Overview of the Physiology of the GI Tract Flashcards
What are the two main layers of smooth muscle that can move material within the lumen of the digestive tract?
- Muscularis mucosae
- Muscularis externa
What are the 4 main layers of the digestive tract wall?
- Mucosa
- Submucosa
- Muscularis externa
- Serosa
What is the mucosa comprised of?
- Epithelial cells
- Exocrine cells
- Endocrine glands
- Lamina propria
- Muscularis mucosae
What is the name given to epithelial cells in the mucosa?
Enterocytes
Exocrine glands in the mucosa will release what?
Regulatory peptides and amines
What is the submucosa composed of?
Loose connective tissue
Largery lymph and blood vessels
Glands
Nerve network - submucosal plxus
What is the name of the nervous system associated with the GI tract?
Enteric nervous system
What is the enteric nervosu system composed of?
Submucosal plexus
Myenteric plexus (of the muscularis externa)
What is the muscularis externa composed of?
- A circular muscle layer
- Nerve network - myenteric plexus
- Longitudinal muscle layer
What is the nerve network associated with the muscularis externa?
Myenteric plexus
Where is the myenteric plexus found?
Between the two layers of smooth muscle
(circular layer and longitudinal layer)
What is the serosa?
Connective tissue, part of the mesentery
This is the outermost layer
What allows for GI motility?
Mainly - joint action of the circular and longitudinal layers of muscle
Also - action of the muscularis mucosae
What is the collective name of the three bands of muscle that make up the longitudinal muscle?
Teniae coli
What is the main purpose of the muscularis mucosae?
Change absoptive and secretory area
Also aids mixing
What connects smooth muscle cells allowing from spread of electric current?
Gap junctions
In the stomach, small and large intestines how does the electrical activity present?
Slow waves
What are the pacemaker cells that given rise to the slow waves called?
Interstitial cells of Cajal
What mediates the upstroke and downstroke of action potentials for the slow waves?
Upstroke - voltage activated calcium channels
Downstroke - volateg activated potassium channels
The force of contraction of the muscularis externa is directly related to what?
The number of action potentials released by the interstitial cells of Cajal
How many slow wave normally occur per minute in:
a) Stomach
b) Small intestine
c) Large intestine
a) 3
b) 12 in duodenum, 8 in terminal ileum
c) 8 in proximal colon and 16 in the distal colon
Which factors act to inhibit slow wave activity?
Vasoactive internal polypeptides (VIP)
Nitric oxide
What is the function for the myenteric plexus?
Regulates motility and sphincters
In terms of function, what is special about the enteric nervous system?
It can function independently to the brain
(yet, it will respond to cues from the autonomic nervous system and hormones)
What are the 3 main roles of the enteric nervous system?
- Coordinate muscular activity
- Control secretory action
- Coordinate absorptive action
How does the enteric nervous system achieve its roles?
- Sensory neurones - mechano, chemo and thermoreceptors
- Inter-neurones - makes up majority of receptors, co-ordinatign reflexes, motor programs
- Effector neurones - excitory and inhibitory motor neurones supply both muscle layers, secretory epithelium, endocrine cells and blood vessels
Why are there no parasympathetic post ganglion neurones in gut?
Pre-synaptic neurones synapse with the ganglia in the enteric nervous system instead
During peristalsis, which substances cause contraction of either the longitudinal or circular smooth muscle?
Acetylcholine and substance P
(from excitory neurone)
During peristalsis, which substances cause inhibition of either the longitudinal or circular smooth muscle?
Vasoactive internal polypeptide (VIP) and nitric oxide
(released from inhibitory neurone)
The mixing/churning movements of the intestine have different names based on whether they occur in the small or large intestines, what are these?
Small intestine - segmentation
Large intestine - haustration
Name the 6 sphincters of the MAIN GI tract pathway
- Upper oesophageal
- Lower oesophageal
- Pyloric
- Ileocaecal
- Internal anal
- External anal
As well as the 6 main sphincters, what is the other sphincter related to the main GI tract?
Sphincter of oddi
(controls release of bile and pancreatic enzymes into duodenum)
Which three phases are involved in swallowing?
- Oral
- Pharyngeal
- Oesophageal
Which is the voluntary phase in swallowing?
Oral phase
By stimulating mechanoreceptors (as food is pushed towards oropharynx during swallowing) afferent nerve impulses are sent where, by which nerves?
CN IX and CNX
Pons and medulla
The pharyngeal phase of swallowing consists of inhibition of what whilst the food bolus passes?
Ventilation
(breathing)
In the oesophageal phase of swallowing, the swallowing centre within the pons and medulla will trigger what?
- Closure of the upper oesophgeal sphincter
- Initiate primary peristaltic wave (via CN X)
If food becomes lodged in the oesophagus, what is the response from the body?
Local pressure receptors are stimulated
Secondary peristaltic waves are stimulated for a more forceful contraction
Saliva production also increases
What controls peristalsis?
Enteric nervous system
Why does the lower oesophageal sphicter close after the food bolus passes?
To prevent gastro-oesopheal reflux
