GIT conditions and physiology

• Autonomic nervous system (ANS) consists of three parts; the enteric nervous system, the parasympathetic nervous system and the sympathetic nervous system
• The enteric nervous system is the nervous system of the gut itself. It has many nerves (as many as the brain) and so can function by itself without any input from the CNS
• The parasympathetic nervous system sends signals to the enteric nervous system via the vagus nerve. Its main transmitter is acetylcholine. It has mainly excitatory output, from the CNS
• The sympathetic nervous system sends signals from the CNS to the gut and its main transmitter is noradrenaline. It has mainly inhibitory output from the CNS
• The CNS regulates the ANS (links to the gut-brain axis). Gut flora (microbiota) can have an effect on mental health and mood.
• The ANS controls things we dont think about (like sphincters contracting and relaxing) and the CNS has areas which control voluntary control (the things we do think about). The things we think about can affect the movements of the gut e.g thinking about your favourite food can prepare the gut for incoming food which may make you hungry
• Different layers of control

  1. the pacemaker cells within the gut that set a pattern of rhythmic contraction of smooth muscle
• Different layers of control

  1. the pacemaker cells within the gut that set a pattern of rhythmic contraction of smooth muscle
  2. enteric nervous system releases transmitters that affect motor patterns (increasing them, changing the frequency, maaking them bigger, or stopping them)
  3. CNS input
• Inhibitory output from the CNS

  The sympathetic nervous system can slow down or stop certain functions in the body that are not necessary for immediate survival
• GIT secretes enzymes that break down food, yet doesn't damage the cells of the GI wall. The exocrine glands secrete around 7 litres of fluid that consists of enzymes, mucus and water. Most of the fluid must be reabsorbed to prevent dehydration
• Paracrine and endocrine factors are released from the mucosa by neuronal activity, distention and chemical activity. On their release, they regulate water, electrolyte and enzyme secretion. Also influences growth motility as well as absorption from the GIT.
• The basic processes of the GIT

  • provides nutrient supply for the body
  • controlled by coordinated action of the 4 basic processes;
• SECRETION


  Includes both the transfer of water/ions from extracellular fluids (ECF) to the lumen of the GIT AND the release of synthesised material by epithelial cells and associated exocrine glands (also secretes mucus which provides protection)

  • arrival of food (or even the though of it) begins secretions of the digestive enzymes needed
• MOTILITY


  Movement of material along the GIT via coordinated muscle contraction - movement must be fast enough so food doesn't ferment and slow enough to ensure full digestion and absorption --> tightly regulated to ensure the correct time is taken

  • also helps in digestion via mastication (chewing)
• DIGESTION


  Mechanical (mastication/chewing) and chemical (enzyme secretion/HCl in the stomach denaturing proteins) breakdown of food into smaller, more absorbable units
• ABSORPTION


  Active or passive transfer of substances from the lumen of the GIT into the ECF and then into other organs for use

  • not regulated; what you eat, you absorb
  • after absorption, cellular metabolism directs it for use/storage
• Secretion, motility, digestion are tightly regulated, but absorption is NOT
• The epithelial barrier is there to prevent the contents of the GIT getting into our internal environment
• The epithelial barrier is a selective barrier that controls the movement of substances into and out of the body. Also, the barrier is well-maintained as the cells have a high turnover of 2-3 days (cells are replaced every 2-3 days)
• The epithelial layer consists of the lumen (at the top) which is the inside of the GIT tube, columnar epithelial cells, the apical membrane, tight junctions, the lateral membrane and basal membrane
• Apical membrane- at the top (A is the top of the alphabet)

  --> counts as anything above the tight junctions

  BASEal membrane- at the bottom (is the BASE)

  lateral membrane- at the sides (looks like the side of a cell)

  --> the basal/lateral membranes count as anything below the tight junctions
• epithelial layer consists of a single layer of epithelial cells that, via the tight junctions, prevents the entry of things from inside the GIT into our internal environment
• types of epithelial cells:

  columnar- forms columns

  goblet cells- secrete mucus

  enteroendocrine- secrete hormones
• the epithelial barrier has polarity as there are different proteins present at the apical and basolateral membranes.

  Things can only enter the epithelial cell and into the extracellular space (below the basal membrane) via the tight junctions, if there are proteins present there e.g transporter proteins
• immune process;

  the gut forms a boundary between the internal and external environments and this is essential for innate immunity
• The gut contains the largest amount of immune cells in the body. Gut-associated-lymphoid-tissue (GALT), whose main function is oral tolerance

  --> this is important so we don't form an intolerance to everything we eat; the gut would naturally do this as the things we eat are foreign to the gut.
• regions of the GIT

  mouth + pharynx, salivary glands, oesophagus, stomach, pancreas, liver, gall bladder, small intestine & large intestine
• mouth + pharynx


  where chewing (mastication) begins; initiation of the swallowing reflex

  • where the process of digestion begins by mastication, making food into smaller pieces

  --> helps digestion, but not essential for it
• salivary glands


  secretes amylase, mucus and salt + water

  amylase - enzyme that digests polysaccharides into sugars --> activates taste chemoreceptors (that makes food taste pleasurable)

  mucus - for lubrication

  salt + water - to moisten food into a soft/flexible bolus. is moved into the back of the mouth cavity + pharynx and then into the oesophagus
• oesophagus


  secretes mucus to provide lubrication as the material (food) is moved down into the stomach (food moves down my a motor pattern called peristalsis)

  • is 25cm in length (skeletal muscle at the top, smooth muscle at the bottom 2/3rds)
• stomach


  where digestion occurs, and very little absorption (only small lipid-soluble molecules)

  purpose of the stomach is to store, mix dissolve and continue digestion of food

  also acts as a sieve as smaller particles are moved down into the small intestine

  • secretes mucus (for lubrication + epithelial cell protection), HCl and pepsins
• stomach


  HCl in the stomach provides an acidic environment inside the gastric lumen, this alters the ionisation of polar molecules (esp. proteins) disrupting the extracellular tissue on the food itself, allowing enzymes to enter and solubilising the food. Also acts as a microbe by killing bacteria that doesn't like acidic environments. HCl also activates pepsiongens into pepsins (which are protein-digesting enzymes)
• stomach


  actions of the stomach, plus HCL + digestive enzymes produce chyme. It is composed of fragments of proteins, polysaccharides, droplets of fat, water and other small molecules
• pancreas


  secretes digestive enzymes and bicarbonate solution. The bicarbonate solution is used to neutralise the stomach released into the small intestine from the stomach
• liver


  secretion of bile is the most important for digestion

  • bile helps to solubilise fats, to allow them to be digested by lipase (bile also contains phospholipid and cholesterol that helps???)
  • bicarbonate ions again help neutralise stomach acid
• gall bladder


  stores and concentrates bile between meals + releases bile into small intestine via the sphincter of oddi
• small intestine


  main location of digestion and absorption of most nutrients. secretes enzymes, mucus and salt + water

  these enzymes are important in digestion of all food classes

  salt + water maintain the fluidity in the gut lumen

  mucus for lubrication
• large intestine


  secretes mucus

  large intestine roles are it stores and concentrates undigested matter (ready for expulsion), absorbs salt + water from previous regions in the GIT and defecation
• small intestine


  made up of 3 parts; duodenum (30 cm in length) where majority of digestion occurs, jejunum (1.2 metres in length) where carbs + proteins are absorbed and ileum (1.5 metres in length) where fats, ions and fluids are absorbed
• the small intestine microstructure has been designed for its function - there are permanent circular folds within the gut wall, which helps increase the surface area for absorption.

  • the series of circular folds are called plicae (about 800 of them in total)