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Home , Autonomic nervous system, Myenteric plexus, Peristalsis

THE DIGESTIVE SYSTEM: Lab 12:

Dalay Olson Ph.D Office: Jackson Hall 3-120 Office hours: Thursday 11-1pm Salivary glands Oral cavity

Pharynx Lecture 1 Upper GI

Stomach Lecture 2 Gastric Physiology

Small intestine Today’s Lab, Lecture 3 & 4 SI, liver, pancreas and LI LEARNING OBJECTIVES

1. Identify the layers of the gut wall and describe the function of each layer 2. Explain the modulation of the enteric by the of the 3. Explain how the serves as a control center for short 4. Explain the relationship between the basal electrical rhythm (BER) in gut , the production of action potentials by additional stimuli and the production of tension in the gut muscularis. Muscularis Externa: Longitudinal muscle Circular muscle IMPORTANT GUT LAYERS

Mucosa: Epithelium Lamina propria Muscularis mucosae Serosa: Epithelium Connective Tissue : Enteric Nervous System: ORGANIZATION OF THE ENTERIC NERVOUS SYSTEM Extrinsic neuronal input NERVOUS SYSTEM BRANCHES AND THEIR EFFECTS ON MOTILITY

Enteric Nervous System (ENS) – Can serve as a control center for local reflexes – Responds to extrinsic neuronal input from SNS and PNS.

Sympathetic nervous system (SNS) – Postganglionic release – Tends to inhibit digestive activity

Parasympathetic nervous system (PNS or Vagus ) – Postganglionic neurons release – Tends to stimulate digestive activity – The is the most important parasympathetic contributor. MOTILITY SEGMENTATION Peristalsis

2 MAJOR JOBS OF INTESTINAL MUSCLE

Thoroughly mixing food molecules

Moving contents along intestine Segmentation TYPES OF MOTILITY IN THE SI

Extrinsic neuronal input • Sympathetic innervation inhibits motility in the gut. • Parasympathetic innervation activates motility in the gut. • Modulates the resting membrane potential of a pacemaker cell to inhibit or activate motility

Myogenic • Individual smooth muscle contracts in response to stretch. • Increased stretch opens mechanically gated Ca2+ channels  increased force of contractions • Independent of ENS

Intrinsic neuronal input • Mechanoreptors sense stretch and activate the ENS. Activated intrinsic enteric neurons activate smooth muscle cells to contract. • Dependent on ENS (short reflex)

Spontaneous • Contractions of the smooth muscle occur as a result of the oscillating membrane potential (BER). • Contractions occur even when food is not present in the intestine. • The (MMC) is activated in fasting individuals. • Contractions help sweep any undigested food through the digestive system. Spontaneous

PACEMAKER CELLS! SMOOTH

SMOOTH MUSCLE CELLS! EXPERIMENTAL CONDITIONS

• The 95% O2 and 5% CO2 • Krebs/Ringer Bicarbonate solution (Physiological interstitial fluid) • Experiment is running at 37˚Celsius (body temperature)

• What are we measuring? • Displacement. How much is the tissue moving. The machine isn’t calibrated so we can’t calculate force. We are only measuring a relative change in position. An increase in the displacement is an increase in contraction. Myogenic Reflex INTRINSIC REGULATORS OF THE SMALL INTESTINE

Stretch

Ca2+

Ca2+ Channel

Opens up Ca2+ channels that increase strength of contraction

Enteric Nervous System (ENS) – Can serve as a control center for local reflexes –Stretch activates sensory that increase motility through activation of the enteric nervous system. –Stretch activates smooth muscle contraction by increasing Ca2+ permeability. Myogenic Reflex SURVIVAL GUIDE

Stretch

Ca2+

Opens up Ca2+ channels that increase strength of contraction SURVIVAL GUIDE

Parasympathetic Sympathetic What does Acetylcholinesterase do? What would happen if you didn’t have AchE?

Acetylcholine activates motility SURVIVAL GUIDE

Parasympathetic Sympathetic

Epinephrine inhibits motility BREAK UNTIL PHYSOSTIGMINE IS ADDED SURVIVAL GUIDE

How does extracellular fluid composition affect depolarization? K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ Increasing extracellular [K+] depolarizes the cell by preventing K+ from exiting. SURVIVAL GUIDE

What effect would blocking the K+ channel have on membrane potential?

How would this change gut motility?