case 1 Flashcards
(156 cards)
layers of the GI tract
Serosa, longditudinal muscle, circular muscle (myenteric plexus), submucosa (meissners plexus), mucosa, epithelial lining.
• Electrical signals that initiate muscle contractions can travel readily from one fibre to the next within each bundle using gap junctions.
enteric nervous system
- The enteric nervous system is part of the autonomic nervous system that is found in the lining of the GI Tract, beginning at the oesophagus and extending down to the anus.
- It is involved in the coordination of reflexes (although it receives innervation by the autonomic nervous system, it can work independently of the brain and the spinal cord).
two main plexuses of the enteric nervous system
- A submucosal plexus/ Meissner’s plexus that lies in the submucosa - controls mainly gastrointestinal secretion and local blood flow.
- A myenteric plexus lying between the longitudinal and circular muscle layers - controls motility.
This also secretes vasoactive intestinal polypeptide. The resulting inhibitory signals are especially useful for inhibiting some of the intestinal sphincter muscles that impede movement of food.
neurons in the enteric nervous system
afferent neurons, interneurons and efferent neurons.
Sensory neurons report on mechanical and chemical conditions.
Through intestinal muscles, the motor neurones control peristalsis and churning of intestinal contents.
two types of movement in the GI tract
propulsive and mixing movements
Propulsive movements-peristalsis
stimulation cause contractile ring in circular muscle which spreads along gut. Stimulus-distention of gut. stretch stim ENS to contract behind it. also stim by irritation of epithelial lining. Parasymp NS to gut elicit peristalsis. requires an active myenteric plexus. starts orad side, moves toward segment, pushing in anal direction 5cm before dying out. there is receptive relaxation down stream allowing food propelled through.
4 stages of swallowing
- Cephalic Stage
- Oral Stage (Voluntary stage)
- Pharyngeal stage – involuntary and constitutes passage of food through the pharynx into the oesophagus
- Oesophageal stage – involuntary phase that transports food from pharynx to the stomach
cephalic stage
• This is the point where one is thinking about having a meal:
All of this is part of the process of which would induce the activity of swallowing
oral stage
- Chewing (mastication)
- Salivation – lubricate the bolus and begin the process of digestion (discussed later)
- Movement of bolus
The bolus is pushed against the hard palate.
The rugae on the hard palate help move the bolus posteriorly into the back of the mouth into the pharynx.
mastication
first step digestion, food ground by teeth, inc surface area food more efficient breakdown by enzymes, food positioned by cheek and tongue between teeth for grinding, 4 muscles-masseter temporalis, lat and medial pterygoid. innervated mandibular branch trigeminal. Stimulation of specific reticular areas in the brain stem taste centres will cause rhythmical chewing movements.
Also, stimulation of areas in the hypothalamus, amygdala, and even the cerebral cortex near the sensory areas for taste and smell can often cause chewing food made softer and warmer and process salivation begins.
teeth and mastication
- The anterior teeth (incisors) provide a strong cutting action.
- The posterior teeth (molars) provide a strong grinding action.
process of mastication - chewing reflex
presence of bolus in mouth initiates reflex inhibition of muscles of mastication so jaw drops, the drop makes stretch reflex rebound contraction. this compresses the bolus against lining of mouth making inhibition again to jaw drop, this causes break down of food, important for digestion carbs as have indigestible cellulose membranes.
pharyngeal stage
- After the voluntary stage, the bolus of food enters the posterior mouth and pharynx.
- Here, it stimulates the epithelial swallowing receptor areas all around the opening of the pharynx, especially on the tonsillar pillars.
- Impulses from these pass to the brain stem to initiate a series of automatic pharyngeal muscle contractions:
contractions of the soft patate and palatopharyngeal folds in the pharyngeal stage
1)The soft palate is pulled upward to close the posterior nares, to prevent the reflux of food into the nasal cavities (nasopharynx) 2)The palatopharyngeal folds on each side of the pharynx are pulled medially to approximate each other. In this way, the folds form a sagittal slit through which the food must pass into the posterior pharynx. This slit performs a selective action, allowing food that has been masticated sufficiently to pass with ease. Because this stage lasts less than 1 second, any large object is usually impeded too much to pass into the oesophagus.
