GIT 200L PHYSIOLOGY EKSU 2024.docx

**[GIT PHYSIOLOGY -- B.sc 200L.]** ***[COURSE OUTLINE]***: - *[Gastrointestinal Tract]*: (i) Functional of GIT (ii) Innervations and Visceral Sensations (iii) GIT Motility (Movements) (iv) GIT Secretions and Control (v) GIT Blood circulation. - \(i) GIT hormones (ii) GIT Biliary S...

**[GIT PHYSIOLOGY -- B.sc 200L.]** ***[COURSE OUTLINE]***: - *[Gastrointestinal Tract]*: (i) Functional of GIT (ii) Innervations and Visceral Sensations (iii) GIT Motility (Movements) (iv) GIT Secretions and Control (v) GIT Blood circulation. - \(i) GIT hormones (ii) GIT Biliary System: Secretions and Control, (iii) Composition of Bile, adaptability of biliary secretions to digestion and absorption of food. - Digestion and Absorption of different classes of food i.e. Carbohydrate, Protein Nucleic acids, Fat, Water etc. in the GIT *[At the end of the Lectures on this section, students shall be able to discuss and appreciate]*: (i). Physiological and Functional Anatomy of GIT; (ii). Innervations, Nervous control and sensations of GIT; (iii). Motility or Mechanical activities of GIT; (iv). GIT blood Flow (Splanchnic Circulation). (v). GIT hormones, Secretions and Control. (vi). Digestion and absorption of different types of food. (vii). Composition of Bile, adaptability of biliary secretions to digestion and absorption of food. **[\ ]** **[THE PHYSIOLOGICAL AND ANATOMICAL FUNCTIONS OF THE GASTROINTESTINAL TRACT.]** The Gastrointestinal System. - Gastrointestinal System is the: (i) GIT and (ii) Associated glandular and other organs i.e. (a)Teeth (b) Tongue (c) Salivary glands (d) Liver (e) Gall bladder and (f) Exocrine part of Pancreas - The GIT consists of a hollow tube that starts from the mouth and ends up in the anus with a length of about 30 ft. It opens into the external environment on both ends. - The GIT can be divided from above downwards into (i) Mouth (ii) Pharynx (iii) Oesophagus (iv) Stomach (v) Duodenum (vi) Jejenum/ileum (vii) Colon (viii) Rectum and (ix) Anus. - \(i) --(ix) above are ***Primary digestive organs*** while (a) --(f) as listed above are ***accessory digestive organs*.** - The ***[main functions]*** of the **[GI system]** are: 1. Ingestion or Consumption of food substances; 2. Breaking these foods into small particles; 3. Transportation of these small food particles to different areas of the digestive tract; 4. Secretion of necessary enzymes and other substances for digestions; 5. Digestion of food particles; 6. Absorption of the digested products (Nutrients); 7. Removal of unwanted substances from the body. In order to perform these functions, the GI system carries out the under-listed activities broadly grouped into five (5) main headings: 1. Motility 2. Secretion 3. Digestion 4. Absorption and 5. Excretion of unwanted materials. **[MOTILITY]**: This is movement of GIT, which help to mix and grind the contents of the GIT and propel same along the length of the tract and in an **[oro-caudal]** direction i.e. from mouth to anus. **[SECRETION:]** This is the process by which the glands that are associated with the GI system pour H~2~O and other substances (enzymes, mucus, electrolytes e.t.c into the GIT. **[DIGESTION:]** This is the process by which ***[ingested]*** large food molecules are chemically broken down into much smaller molecules that can pass across the wall of the GIT into the blood steam. Digestion consists of: (i) Mechanical brake down of food by Mastication (Chewing) and (ii) Chemical digestion of food into small molecules by enzymes present in secretions produced by glands and accessory organs of the digestive system. **[ABSORPTION:]** This is the process by which nutrients molecules (the end products of digestion) are transported from the gut lumen into the blood steam. **[ELIMINATION/EXCRETION]**: This refers to the passage of faeces out of the body through the anus in form of faeces. Faecal material includes undigested and unabsorbed dietary food products, colonic bacteria and their metabolic products such as heavy metals like Fe^+^, Cu^+^ and several organic anions or cations and drugs. - **Accessory organs:** Various secretions are poured into the alimentary canal, some by glands in the lining membranes of the organs, e.g. gastric juice secreted by glands in the lining of the stomach, and some by glands situated outside the tract. The latter are the accessory organs of digestion and their secretion pass through ducts to enter the ducts. They consist of: - three pairs of Salivary glands; - the Pancreas; - the liver and biliary tract. The organs and glands are linked physiologically as well as anatomically in that digestion and absorption occur in stages, each stage being dependent upon the previous stage or stages. **[FUNCTIONAL ANATOMY OF THE GASTROINTESTINAL OF THE GIT STYSTEM.]