ALHT106 Lecture Notes - Lecture 2: External Anal Sphincter, Internal Anal Sphincter, Nerve Plexus
1. Discuss the structure and function of the digestive system and accessory organs
2. Discuss the hormonal and neural regulation of digestive functions
-Describe the location, structure and function of key digestive and accessory organs
(pancreas, liver and gall bladder)
-Explain the major processes occurring during digestive system activity (motility,
secretion, digestion, absorption, elimination) including the swallowing reflex
-Describe the control of digestive system function and gastric secretions
-Hormonal: Gastrin, secretin, CCK and GIP
-Neural: Enteric nervous system, short and long reflexes
-Describe the functions of HCl and explain how nutrients are digested
-Role of molecules such as proteases, lipase, amylase, and intrinsic factor
-Briefly explain how fats, proteins and carbohydrates are absorbed
-Describe the role of the large intestine in defection
-Summarise the age-related changes that occur to the GI tract (smooth and skeletal
muscle tone, cell division rates) and their consequences
Digestive system (Gastrointestinal (GI) system):
‘Mouth to anus- an open tube exposed to external environment. substances must cross cell
Membranes before they are internal
Processes- ingestion, secretion, mixing/movement, digestion, absorption, excretion
Muscular tube- oral cavity (mouth), pharynx, oesophagus, stomach, small intestine, large intestine
(open to anus)
Accessory structures assist in digestion/absorption: teeth, tongue, glands (salivary, liver, gall
bladder, pancreas)
Glands produce secretions into lumen/surface- water, enzymes, buffers, organic and inorganic
substances
Digestive functions:
Motility:
-smooth muscle maintains a low level of tone
-Muscle contractions- mixing, propulsion forward (peristalsis), compacting churning
-increase surface area for chemical attack (enzymes)
Secretion:
-epithelial/gland release of water, acids, enzymes, buffer, salts
Digestion:
-biomechanics breakdown of food (carbs, protein, fat) by enzymes
-Mechanical- tearing, crushing
-Complex to basic unit- suitable for absorption
Absorption:
-uptake of organic molecules, electrolytes, vitamins, water across intestinal epithelia into
interstitial fluid, blood and lymph
Excretion:
-removal of wastes and indigestible reside (faeces); defecation
find more resources at oneclass.com
find more resources at oneclass.com
The digestive system:
Mouth:
-mastication and initial stages of CHO digestion (enzymes)
-swallowing
Stomach:
-initial stages of protein digestion
-acid, enzymes, horns, mucus
-Churning, peristalsis
Small intestine:
-protein, CHO, fat and nucleic acid digestion
-Enzymes, alkali, mucus, hormones
-main site of absorption of nutrients
-segmentation, peristalsis
Large intestine- peristalsis:
-final digestion and absorption of nutrients
-Mucus
-water absorption
-waste concentration
Peristalsis- type of mechanical process:
Depolarisation of self excitatory smooth muscle cells
Rhythmic waves of contraction
Moves bolus forward- 1 direction
Circular muscles of muscular contract behind the blows, circular muscles
ahead are relaxed
Longitudinal muscles contract to shorten tube
NB segmentation- different type of mechanism:
-churning, not rhythmic
-no set pattern
-no one direction
Mouth, pharynx, salivary glands:
Sensory analysis of food, mastication and swallowing
Moistening and lubrication- mucus, saliva, water, salt
Digestion by enzymes- lingual lipase- lipids, salivary amylase- CHO
Mechanical processes (chewing, swallowing) by teeth, tongue, palate, pharynx to prepare a blouse
No absorption of nutrients- (is possible for lipid soluble drugs)
Oesophagus:
Straight hollow muscular tube 25cm
Posterior to trachea, passes into abdominal cavity through diaphragm
Has glands that secrete mucus
Mix of skeletal and smooth muscle
Muscle tone in superior section reduces entry of air
Muscle tone in inferior section reduces back flow from stomach
Function = lubricate and move blogs to stomach by peristalsis
Stomach:
J shaped organ
Highly foldable rug which flatten when stretched
3 layers of muscle for churning
Secretion:
-HCl pH 1.5-2
-Glands secrete a mucous carpet that protects lining enzyme- pepsin
find more resources at oneclass.com
find more resources at oneclass.com
Function= lubrication, churning, first protein digestion, food solubilisation, no absorption, kill
microbes
Small intestine- duodenum, jejunum, ileum:
Duodenum 25cm- ‘mixing bowl’ for acid neutralisation
Jejunum 2.5m- most chemical digestion, most nutrient absorption
Ileum 3.5m- terminates with ileocecal valve to Li
Mixing by segmentation- not rhythmic motility mix chyme with intestinal, bile and pancreatic juice
peristalsis
Secretions:
-enzymes for protein digestion— peptidases
-enzymes for lipid digestion- lipases
-CHO digestion- amylases, brush border enzymes
Propulsion to large intestine
Function= lubrication, motility, digestion (CHO, protein, fat), nutrient absorption (90%)
Pancreas:
Accessory gland- exocrine: contributes secretions- enzymes, bicarbonate
- endocrine: secretes hormones
Liver:
Site of essential metabolic and synthesising functions
secretions- bile
storage of nutrients eg glycogen
Gall bladder:
Receives bile (emulsifying agent) from liver, stores, concentrates and releases bile to SI
Large intestine- cecum, colon, rectum:
Cecum- pouch
Colon- ascending, transverse, descending, sigmoid
Rectum- storage prior to defecation
Reduces meal residue to faeces (25% solids (bacteria, fibre, fats, protein, salts, epithelium), 75%
water)
Resident bacteria breakdown undigestible carbohydrates, produce vitamins (K, B5, biotin)
Movement- haustral contraction (every 30 mins) and mass movement (1-3 times per day)
Secretion- mucus
Function= lubrication, water reabsorption, waste compaction, motility, vitamin absorption, storage
The anal sphincters:
Internal anal sphincter- circular smooth muscle not under voluntary control
External anal sphincter- encircles the distal portion of the anal canal
- ring of skeletal muscle under voluntary control
Function= release of faeces
Overall regulation of digestion:
Requires communication between different parts of the digestive tract
-ensures the presence of sufficient secretions when food present
-helps avoid overabundance of secretions in absence of food
-two types of mechanisms- endocrine (hormones) and neural (ENS) (hormones released into
blood that affect cells lining GIT)
Enteric nervous system (ENS)- 2 major nerve plexus (networks)
-allows for autonomous behaviour of the digestive system
-CNS control is not required for digestive functioning
Submucosal nerve plexus:
-associated with mechano- and chemo-receptors in the mucosa
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Discuss the structure and function of the digestive system and accessory organs, discuss the hormonal and neural regulation of digestive functions. Describe the location, structure and function of key digestive and accessory organs (pancreas, liver and gall bladder) Explain the major processes occurring during digestive system activity (motility, secretion, digestion, absorption, elimination) including the swallowing re ex. Describe the control of digestive system function and gastric secretions. Neural: enteric nervous system, short and long re exes. Describe the functions of hcl and explain how nutrients are digested. Role of molecules such as proteases, lipase, amylase, and intrinsic factor. Brie y explain how fats, proteins and carbohydrates are absorbed. Describe the role of the large intestine in defection. Summarise the age-related changes that occur to the gi tract (smooth and skeletal muscle tone, cell division rates) and their consequences. Mouth to anus- an open tube exposed to external environment. substances must cross cell.
