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Summary Nutritional physiology
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summary of nutritional physiology NP02 through NP10
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Lecture NP02: Anticipation to a meal; cephalic/oral/gastric phaseCephalic phase
Anticipation of a meal, visual olfactory (smell) and auditory senses (memories) activate several
gastrointestinal organs.
- BEFORE the food is in your mouth.
- The cephalic phase start in the brain. → everything is controlled by the dorsal vagal complex
(DVC) → all different stimuli trigger your brain → motor neurons send signals to different parts
of the body, for example motor behaviour: physical/structural behaviour
- The brain signals the stomach to get ready before food is present, even without food in your
mouth, the stomach starts to enlarge already to accommodate the food that is coming.
- Sphincter of Oddi: point where the gallbladder and pancreas meet around the duodenum (first
part of small intestine). → during the cephalic phase the sphincter of oddi also relaxes.
- Next to motor behaviour is there also Secretory behaviour: stimuli that trigger the cells of the
stomach to secrete acid (HCL), pepsinogen (pepsin is active form for protein digestion), and
intrinsic factor (linked with preserving vitamin B12)
- ! taste is NOT part of the cephalic phase. → because this is already part of the oral phase as
the food is in the mouth.
- Effector responses during the cephalic and oral phases are mediated by the parasympathetic
nervous system.
Cephalic phase is made up of
1. Stimuli smell, visual, memory, hearing)
2. Transmission pathway: When there is a stimuli the Cortex, hypothalamus and limbic system
of the brain send out a signal via the dorsal vagal complex to increase the parasympathetic
outflow via vagus nerve: is a central nerve important for the communication in the cephalic
phase between the brain and changes in the gastrointestinal system.
3. Effector response: Salivary secretion, gastric secretion (acid, pepsinogen), pancreatic enzyme
secretion, gallbladder contraction, relaxation of the sphincter of Oddi.
The cephalic phase gastric acid secretory responses are meal dependent → meaning that a self-
selected meal (something you really like) results in more acid (HCL) secretion.
1
,Everything starts in the brain and then the Dorsal vagal complex (DVC) mediates the response. → there
is a release of acetylcholine (Ach) which is a neurotransmitter → Ach binds to specific cells of the
stomach: binding to Chief cells (blue) induces the release of pepsinogen, and binding to the Parietal
cells (green) it reduces the release of acid chlorine (HCL). → HCL converts pepsinogen into pepsin
which is the active form of the protein digesting enzyme.
Next to the secretion of the neurotransmitter acetylcholine, there is also secretion of the hormone
Gastrin releasing peptide (GRP) → GRP binds gastrin releasing cells (endocrine cells) → Gastrin is then
released in the blood circulation and moves to the top part of the stomach where it binds to both chief-
and parietal cells and stimulates even more secretion of pepsinogen and HCL (=positive feedback).
During digestion there is about 2.5 hours of gastric acid secretion. Most of which happens during the
gastric phase.
Ghrelin is the ONLY hormone that stimulates the central nervous system to take in more food.
Once the food is in the mouth the cephalic phase is over and the oral phase starts.
Oral phase:
- The oral cavity prepares food for transfer to the pharynx and activates the digestive process.
- The teeth are important for the mechanical digestion.
- In the oral cavity/mouth there are a lot of glands which are important in the production of
saliva.
- Saliva made of different components:
o Mucins: lubrications, making an emulsion → food mixed with saliva is called the bolus.
o Amylase → digestion of starch, salivary amylase activity is maintained in the stomach
by buffers and substrate protection. Enzyme activity is expressed in units. → every
enzyme has an optimal pH (highest activity). For amylase the highest activity is at pH
7-8 → in the stomach the pH is much lower (around 2) which means the activity of
amylase decreases. → the starch in the mouth forms a ringlike structure around the
amylase to protect the amylase in the stomach from the low pH.
o Lipase → digestion of fat
o Lysozyme → antibacterial
o IgA → immune protection
o Nerve growth factor (unknown function)
o Epidermal growth factor (unknown function)
- After the oral phase the food/bolus passes through the oesophagus to the stomach, this
happens with the help of peristaltic movement.
2
, The oesophagus has 2 sphincters: upper esophageal sphincter (UES) = between
mouth/pharynx and oesophagus, and Lower esophageal sphincter (LES) = between the
oesophagus and the stomach.
In resting phase: the lower sphincter is always closed (to prevent reflux) and the upper
sphincter is a bit open (not completely).
4 seconds after swallowing: the food does not just drop into/through the oesophagus, but it is
slowly pushed through the oesophagus to the stomach. Acetylcholine stimulate contraction,
and nitric-oxide (NO) and Vasoactive intestinal peptide (VIP) that stimulate relaxation. → All
these neurotransmitters are secreted very close to each other until the food bolus enters the
stomach.
Questions
Which response is NOT involved in the cephalic phase response of the body? → contraction of
the sphincter of Oddi. Because it is actually relaxed so the enzymes can pass.
What is correct about the cephalic phase of digestion (anticipation to a meal)? → Involves
neural components as first trigger AND may be stimulated by the smell of food AND induces
gastric acid secretion.
Which type of stimulus is not a trigger of the cephalic phase response of the body? → Taste
Gastric phase
- Happens in the stomach.
- Mammals have similar intestinal structure
The stomach
3
, The picture shows how much muscle there is in the stomach: circular muscles, oblique muscles →
important for the digestion of the food; mixing (mechanical digestion)
Cardia: First part of the stomach
Fundus: Upper part of the stomach
Body: Middle part of the stomach
Anterum: Lower part of the stomach
Pylorus: Final part of the stomach
Pyloric sphincter: sphincter from stomach to duodenum (small intestine)
The stomach is divided into 3 sections: based on morphology and function
1. Lower oesophageal sphincter (LES) and Cardia
a. Involved in secretion of mucus and bicarbonate (HCO3-) → bicarbonate prevents this
part of the stomach of having a very low pH
b. The mucus and bicarbonate production prevent reflux and accommodate the entry
of food, it also regulates belching (burping).
2. Fundus and Body
a. Secretion of high amount of acid (H+), intrinsic factor, mucus, bicarbonate,
pepsinogens and lipase (enzymes for digestion of the nutrients).
b. Function: food reservoir, tonic force during emptying (because of the many muscles)
3. Antrum and Pylorus
a. Involved in secretion of mucus and bicarbonate (HCO3-)→ bicarbonate prevents this
part of the stomach of having a very low pH
b. Function: mixing and grinding of food. Sieving of the food: smaller particles can pass
through to the small intestine and bigger particles have to stay in the stomach to be
further digested. Only particles below 2mm are allowed into the intestine. Also
important in the regulation of emptying the stomach.
In the cephalic phase the DVC sent out signals to the stomach to “get ready for food” → this is still
ongoing during the gastric phase, but now the stomach communicates back to the brain → in the
stomach are mechano-receptors (distension) that react upon the stretching of the stomach and send
a signal back to the vagal system and from the vagal system to the brain → the stomach is ‘asking’
the brain to amplify the signal because it has started digestion: it need more acid and enzymes →
vagal efferent induces: relaxation of the stomach (accommodate more food), more acid secretion,
more pepsinogen secretion → antral contraction, gastrin secretion (is released in the blood
circulation and goes back to the stomach cells so more acid is secreted), there is also more enzymatic
4
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