** - The layers of the walls of the GIT canal follow a consistent pattern from the Oesophagus onwards. - This basic structure does NOT apply to the mouth and pharynx. - From the oesophagus onwards, modifications to structures are found which are associated with specific functions. **[WALLS OF THE GIT]**: The GIT is formed by four (4) layers which are from inside out, (i) Mucosa layer (ii) Submucosa layer (iii) Muscular layer (iv) Serous or Fibrous layer. 1. **[MUCUS LAYER:]** This is the innermost layer of the GIT. It faces the hollow cavity. It has three (3) layers- (a) Epithelial Lining (b) Lamina Propia (c) Muscularis mucosa. - \(a) **[Epithelial Lining]:** (i) It is in contact with the contents of the GIT with different types of cells in different parts of the GIT; (ii) ***[Stratified Squamous cells]*** are present in the inner surface of (a) Mouth (b) Tongue (c) Pharynx and (d) Oesophagus. (iii) The Mucus membrane of (a) Stomach (b) Small intestine and (c) Large intestine are lined by ***Columnar*** ***epithelial cells line.*** \(b) **[Lamina Propia]:** This layer is formed by *connective tissue*, which contains *fibroblasts, macrophages*, *lymphocytes* and *eosinophils*. \(c) **Muscularis mucosa:** This layer contains a thin layer of smooth muscle fibres. It is absent in the mouth and pharynx BUT present from oesophagus onwards. **2. [ SUBMUCUS LAYER]**: This layer is present in ALL parts of GIT *except* the mouth and the pharynx. It contains (a) (i) Loose collagen fibres (ii) elastic fibres (iii) reticular fibres and (iv) few cells of connective tissues (b) Blood vessels, lymphatic vessels and nerve plexuses are also present in this layers. 3. **[MUSCULARIS LAYER]**: *Muscularis layer* are present in (i) lips and (ii) cheek while *smooth muscle fibres* form the stomach and the intestines. ***[In the Stomach]***, smooth muscle fibres are arranged in three (3) layers: (i) Inner oblique layer; \(ii) Middle circular layer and \(iii) Outer longitudinal layer. ***[In the Intestine]**,* Smooth muscle fibres are arranged two (2) layers: i. Inner circular layer. ii. Outer longitudinal layer. Auerbach's Nerve plexus is present in between the Circular and Longitudinal fibres. The **smooth muscle fibres** are present in the inner circular layer of anal canal constituting the **internal anal sphincter**. **Skeletal muscle** fibres form the **external anal sphincter**. 4. **[SEROUS OR FIBROUS LAYER.]** This is the outermost layer of the GIT and it either serous or fibrous in nature. i. The serous layer is also called serosa or serous membrane and it is formed by connective and mesoepithelial cells. It covers the (i) Stomach (ii) Small intestine and (iii) Large intestine. ii. The fibrous layer is also called fibrosa is formed by connective tissue to cover the pharynx and oesophagus. **Extrinsic nerve supply to GI tract. T5 = 5th thoracic segment of spinal cord L1 = 1st lumbar segment of spinal cord S2 = 2nd sacral segment of spinal cord** ***[INTRINSIC NERVE SUPPLY]*** called Enteric Nervous System that (i) Controls **ALL** the (a) *Secretions* and (b) *Movements* of GIT; ii. It is present within the wall of GIT from Oesophagus to the Anus; iii. The Nerve fibres are interconnected to form **two (2) major** **networks** called 1. *Auebach's Plexus.* 2. *Meissner's Plexus.* - These nerve plexus contain (i) Nerve cell bodies. \(ii) Processes of nerve cells. \(iii) Receptors i.e*. Stretch receptors and Chemoreceptors.* ***[Auerbach's Nerve Plexus:]*** is also known as ***[myenteric nerve plexus]*** present between the ***inner circular muscle layer*** and ***outer longitudinal muscle layer.*** - The *major function* of the auerbach's plexus is *to regulate the* *movements* of the GIT. - Some fibres of the auerbach's plexus *accelerate the movements of the GIT* by *secreting the excitatory neurotransmitter substances like (i) Ach, (ii) serotonin and (iii) substance P.* - Other fibres *slows down the GIT motility by secreting inhibiting substances like (i) Vasoactive Intestinal Polypeptides (VIP) (ii) Neurotensin and (iii) encephalin.