___ ____ that occur when the pacesetter potential reaches threshold.
number of AP’s
why is it that the greater the number of AP’s the more intense the strength of contraction in SM?
greater #AP’s –> greater [Ca+2] in cytosol –> greater amount of crossbridges form –> more intense contraction strength
t/f: pacesetter cells initiating AP’s do not occur along the whole length of the small intestine
False, they do occur along the whole length of the small intestine however their activity does vary along the length
describe the activity of pacesetter cells along the length of the small intestine
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)
Define peristalsis. What controls this event - and where is such structure found?
- 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
What is the most common movement in the small intestine?
segmentation
Define and describe segmentation.
- local mixing movement of chyme with intestinal juices
- caused by closely spaced alternate contraction and relaxation of circular muscle layer
Enteric reflexes in small intestine respond to the presence of what? What is the effect of this?
presence of chyme –> increase intestinal motility
Segmentation depends on ____, which sends impulses back to the enteric plexus and CNS.
distension (stretch)
Where are the pacesetter SM cells of the small intestine located?
in the musculus externa of the GIT
What does MMC stand for?
Migrating Myoelectric Complex
Briefly explain what the MMC is.
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.
Waves of activity due to the MMC appear, and propagate (progress) toward which locations?
Appear in the duodenum and progress toward the terminal ileum.
How often do waves of activity due to the MMC repeat?What does this time period represent?
Every 75-90 minutes. This is the time it takes for the “wave” to travel from duodenum to terminal ileum.
How many phases are there in the MMC?
4
What’re the 4 phases of the MMC?
- Quiescent phase - slow waves, few AP’s/ contractions.
- Irregular AP’s & contraction - gradual increase of both intensity and frequency of AP’s.
- Intense activity - for 3-6 mins
- Rapid decline of activity - blends into phase 1
T/f: A new MMC only begins as the previous finishes.
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.
What is the primary function of the MMC?
Sweep the bowel clean
What is a secondary function of the MMC?
The MMC inhibits the migration of colonic bacteria into the terminal ileum.
What hormone is responsible for MMC?
Motilin
What event abolishes the MMC?
Eating
What’re the 3 major functions of gastric motility?
- Allow stomach to act as food reservoir.
- Fragment food into smaller pieces and mix it with digestive secretions.
- Empty gastric contents into the duodenum at a CONTROLLED rate.
This phrase relates to the stomach.
Contractions in the ____ and ____ regions are weak, whereas in the ____ they are vigorous.
fundus and body; antrum
Detail the layers of smooth muscle surrounding the stomach.
- Outer - longitudinal
- Middle - circular
- Inner - oblique
How often to gentle mixing waves occur in the stomach?
What is the purpose of these waves?
Every 15-20 seconds. The purpose of gentle mixing waves is to mix bolus with gastric juice, forming chyme.
Vigorous waves of contraction occur in the smooth muscle of the stomach, in which direction?
From the body of the stomach toward the pyloric region
Where do ‘intense’ waves of contraction occur in the stomach? What is the purpose of these?
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.
Where do gastic contractions of the stomach begin?
In the middle of the body of the stomach - this region is known as the pacemaker zone.
What is an ‘antrum’?
a natural cavity, thus all the space in the stomach is the ‘antrum’ of the stomach.
How often does ‘major reflex mixing’ occur in the stomach?
3 times/ minute
Gastric potentials are generated by a ____ zone.
pacemaker
The shape of a gastric electrical potential resembles the electrical potential of what?
cardiac muscle. However - the key difference is that the timebase for the gastric electrical potential is in seconds rather then ms!
In gastric smooth muscle, the longer the cell remains ______ ___ ______ - the greater the force of contraction.
depolarised above threshold
The following refers to the electrical activity underlying gastric contractions:
What causes the steep depolarisation?
Calcium influx through voltage gated calcium channels.
The following refers to the electrical activity underlying gastric contractions:
What causes the slow plateau phase post depolarisation?
Calcium and sodium influx through slower voltage gated channels.
The following refers to the electrical activity underlying gastric contractions:
What causes repolarisation of the gastric SM cell?
K+ efflux.
Gastric emptying is regulated by both ____ and ____ mechanisms.
neural and hormonal
Which cells are responsible for the detection of acidity, osmotic pressure and fat content of chyme?
Duodenal and jejunal mucosal cells.
Stimulation of the duodenal and jejunal mucosal cells (indicating presence of fat/ acidity) will _____ the rate of gastric emptying.
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.
