[MOD10A - PHYSIO] GIT1: Motility_2028

What is the main function of the gastrointestinal tract?

Provides the body with water, electrolytes, vitamins, and nutrients.

Which processes are required for the GI tract to function?

Movement of food, secretion of digestive juices, digestion, absorption, circulation of blood, and control by nervous and hormonal systems.

What is the function of the esophagus?

Simple passage of food.

What is the function of the stomach?

Temporary food storage.

What is the function of the small intestine?

Digestion and absorption.

True or False: The GI tract is a continuous one-way tract with no alternate routes.

True.

What maintains the one-way movement of food in the GI tract?

Peristaltic movement and tonic contraction of the intestinal walls.

True or False: The peristaltic movement and tonic contraction prevent backflow of material even against gravity.

True.

List the layers of the intestinal wall from outermost to innermost.

Serosa, longitudinal smooth muscle, circular smooth muscle, submucosa, mucosa.

What is the deeper layer of the mucosa called?

Muscularis mucosa or mucosal muscle.

What is the length of individual smooth muscle cells in the GI tract?

200 to 500 micrometers.

What is the diameter of individual smooth muscle cells in the GI tract?

2 to 10 micrometers.

How are smooth muscle cells arranged in the GI tract?

In bundles of up to 1000 fibers.

What electrically connects the smooth muscle bundles in the GI tract?

Gap junctions.

What is the function of gap junctions in GI smooth muscle?

Allow low resistance movement of ions from one muscle to another.

True or False: Electrical signals travel rapidly along the length of smooth muscle fibers.

True.

What structure do smooth muscle bundles form when they fuse together?

A branching lattice work.

What does it mean when the GI muscle is described as a syncytium?

An action potential travels in all directions in the muscle.

True or False: Each smooth muscle cell in the GI tract has multiple nuclei.

False. It is a single cell body with multiple nuclei.

How does an action potential travel in GI smooth muscle?

It spreads in all directions due to the syncytial nature of the muscle layer.

What are the two basic types of electrical activity in GI smooth muscle?

Slow waves and spike potentials.

What do slow waves represent?

Rhythmic GI contractions.

True or False: Slow waves are action potentials.

False. They are slow undulating changes in resting membrane potential.

What is the range of slow wave frequency in the stomach?

3 per minute.

What is the range of slow wave frequency in the duodenum?

12 per minute.

What is the range of slow wave frequency in the terminal ileum?

8-9 per minute.

What is the possible cause of slow waves in GI smooth muscle?

Interstitial cells of Cajal, which act as pacemakers.

True or False: Slow waves directly cause muscle contraction throughout the GI tract.

False. They do not cause contraction by themselves, except in the stomach.

How do slow waves contribute to muscle contraction?

They excite the appearance of intermittent spike potentials, which trigger contraction.

What is the nature of spike potentials?

They are true action potentials.

What is the threshold for spike potentials to occur?

When the resting membrane potential becomes more positive than -40 mV.

What is the normal resting membrane potential of GI smooth muscle?

-50 to -60 mV.

When do spike potentials appear in relation to slow waves?

When slow waves temporarily rise above -40 mV.

What happens when slow waves have higher amplitude?

They result in a greater spike frequency.

What ion channels are responsible for GI smooth muscle action potentials?

Calcium-Sodium Channels.

What ions enter the cell during a GI smooth muscle action potential?

A large number of calcium ions and a smaller number of sodium ions.

What is the normal resting membrane potential of GI smooth muscle?

-56 millivolts.

What happens during depolarization?

The membrane potential becomes less negative, making muscle fibers more excitable.

What factors contribute to depolarization of GI smooth muscle?

Stretching of the muscle, acetylcholine (Ach) from parasympathetic nerves, and specific GI hormones.

What happens during hyperpolarization?

The membrane potential becomes more negative, making the muscle fibers less excitable.

What factors contribute to hyperpolarization of GI smooth muscle?

Norepinephrine, epinephrine, and sympathetic nerve stimulation.

What ion is primarily responsible for GI smooth muscle contraction?

Calcium ions.

How does calcium cause smooth muscle contraction?

It triggers attractive forces between myosin and actin filaments.

True or False: Slow waves directly cause calcium entry and muscle contraction.

False. Slow waves do not cause calcium entry and thus do not cause contraction.

What type of potential leads to significant calcium entry and contraction?

Spike potentials.

What is tonic contraction in GI smooth muscle?

A continuous contraction not associated with slow waves, often caused by repetitive spike potentials.

What is the primary cause of tonic contraction?

Continuous calcium entry into the cell that is not associated with membrane potential changes.

When does the Migrating Motor Complex (MMC) occur?

During fasting, between periods of digestion.

How does GI electrical and motor activity change during fasting?

It follows cycles of motor activity that migrate from the stomach to the distal ileum.

What is the name of the cycles of motor activity that occur during fasting?

Migrating Motor Complex (MMC).

What are the three phases of the MMC?

Phase I: Quiescent period, Phase II: Irregular electrical and mechanical activity, Phase III: Burst of regular activity.

What hormone initiates the MMC?

Motilin.