* ***[Meissner's Nerve Plexus]:*** This is also known as ***Submucus*** ***nerve plexus*** situated in between the **muscular layer** and **submucus layer** of the GIT - The major function of the meissner's plexus is (i) the regulation of secretory function of the GIT and (ii) Constriction of blood vessels of GI **[EXTRINSIC NERVE SUPPLY:]** Extrinsic nerves that control the enteric (intrinsic) nervous system of GIT are from the Autonomic Nervous System (ANS) i.e sympathetic and parasympathetic divisions of the ANS supply the GIT. ***\*\*Enteric nervous system is controlled by extrinsic*** ***nerves supply from the ANS\****\*.  ***[Sympathetic Nerve Fibres:]*** [Thoraco-Lumbar outflow] (RED COLOUR NERVES) - The preganglionic Sympathetic nerve fibres to GIT arise from the lateral horns of spinal cord between 5^th^ thoracic and 2^nd^ lumbar segments (T5-L2). - From T5-L2, the fibres leave the spinal cord, pass through the ganglia of spinal cord and that of sympathetic chain without having any synapse and then terminate in the **celiac** and **mesentery ganglia --** *both superior and inferior*. - The postganglionic fibres from these ganglia are then distributed throughout the GIT. - Sympathetic autonomic nerve fibres (i*) inhibit the GIT movements (ii) decrease the secretion and (iii) causes the constrictions of the sphincters by secreting neurotransmitter **Noradrelanine** (Nadr)* ***[Parasympathetic Nerve Fibres]*. *--*** [Cranio-Sacral outflow] (B[LUE COLOUR NERVES).] - Parasympathetic nerve fibres to GIT pass through some of the ***cranial nerves*** and ***sacral nerves*.** - The preganglionic and postganglionic parasympathetic autonomic nerve fibres to mouth and salivary glands pass through **facial (CN7),** **glossopharyngeal nerves (CN9)** and **Vagus (CN10).** - Preganglionic parasympathetic nerve fibres to (i) oesophagus, stomach (iii) small intestine and (iv) upper part of large intestine pass through the **vagus nerve.** - Preganglionic nerve fibres supplying lower parts of the of large intestine arise from 2^nd^, 3^rd^, 4^th^ sacral segments (S2, S3, and S4) of spinal cord and pass through **pelvic nerve**. - All these preganglionic parasympathetic nerve fibres synapse with the postganglionic nerves in the myenteric and submucus plexuses of the GIT. - Parasympathetic nerve fibres accelerate the movements and increase the secretions of GIT by secreting the neurotransmitter ***Acetycholine*** (Ach). - The GI system therefore contains both afferent and efferent nerve fibres that serve both local and central reflex arc **Actions of sympathetic & parasympathetic divisions of ANS on GIT i.e Extrinsic Nerve Supply & Sensations of GIT)** S/N Effector organ Sympathetic Parasympathetic ----- --------------------- -------------------------------------------- ------------------------------------------------------- -------------- 1 Salivary Glands Decrease in Secretion and Vasoconstriction Increase in Secretion and Vasodilatation 2 GIT Motility Inhibition Acceleration Secretion Decrease Increase Sphincters Constriction Relaxation Smooth muscles Relaxation Constriction 3 Gall bladder Relaxation Contraction 4 Neurotransmitter(s) Nadr Ach 5 Cranial Outflow Thoraco-lumbar (T5-L2) Cranio-sacral (Nuclei of CN7, CN 9 & 10); & (S2 &S3). **[THE PERITONEUM]** - The peritoneum is a continuous membrane that lines the abdominal cavity and covers the abdominal organs (abdominal viscera). - It acts to support the viscera, and provides pathways for blood vessel and lymph to travel to and from the viscera. - It is the largest serous membrane in the body. Peritoneal fluid lubricates the surface of the abdominal cavity. ***[Layers]***: The peritoneum contains of two (2) layers: (i) the superficial parietal layer and (ii) the deep visceral layer. - The peritoneal cavity contains the (i) omentum (both greater and lesser) (ii) ligaments and (iii) mesentery -- double layer of peritoneum attaches the vasculatures and nerves to the intraperitoneal organs.; - Intraperitoneal organs are (i) stomach (ii) spleen (iii) liver (iv) 1^st^ & 4^th^ parts of the duodenum, jejenum, ileum, transverse colon and sigmoid colon. - The peritoneal cavity is not closed in the females to allow the uterine tube to open into the peritoneal cavity, providing a potential pathway between the female genital tract and the abdominal cavity. - Retroperitoneal organs lie behind the posterior sheath and include the (i) aorta, (ii) esophagus (iii) 2^nd^ & 3^rd^ part of duodenum (iv) - ascending and descending colon (v) pancreas (vi) kidneys (vii) ureters, and adrenal glands. **[GASTROINTESTINAL MOTILITY:]** These are the mechanical activities of the GIT. *[Course outlines]*: At the end of this topic, students should be able to: 1\. List the major forms of motility in the GIT and their roles in digestion and secretion; distinguish between