57 Questions
What is the role of the gastrointestinal (GI) system in maintaining homeostasis?
Regulating the internal environment and providing essential elements for cells
What essential elements do cells need from the internal environment?
Nutrients and oxygen
What is the main function of the enteric nervous system in gastrointestinal function?
Regulating local GI activity independently of the central nervous system
What are the types of neurotransmitters secreted by enteric neurons?
$ ext{Acetylcholine and serotonin}$
What controls gastrointestinal motility?
$ ext{Hormones, neural signals, and local factors}$
"Splanchnic circulation" refers to:
$ ext{Blood flow to the abdominal organs}$
What event led to foundational discoveries about digestion?
A shotgun wound creating a window to the stomach
What is the primary function of the GI system?
Break down complex food into smaller, absorbable molecules
Where does digestion primarily occur?
Small intestine and large intestine
What are fats broken down into?
Monoglycerides and free fatty acids
Which organ produces bile for fat digestion?
Liver
What is the primary site of nutrient absorption?
Small intestine
What is the function of sphincters in the GI tract?
Control the movement of food between regions of the GI tract
Which organ secretes digestive enzymes into the duodenum?
Pancreas
Which type of muscle has spindle-shaped, non-striated fibers and is involuntary?
Smooth muscle
What causes contraction in phasic smooth muscles?
Action potential triggering an increase in cytosolic Ca$^{2+}$
Which type of smooth muscle contracts in bursts 'intermittently'?
Phasic smooth muscle
What is the role of myosin light-chain kinase (MLCK) in smooth muscle contraction?
Phosphorylates myosin head increasing the activity of ATPase
Which type of smooth muscle contracts as a single unit and is composed of many smooth muscle fibers that become excited and contract together?
Single-unit smooth muscle
What prevents myosin from binding to actin at rest in smooth muscles?
Myosin light chain (MLC)
Which type of muscle contraction is initiated by self-excitable cells called pacemakers?
Myogenic
What characterizes the electrical activity of smooth muscles in the GI tract?
Oscillating repolarization and depolarization
What is the function of longitudinal muscles in the GI tract?
Shorten the segment of the intestine and expand the lumen
What makes myogenic contractions more important in the GI tract?
Presence of intracellular Ca2+ release and more gap junctions
What ion is responsible for the slow waves in smooth muscle?
$Na^+$ influx
What are Interstitial cells of Cajal (ICC) specialized non-contractile cells known for?
Generating slow wave potentials
What type of muscle contraction can be initiated in response to nerve signals or intrinsically within the muscle itself?
Neurogenic
What characterizes the electrical activity of smooth muscles in the GI tract?
Oscillating depolarization and repolarization
What is the function of circular muscles in the GI tract?
Reduce the diameter of the lumen and increase its length
Which process of the GI system involves the movement of food through the GI tract?
Motility
What is the primary site of absorption in the GI system?
Small intestine
Which organ is responsible for initiating protein digestion?
Stomach
What are fats broken down into during digestion?
Monoglycerides and free fatty acids
What is the function of the pancreas in the GI system?
Secreting digestive enzymes into the duodenum
What characterizes the primary function of the large intestine in the GI system?
Reabsorbing electrolytes and fluid
What are accessory organs of the GI system responsible for?
Producing secretions to aid in digestion
What is the primary ion responsible for the slow waves in smooth muscle?
Na$^+$
What characterizes the electrical activity of smooth muscles in the GI tract?
Oscillating slow waves
What is the role of the Enteric Nervous System (ENS) in smooth muscle function?
Coordinating excitatory and inhibitory motor neurons
What type of muscle contraction can be initiated by self-excitable cells called pacemakers?
Myogenic contraction
What characterizes the function of longitudinal muscles in the GI tract?
They shorten the segment of the intestine and expand the lumen.
What is the function of circular muscles in the GI tract?
They reduce the diameter of the lumen and increase its length.
What type of muscle contraction is more important in the GI tract due to intracellular Ca$^{2+}$ release and more gap junctions?
Myogenic contraction
What specialized non-contractile cells act as pacemakers and generate action potentials in smooth muscles?
Interstitial cells of Cajal (ICC)
In what way does smooth muscle in the GI tract function during contraction?
As a syncytium acting as a single unit
Which layer of the GI wall is responsible for shortening the segment?
Muscularis
What type of smooth muscle contraction is associated with continuous entry of Ca+2 ions?
Tonic
Which type of smooth muscle is composed of discrete, separate muscle fibers that operate independently?
Single-unit
Which part of the GI tract contains phasic smooth muscles?
Stomach
What characterizes the electrical activity of smooth muscles in blood vessels and airways?
