DIgestive System

What is the function of the GIT?

convey food along the GIT, allowing it to be broken down into small molecules which can be absorbed into the circulation

source of nutrients that feed cells of the body

What are the 6 activities of the GIT?

1. ingestion → through the mouth

2. secretion → chemical breakdown using glandular activity (exocrine and endocrine)

3. motility → due to musculature of the tract → muscles help with propulsion to move the contents down the tract and for its physical breakdown (mix food with enzyme)

4. digestion → process of breaking down into smaller molecules

5. absorption → transfer to blood circulation

6. defecation

What is homeostasis?

at all levels of organization, body activities are directed at maintaining optimal and relatively constant internal conditions

What is the main role of the GIT?

• provide nutrients to the body

• we take food from external environment → break down until they are absorbable → delivered through all the cells to they can perform their functions, growth and repair (provides them with energy)

what percentage of molecules can the GIT absorb at its best?

99% of carbohydrates

95% of fat

92% of protein

Why can they absorb this much?

because the propulsive, secretory and absorptive activities are highly coordinated by neuronal and hormonal mechanisms

• all intreated for high functional efficiency

What is the simplest description of the GIT structure?

GIT can be through of as a tube that connects the external environment on each end

• lumen Is extracellular environment because it connects the two extracellular environments

• has a barrier function because we don’t want bacteria, fungi that come in with food to be able get into blood easily

• there will also be specializations to increase absorption

What is the length of the GIT in humans?

x3 the length of our body ~ 4.5 m in the living adult

however in a cadaver it may be longer because there is always muscular tone contributing to some level of contraction of the tract

What is another way the GIT can grow?

its internal surface are

• If we take a crossection we can see that the total internal surface area is much larger than external area

• equivalent to size of tennis court

• form follows function → inc surface area for optimal absorption

How can you increase surface area?

• by having invaginations → infolding

• can also have outpushing (finger like projections)

Describe the differentiation of the GIT

compared to worms we have a bunch of different parts of the GIT

• own organ or regions with their own functions

What are the 4 main layers of the gut wall?

• mucosa → epithelial cells located

• sub mucosa

• muscular externa → 2 layers of muscle

• serosa

What is the serosa?

• thin, tough layer of CT

• protects the tract

• continuous with peritoneum which holds GIT in place

What is the muscularis externa?

muscles important for motility and mixing

• longitudinal fibers → if you contract this fibre it will shorten the tube

• circular fibers → if you contract it will decrease size of lumen

depending on where we are in tract determines the type of muscle

• in the upper 1/3 and very end → striated muscle

• the rest (middle) → smooth muscle

• both are controlled in very different ways

What is the submucosa?

• loses connective tissue containing lymphatics and blood vessels

What is the mucosa?

• 3 layers

• thin layer of smooth muscle called the muscular mucosae

• lamina propria → loose connective tissue

• epithelial layer → epithelial and glandular cells, absorptive cells

What is the enteric nervous system (ENS)?

• its neurons reside within the wall of the GIT

• it is independent from the CNS → can work without input from the CNS

• has all the different types of neurons it requires to function

• integrative → something that happens at level of stomach can send signals elsewhere in the gut

• important for initiating, programming, regulating, coordinating activities of muscular, secretory and absorptive elements of the GIT

what are the spaces between the two muscle layers called?

• submucosal plexus

• myenteric plexus

• they are collection of nerve cell bodies

What types of neurons are included?

• sensory neurons

  • receptors in the mucosa or muscle

  • can sample lumenal content

  • sensory receptors

• motor/effector neurons

  • activate the secretory and muscle cells

  • excitatory or inhibitory

• interneurons

  • expand the responses to stimuli in the GIT allows for integration

describe the effector neurons

• they synapse of musculature or secretory cells

• excitatory effector neuron is going to release acetylcholine onto muscarininc receptors → depolarization → contraction/secretion

  • can be blocked by atropine

• inhibitory effector neurons releases Non-Adrenergic/ Non-Cholinergic peptides (ie. Nitric Oxide) → hyper polarize → DEC function

Why is the ENS called the intramural system?

Short, Enteric (intramural) reflexes

we can sense a stimulus with sensory neurons → send signals via nerve plexus → activate effector neurons → send signals to smooth muscle or gland cell to get a response

• everything involved resides in the gut wall

• not dependent on CNS

What is the ANS?

parasympathetic innervation and sympathetic innervation

• the effects are always on the neurons of the ENS

• parasympathetic = preganglionic nicotinic excitatory fibers (release Ach)

  • input to GIT comes from vagal nerve and pelvic nerve

  • leads to dilation of blood vessels

• sympathetic = postganglionic inhibitory fibres (release NE)

  • goes to chain of ganglia form length of spinal cord before innervating ENS

  • leads to constriction of blood vessels

What do sensory neurons allow for?

long reflexes

• input from autonomic neurvous system