peristalsis and segmentation. - The peritoneum is a continuous membrane that lines the abdominal cavity and covers the abdominal organs (abdominal viscera). - It acts to support the viscera, and provides pathways for blood vessel and lymph to travel to and from the viscera. - It is the largest serous membrane in the body. Peritoneal fluid lubricates the surface of the abdominal cavity. ***Layers***: The peritoneum contains of two (2) layers: (i) the superficial parietal layer and (ii) the deep visceral layer. - The peritoneal cavity contains the: (i) omentum (both greater and lesser) (ii) ligaments and (iii) mesentery -- double layer of peritoneum attaches the vasculatures and nerves to the intraperitoneal organs.; - Intraperitoneal organs are (i) stomach (ii) spleen (iii) liver (iv) 1^st^ & 4^th^ parts of the duodenum, jejenum, ileum, transverse colon and sigmoid colon. - The peritoneal cavity is not closed in the females to allow the uterine tube to open into the peritoneal cavity, providing a potential pathway between the female genital tract and the abdominal cavity. - Retroperitoneal organs lie behind the posterior sheath and include the (i) Aorta, (ii) Esophagus (iii) 2^nd^ & 3^rd^ part of duodenum (iv) Ascending and Descending colon (v) Pancreas (vi) Kidneys (vii) Ureters, and (viii) Adrenal glands. **[GIT MOTILITY: ]** - The digestive and absorptive functions of the GI system depend on a variety of mechanisms that: (i) soften the food (ii) propel same through the length of the GIT and (iii) mix same with bile from the gallbladder and digestive enzymes of salivary glands and pancreas. - So many of these mechanisms depend on intrinsic properties of the GI smooth muscle and other involve in the operations of: (i) reflexes involving the neurons intrinsic to the GIT, (ii) reflexes involving the CNS, (iii) paracrine effect of messengers and (iv) GIT hormones. - These mechanical activities (GIT motility) occur in different regions of the GIT. They include: 1.. Chewing (mastication) in the mouth; 2.. Swallowing (deglutition) in the mouth and esophagus; 3.. Gastric motility & emptying 4.. Motility of the small intestine; 5.. Motility of the Colon; 6.. Defeacation (passage) of faeces. The GIT movements are facilitated/controlled by the GIT *(i). Intrinsic* *nervous system;* (ii). *Extrinsic nervous systems;* (iii). *GIT hormonal secretions* and (iv). *Gated or timed the GIT sphincters*. GIT sphincters are seven (7) oro-caudally. The sphincters are: 1. *Upper oesophageal;* 2. *Lower oesophageal;* 3. *Pyloric;* 4. *Ileo-caecal valve (regulates entrance from the intestine to the ileocaecal part);* 5. *Houston's valves (the semilunar transverse folds of rectal wall that protrude into the rectum);* 6. *Internal sphincter of anus;* 7. *External sphincter* *of anus*. - Two types of movement occur in the GIT: (1) ***[Propulsive movement]*** which cause food to move forward along the GIT to move forward along the tract at an appropriate rate to accommodate digestion and absorption i.e *Peristalsis* and (2) ***[Mixing movements]*** which keeps the intestinal contents thoroughly mixed at all times i.e. Churning & Local intermittent constrictive contraction. **[PERISTALSIS]**:  - Peristalsis is a reflex response that is initiated when the gut wall is stretched by the content of the lumen, and it occurs in ALL parts of GIT from the esophagus to the rectum. It is a wave of contraction followed by wave of relaxation of muscles of GIT which travel in ***[abora]*l** direction. - The stretch initiates a circular contraction behind the stimulus and an area of relaxation in front. - The wave of contraction then moves in an oro-caudal direction, propelling the contents of the lumen forward at the rate of 2-25cm/s.Peristaltic movement can be increased or decreased by autonomic input to the gut BUT its occurrence is independent of extrinsic innervation from ANS. - Peristalsis is NOT blocked by removal and resuture of a segment of intestine in its original position and it can only be blocked only if the segment is reversed reversed before it is sewn. - Peristalsis is an example of the enteric nervous system. Local stretch releases (i) Serotonin which activates sensory neurons that activate the myenteric plexus. - Cholinergic neurons passing in a retrograde direction in this plexus activates neurons that release substance P, acetylcholine, causing smooth muscle contraction behind the bolus. - Cholinergic neurons passing in an anterograde direction activate neurons that secrete (i) NO and (ii) Vasoactive Intestinal Polypeptide (VIP), producing the relaxation ahead of the stimulus. *[Examples of peristalsis include]*: 1.. Deglutition in the mouth, pharynx, esophagus; 2.. Gastric motility & emptying; and Vomiting (reverse peristalsis; 3.. Segmentation and Tonic contractions in the small and large intestine; 4.. Migrating Motor Complexes (MMCs) in small intestine; 5.. Mass action complex (occurring about 10 times a day; 6.. Defaecation; 7.. Evacuation of gases from the GIT- Belching & Flatulence. **[(A). MOVEMENT IN THE MOUTH:]** This is *[Mastication and Deglutition.]