Motility I PDF

Motility I. Chewing and Deglutition ANATOMY Main parts of the digestive system: There are different parts, involved in performing the different functions of the digestive system: motility, absorption, digestion and metabolism. - - # = MOTILITY FUNDAMENTALS Within the intestinal wall, we find several...

Motility I. Chewing and Deglutition ANATOMY Main parts of the digestive system: There are different parts, involved in performing the different functions of the digestive system: motility, absorption, digestion and metabolism. - - # = MOTILITY FUNDAMENTALS Within the intestinal wall, we find several layers: serosa, muscularis externa (circular+longitudinal muscle), submucosa (+ its muscular layer: muscularis mucosae), mucosa and mucosal glands. The mesentery is the fibrous connective tissue which keeps all the intestines together. Between these layers, innervating muscle, there are 2 very important plexuses: for motility ~Provision Abori Myenteric plexus (Auerbach) blood for supply and Submucosal plexus (Meissner) gland The submucosal plexus is located in the submucosa, and innervates the muscularis mucosae. The myenteric plexus is in the muscularis externa layer, in between the circular and longitudinal muscles, which it innervates. FOR MIXING AND = more = more secretion Myenteric plexus The myenteric plexus is in between the circular and longitudinal layer of the muscularis externa. The mechanoreceptors are the ones who modulate the actions of this plexus, through the autonomic nervous system (as this is smooth muscle). The two muscle layers are interconnected by fibres, forming a sort of syncytium. - Diameter = 2-10 microns - Length = 200-500 microns They convey the action potential which reaches them through CNS neurons. Electric activity of GI smooth muscle cells 3 types of behaviours: i. Spike depolarizations ii. Plateau depolarizations iii. Slow waves The dotted line marks the threshold of contraction INTERSTIVAL CELLS Of CAJAL /specialized mooth muscle alls) are the "PEACEMAKERS of The HEAR"- generate and propagate sbw waves When at rest, we have slow waves, whose depolarization depends on Ca2+ influx and their hyperpolarization, on K+ efflux. There is always a basal muscle tone through these slow waves. If stronger contractions are needed, then depolarization is induced. Causes of depolarization: 1. Mechanic distension, one of the main stimuli for muscle contraction of the GI tract 2. Acetylcholine, the main neurotransmitter of the PS nervous system 3. Hormones, which also aid in depolarization Causes for hyperpolarization: 1. Noradrenaline 2. Adrenaline and noradrenaline combined (sympathetic nervous system) Patterns of contraction - Slow waves give place to a rhythmic contractions (at rest) - We can also have tonic contractions, which are sustained, non-rhythmic and segmental lat rest) 3 #is is PATHOLOGICAL Sympathetic vs Parasympathetic GI innervation The PS system allows the GI tract muscles to function, stimulates them, while the sympathetic system inhibits peristalsis and the contraction of muscles in these viscera. The neurotransmitter in the PS systems is Ach, while in the sympathetic it’s A and NA. Myenteric and Submucosal plexuses - Sympathetic fibres = arrive as postganglionic neurons PS fibres = arrive as preganglionic and will synapse at the walls of the organ. We can also find afferent fibres Similarities and differences between the plexuses: Location Functions: the myenteric plexus allows for motility (main function), while the submucosal plexus activates gland secretion + vasoconstriction or dilation of vessels in the wall, to increase or decrease blood supply Effects: The myenteric plexus: - Causes excitations (Ach) to increase tone, intensity of rhythmic contractions, frequency and speed of contraction in the muscularis externa - Causes relaxation (VIP, vasoactive intestinal peptide) of sphincters (pyloric) and the ileocecal valve, allowing the passage of the bolus ↑ RELAX INTERNAL ANAL SPHINCTER The submucosal plexus has a more local, region-specific effect, causing: - Contractions of submucosal muscle (= changes on the absorption surface) - Local secretion - Local absorption Types of movement Mixing contractions. Segmentation: these contractions are propulsive too, but of low efficiency. Their main purpose is mixing the contents in the chyme and spreading the chyme along the epithelial surface, in order to increase absorption. why MOVEMENT IS D - Because of the segmentation, the chyme is divided into different subchymes Due to alternative contractions, the chymes are now mixed. It allows us not to move the substances, but mix them Propulsive contractions, mediated by the PS nerves in the myenteric plexus. Here, there is an upstream contraction and downstream relaxation, which allows the bolus to advance. The mission of the propulsive contraction is to move the chyme or bolus further along the tube. - In the proximal tube, it advances rapidly: 0.5-2 cm/s - In the distal tubule, it slows down: 1 cm/minute It takes the bolus 3 to 5h to go from the pylorus to the ileocecal valve. CHEWING AND DEGLUTITION Oral phase VOLUNTARY Chewing or mastication. Chewing occurs thanks to: ○ Jaw elevators - Temporal - Masseter ○ Jaw depressors - Lateral and Medial Pterygoideus - Digastric INSERT - Mylohyoid ↑ - Geniohyoid (innervated by C1) Most are innervated by the 3rd branch of the trigeminal nerve (V3): Mandibular nerve cranios v3 verve CRANIAL NERVE V3 ? first Cervical Nerve > Homogenization. Another function, apart from the mechanical one of chewing, is the homogenization. Occurs thanks to: ○ Salivary glands are in charge of chemical homogenization through the secretion of enzymes such as salivary amylase. - Sublingual and Submandibular glands (VII CN, facial) - Parotid gland (IX cranial nerve, Glossopharyngeal. The facial nerve courses through the parotid gland but does NOT provide innervation. ○ Muscles of the tongue are in charge of mechanical homogenization i. Intrinsic lingual muscles. We find 4: - Superior longitudinal - Vertical - Transverse - Inferior longitudinal All innervated by the XII cranial nerve (hypoglossal) ii. Extrinsic lingual muscles. We also find 4: - Genioglossus INNERVATED BY HYPOGLOSSAL - Hyoglossus - Styloglossus - Geniohyoid 3 Taste. Sensory inputs. The tongue is also involved in sensation, both in general and special sensation. Special senses involve specialized sense organs (ex: taste (tongue), smell (nasal passages)). General senses are all associated with touch and lack special sense organs. Touch receptors are found throughout the body, but particularly in the skin. ○ Touch (general sensory). Somatic - Posterior ⅓ of the tongue → IX cranial nerve, Glossopharyngeal - Anterior ⅔ of the tongue → Depends on a branch of the mandibular nerve V3 (Lingual nerve) (come from the trigeminal) ○ Taste (special sensory) - Beginning of the pharynx → X cranial nerve, vagal branch (Internal laryngeal nerve) - Posterior ⅓ of the tongue → IX cranial nerve, Glossopharyngeal - Anterior ⅔ of the tongue → depends on a branch coming from the Facial nerve / CN VII (Chorda tympani) > - Pharyngeal phase Involuntary phase, unlike the oral phase; consists in the movement of the bolus from the oropharynx into the oesophagus. In this phase, events are carried out to fulfil 3 functions: 1. Coordination of the passage of the bolus. 2. Preventing food from entering the nasopharynx 3. Preventing food from entering the lungs 1. Pharyngeal phase I The nasopharynx is sealed off from the oropharynx and laryngopharynx by elevation of the soft palate and its uvula. In a healthy person, it’s very well coordinated. How does it work? Elevation of the uvula Elevation of the palate Tension of the palate This occurs through 2 muscles: Palate elevator (Levator veli palatini) and the Tensor veli palatini. Some of the nerves here provide sensory fibres (IX), others are motor (X and trigeminal nerve branch V3 (mandibular nerve)). - The reflex begins when the IX CN (glossopharyngeal) senses the presence of food touching the innervated structures - The info is relayed and nerves X and V3 will stimulate the mentioned muscles, which they innervate, to elevate and tense the palate + uvula -- 2. Pharyngeal phase II Elevation of the Pharynx Elevation of the Larynx Every time we perform deglutition, this occurs: the larynx moves up and also, anterior, to aid the closing of the epiglottis in the next phase. The pharynx moves up (therefore shortening) and widens its small diameter in order to receive the bolus. We can find 2 pharyngeal muscle groups: Constrictors/outer circular muscles: - Superior constrictor (X CN) - Middle constrictor (X CN) - Inferior constrictor (X CN) Elevators/inner longitudinal layer: - Stylopharyngeus → IX CN - Salpingopharyngeus → X CN - Palatopharyngeus → X CN Pharynx and larynx will elevate due to the contraction of the inner longitudinal layer of muscles (named according to their origin) The pharyngeal constrictors have no function in this phase. In the next stages, they will contract to push the bolus posterior, into the esophagus. Additionally, they will contract sequentially, generating a sort of peristaltism which’ll move the food into the esophagus. 