What is the basic function of the mouth and oropharynx?
Chops and lubricated food, starts carbohydrate digestion, propels food to the oesophagus
What is the basic function of the oesophagus?
It is a muscular tube that propels food to the stomach
What is the basic function of the stomach?
It stores/churns food, continues carbohydrate and initiates protein digestion, regulates delivery of chyme to the duodenum
What enzyme is present in the stomach?
Pepsin, helps to break down food
What are the 3 sections of the small intestine?
Duodenum, jejunum and ileum
What is the basic function of the small intestine?
Principal site of digestion and absorption of nutrients
What are the 3 sections of the large intestine?
Caecum, appendix and colon
What is the basic function of the large intestine?
Colon reabsorbs fluid and electrolytes, stores faecal matter before delivery to the rectum
What is the basic function of the rectum and anus?
Storage and regulated expulsion of faeces
What are the accessory structures?
Salivary glands, pancreas, liver and gall bladder (hepatobiliary system)
Where is fat digestion initiated?
Small intestine
What are the 4 activities of the alimentary canal?
Motility, secretion, digestion and absorption
What occurs during motility?
Mechanical activity mostly involving smooth muslce (skeletal muslce at mouth, pharynx, upper oesophagus and external anal sphincter)
What occurs during secretion?
Into the lumen of the digestive tract occurs from itself and accessory structures in response to the presence of food, hormonal and neural signals. Secretion is required for: digestion, protection and lubrication
What occurs during digestion?
Chemical breakdown by enzymatic hydrolysis of complex foodstuffs to smaller, absorbable untis
What occurs during absorption?
The transfer of the absorbable products of digestion (with water, electrolytes and vitamins) from the digestive tract tot eh blood, or lymph - largely mediated by numerous transport mechanisms
What is contained within the mucosa?
Epithelial cells Exocrine cells Endocrine gland cells Lamina propria (capillaries, enteric neruones, immune cells (lymphocytes and pathogens) Muscularis mucosae
What is the function of the muscularis mucosae?
Thins smooth muscle layer that has the function of changing the shape of the entire mucosa and is important in the mixing the contents of the lumen
What is contained within the submucosa?
Elastic connective tissue, larger blood and lymph vessels, glands and the submucous plexua
What are the 2 parts tot he muscularis externa?
Circular smooth muscle and longitudinal muscle layer
What does the circular smooth muscle do?
Controls the diameter of the lumen
What does the longitudinal smooth muscle layer do?
Controls the length of the lumen
What is the serosa?
Connective tissue
What type of cells are contained within the serosa?
Squamous cells that secrete lubricants that help the movement of flood through the intestine
What occurs when the circular muscle contracts?
The lumen of the tract becomes narrower and longer
What occurs when the longitudinal muscle contract?
The lumen becomes shorter and fatter
What is a single unit muscle?
The muscle is comprised of individual smooth muscle cells but they are electrically linked via gap junctions
What is the function of gap junctions?
They allow the spread of electrical currents from cell to cell forming a functional syncytium
What modulated spontaneous electrical activity via pacemaker cells?
Intrinsic (enteric) and extrinsic (autonomic) nerves
Numeous hormones
What is a slow wave?
Rhythmic patterns of membrane depolarisation and repolarisation that spread from cell to cell via gap junctions
What does the slow wave electrical activity determine?
The max frequency, direction and velocity of rhythmic contractions
What is the name of the pacemaker cells in the GI tract?
Interstitial cells of Cajal (ICCs)
When does contraction occur in smooth muscle cells?
Only occurs if the slow wave amplitude is sufficient to trigger SMC action potentials
What mediates the upstroke of SMC action potentials?
Voltage-activated Ca2+ channels (L-type Ca2+ channels)
What mediates the downstroke of SMC action potentials?
Voltage-activated K+ channels
Where are ICCs located?
Between the longitudinal and circular muscle layers and in the submucosa
What do slow waves determine?