Tonic contractions
What is the primary role of myosin light-chain kinase (MLCK) in smooth muscle contraction?
Initiating cross-bridge cycling between actin and myosin
What is the role of slow waves in gastrointestinal smooth muscle?
Generating rhythmic contractions
What is the primary function of the myenteric plexus in the enteric nervous system?
Regulating gut motility
What is the main function of the hormone gastrin in gastrointestinal motility?
Stimulating gastric acid secretion
What is the effect of sympathetic nervous system activation on gastrointestinal smooth muscle?
Inhibition of motility and secretion
Which type of movement in the GI tract involves mixing and churning actions to facilitate digestion and absorption?
Segmentation
What is the primary function of the large intestine in the GI system?
Absorption of water and electrolytes
Study Notes
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Smooth muscles in the body can be stimulated to contract even if the stimulus arrives at only one muscle, as the signal will spread to the rest through gap junctions.
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Muscles like the ciliary muscle and iris of the eye, and the piloerector muscle, are examples of this. Other muscles include those in the uterus and GI tract.
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Muscle contraction can be initiated in response to nerve signals (neurogenic) or intrinsically within the muscle itself (myogenic).
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Myogenic contraction is initiated by self-excitable cells called pacemakers that generate an action potential (AP) regardless of external innervation.
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In the case of smooth muscles in the GI tract, they are divided into two types based on their function: longitudinal and circular.
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Longitudinal muscles shorten the segment of the intestine and expands the lumen, while circular muscles reduce the diameter of the lumen and increase its length.
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The presence of intracellular Ca2+ release and more gap junctions makes myogenic contractions more important in the GI tract.
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The Enteric Nervous System (ENS) is important for the functions of smooth muscles in the GI tract as it contains excitatory and inhibitory motor neurons.
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Smooth muscle in the GI tract functions as a syncytium, meaning it acts as a single unit when contracting.
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The electrical activity of smooth muscles in the GI tract is characterized by slow waves and spike potentials.
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Slow waves are oscillating depolarization and repolarization in the resting membrane potential of the muscle, but they do not generate contractions on their own.
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The frequency and amplitude of slow waves vary between different parts of the GI tract.
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The RMP value of smooth muscle is less stable than that of skeletal or cardiac muscle and is characterized by spontaneous gradual alternating hyperpolarizing and depolarizing swings in potential, called slow wave potentials.
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The ion responsible for the slow waves in smooth muscle is Na+ influx, not Ca2+.
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The Interstitial cells of Cajal (ICC) are specialized non-contractile cells that act as pacemakers and generate APs in smooth muscles.
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ICCs are abundant in the myenteric plexuses and form a network with each other and smooth muscle cells, allowing the GI system to function normally even when disconnected from the CNS.
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The GI wall has five layers: serosa, muscularis, submucosa, mucosa, and mucosal muscle.
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The muscularis layer consists of two types of muscle: longitudinal and circular.
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Longitudinal muscle layers shorten the segment, while circular muscles constrict and decrease the lumen's diameter.
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The mucosa contains smooth muscle fibers and sparse bundles of smooth muscle fibers.
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Smooth muscles are non-striated, spindle-shaped, and have smaller and shorter fibers than skeletal muscles.
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Smooth muscles use cross-bridge cycling between actin and myosin to develop force and calcium ions to initiate contraction.
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Smooth muscles can be classified as phasic (contracts in bursts) or tonic (continuously contracted) depending on their contractile activity and how cytosolic Ca+2 increases.
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Tonic contractions can be caused by repetitive spike potentials, hormones, or continuous entry of Ca+2 ions and are not associated with slow waves.
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Single-unit smooth muscles are composed of discrete, separate muscle fibers that operate independently and do not contract in response to stretch or without neural input.
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Stomach and intestine smooth muscles are classified as multi-unit and tonic, and they contract spontaneously in response to stretch in the absence of neural or hormonal influence.
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The lower esophageal sphincter is the only smooth muscle in the esophagus and is responsible for preventing reflux of stomach contents into the esophagus.
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The GI tract, gastric antrum, small intestine, and esophagus are examples of phasic smooth muscles.
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Blood vessels and airways are examples of tonic smooth muscles.
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Muscles are usually partially contracted at all times and only relax when there's an inhibitory signal coming from the nervous system.
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Smooth muscles also play a role in blood pressure regulation and are connected to each other by gap junctions.
Test your knowledge of the anatomy and physiology of the gastrointestinal (GI) tract, including the sphincters and layers of the GI wall. This quiz covers the control of secretion from accessory organs, the sphincter of Oddi, and the four main layers or five layers of the GI wall.
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