Gastro Flashcards
Length of GIT
~4.5 m in a living adult
Components of the GIT
- Mouth
- salivary glands
- esophagus
- liver
- stomach
- gallbladder
- pancreas
- large intestine
- small intestine
- rectum`
Gut Wall Structure (mid-esophagus to anus)
- Serosa
- Muscularis Externa
- Submucosa
- Mucosa
Serosa
thin, tough layer of connective tissue (continuous in places with abdominal mesentery)
Muscularis Externa
outer layer longitudinal fibres (when it contracts, the GIT shortens), inner layer circular fibres (when it contracts, lumen narrows); (musculature in oral cavity, pharynx, upper 1/3 esophagus and external anal sphincter is striated; the rest is smooth)
Submucosa
loose connective tissue, housing neuronal network, lymphatics, blood vessels
Mucosa
- muscularis mucosae (sm. muscle)
- lamina propria (loose connective tissue)
- epithelial layer (secretory - exocrine and endocrine - and absorptive cells).
3 activities of the GIT:
- Motility(muscular activity) –> Propulsion and Physical Breakdown
- Secretion(glandular activity - exocrine and endocrine) –> chemical breakdown
- Absorption –> transfer to blood circulation
Endocrine
released in the blood stream, carried throughout the body
Exocrine
released substances onto the tissue to act locally.
GIT Digestive/ Absorptive Capacity (exquisitely efficient)
Carbs - 99%
Fat - 95%
Protein - 92%
Enteric Nervous System (ENS)
- its neurons reside within the wall of the GIT
- is an addition to the automatic nervous system
- independent, integrative nervous system
- initiates, regulates, programs, coordinates (activities of muscular and secretory and absorptive elements)
Plexus
- collections of neural cell bodies
- they contain all the elements needed for reflex: sensory neurons, effector neurons, and interneurons
Sensory Neurons
w/ receptors in mucosa or muscle… stretch receptors, chemoreceptors, osmoreceptors
Effector Neurons
activate the secretory and muscle cells
Interneurons
expand the responses to stimuli in the GIT
Submucosal and Myenteric Plexus
Though anatomically distinct, the 2 plexuses behave as ONE functional unit.
- Myenteric has all neurons (between the longitudinal and circular muscle layers)
- Submucosal has only sensory and effector
Acetylcholine
Excitatory Neuron transmitter. Can be blocked by atropine
Non-Adrenergic and Non-Cholinergic (NANC)
Inhibitory transmitters
Ultimate activity (or what happens to the cell)
algebraic sum of influences from the nerves.
Short, Enteric Reflexes
-occur in the GIT wall
e.g.
Stimulus –> chemoreceptors, osmoreceptors, mechanoreceptors –> nerve plexus –> smooth muscle or gland cell –> response.
Parasympathetic(preganglionic)
- Nicotinic excitatory
- Signal comes straight from the CNS and synapsing with the ENS
- Parasympathetic neurons reach the wall of the GIT as preganglionic fibres, synapsing (via nicotonic ACh receptors) with enteric neurons (both excitatory and inhibitory) exerting an excitatory effect.
Sympathetic(post ganglionic)
- NA inhibitory
- ganglia outside the CNS and GIT wal. Will synapse outside the wall, before they send a signal to the GIT, which will also affect the ENS.
- Sympathetic neurons reach the wall of the GIT as postganglionic fibres, synapsing (via NA receptors) with enteric neurons (both excitatory and inhibitory) exerting an inhibitory effect
- also innervates smooth muscle in blood vessels in the wall, causing vasoconstriction
Autonomic Innervation of GUT wall
- modulates the ENS activity via Long, Extrinsic reflexes -synaptic connections with ENS neurons
Long Reflexes
- Stimulus is detected by the receptors. This info is sent to the nerve plexus. At the nerve plexus the algebraic sum will be compared and processed to determine to ultimate effect on the smooth muscle or gland. Whole process is called a short enteric reflexes. Happening within the wall of the GIT.
- Sensory afferent going up to the CNS sending information about the state of stretch or chemical composition of the contents.
- Sight, smell, taste of food, and emotional state affect the CNS
- Messages sent to nerve plexus via efferent autonomic neurons(Para and symp nerves)
- then smooth muscle or gland to make a response.
Hormonal Regulation
a. Non-GIT hormones may influence growth and development of GIT
b. GIT hormones may influence activities outside the GIT
c. GIT hormones regulate activities inside the GIT
Ghrelin
-produced by cells in the stomach. will travel in the blood stream to the hypothalamic feeding center to stimulate appetite
Leptin
- released by fat cells with overeating.
- induces satiety
- decreases appetite
Diffuse Endocrine System (DES)
- The regulation of gut activity done by endocrine cells, that are spread throughout the gastro intestinal system wall, in the epithelial layer
- largest, most diversified endocrine system in the body
- scattered among other cells in mucosa
5 most important GI hormones
- gastrin
- cck
- secretin
- GIP
- VIP
Paracrine
release of hormone by endocrine cell to neighboring cell
Movement of Hormones from mucosa
- Released from mucosa into portal blood –> liver –> heart –> systemic circulation –> target cell
- Have multiple targets (excitatory or inhibitory)
- Interact with one another and with neurotransmitters
a. Synergistically (they potentiate one another)
b. Antagonistically (inhibit
Peptide Hormones working synergistically
Peptide hormones work with one another or with other transmitters. This effect can either increase or potentiate one another
Peptide Hormones working antngonistically
Combination of hormones and neurotransmitters can inhibit the response.