* ***[MASTICATION]**:* This is chewing, the first mechanical process in the GIT by which the food particles are cut/torn into smaller particles and crushed or ground into a **small bolus**. Saliva: - Mixes with the food substances properly; - Lubricate and moisten the dry food for easy deglutition; - Allows the taste of the food to be appreciated; - The muscles of mastication are (i) Masseter muscle (ii) Temporal muscle, (iii) Pterygoid muscle (iv) Buccinator muscle. These muscles allow (a) opening and closure of mouth (b) rotational movement of jaw (c) Protraction and retraction of jaw for proper chewing and grinding of foodstuff. - Mastication is almost a reflex process BUT can be carried out voluntarily. The centre of control for mastication is in Medulla and Cerebral cortex. The muscles of mastication are supplied by the mandibular division of cranial nerve 5 (CN5) -- the Trigeminal nerve. ***[DEGLUTITION]***: (Swallowing). - Swallowing is a reflex response that is triggered by *afferent* impulses in the trigeminal (CN5), glossopharyngeal (CN9) and vagus (CN10) nerves. - These impulses are *integrated in the nucleus of the tractus solitarius and the nucleus ambiguus.* - The *efferent f*ibres pass to the **pharyngeal musculature and the tongue via the *trigeminal (CNV), facial (CNVII)*** an*d **hypoglossal (CNXII) nerves.*** [**ORAL STAGE (FIRST STAGE**)] This is a *[voluntary stage]*. Bolus of food moves from the mouth into the pharynx by in four (4) sequences: i. Bolus is put over the postero-dorsal surface of the tongue- the preparatory position; ii. Anterior part of tongue is retracted and depressed; iii. Posterior part of tongue is elevated and retracted against the hard palate to push the bolus backwards into the pharynx. iv. Forceful contraction of tongue against the palate produces a positive pressure in the posterior part of oral cavity to push the food into the Pharynx. **[PHARYNGEAL STAGE (SECOND STAGE]** This is an [*involuntary stage*.] Since pharynx is communicated with the mouth, nose and larynx, and esophagus, during this 2^nd^ stage of deglutition, bolus from the pharynx can enter into four (4) paths: (i) back into the mouth (ii)upward into nasopharynx (iii) forward into larynx and (iv) downward into the esophagus- the only physiological path. However, (i) -- (iii) are prevented by: -.(a) Position of tongue against the soft palate (roof of the mouth) and high intraoral pressure developed by the movement of tongue; - (b)Elevation of soft palate along with its extension with Uvula; - (c). Movement of bolus into the larynx which is prevented by: (i) Approximation of the vocal cords (ii) Forward and upward movement of larynx (iii) Backward opening of epiglottis to seal the opening of the larynx- the glottis. - The actions (i) to (iii) as listed cause temporary arrest of respiration for a few seconds- the deglutition or swallowing apnoea- during pharyngeal stage of deglutition. - \(d) Entrance of bolus into the Esophagus is achieved by combine efforts and action of the following factors: 1. Upward movement of larynx stretches the opening of esophagus; 2. Simultaneously, upper 3- 4cm of esophagus relaxes i.e. the Upper Esophageal Sphincter (UES) or Pharyngo-esophageal sphincter; 3. At the same time, peristaltic contractions start in the pharynx due to the contractions of pharyngeal muscles; 4. Elevation of larynx also lifts the glottis away from the bolus passage. All the actions listed work above act together to facilitate easy movement of the bolus into the esophagus. The process is purely involuntary and takes 1-2s. **[ESOPHAGEAL STAGE (THIRD STAGE]**): - This stage is *[purely involuntary]* and food bolus from the esophagus enter the stomach by esophageal peristaltic waves. - When bolus reaches the esophagus, two peristaltic waves are initiated (1) Primary peristaltic contractions and (2) Secondary peristaltic contractions.  *[Primary Peristaltic Contraction.]