The basic electrical rhythm (BER)
What is the BER frequency in the stomach?
3 slow waves per minute - will only initiate an AP if it reaches the threshold via the action of a stimulus
What is the BER frequency of the small intestine?
12 in the duodenum and 8 in the terminal ileum, this drives luminal contents in the aboral direction
What is the BER frequency of the large intestine?
8 in the proximal colon
16 in the distal (sigmoid) colon which favours retention of luminal contents facilitating reabsorption of water and electrolytes
Where can parasympathetic fibers be released from?
Vagus nerve (medulla oblongata) Sacral spinal cord (S2-4) - pelvic nerves
Where are the synapses within the parasympathetic system?
Preganglionic fibres synapse with ganglion cells within the ENS
What occurs when the parasympathetic nervous system is excited?
Increased gastric, pancreatic and small intestinal secretion, blood flow and smooth muscle contraction
What inhibitory influences does the parasymathetic system impose?
Relaxation of some sphincters and receptive relaxation of stomach
Where does the sympathetic nervous system synapse?
Preganglionic fibers synapse in the prevertebral ganglia (abdomen). The postganglionic fibers will then innervate mainly enteric neurones but also other structures
Where can sympathetic nerves originate from?
Thoraco-lumbar region
What are the prevertebral ganglia in the abdomen?
Celiac
Superior mesenteric
Inferior mesenteric
What are the exitatory influences of the sympathetic innervation on the GI tract?
Increased sphincter tone
What are the inhibitory influences of sympathetic innervation on the GI tract?
Decreased motility, secretion and blood flow
What does the myenteric plexus regulated?
Motility and sphincters
What does the submucous plexus regulate?
Mainly epithelia and blood vessels
What are sensory neurones?
Mechanoreceptors
Chemoreceptors
Thermoreceptors
What is the function of interneurones?
To co-ordinate reflexes and motor programmes
What is the function of effector neurones?
Excitatory and inhibitory motor neurones supplying both smooth muscle layers, secretory epithelium, endocrine cells and blood vessels
What is a local reflex?
Occurs entirely within the walls of the GI tract (mechanical receptor detects distension, signals to interneurone which then sends this to the motor neurone)
What is an example of a local reflex?
Peristalsis
What is a short reflex?
Sensory nerurone detects change and sends signal to prevertebral ganglion where it synapses. This then sends the signal to the interneurone back within the walls of GI tract where it synapses with the effector neurone
What is an example of a short reflex?
Intestine-interstinal inhibitory reflex
What is a long reflex?
Same as short reflex but the sensory neruone is not contained within the GI walls and it will synapse in the CNS with the vagus nerve
What is an example of a long reflex?
Gastroileal reflex
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 occurs in the propulsive segment?
Longitudinal muslce relaxes (release of VIP and NO from inhibitory motorneurone)
Circular muslce contracts (release of ACh and substance P from excitatory motorneurone)
What occurs in the receiving segment?
Longitudinal muscle contracts (release of ACh and substance P from excitatory motorneurone)
Circular muscle relaxes (release of VIP and NO from inhibitory motorneurone)
What is energy homeostasus?
The physiological process whereby energy intake is matched to energy expenditure over time
What does energy homeostasis promote?
Body fuel stability, energy is primarily stored as fat
What are the causes and consequences of metabolic stress?
Metabolic syndrome Central obesity Dyslipidemia Insulin resistance Type 2 diabetes CV disease
How is BMI calculated?
Weight/m2
What are the BMI vales?
up to 25 = thin/acceptable
25-29.9= overweight
30.0-39.9 = obese
40 or above morbidly obese
What are the major factors influencing obesity?
Genetics
Environment - sedentery lifestyle
What are the long term consequences of obesity?
Stroke (hypertension) Respiratory disease (sleep apnea) Heart disease (lipids, diabetes, hypertension) Osteoarthritis Gallbladder disease Dementia (alzheimers) NAFLD (fatty liver) Diabetes Cancer (uterus, breast, prostate, colon) Hyperuricemia, gout
Why do we need fat?
