1 Large intestine & Gastrointestinal motility

What type of muscle is primarily responsible for gastrointestinal (GI) motility?

• Mainly smooth muscle

• Includes circular, longitudinal layers and muscularis mucosa

Where is skeletal muscle involved in gastrointestinal motility?

• Mouth

• Pharynx

• Upper oesophagus

• External anal sphincter

What happens during longitudinal muscle contraction in the GI tract?

• Lumen becomes shorter

• Lumen becomes fatter

What happens during circular muscle contraction in the GI tract?

• Lumen becomes narrower

• Lumen becomes longer

What is the function of muscularis mucosae contraction?

• Changes absorptive area of mucosa

• Alters secretory surface of mucosa

Which muscle layers contribute to the mechanical actions of the GI tract?

• Circular smooth muscle

• Longitudinal smooth muscle

• Muscularis mucosa

Where are the neurons of the enteric nervous system (ENS) located?

• Cell bodies in ganglia

• Found in myenteric (Auerbach’s) and submucous (Meissner’s) plexuses

• Interganglionic fibers connect the two plexuses

What is the enteric nervous system?

• Entirely located within GI tissue (intrinsic)

• reflex circuit that operates independently of rest of nervous system. is strongly modulated by hormones and extrinsic nerve input

What are the sensory components of the ENS?

• Sensory neurons

• Mechanoreceptors (detect stretch)

• Chemoreceptors (detect chemical environment)

• Thermoreceptors (detect temperature)

What is the role of interneurons in the ENS?

• Coordinate reflexes

• Organize motor programs

• Act as relay between sensory and motor neurons

What is the function of effector neurons in the ENS?

• Excitatory motor neurons

• Inhibitory motor neurons

What is the enteric nervous system comprised of?

• sensory neurones

• intemeurons

• effector neurons

Where do parasympathetic preganglionic fibres synapse(connect)  in the GI tract? 5 steps

• Preganglionic fibres come from the brainstem or spinal cord

• They travel to the GI tract and synapse with postganglionic neurons

• These postganglionic neurons are part of the enteric nervous system (ENS)

• ENS acts as a local control center for gut function

• Acetylcholine (ACh) is released at the synapse to activate postganglionic neurons

What neurotransmitter is released by parasympathetic preganglionic fibres?

Acetylcholine (ACh)

What are the excitatory effects of parasympathetic input on the GI tract?

• Increased gastric secretion

• Increased pancreatic secretion

• Increased small intestinal secretion

• Enhanced blood flow

• Increased smooth muscle contraction

What are the inhibitory effects of parasympathetic input on the GI tract?

• Relaxation of some sphincters

• Receptive relaxation of the stomach

What does the sympathetic nervous system do in the gastrointestinal (GI) tract?

• Step 1: Preganglionic fibres originate in the spinal cord.

• Step 2: These fibres release acetylcholine (ACh) at the synapse in prevertebral ganglia (outside the spinal cord).

• Step 3: Postganglionic fibres begin in the prevertebral ganglia.

• Step 4: These postganglionic fibres release noradrenaline (NA).

• Step 5: Noradrenaline acts mainly on enteric neurons (within the ENS), not directly on muscle.

What type of activity does smooth muscle in the GI tract exhibit?

• Capable of spontaneous electrical activity

• Also shows spontaneous contractile activity

How are adjacent smooth muscle cells in the GI tract connected?

• Connected by gap junctions

• Allow spread of electrical currents from cell to cell

• Forms a functional syncytium=Hundreds of cells contract at the same time

What modulates the spontaneous activity of GI smooth muscle?

• Intrinsic nervous system (enteric nervous system)

• Extrinsic nervous system (autonomic nervous system)

• Hormones

What are slow waves in GI smooth muscle?

• Rhythmic patterns of membrane depolarization and repolarization

• Spread through cells via gap junctions

• Set the basic rhythm of contractions

What drives slow wave electrical activity in the GI tract?

• Driven by interstitial cells of Cajal (ICCs)

• ICCs act as pacemaker cells

When does smooth muscle contraction occur in relation to slow waves?

• Contraction only if slow wave amplitude triggers action potentials

• Action potentials are mainly through voltage-activated Ca²⁺ channels

• Contraction force depends on the number of action potentials fired

How do ICCs communicate with smooth muscle cells?

• ICCs form gap junctions with each other

• Also form gap junctions with smooth muscle cells

Where are the interstitial cells of Cajal (ICCs) located?

• Between the longitudinal and circular muscle layers

• Also in the submucosa

What determines the Basic Electrical Rhythm (BER) in the GI tract?

• BER is determined by slow waves

• Not all slow waves trigger contraction

What factors influence whether slow wave amplitude reaches the threshold for contraction?

• Neuronal stimuli

• Hormonal stimuli

• Mechanical stimuli

How does the frequency of the Basic Electrical Rhythm (BER) vary along the GI tract?

• Stomach: ~3 slow waves per minute

• Small intestine:

  • Duodenum: ~1–12 waves per minute

  • Terminal ileum: ~8 waves per minute

• Large intestine:

  • Favors retention of contents to aid water and electrolyte absorption

What is the functional significance of the BER frequency differences along the GI tract?

• Small intestine waves tend to drive luminal contents in the aboral direction (away from the mouth)

• Large intestine waves favor retention of luminal contents, facilitating absorption of water and electrolytes

What is peristalsis in the GI tract? 4

• A series of wave-like muscle contractions

• Moves food through the digestive tract

• Triggered by distension of the gut wall

• Involves contraction of longitudinal muscle layers

What is tonic contraction in the GI tract?

• Sustained contractions

• Found in the sphincters of the GI tract

What is segmentation in the GI tract?

• Rhythmic contractions of the circular muscle layer

• Function to mix and divide luminal contents

3 major motility patterns in the GI

1 peristalsis
2 tonic contraction
3 segmentation