What secretions increase during the MMC?

Gastric, bile, and pancreatic secretions.

What is the purpose of the MMC?

To clear the stomach and small intestine of luminal contents in preparation for the next meal.

What are the two main plexuses of the enteric nervous system?

Myenteric (Auerbach's) plexus and Submucosal (Meissner's) plexus.

What is another name for the Myenteric plexus?

Auerbach's plexus or Outer plexus.

Where is the Myenteric plexus located?

Between the longitudinal and circular muscle layers.

How far does the Myenteric plexus extend?

The entire length of the GIT.

What are the effects of stimulating the Myenteric plexus?

Increases tonic contraction (gut tone), rhythmic intensity, rate of contraction rhythm, and conduction velocity for rapid peristalsis.

True or False: The Myenteric plexus is entirely excitatory.

False. Some of its neurons are inhibitory.

What inhibitory neurotransmitter is released by the Myenteric plexus?

Vasoactive Intestinal Peptide (VIP).

Why are inhibitory signals from the Myenteric plexus important?

They regulate sphincter muscles.

What is another name for the Submucosal plexus?

Meissner's plexus or Inner plexus.

Where is the Submucosal plexus located?

In the submucosa.

What does the Submucosal plexus mainly control?

Secretion and blood flow.

What are the three local functions controlled by the Submucosal plexus?

Local secretion, local absorption, and local contraction of submucosal muscle.

Where do sensory nerve endings from the GI epithelium send signals?

To the prevertebral ganglia of the sympathetic nervous system, spinal cord, and brainstem.

What are the two main types of neurotransmitters in the enteric nervous system?

Excitatory (Acetylcholine) and Inhibitory (Norepinephrine & Epinephrine).

What is the effect of parasympathetic stimulation on the enteric nervous system?

Increases GI activity.

What is the origin of the sacral parasympathetic nerves supplying the distal large intestine and anus?

S2-S4.

What is the effect of sympathetic stimulation on the GI tract?

Inhibits GI activity.

Where do sympathetic fibers that control the GI tract originate?

T5 to L2.

Through which ganglia do sympathetic fibers pass before reaching the GI tract?

Celiac and mesenteric ganglia.

How does sympathetic innervation compare to parasympathetic innervation of the GI tract?

Sympathetic innervation extends throughout the GI tract, while parasympathetic innervation is more concentrated near the oral cavity and anus.

What neurotransmitter is primarily secreted by sympathetic nerve endings in the GI tract?

Norepinephrine.

What stimulates afferent nerve fibers in the GI tract?

Irritation of the mucosa, gut distension, and specific chemical substances.

What is the effect of afferent nerve fiber stimulation in the gut?

It can cause excitation or inhibition of movements/secretion.

What are the three types of reflexes essential to GI control?

Gut wall reflexes, Gut to prevertebral sympathetic reflexes, and Gut to spinal cord/brainstem reflexes.

What do gut wall reflexes control?

Secretion, peristalsis, and mixing.

What is the function of the gut to prevertebral sympathetic reflexes?

They transmit signals over long distances.

What is an example of a gut to prevertebral sympathetic reflex?

Gastrocolic reflex (signals from the stomach cause evacuation of the colon).

What is an example of a gut to spinal cord/brainstem reflex?

Defecation reflex.

How do GI hormones exert their effects?

They are released into the portal circulation and act on target cells.

True or False: The effects of GI hormones continue even if nervous connections are severed.

True.

Where is Gastrin secreted, and what is its function?

Secreted by G cells in the antrum; stimulates gastric acid secretion and mucosal growth.

Where is Cholecystokinin (CCK) secreted, and what does it do?

Secreted by I cells in the duodenal and jejunal mucosa; contracts the gallbladder to expel bile and slows gastric emptying.

Where is Secretin secreted, and what is its function?

Secreted by S cells in the duodenum; promotes pancreatic secretion of bicarbonate.

What is the function of Glucose-dependent insulinotropic peptide (GIP)?

Decreases stomach motor activity.

Where is Motilin secreted, and what does it do?

Secreted by the stomach and upper duodenum during fasting; increases GI motility and stimulates interdigestive myoelectric complexes every 90 minutes.

What is the function of Cholecystokinin (CCK)?

Induces the secretion of both bile (via contraction of gallbladder) and pancreatic contents.

What is the function of Secretin?

Induces the secretion of bicarbonate and pepsin.

What is the basic propulsive movement of the GIT?

Peristalsis.

What is peristalsis?

A contractile ring that appears in the circular muscle and spreads along the gut tube.

What is the usual stimulus for peristalsis?

Gut distention.

What happens when the gut is distended?

The gut contracts 2-3 cm proximally, initiating peristalsis.

In which direction do peristaltic waves move?

Towards the anus.

What is the term for peristaltic movement directed towards the anus?

Law of the gut.

What is receptive relaxation?

The relaxation of the gut several cm distally as peristalsis is initiated.

What happens in the absence of the myenteric reflex?

Peristalsis and receptive relaxation are greatly repressed.

What is the main cause of mixing in the intestines when blocked by a sphincter?

Peristaltic contractions.