Physiology and Pharmacology 1 - intro to GI tract Flashcards
What is the alimentary canal?
series of hollow organs running from the mouth to the anus (oral to aboral) that are separated by sphincters controlling movement
4 functions of the mouth and oropharynx
chop foodlubricate itstart carbohydrate (and limited fat) digestionpropel food to oesophagus
Purpose of oesophagus
Propels food to the stomach
3 functions of stomach
stores and churns foodcontinues carbohydrate and initiates protein diffusionsregulates delivery of chyme (semifluid mass of partly digested food) to the duodenum
3 parts of the small intestine
DuodenumJejunumileum
Purpose of small intestine
Principle site of digestion and absorption of nutrietns
3 main parts of the large intestine
AppendixCecumColon (ascending, transverse, descending, sigmoid)
2 functions of large intestine
colon reabsorbs fluids and electrolytes Stores faecal matter before delivery of faeces
Purpose of rectum and anus
Storage and regulated expulsion of faeces
Accessory glands of the GI tract?
Salivary glandpancreasliver and gallbladder
What are sphincters
sphincters are reinforced circles of smooth muscle which act as unidirectional valves creating movement in one direction
4 activities of the alimentary canal
Motilitysecretiondigestionabsorption
What is motility
Mechanical activity mainly involving smooth muscle
4 places skeletal muscle is present
Mouth, pharynx, upper oesophagus and external anal sphincter
What does secretion occur due to
the presence of food, hormonal and neural signals
what are the 3 things secretion is required for
Digestionprotectionlubrication
what is digestion?
chemical breakdown by enzymatic hydrolysis of complex foodstuffs to smaller, absorbable units
Where does physical digestion occur (3)
Mouthstomachsmall intestine
What is absorption
transfer of the absorbable products of digestion (with water, electrolytes and vitamins) from the digestive tract to the blood, or lymph
overal length of the digestive tract
7-10m
What are the 4 generalised layers of the digestive tract
MucosaSubmucosaMuscularis externaSerosa
What is the 5 things the mucosa is made up from
epithelial cells exocrine cellsendocrine gland cellslamina propria (capillaries, enteric neurones, immune cells)muscularis mucosae (thin layer of muscle that can regulate the surface area of the mucosa)
What is the 4 things the submucosa is made of
connective tissuelarger blood and lymph vesselsglandsnerve network (sub-mucous plexus)
What are the 3 things the muscularis externa is made up of
circular muscle layernerve network (myenteric plexus)longitudinal muscle layer
What is the serosa made up from
Connective tissuereleases a sticky fluid that can help the intestines slide across each other and the peritoneum
What are the 3 types of smooth muscle in the GI tract
Circularlongitudinal muscularis mucosae
What is the name of the 3 bands of longitudinal layers that are present in the colon
Taeniea coli
Where is an extra oblique layer of muscularis mucosae present
Stomach
What happens when circular muscle contracts
The lumen becomes narrower and longer
What happens when longitudinal muscle contracts
The intestine becomes shorter and fatter
What happens when the muscularis mucosae contracts
There is a change in the absorptive and secretory area of mucosa (folding) and mixing activity
What allows the spread of electrical currents from cell to cell forming a functional syncytium
The coupling of adjacent smooth muscle cells by gap junctions
What drives spontaneous activity across the synctium
specialised pacemaker cells
What modulates the spontaneous activity across the synctium
Intrinsic (enteric) and extrinsic (autonomic) nervesNumerous hormones
What does spontaneous electrical activity occur as in the stomach, small intestine and large intestine
slow waves
What are slow waves
rhythmic patterns of membrane depolarisation and depolarisation that spread from cell to cell via gap junctions
What do slow waves determine?
determines max. frequency, direction and velocity of rhythmic contractions
What cells drive slow waves
Interstital cells of Cajal
What mediates upstroke and downstroke of slow waves
Up = voltage activated Ca2+ channels, downstroke = voltage activated K+ channels
What must happen for contraction of SMCs to occur
Slow wave amplitude must trigger SMC action potential
What is force of contraction related to
Number of action potentials discharged
Where are ICCs located
between the longitudinal and circular muscle layers and in the submucosa
What mediates the basic electrical rhythm
Slow waves
What factors determine whether slow wave action potentials reach threshold
neuronal stimulihormonal stimulimechanical stimuli (these tend to depolarise SMCs rather than influence slow waves directly)
Basic Electrical Rhythm (BER) of:Stomach?Small intestine?Large intestine?
3 slow waves per minute12 in duodenum and approx. 8 in terminal ileum 8 in proximal colon to 16 in sigmoid colon (favours retention of contents to allow reabsorption)
Where are the ganglia of the ENS mainly found? (2)What is each plexus main function?
Myenteric (Auerbach’s) plexus - mainly regulates motility and sphinctersSub-mucous (Meissner’s) plexus - mainly modulates epithelia and blood vessels
What exerts a strong regulatory influence on the ENS?