Energy storage
Prevention of starvation
Energy buffer during prolonged disease`
Why is it difficult to lose weight once gained?
Increased body fat alters the brain function and long term obesity changes the brain to view the extra weight as normal and dieting as a threat to body survivial
How does the CNS influence energy balance and body weight?
Behaviour - feeding and physical activity
ANS activity - regulates energy expenditure
Neuroendocrine system - secretion of hormones
What neural center is responsible for control of energy intake and body weight?
Hypothalamus
Lesioning ventromedial hypothalamus = obesity
Lesioning lateral hypothalamus = leanness
What 3 concepts underly the control system that controls energy intake + body weight?
Satiety signalling
Adiposity negative feedback signalling
Food reward
What is satitation?
The sensation of fullness generated during a meal
What is satiety?
The period of time between termination of one meal and the initiaion of another
What is adiposity?
The state of being obese
How do satiation signals change during meals?
They increase during meals to limit the meal sizes
What is cholecystokinin (CCK)?
A hormone secreted from enteroendocrine cells in the duodenum and jejunum.
What is the function of CCK?
It is released in proportion to lipids and proteins in meals. It signals via sensory nerves to hindbrain and stimulates the hindbrain directly (nucleus of solitary tract (NTS))
What is Peptide YY (PYY)?
A hormone secreted from endocrine mucosal L-cells of the GI tract
What is the function of PYY?
Levels increase rapidly post-prandially. This inhibits gastric motility, slows emptying and reduces food intake (hypo)
What is glucagon-like peptide 1 (GLP-1)?
A product of pro-glucoagon gene that is released frfom L cells in response to food ingestion
What is the function of GLP-1?
Inhibits gastric emptying and reduces food intake (hypo, NTS)
What is oxyntomodulin (OXM)?
A protein from pro-glucagon and released from oxynitic cells of the small intestine after a meal
What is the function of OXM?
It acts to supress appetite
What is obestatin?
A peptide produced from a gene that encodes ghrelin and released from cells lining the stomach/small intestine
What is the function of obestatin?
It reduces food intake and also may act to antagonise the actions of ghrelin
What is ghrelin?
An octanolyated peptide that is produced and sereted by oxynitic cells in the stomach
When do ghrelin levels rise and fall?
Ghrelin levels increase before meals before meals and decrease after meals. Levels are raised by fasting and hypoglycaemia
What is the function of ghrelin?
Peripheral ghrelin stimulates food intake (hypo) and decreases fat utilization
What signals increase food intake?
Glutamate, Gaba and opioids
What signals supress food intake?
Monoamines
What is leptin?
A hormone made and released from fat cells
What is inslin?
A hormone made and released from pancreatic cells, the levels int he blood increase as more fat is stored
What is the function of hormones in feeding?
It informs the hypothalamus to alter energy balance - eat less and increase energy burn
What is the function of leptin?
Circulates in proportion to body adiposity and has a specific transport system for leptin to enter the brain as there are high levels of leptin receptors (Ob-Rb) in the hypothalamus.
What are the biological roles of leptin?
Food intake/energy expenditure/fat deposition
Peripheral glucose homeostasis/insulin sensitivity
Maintenance of immune system
Maintenance of reproductive system
Angiogenesis
Tumorigenesis
Bone formation
What is an approved weight loss drug?
Orlistat which inhibits pancreatic lipase decreasing triglyceride absorption. This reduces the efficiency of fat absorption in the small intestine
What is an example of surgery that can be used for weight loss?
Gastric by-pass produces substantial weight loss and can completely resolve type 2 diabetes
By-pass restricts calorie intake and induces malabsorption of nutrients
What are thermogenic adipocytes?
They increase energy expenditure by uncoupling of oxidative metabolism from ATP production. The uncoupling of protein 1 (fatty acid activated protein) short circuits the proton gradient in the mitochondria and accelerates fuel oxidation producing heat