motility in the GI tract Flashcards
what does segmentation allow
rhythmic contraction and relaxation of the intestine.
mixes chyme thoroughly in both directions (unlike peristalsis)
where is skeletal muscle found
in only the upper third of the oesophagus
is contraction of the skeletal muscle voluntary
yes
where is smooth muscle found
all of GI tract except upper third of the oesophagus
when does contraction of smooth muscle occr
in response to neuronal and hormonal or paracrine input
what stimulates skeletal muscle to contract
neurons
what do motor neurons do
innervate skeletal muscle via axons branching and forming junctions with the muscle.
what do axon terminals contain
vesicles which contain neurotransmitter.
what happens at the neuromuscular junction
Action potential in motor
neurone leads to ACh
release at cleft
ACh binds to receptors on
motor end plate (muscle
plasma membrane under axon
terminal), opening ion channels
and resulting in depolarisation
Propagation of action potential in muscle cell
plasma membrane
excitation-contraction coupling
AP in t-tubule triggers
Ca2+ entry and increased
[Ca2+]i.
Ca2+ binding to troponin
allows force generation and
contraction.
Removal of Ca2+ from the
cytosol is required for
relaxation.
what is the type of muscle in the small intestine and large intestine
smooth muscle
how are smooth muscle cells connected
by gap junctions so are electrically coupled
why do smooth muscles appear to be smooth
they lack striations/banding pattern seen in skeletal muscle
what allows coordinated control of smooth muscle
depolarisation spreads through adjacent sections
what initiates segmentation
the electrical activity of ‘interstitial cells of cajal’ (pacemaker cells) in circular smooth muscle.
what regulates contraction of smooth muscle
neurotransmitters released by autonomic nerve endings. No specialised motor end plate but swollen regions of axon that contain NT.
can also be regulated by hormonal or mechanical input via receptor activation.
what can a neurotransmitter be
either excitatory or inhibitory to smooth muscle contraction
what type of input does the skeletal muscle receive
only excitatory input
how does E-C coupling work in smooth muscle
no troponin involved
instead, contractile proteins are regulated by Ca2+/CaM activation of Myosin Light Chain Kinase resulting in phosphorylation of myosin
example of excitatory neurotransmitter which leads to contraction
acetylcholine and 5-HT
example of inhibitory neurotransmitter which leads to relaxation
VIP and nitrous oxide (NO)
how long does small intestine transit last
its relatively constant and little affected by diet or exercise
how quick is transit in duodenum last
its rapid, slowing down as chyme processes through small intestine
where does most digestion and absorption occur
in the duodenum and jejunum
how long is transit time and rate of absorption in the ileum
both are slower, allows absorption of more slowly digested substances
what happens in the fasted state
interdigestive peristaltic activity occurs.
This is known as the migrating myoelectic complex (MMC) and begins in the stomach
what happens as the MMC reaches the ileum
one starts at the stomach again which allows undigested material to move towards to large intestine
what initiates the MMC
an increase in motilin (hormone) which increases neuronal activity
what happens in a fed state
segmentation
what affects the number of contractions
physical nature of the food
does solid food induce more contractions than equicalorific liquid
yes, 2x as many
conditions that increase small intestine transit time
- Diarrhoea
- Thyrotoxicosis (hyperthyroidism)
- Irritable bowel syndrome
- Chronic Pancreatitis
conditions that decrease small intestine transit time
- Constipation
- Myxoedema (hypothyroidism)
- Pseudo-obstruction
- Ileal Resection
- Partial gastrectomy
- Jejunal bypass
- Diabetes (constipation affects blood sugar control & high blood sugar causes constipation)
what do anti-spasmodics do
reduce smooth muscle contraction
e.g. anti-muscarinic antagonists (hyoscine butylbromide – Buscopan, mebeverine)
what do anti-motility drugs do
reduce transit through the GI tract by decreasing activity of the myenteric plexus e.g. loperamide (opioid agonist)
what do pro kinetics do
stimulate GI tract activity by increasing contraction
e.g. drugs that act on 5-HT receptors e.g. Tegaserod (5-HT4 agonist), metoclopramide (acts at 5-HT receptors and dopamine receptors)
what do laxatives do
stimulate intestinal movement by increasing bulk, adding lubrication or acting as a local irritant to the mucosal layer.
