2016_winter_lab_6 - BBio 351: Principles of Anatomy &...

BBio 351: Principles of Anatomy & Physiology I Winter 2016
Lab 6: Earthworm action potentials
created by Doug Wacker with materials provided by ADInstruments;
revised by Greg Crowther
Today’s objectives
 Make extracellular recordings of action potentials from anesthetized earthworms.
 Use these recordings to explore four core concepts of neurophysiology: threshold, “all-
or-none” responses, nerve conduction velocity, and refractory period.
Supplies/equipment
 LabChart 7.1 software
 PowerLab Data Acquisition Unit with Bio Amp
 Stimulator Cable (BNC to Alligator Clips)
 5 Lead Shielded Bio Amp Cable
 Shielded Lead Wires (3 Alligator Clips)
 Three silver wires, chlorided at one end
 Modeling clay
 Petri dish
 Millimeter ruler
 Tissues
 Blunt probe
 Dissection tray with wax or pad
 Dissection pins
 Eyedropper or Pasteur pipette
 One earthworm ( Lumbricus spp.), at least 60 mm in length
 10% ethanol in earthworm saline
Pre-lab assignment
By the start of your lab section (Thursday at 8:45am, or Thursday at 1:15pm) you should (1)
print or download this lab handout, read it over, and bring it to lab; and (2) complete the pre-lab
assignment posted to Canvas.
Background (see also sections 4.1 to 4.4 of your textbook)
A characteristic of the nervous system is that signals are conducted from place to place by
nerve impulses – action potentials that travel along a nerve fiber, or axon. All sensation,
thought, and movement is mediated by coded patterns of nerve impulses. The properties of
action potentials in single fibers are therefore of fundamental physiological importance.
Annelids, such as the common earthworm ( Lumbricus spp.), and arthropods have a ventral
nerve cord that is analogous to the dorsal spinal cord of chordates. The earthworm possesses
a medial giant nerve fiber and two lateral giant nerve fibers (that lie on either side of the medial
one) that run the length of the animal. The earthworm also has a multitude of small nerves that
do not run the length of the animal (there are about 500-1000 neurons per segment, or about
100,000 neurons altogether). Each giant fiber is actually made up of multiple large cells (one
per segment) that are electrically coupled to each other through gap junctions . This results in
the rapid conduction of action potentials from cell to cell so that each giant fiber behaves almost
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BBio 351: Principles of Anatomy & Physiology I Winter 2016