* 1. This is when the bolus reaches the upper part of the esophagus to trigger the peristaltic movement; 2. The peristaltic contractions pass down the through the rest of the esophagus propelling the bolus towards the stomach as a result of the pressure thereby developed; 3. The stretching of the closed esophagus by the elevation of the of the larynx would lead to NEGATIVE PRESSURE in the upper part of the esophagus and immediately become POSITIVE to increase up to 10-15cm of H~2~O. *[Secondary Peristaltic Contraction.]* 1. If the 1^0^ peristaltic movement fails to propel the bolus into the stomach, the 2^0^ peristaltic contractions appear and push the bolus into the stomach; 2. The 2^0^ peristaltic contractions are induced by the distension of upper esophagus by the bolus which pass down like the 1^0^ contraction producing a POSITIVE pressure. 3. The distal 1/3^rd^ of the esophagus (i.e 2-5cm) is the ***[esophageal] sphincter*** which is ***constricted*** always. 4. The walls of the esophagus are lubricated by mucus which assist the passage of the bolus during the peristaltic contraction of the muscular wall. ***[DEGLUTITION REFLEX]***: - Deglutition is a voluntary and involuntary reflex that later becomes involuntary and it is carried out by the ***[Deglutition Reflex.]*** **[*Stimulus*]** = Bolus in the oropharyngeal region stimulates the receptors therein. ***[Afferent Fibres]*** = Receptors in the oropharyngeal region pass via glossopharyngeal nerve fibres to the deglutition centre. **[*Centre* =]** deglutition centre in the floor of the the 4^th^ ventricle in the Medulla Oblongata in the brain. ***[Efferent]*** = efferent from the deglutition centre travel via glossopharyngeal and Vagus (parasympathetic motor fibres) to reach the soft palate, pharynx and esophagus. The glossopharyngeal nerve is concerned with pharyngeal stage while Vagus nerve is concerned with esophageal stage, ***[Response/Effector]**:* The reflex causes upward movement of soft palate to: i. Close Nasopharynx; ii. Close Respiratory passage that bolus enters the esophagus; iii. The peristalsis now occurs in esophagus to push the bolus into Stomach though the relaxed cardiac sphincter. iv. The constriction of the cardiac sphincter of the stomach prevents reflux of gastric acid into the esophagus. **[BASIC ELECTRICAL ACTIVITY & REGULATION OF MOTILITY. ]** The smooth muscle of the GIT has spontaneous rhythm \(B) [ **MOVEMENTS OF THE STOMACH (GASTRIC MOTILITY & EMPTYING:**] - Food is stored in the stomach, mixed with acid, mucus and pepsin. - It is then released at steady rate into the duodenum. - Activities of the smooth muscles of stomach increase during digestion (when the stomach is filled with food) and when the stomach is empty. - There are ***[three (3) types]*** of movements in the stomach: 1. Hunger contractions are of three (3)- types I, II & III. 2. Receptive relaxation. 3. Peristalsis. \(1) [ ***HUNGER CONTRACTIONS**.*] - Hunger contractions are the movements of empty stomach related to hunger sensations and are peristaltic waves superimposed over the contraction of gastric smooth muscle as a whole; - Hunger contractions of empty stomach involves the entire stomach while digestive peristaltic contractions usually occur in the body and pyloric part of the stomach. +-----------------------------------+-----------------------------------+ | Type I Contractions | - First to appear on empty | | | stomach when the tone of the | | | gastric muscle is low as last | | | for about 20s at an interval | | | of about 3-4 secs. ; | | | | | | - Tone of muscles does not | | | increase between | | | contractions; | | | | | | - Pressure produced by these | | | contractions is about 5cm of | | | H~2~0. | +===================================+===================================+ | Type II Contractions | - Appear when the tone of the | | | gastric muscle is stronger | | | when food intake is postponed | | | even, after the appearance of | | | type I contractions; | | | | | | - It lasts for 20s but the | | | pause between contraction is | | | reduced; | | | | | | - Pressure produced by these | | | contractions is 10-15cm of | | | H~2~O. | +-----------------------------------+-----------------------------------+ | Type III Contractions | - Appear when the hunger | | | becomes severe ant the tone | | | increases to a great extent. | | | Very rare in man because food | | | is taken before the | | | appearance of these | | | contractions; | | | | | | - It lasts for 1-5 mins and | | | produce a pressure of 10-20cm | | | of H~2~O. | | | | | | - When the stomach is empty, | | | the Type I \< Type II \< | | | Type III. | | | | | | - As soon as food is consumed | | | and in the stomach, the | | | hunger contractions | | | disappear. | +-----------------------------------+-----------------------------------+ 2**. *[RECEPTIVE RELAXATION]*.