Question

i have a med assignment to do. this is the background and the questions. 100% rating if all parts are answered, make sense and are relevant. i just need to make sure my answers are correct (hence why im posting it here!)

BACKGROUND A 30cm length of distal small intestine, attached to a short length of colon, is surgically resected from a human patient undergoing surgery for a cancerous tumour in the right colon. The small intestine is separated from the rest of the excised gut and placed in an organ bath filled with warmed, oxygenated Krebs solution, which effectively replaces the blood supply and keeps the tissue alive and active for several hours. (i). The physiologist pushes a soft wad of cotton wool (2cm diameter, 3 cm long) into the proximal end (i.e: oral end) of the tubular specimen of small intestine. It is big enough to slightly distend the gut. After a delay of a few seconds, the gut starts to produce a squeezing movement, which propels the cotton wool bolus slowly along the intestine until, a few minutes later, it is pushed out of the other (aboral/anal) end of the preparation. (ii). The physiologist then pushes the wad into the other (aboral/anal) end of the specimen. It remains still for a minute or two, then is expelled from the aboral/anal end, without being propelled along the tubular intestine: the gut has rejected it. (iii). A commonly used toxin extracted and purified from puffer fish – “tetrodotoxin” (TTX) - is added to the bath to reach a final concentration of 10-6 moles/litre, which is sufficient to block all action potentials in nerves - and hence all neural activity. A few minutes later, the physiologist pushes the cotton wool wad into the oral end of the specimen again. It doesn't get propelled. However, the gut can clearly be seen to be moving rhythmically – at about 7 movements per minute - but this does not propel the cotton wool.

QUESTIONS 1A: Name the motor pattern that pushes the wad of cotton wool along the intestine in (i). (1 mark)

1B: Explain how muscles push the wad along the intestine in (i), in a coordinated fashion, when the gut is clearly not connected to the spinal cord or brain. Make sure that you describe the types and layers of muscles involved. (2 marks)

1C: Describe how different types of neurons in the specimen (sensory neurons, interneurons, inhibitory motor neurons, excitatory motor neurons) are activated in neural pathways to give rise to this distinctive motor pattern in (i) including why the wad of muscle moves. Name which division of the autonomic nervous system these neurons belong to. (4 marks)

1D: Explain why the wad of cotton wool was not propelled up the intestine when it was pushed into the aboral end of the specimen in (ii). (1 mark)

1E: After tetrodotoxin was added to the bath it blocked all neural activity in (iii). The bolus was now not propelled down the gut, showing that this motor activity required functioning nerve cells. Now explain why, in the presence of tetrodotoxin, the gut continues to move at all. (2 marks)

Answer 1

1 A peristaltic movement

1 B enteric nervous system is a part of autonomic nervous system responsible for peristaltic movement in the small intestine. The muscles involved in peristalsis

Peristalsis, involuntary movements of the longitudinal and circular muscles, primarily in the digestive tract. In the small intestine, local stimulation of the intestinal smooth muscle by the presence of cotton wad causes contractions that tend to travel from the stimulated point in both directions. Under normal circumstances, the progress of the contractions in an oral direction is quickly inhibited, while the contractions travelling away from the mouth tend to persist

1 C and 1 D

Peristalsis is a manifestation of two major reflexes within the enteric nervous system that are stimulated by the cotton ball in the lumen

Mechanical distension and mucosal irritation stimulate afferent enteric neurons. These sensory neurons synapse with two sets of cholinergic interneurons, which lead to two distinct effects:

One group of interneurons activates excitatory motor neurons above the cotton wad - these neurons, which contain acetylcholine and substance P, stimulate contraction of smooth muscle above the cotton wad
Another group of interneurons activates inhibitory motor neurons that stimulate relaxation of smooth muscle below the cotton wad. These inhibitor neurons use nitric oxide, vasoactive intestinal peptide and ATP as neurotransmitters

1E tetrodoxin inhibits voltage sensitive sodium channels of neurons however it's effect on the enteric neurons is less pronounced than other neurons hence though there is movement in the gut it is not effective enough to move the bolus