Hormones and extrinsic nerves (however the ENS is a complete reflex circuit that can operate independently)
How does the ENS co-ordinate muscular, secretive and absorptive activities via? (3)
sensory neurones (mechanoreceptors, chemoreceptors, thermoreceptors)interneurones (the majority co-ordinating reflexes and motor programs)effector neurones (Excitatory and inhibitory motor neurones supplying both smooth muscle layers, secretory epithelium, endocrine cells and blood vessels)
What are some of the key transmitters that regulate peristalsis?
5-HTAChNOSPVIP
What do pre-ganglionic fibres of the parasympathetic autonomic innervation of the GI tract synapse with?
Ganglion cells within the ENS
Excitatory (3) and inhibitory (2) influences of the parasympathetic GI supply?
Excitatory influences:increased gastric, pancreatic and small intestinal secretion blood flowsmooth muscle contractionInhibitory influences:relaxation of some sphinctersreceptive relaxation of stomach
Where do preganglionic fibres of the sympathetic supply to GI tract synapse?What do postganglionic fibres mainly innervate?
In the prevertebral ganglisMainly enteric neurones but also other structures
Excitatory (1) and inhibitory (3) effects of the sympathetic nervous system on the GI tract?
Excitatory influences: increased sphincter toneInhibitory influences:decreased motility, secretion and blood flow
Example of GI local reflex?
Peristalsis
Example of GI short reflex?
intestino-intestinal inhibitory reflex (local distension activates sensory neurones exciting sympathetic pre-ganglionic fibres that cause inhibition of muscle activity in adjacent areas)
Example of GI long reflex?
gastroileal reflex (increase in gastric activity causes increased propulsive activity in the terminal ileum)
What is peristalsis?
a wave of relaxation, followed by contraction, that normally proceeds along the gut in an aboral direction - triggered by distension of the gut wall
What 2 transmitters cause the longitudinal/circular muscle to relax during peristalsis?
VIP (vasoactive intestinal peptide)NO (nitric oxide)
What 2 transmitters cause the longitudinal/circular muscle to contract during peristalsis?
AChSubstance P
What is segmentation
(mixing, or churning, movements) - rhythmic contraction of the circular muscle layer that mix and divide luminal contentsoccurs in the small intestine (in the fed state) and in the large intestine (where it is called haustration)
what is colonic mass movement
powerful sweeping contraction that forces faeces into the rectum - occurs a few times a day
What is migrating motor complex (MMC)?
Powerful sweeping contraction from stomach to terminal ileum
Where does tonic contraction (sustained contractions) occur in the GI tract at low pressure and high pressure?
Low pressure - organs with a major storage function e.g. stomachhigh pressure = sphincters
How many sphincters does the GI tract have (excluding the sphincter of oddi)?
6Specialised circular, generally smooth muscle act essentially as one way valves by maintaining a positive resting pressure relative to two adjacent structures (e.g. oesophagus and stomach)in general, stimuli (pressure) proximal and distal to a sphincter cause opening and closing, respectively
Name the 6 sphincters (excluding the sphicter of do)?
upper oesophageal sphincter (UOS) - skeletal muscle - relaxes to allow swallowing, closes during inspirationLower oesophageal sphincter (LOS) - relaxes to permit entry of food to the stomach, closes to prevent reflux of gastric contents to the oesophagusPyloric sphincter - regulates gastric emptying, usually prevents duodenal gastric refluxileocaecal valve - regulates flow from ileum to caecum, distension of ileum opens, distension of proximal colon closes (this is important as the large intestine is filled with commensal bacteria - get serious inflammatory conditions if the bacteria proliferates in the small intestine)Internal (smooth muscle) and external (skeletal muscle) anal sphincters - are regulated by the defaecation reflex (see later)
Which sphincter is composed of skeletal muscle?
The external anal sphincter
How many teeth do adults and children have?
Adults = 32Children = 20 milk teeth
Scientific name for swallowing?
Deglutition
What are the 3 phases of swallowing?
Oral (or voluntary)PharyngealOesophageal
What happens during the oral phase of swallowing?
Mouth closes Tongue moves to hard palate pushing the bolus into the oropharynxStimulation of mechanoreceptors starts the swallowing reflex
What is the swallowing reflex?
Mechanoreceptors stimulated -> afferent nerve impulses via CN IX naad CNX to pons and medulla -> Efferent nerve impulses via CN VII, IX, X and XI to skeletal muscles of pharynx and larynx
What happens during the pharyngeal phase of swallowing?
Ventillation is inhibited, laryngeal muscles closes glottides and raises larynxContraction of superior and middle pharyngeal constrictors propel bolus into hypo pharynxBolus forces epiglottis over larynxBolus enters oesophagus through open oesophageal sphincterGlottis reopens, ventillation recommences
What happens during the oesophageal phase of swallowing?
Swallowing centre (pons and medulla) triggers closure of the upper oesophageal sphincter and a primary peristaltic wave (via vagus)Wave is mediated by skeletal muscle in the upper oesophagus and smooth muscle in the distal regionsPeristalsis (in smooth muscle) is co-ordinated by the enteric nervous systemThe lower oesophageal sphincter opens within 2-3s of the initiation of a swallow
How is sticky food dislodged from the oesophagus?
Stimulates local pressure receptorsCauses a secondary peristaltic wave (more forceful than primary and locally triggered)Causes increased salivary production
