Motility In The Gi Tract Notes

Lecture 3

Motility in the GI Tract

• Regulation of GI function

  • Transmitters

    • Intrinsic NS

      • Excitatory= ACh and Substance P

      • Inhibitory= Vasoactive intestinal peptide (VIP) & Nitric Oxide (NO)

    • Extrinsic NS

      • Parasympathetic= ACh

      • Sympathetic= Noradrenaline

  • Types

    • Hormonal regulation

      • Endocrine cell A

        1. Circulating hormones (Endocrine)

        2. Via blood

        3. Act on secretory cells or even paracrine cells

      • Endocrine cell B

        1. Paracrine (locally) act on secretory cells

    • Neural regulation

      • CNS

      • Enteric nerves

      • Act directly on smooth muscle, or endocrine cell A

  • Phases of GI control

    • CEPHALIC phase

      • Sight, smell, taste and chewing

    • GASTRIC phase

      • Distension, acidity etc.

    • INTESTINAL phase

      • Distension, acidity, osmolarity etc.

• Musculature of GI tract

  • Motor activity of GI performs 3 functions

    • Non-propulsive or Segmentation movements

      • “Churning” to promote digestion

      • Most frequent type of movement in small intestine

      • Rhythmic contraction and relaxation of circular muscle layer

      • Mixes chime & brings fresh chime into contact with mucosal surface

    • Peristaltic movements

      • Relatively low frequency

      • Progressive contraction of successive sections circular muscle layer

      • Propel chime short distance, allows time for digestion/absorption

      • Contraction behind & relaxation in front of bolus of food

    • Reservoir function

      • Lower oesophageal sphincter (LES) & pyloric sphincter help function

      • Upper part stomach relaxes keeping reservoir function (churning)

      • Only food smaller than 2mm in diameter can pass through pyloric

• Properties of GI smooth muscle

  • Structure

    • Small (4-10µm wide, 50-220µm long)

    • No sarcomeres, no striation

    • Lattice like arrangement of contractors in a cell

  • Regulation of contraction in smooth muscle by calcium

    • Calmodulin bonds with Ca2+, which can then activate kinase

    • Myosin light-chain kinase activated

    • Myosin with non-phosphorylated light chain phosphorylated with help of ATP (turned to ADP), making activated proteins

  • Slow waves and the control smooth muscle activity

    • Contractile activity of SM triggered by increased intracellular Ca2+ conc.

    • How do we get Ca2+ into the cells?

      • Voltage-independent Ca2+ channels

        1. Not distinct channels

        2. Not regulated by voltage, but by other signals

      • Voltage-gated Ca2+ channels

        1. Voltage change opens channel

        2. Allows Ca2+ in, act on Ryanodine receptor

        3. “Ca2+ induced Ca2+ release” from SR

      • G-protein coupled receptors

        1. Agonist acts on receptor

        2. Intracellular signal (IP₃) formed

        3. Acts on IP₃-gated Ca2+ channel & Ca2+ released from SR

  • Organisation of GI smooth muscle

    • Single unit: Muscle fibres act together as functional unit

      • Co-ordinated fashion

      • Not all innervated (by autonomic neuron), but cells connected by gap junctions to allow excitation...

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