general structure of the gut wall of the DS

What are the 4 layers of the alimentary canal?

• Mucosa

• Submucosa

• Muscularis externa

• Serosa/Adventitia

What is the innermost layer of the digestive tract/alimentary canal?

Mucosa

What does the mucosa (mucous membrane) consist of?

• An epithelium

• Lamina propria

• Muscularis mucosae

What is the mucosal epithelium histology? Mucosal

• Mucosal epithelium may be simple or stratified

• Stratified squamous epithelium is generally protective

• Simple columnar epithelium is rich in mucus-secreting cells

What is the structure of the lamina propria?

Highly vascular loose connective tissue

Rich in lymph vessels

Contains dispersed lymphoid cells (MALT) - defend from bacteria

Capillaries for nourishment of epithelium and absorption

Smooth muscle cells

Sensory nerve endings

Mucous secreting cells

What is the function of capillaries located in the layer of SI?

Nourish the epithelium and absorb digested nutrients

Which part of SI does absorption work well?

Ileum (terminal part)

What is the function of lymphoid cells of MALT (mucosa-associate lymphoid tissue)?

Critical first line of defense

Initiates local immune responses at mucosal surfaces - immune activation, pathogen destruction, immune tolerance

What is the muscularis mucosae composed of?

Elastic fibres

Thin circular layer of smooth muscle cells (inner and outer longitudinal layers)

What is the submucosa comprised of?

Thick irregular layer of connective tissues

Contains blood vessels, lymph vessels, glands and MALT tissue

Lymphoid follicles and nerve fibres that supply surrounding tissues of GI tract wall

Elastic fibres - help stomach regain shape after eating

Meissner’s plexus - intrinsic innervation

What is the structure of the muscularis externa?

Visceral smooth muscle cells (inner circular and outer longitudinal layer)

What role does ME play in digestion?

• Responsible for peristalsis and mechanical digestion

• Coordinated contractions move food along the digestive tract

• Controlled by the enteric nervous system (ENS)

• Myenteric plexus regulates muscle activity

How does ME regulated?

By ENS - where large myenteric nerve plexus lies between 2 muscle layers

How is the motility of SMC in ME coordinated?

By circuits of enteric sensory neurons, interneurons and motor neurons in the plexus

What stimulus do the sensory neurons detect to induce gut motility?

Mechanical stimuli - the presence of food causes distension (stretching) of the stomach and intestinal walls

Chemical stimuli - sensory neurons detect nutrients

What causes peristalsis? How is it initiated and coordinated?

Initiated by pacesetter cells that spontaneously depolarise

Electrical signals spread via gap junctions

Sensory neurons detect stimuli

ENS coordinates response via interneurons and motor neurons

Results is wave-like contraction of smooth muscle

Moves along he digestive tract

What does ENS consist of?

• Approx 100 million neurons derived from neural crest cells

• Complete neural reflex circuits regulating gut motility and secretory processes of the DS

How does ENS work?

• e.g. in the control of peristalsis

• Extrinsically innervated

What are the local sensory-motor reflex circuits in the ENS?

• Sensory neurons that monitor changes in the gut lumen activity e.g. distension of the gut wall, specific nutrients, pH

• Interneurons (activated by the sensory neurons) relay action potentials that in turn activate enteric motor (effector) neurons.

• Motor neurons then stimulate/inhibit a wide range of effector cells including pacesetter cells of the ME, secretory cells, submucosal blood vessels

What is the Serosa/adventitia composed of?

• n layer of connective tissue

• Covered by simple squamous epithelium - mesothelium

• Contains blood and lymph vessels

• Adipose tissue

Layers of gut wall (inner→outer)

Mucosa → submucosa → muscularis externa → Serosa/adventitia