** - This is the relaxation of the upper part of the stomach when food enters the stomach from the esophagus. - It involves the fundus and upper part of the body of the stomach,The significance is to accommodate the food easily (***receptive relaxation)*** without much increase in the pressure of the stomach. - This is the **accommodation** of the stomach. - Receptive relaxation is, in part, vagally mediated and trigered by the movement of the pharynx and esophagus. Intrinsic reflexes also lead to relaxation as the stomach wall is stretched. - Peristaltic waves controlled by the gastric Basic Electrical Rhythm (BER), sometimes called ***antral systole***, begin thereafter and sweep towards the pylorus. It can last up to 10s and occurs 3-4 times per minute. - When food enters the stomach, peristaltic contraction starts, and starts from the lower part of the body of stomach, passes through the pylorus till the ***pyloric sphincter***. - The contraction increases in tone from the greater and lesser curvatures and travels towards the pylorus and ends with the contraction of the pyloric sphincter. - Each peristaltic wave takes about 1-3 min to travel from the point of origin to the terminating point. - The gastric peristaltic contractions are called ***digestive peristalsis*** because it is responsible for the mixing with gastric juice and grinding the food particles and permitting small, semiliquid portions of it to pass through the pylorus and enters the duodenum for digestive activities. **[FILLING THE STOMACH]**: - The food taken is arranged in different layers in the stomach: i. The first eaten food is placed against the greater curvature in the fundus and body of the stomach; ii. The successive layers of food particles lie nearer the lesser curvature, until the last portion of food eating lies near the upper end of lesser curvature, adjacent to cardiac sphincter; iii. The liquid remains in the lesser curvature and flows towards the pyloric end of the stomach along the V-shaped groove, made up of smooth muscle called ***magenstrasse.*** Large amount of fluid flows around the entire food mass and is distributed over the interior part of stomach, between the wall of stomach and the food mass. [**STOMACH EMPTYING AND FACTORS AFFECTING**:] This is the process by which the stomach empties the chyme (Acidic, semisolid, partially digested food in the stomach) from the stomach into the intestine. Swallowed food remains in the stomach for about 3-4 hrs for digestion and formation of chyme. - The slow emptying is necessitated to facilitate the final digestion of and maximum (about 80%) absorption of the digested food materials from small intestine. - Gastric emptying occurs due to the peristaltic waves in the body and pyloric part of the stomach and simultaneous relaxation of the pyloric sphincter. In the regulation of the gastric emptying, the antrum, pylorus, and upper duodenum actually function as a unit. Contraction of the antrum is followed by sequential Contraction of the pyloric region and the duodenum. - In the antrum, partial contraction ahead of the advancing gastric contents prevents solid masses from entering the duodenum BUT they are mixed and crushed instead. - The more liquid gastric contents are squirted a little at a time into the small intestine at regulated interval. - Regurgitation from the duodenum is prevented by prolonged (i) contraction of the pyloric segment and (ii) stimulation action of cholecystokinin (CCK) and secretin on the pyloric sphincter. - Stomach emptying is influenced by (i) various of the dietary gastric content and types of food (ii) hormonal factors and (iii) nervous factors: ***[FACTORS AFFECTING STOMACH EMPTYING:]*** S/N Factors affecting Gastric emptying Explanations ----- ---------------------------------------------------- ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1 Volume of gastric content \(i) For any type of meal, gastric emptying is directly proportional to the volume. (ii)The more the stomach content, the faster same is emptied into the intestine. 2 Consistency of gastric content (degree of density) (i)Inert liquids e.g H~2~O leave the stomach rapidly. (ii)Solid leave food leave the stomach after been converted to chyme. (iii) Undigested solid particles are not easily emptied. 3 Chemical composition or types of food (i)Carbohydrates are emptied faster than the Proteins (ii) Proteins are emptied faster than Fats. 4 pH of gastric content Gastric emptying is directly proportional to the pH of the chyme. 