3. Pharyngeal phase III Adduction of the vocal cords Deglutition apnea Epiglottis closure Elevation and anteversion of the larynx During this phase, respiration is inhibited (apnea), and the epiglottis closes and blocks off the upper airway to prevent the food bolus and liquids from entering the airway and being inhaled. If food does enter the airway, the coughing reflex is triggered. 4. Pharyngeal phase IV Pharyngeal peristalsis: propulsion of the bolus. Once the bolus has entered and everything is properly closed, it is important to push the bolus downstream, so it's propulsed. This occurs through the contraction of the outer circular muscles: superior, middle and inferior pharyngeal constrictors Relaxation of the upper esophageal sphincter (Cricopharyngeal muscle), in order to allow the passage of food ↳ if this contracts wrong time =- zenker's diverticulum Esophageal phase Like the previous phase, it’s also involuntary. The food bolus is forced inferiorly from the pharynx into the esophagus after the sequential contraction of the three pharyngeal constrictor muscles (pharyngeal peristalsis). Once the food bolus has fully entered the esophagus, the upper esophageal sphincter will close again. 1. Esophageal phase I The food bolus will move through the esophagus via peristalsis. Swallow-induced peristalsis is called primary peristalsis, and the peristalsis elicited by esophageal distention is called secondary peristalsis. Primary Esophageal peristalsis: it’s a continuation of the previous pharyngeal one. The passage of the bolus occurs in approximately 5-10 seconds. The esophagous has a striated muscle and a smooth muscle segment: - The striated muscle in the upper thirds, is innervated by the IX and X cranial nerves (vagal motor endings from the nucleus ambiguus). The CNS is required to activate both 1ary and 2ar peristalsis + control the sequential nature of contraction in these striated portions - In the inferior third, we find smooth muscle. Within it, the myenteric plexus, receiving stimuli from the X CN (vagus nerve) (Cell bodies of vagal efferent fibres that innervate the smooth muscle esophagus are largely in the dorsal motor nucleus). In this smooth muscle portion, it has been seen that the CNS is required for activation of primary peristalsis, BUT peristalsis can also occur independently from it, which highlights the importance of neuromuscular mechanisms intrinsic to the esophageal wall. There is central but also peripheral neurogenic control: peristalsis can be induced by local distention and electrical stimulation of an esophagus devoid of extrinsic innervation. Secondary Esophageal peristalsis is activated if the bolus cannot be moved refluxed esophageal /esophage) food through primary esophageal peristalsis. Activated through the firing of local mechanoreceptors, which detect the distension caused by the bolus. - Above the bolus level: circular layer contraction, longitudinal layer relaxation - Below the bolus level: circular layer relaxation, longitudinal layer contraction ↑ SOMATIC Efferent - VISCERAL EFFERENT when some / volus is left behind after the first peristalsis ware has passed when contents from stomach Neurotransmitters. 2ary peristalsis - Relaxation: VIP (vasoactive intestinal polypeptide) and NO (Nitric oxide) - Activators: Acetylcholine and substance P. Released to circular or longitudinal layers, depending on the level in relation to the bolus. 2. Esophageal phase II Relaxation and opening of the Lower esophageal sphincter (cardias) DIAPHRAGM has - COSTAL part : Cattaches to rita Crural - part /attaches to WHEN & vertebra Bott THEY CONTRACT , MOVE DOWNWARD HORACIC CAVITY = EXPANSION The esophagous descends and pierces the diaphragm at the level of the sphincter. The costal diaphragm and phreno-esophageal membrane are important in the closure of this gastro-esophageal sphincter (cardias), as there are no muscles in this sphincter. It’s not an anatomical sphincter, but a physiological one. Once the bolus has entered, the lower esophageal sphincter will close to prevent regurgitation of stomach contents therefore protecting the esophagus from acid reflux.