Functions of the GI tract 1. Ingestion 2. Digestion 3. Absorption 4. Egestion Motility - mechanical breakdown of food, propulsion of food through gut Secretion - secretion of enzymes, water & ions Control - of motility and secretion by nervous system and hormones

Muscularis Mucosae Gland in submucosa Serosa Submucosa Lymph node Lamina propria Villus Epithelium Longitudinal Circular

circula r circula r Longitudinal Submucosal plexus (Meissner’s) Myenteric plexus (Auerbach’s)

Form hollow tubes  not contracting against skeleton Form a syncitium - electrically coupled, joined by gap junctions  contractions synchronous Actin:myosin ratio 15:1 (skeletal muscle 2:1) Contractile elements not arranged in sarcomeres  not striated Stimulated by neurotransmitter released from varicosities Have slow wave activity 5-10  m 200  m Properties of GI smooth muscle

0 -60 Membrane potential (mV) Tension Tension Membrane potential (mV) Acetylcholine

slow waves are changes in resting membrane potential (E m ) 3-12 cycles per minute depending on area of GI tract - 3/min in stomach, 12/min small intestine always present but do not always cause contractions Frequency of contractions dictated by frequency of slow waves slow wave frequency and height modulated by - body temp & metabolic activity intrinsic & extrinsic nerves (increased by Ach, SP; decreased by noradrenaline, NO, VIP), circulating hormones (esp CCK, motilin) Slow waves in GI smooth muscle

Contraction of GI smooth muscle Out In Ca 2+ (Ca 2+ ) 4. calmodulin Inactive myosin light chain kinase Active myosin light chain kinase MUSCLE CONTRACTION Myosin -(PO 4 ) 2 Calcium activates contraction Depends on influx of calcium from extracellular space through calcium channels Calcium- calmodulin complex activates myosin light chain kinase Contraction explained by sliding filament theory

Innervation of the GI tract 1. Intrinsic nerve plexuses Located in the submucosa (submucosal or Meissner’s plexus) and between circular and longitudinal muscle layers (myenteric or Auerbach’s plexus) Control Motility - Myenteric plexus Secretion - Submucosal plexus through release of neurotransmitters Excitatory - Acetylcholine, Substance P Inhibitory - VIP, nitric oxide Excitatory - Acetylcholine

2. Extrinsic nerves Innervation of the GI tract Parasympathetic innervation - via preganglionic fibres in vagus and pelvic nerves - synapse on ganglionic neurons in enteric nervous system - excitatory through release of acetylcholine Sympathetic nervous system - postganglionic fibres from coeliac, superior and inferior mesenteric ganglia - inhibitory through release of noradrenaline

Myenteric plexus Submucosal plexus Enteric nervous system CNS Sympathetic ganglia Vagal nuclei Sacral spinal cord Preganglionic fibres Postganglionic fibres Preganglionic fibres Parasympathetic n.s Sympathetic n.s. Smooth muscle Secretory cells Blood vessels Endocrine cells

4. Noradrenaline release 5. Sympathetic stimulation more hyperpolarised smooth muscle,  Less excitable and fewer contractions StimulusEffect on muscle Effect of different stimuli on muscle contraction 1. Stretch of GI tract wall 2. Acetylcholine release 3. Parasympathetic stimulation more depolarised smooth muscle,  more excitable. Leads to action potential generation and smooth muscle contraction

The splanchnic circulation Large blood volume (30% cardiac output) Large reservoir - holds 30% blood volume under normal conditions Blood flow through splanchnic circulation controlled by many factors 1. Local nervous system (enteric NS) causes “post-prandial vasodilation through VIP release. 2. Parasympathetic stimulation causes vasodilation 3. Sympathetic stimulation on exercise adrenaline release during haemorrhage 4. Local hormone release causes vasodilation gastrin and secretin adenosine released when local pO 2 decreases cause vasoconstriction