5 Osmolar concentration of gastric content. Gastric contents which is isotonic to blood leave the stomach rapidly than the hypertonic or hypotonic due to the "duodenal osmoreceptors" 6 Neural & hormonal mechanism Fats, Carbohydrates and Acids in the duodenum inhibits (i) gastric acid & pepsin secretion and (ii) gastric motility via neural and hormonal mechanisms- *[the enterogastric reflex]*. The messenger involved is probably peptide YY. CCK has also been implicated in the inhibitor of gastric emptying. 1. Presence of chyme in duodenum causes generation of nerve impulses which are transmitted into the stomach by the ***[intrinsic fibres of GIT]*** to inhibit emptying; 2. Impulses from the duodenum pass via ***extrinsic sympathetic fibres*** also to stomach and inhibit emptying; 3. Some impulses from the duodenum travel through *[afferent vagal fibres]* into the brain stem. Brain stem neurons now send *[excitatory efferent vagal fibres]* and stimulates gastric emptying. However, the *[impulses from the duodenum inhibits these brainstem neurons and thereby inhibits gastric emptying periodically.]* 1. (Diagram of EGR) ***Dietary factors which initiates enterogastric reflex:*** 1. Duodenal distention; 2. Irritation of the duodenal mucusa; 3. Acidity of chyme; 4. Osmolality of chyme; 5. Breakdown of protein and Fats. ***[Hormonal factors initiating enterogastric reflex]:*** - Chyme in the duodenum causes the duodenal mucosal to release some hormones to enter the stomach through the blood supply to inhibit the motility of the stomach. - The hormones that inhibits the gastric motility and emptying are: 1. Vasoactive intestinal peptide (VIP). 2. Gastric inhibitory peptide (GIP). 3. Secretin. 4. Cholecystokinin (CCK). 5. Somatostatin. 6. Peptide. ***[VOMITING OR REVERSE PRISTASIS IN THE STOMACH]***. **[MOVEMENT OF SMALL INTESTINE.]** - The ***[Preganglionic Sympathetic nerve]*** fibres to GIT arise from the lateral horns of spinal cord between 5^th^ thoracic and 2^nd^ lumbar segments (T5-L2). - From T5-L2, the fibres leave the spinal cord, pass through the ganglia of spinal cord and that of sympathetic chain without having any synapse and then terminate in the **celiac** and **mesentery ganglia --** *both superior and inferior*. - The postganglionic fibres from these ganglia are then distributed throughout the GIT. - Sympathetic autonomic nerve fibres (i*) inhibit the GIT movements (ii) decrease the secretion and (iii) causes the constrictions of the sphincters by secreting neurotransmitter **Noradrelanine** (Nadr)* ***The [Parasympathetic Nerve Fibres]*. *--*** *[Cranio-Sacral outflow.]* - The Parasympathetic nerve fibres to GIT pass through some of the ***cranial nerves*** and ***sacral nerves*.** - The preganglionic and postganglionic parasympathetic autonomic nerve fibres to mouth and salivary glands pass through **facial (CN7),** **glossopharyngeal nerves (CN9)** and **Vagus (CN10).** - Preganglionic parasympathetic nerve fibres to (i) oesophagus, stomach (iii) small intestine and (iv) upper part of large intestine pass through the **vagus nerve.** - Preganglionic nerve fibres supplying lower parts of the of large intestine arise from 2^nd^, 3^rd^, 4^th^ sacral segments (S2, S3, and S4) of spinal cord and pass through **pelvic nerve**. - All these preganglionic parasympathetic nerve fibres synapse with the postganglionic nerves in the myenteric and submucus plexuses of the GIT. - Parasympathetic nerve fibres accelerate the movements and increase the secretions of GIT by secreting the neurotransmitter ***Acetycholine*** (Ach). - The GI system therefore contains both afferent and efferent nerve fibres that serve both local and central reflex arcs 2. ***[Repeat Diagram of Sympa & Parasympa]*** **Actions of sympathetic & parasympathetic divisions of ANS on GIT (Extrinsic Nerve Supply & Sensations of GIT)** S/N Effector organ Sympathetic Parasympathetic ----- --------------------- -------------------------------------------- --------------------------------------------------------- -------------- 1 Salivary Glands Decrease in Secretion and Vasoconstriction Increase in Secretion and Vasodilatation 2 GIT Motility Inhibition Acceleration Secretion Decrease Increase Sphincters Constriction Relaxation Smooth muscles Relaxation Constriction 3 Gall bladder Relaxation Contraction 4 Neurotransmitter(s) Nadr Ach 5. Cranial Outflow Thoraco-lumbar (T5-L2) Cranio-sacral (Nuclei of CN 7, CN 9 & 10); & (SN2 &S3). 3. DIAGRAM [ ]