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contractions of the vocal cords in the pharyngeal phase
3) The vocal cords of the larynx are closed, and the larynx is pulled upward and anteriorly by the neck muscles. These actions, combined with the presence of ligaments that prevent upward movement of the epiglottis, cause the epiglottis to swing backward over the opening of the larynx.
All these effects acting together prevent passage of food into the nose and trachea. Most essential is the tight approximation of the vocal cords, but the epiglottis helps to prevent food from ever getting as far as the vocal cords.
changes to the larynx in the pharyngeal phase
4) The upward movement of the larynx also pulls up and enlarges the opening to the oesophagus. At the same time, the upper 3-4cm of the oesophageal muscular wall, called the upper oesophageal sphincter (also called the pharyngoesophageal sphincter) relaxes, thus allowing food to move easily and freely from the posterior pharynx into the upper oesophagus. Between swallows, this sphincter remains strongly contracted, thereby preventing air from going into the oesophagus during respiration.
The upward movement of the larynx also lifts the glottis out of the main stream of food flow, so that the food mainly passes on each side of the epiglottis rather than over its surface; this adds still another protection against entry of food into the trachea.
changes to the pharynx in the pharyngeal phase
5) Once the larynx is raised and the pharyngoesophageal sphincter becomes relaxed, the entire muscular wall of the pharynx contracts, beginning in the superior part of the pharynx, then spreading downward over the middle and inferior pharyngeal areas, which propels the food by peristalsis into the oesophagus.
summary of changes in the pharyngeal phase
Trachea is closed.
Oesophagus is opened.
Fast peristaltic wave initiated by the nervous system of the pharynx forces the bolus of food into the upper oesophagus.
The entire process is less than 2 seconds
upper Esophageal sphincter
consists of the cricopharyngeus muscle, the adjacent inferior pharyngeal constrictor, and the proximal portion of the cervical oesophagus. innervated by vagus, innervation of musculature on UES to facilitate opening is 5,7,12 nerve. • The UES remains closed at rest owing to both its inherent elastic properties and neurogenically mediated contraction of the cricopharyngeus muscle. For the UES to open, it is important that the cricopharyngeus muscle has to relax. This occurs due to cessation of vagal excitation. UES opening is also aided by simultaneous contraction of the suprahyoid and geniohyoid muscles that pull open the UES + upward and forward displacement of the larynx + pulling forward of the hyoid bone
neurophysiology of swallowing
At the centre of the swallowing system is the Brainstem Central Programme Generator (CPG). This is mainly in the medulla and extends into the pons and houses a vast array of neurones and interneurons which link together to produce the activity of swallowing. The interneurons consist of excitatory and inhibitory ones.
There are also nuclei which include the dorsal vagal motor nucleus and the nucleus ambiguus alongside the CN nuclei of CN 5, 7, 9, 10 and 12. All of these CN combine together with the interneurons to allow the sequence to take place. The cortex communicates with the brainstem salivatory nuclei. These send signals via motor neurons to the muscles of swallowing. 26 pairs of muscles are required in the entire swallowing process
different types of receptors in swallowing
Sensory receptors in the oropharynx, larynx and oesophagus detect changes and send signals back to the brainstem and the cortex via CN 5, 7, 9, 10 and 12. Chemical receptors – stimulus (acid); response (feedback control)
Thermal receptors – stimulus (hot/cold); response (non-painful sensation) Mechanical receptors – stimulus (distention); response (burning/pain) – MOST POWERFUL INDUCERS OF A SWALLOW. These help mediate the swallowing response.
Swallowing is not a reflex, but a patterned response.
vagal and spinal afferents
send different types of sensations back into the system via the:
Nadose ganglion of the vagus afferents
Dorsal root ganglion of the spinal afferents
The vagus afferents then go via the thalamus and into the cortex via the medulla.
For spinal afferents, the system involved is the anterolateral system (spinithalamic tract) for nociception and mechanoreception.
cortical input in swallowing
- It has been shown through studies that there are many areas of the cortex that help regulate the swallowing response.
- Cortical dysfunction (i.e. in stroke for example) results in dysphagia.
brainstem and swallowing
• During the process of swallowing, the epiglottis closes off the larynx to prevent aspiration.
For this period swallowing period, respiration is stopped.
The salivatory nuclei are situated close to, if not exactly in the same location, as those nuclei controlling breathing.
They work together to carry out this intricate procedure during swallowing.