Propulsion (flow) in the GIT
- Gradients of Pressure Delta P
a. Coordinated contractions of muscular elements in wall of GIT - Variations in Resistance 1/R
a. Normally, little/no resistance
Segmentation
alternation rings of contraction. Most efficient way of mixing food
Peristalsis
- a wave of contraction moving over the wall of the organ, narrowing the lumen and setting up a gradient of pressure favoring aboral movement
- each swallow, a single primary peristaltic wave is generated.
- takes 8-10 secs to be propagated the length of the esophagus
- primary peristalsis is part of the deglutition reflexes.
Phases of Deglutition (swallowing)
- Oral
- Pharyngeal
- Esophageal
Oral Phase of Swallowing
- transport from anterior mouth to pharynx. This involves a series of reflexes coordinated in the Deglutition centre in the medulla oblongata
1. Ability to initiate: voluntary –> cortex
2. Coordinated Movements: reflex = involuntary –> medulla
Pharyngeal Phase of Swallowing
- Involuntary*
- Rapid
- “stereotyped” (all muscles have to work together and at the right time
- Temorospatial Coordination
1. Passage to nose, mouth, and trachea blocked
2. Apnea
3. Upper Esophageal Sphincter (UES) relaxes
4. Pharynx muscles contract
Involuntary Control of Pharyngeal Phase consists of:
- a series of protective relfexes, initiated by stimulation of afferent fibres in the pharynx, organized in Deglutition Centre, closing off nasal, oral, and laryngeal cavities, preventing misdirection of the bolus. Simultaneously, respiration is briefly inhibited.
b. transfer to esophagus, as pharyngeal muscles contract and Upper Esophageal Sphincter relaxes.
Deglutition Reflexes in the Pharyngeal Phase
- Pharyngeal Receptors send afferent to Deglutition centre (medulla)
- efferents from Dg cente go to protective reactions, deglutition apnea, contract pharyngeal constrictor muscles, relax UES.
Upper Esophageal (UES) Movement
Closer - impulses originate in the CNS mediated by the vagus nerve, releasing ACh, causing muscle contraction
Relaxation - cessation of impulses results in muscle relaxation.
*if vagus nerve cut, then no ACh sent to excite muscle/contract
Esophageal Forces
- Gravity - minor importance, mostly for liquids
2. Peristalsis - esophagus contents carried toward stomach
Peristalsis further shit
- Pharyngeal receptors send sensory afferents to the Dg centre. From there the DC either go through the vagus somatic fibers which affect the striated muscles. Or the vagus autonomic fibres join with the enteric system in the more distal part of the esphagus
- Sequential innervation, proximal to distal activation. Wave of activity in striated muscle
- Synchronous activation increased latency in distal esophagus (wave of activity in smooth muscle)
Vagus Somatic Fibres
Sequential activation of the vagus somatic fibres, to push bolus down
Vagus Autonomic Fibres
Vagus autonomic fibres send signals to the enteric system, but at different speeds, so the muscles contract sequentially, creating a peristalsis wave.
When you cut high up in the neck is disconnect the Deglutition Centre
No primary peristalsis
When you cut transthoracically (or just the Vagus autonomic fibres)
Primary peristalsis continues. Distal esophagus has the enteric circuitry necessary for the propagation of the peristaltic wave.
In striated portion of the Esophagus
peristalsis results from the sequential firing of vagal motor neurons, activating progressively more distal regions of musculature
In the smooth muscle portion of the esophagus
once some of the enteric neurons have been activated, they are capable of relaying and activating other enteric neurons (in an orderly fashion), evoking and propagating muscular contraction in the aboral direction, independently of the extrinsic nerves. Thus, the integrity of the enteric innervation is critical to the propagation of the peristaltic wave in the distal esophagus.
Vagus is essential for initiating what?
peristalsis in the proximal esophagus
Intact ENS is essential for?
Continuing and propagating peristalsis in distal esophagus
What happens when the bolus gets stuck in the esophagus
- Secondary peristalsis
- initiated by local distension
- may be mediated by enteric reflexes (short,local) or by long(vagal) reflexes
- Several secondary peristaltic waves may be generated, until the bolus has been displaced.
Lower Esophageal Sphincter (LES)
- muscle is closed at rest. Closed without any innervation. Called myogenic closure because it is mediated by the muscle itself.
- opening of the muscle is called relaxation neurogenic. There needs to be innervation for the muscle to open. Local ENS will release NANC (-) or vagal stimulation will activate inhibitory ENS neurons.
Haitus Hernia
If the LES is entirley displaced into the thorax, intraabdominal pressure increases, but does not increase LES pressure
LES Characteristic
- Intrinsic Physiologic Sphincter - tonically contracted in absence of swallowing
- Anti-reflux mechanism assisted by presence of an INTRAABDOMINAL SEGMENT
Incompetent LES - Sphincter fails to close
“Heartburn” (pyrosis): burning sensation, radiating upwards in the chest towards the neck, due to acid relfux into the esophagus.
Motor functions of the Stomach
- Temporary Storage: 1-2 L
- Physical Disruption and Mixing of Contents: Semi-liquid consistency = chyme
- Propulsion into duodenum : regulated (very important to be highly regulated. This is where most of the absorption occurs).
Components of the Stomach
Proximal - thin walled, for storage
Distal - thick walled, for mixing and propulsion
